Published by Grzegorz KOMARZYNIEC1, The Lublin University of Technology, Institute of Electrical Engineering and Electrotechnologies (1)
Abstract. The HTS transformer inrush current may lead to thermal damage its windings made of HTS 2G tapes. The parameters affecting the value and duration of the inrush current are: the impedance of the transformer windings and the impedance of the power supply line. In the case of HTS transformers, the resistance of the power supply line is the main parameter responsible for attenuation of the inrush current. The paper discusses the measurement results of the HTS transformer inrush current for two values of the power supply line resistance. The results of simulation of the HTS inrush current waveform for various impedances of the power supply line are discussed. The simulations take into account different resistance values as well as the inductance of the line.
Streszczenie. Prąd włączania transformatora HTS może powodować termiczne uszkodzenie jego uzwojeń wykonanych z taśm HTS 2G. Parametrem wpływającym na wartość i czas trwania prądu włączania są impedancja uzwojeń transformatora i impedancja sieci zasilającej. W przypadku transformatorów HTS rezystancja sieci zasilającej jest głównym parametrem odpowiedzialnym za tłumienie prądu włączania. W pracy omówiono wyniki pomiarów prądu włączania transformatora HTS dla dwóch wartości rezystancji linii zasilającej. Omówiono wyniki symulacji przebiegu fali prądu włączania transformatora HTS dla różnych wartości impedancji sieci zasilającej. W symulacjach uwzględniono różne wartości rezystancji jak i indukcyjności sieci. (Wpływ indukcyjności sieci zasilającej na prąd włączania transformatora HTS)
Słowa kluczowe: prąd włączania, transformator, nadprzewodnictwo, sieć zasilająca. Keywords: inrush current, superconductivity, power supply network.
Introduction
One of the problems associated with the operation of superconducting transformers (HTS) is the phenomenon of inrush currents occurring with sudden surges of voltage at the transformer terminals.
The basic operational problem of HTS transformers is the necessity of uninterrupted maintenance of superconducting windings (HTS) at cryogenic temperature and preventing the loss of superconducting state in them. A high inrush current with a sufficiently long duration may cause the HTS windings to move to a resistive state. A state in which the HTS windings leave the superconductivity should be treated as an emergency condition of the HTS transformer’s operation, hindering its switching on and creating a risk of possible interruption of winding continuity as a result of their thermal damage. The high density of currents in the second-generation high-temperature superconductor wires (HTS 2G) and the small area of heat exchange with the cooling medium make these conductors very susceptible to thermal damage [1] [2].
Inrush current
The problems related to the occurrence of the inrush current of HTS transformers are: high amplitude of unidirectional current impulses, long decay time of the current wave and high content of higher harmonics [3].
The first impulse of the transformer inrush current may reach values 20÷40 times higher than the value of its rated current [4][5]. High prices of superconducting winding wires impose critical values of transformer winding currents being only slightly higher than their rated currents. As such, the occurrence of the HTS transformer inrush current leads to the loss of the superconducting state of its windings a large number of cases.
The time of decay of the inrush current wave can range from several periods of supply voltage, for low power transformers, to several thousand periods for large units. It can be associated with long-term loss of the superconducting state of the transformer windings.
In case of conventional transformers, i.e. with copper or aluminum windings, the unidirectional inrush current impulses are calculated from the following dependence (1) [6]:
.
Reactance X, which is a measure of the inertia of the circuit, equals the sum of the reactance of the primary transformer winding X1 and the reactance of the power supply line Xs. The inrush current for the entire duration of its wave is damped by the constant resistance of the primary transformer winding, R1(if changes in resistance related to the heating of the winding are ignored), and the resistance of the power supply line, Rs.
.
Fig.1. The course of the inrush current impulse and changes in resistance of the HTS transformer windings during its duration, as well as the voltage of the power supply line and the magnetic flux in the transformer core; RHTS – primary winding resistance of the superconductor transformer, e – supply voltage, φ – low in the transformer core, iHTS – unidirectional inrush current impulse, Φn – core saturation flow value Φr – residual magnetic flux value at the moment of transformer switching on, Ic – the critical current of the transformer primary winding, Icw – the current at which the winding returns to the superconducting state
When analyzing the damping phenomenon of the HTS transformer inrush current, during one impulse of the inrush current, three intervals should be distinguished (Fig. 1): I – the current impulse has not exceeded the critical value of the winding current, the winding is in the superconducting state and its resistance is equal to zero (R1HTS=0 Ω), II – the current impulse has exceeded the critical value and the winding has changed to a resistive state (R1HTS>0 Ω), III – the current impulse is lower than the critical value and the winding has returned to superconductivity (R1HTS=0 Ω).
It therefore follows that in the I and III intervals, the inrush current is only damped by the resistance of the supply network Rs. In the interval II, the resistance damping the inrush current impulses is the sum of resistance of the power supply line Rs and resistance of the primary winding R1HTSin its resistive state.
Power supply line resistance has a greater impact on the value and duration of the HTS transformer inrush current than in the case of a conventional transformer. During the rise of the inrush current impulse, when the HTS transformer windings are in the superconducting state (interval I, Fig. 1), the current is only damped by the resistance of the power supply line. The value of this resistance determines whether the current impulse exceeds the critical value for the HTS winding and how long it will last.
Investigation of HTS transformer
A single-phase HTS transformer with a power of 13.8 kVA has been tested (Fig. 2) [7]. The rated voltage of the primary (HV) and secondary (LV) windings is 230 V and 60 V, respectively. The rated current of the primary (HV) winding is 60 A, and that of the secondary (LV) winding is 230 A. The nominal parameters are to be found in Table I. The transformer’s primary winding was made with the SCS4050-AP superconducting tape, with a minimum critical current of 87 A at 77 K, in the own field. The secondary winding was made with SCS12050-AP tape with a minimum critical current of 333 A. Parameters of windings are given in Table II.
The measurements were made in a circuit as shown in Figure 3. The resistance of the power supply line was being changed by including resistors of 4 mΩ and 3 Ω in the current circuit. Circuit parameters are given in Table III.
Fig.2. Superconducting transformer with power 13,8 kVA
Table I. Transformer’s nominal data
Table II. Windings parameters
.
Fig.3. Electrical circuit for measurement of the inrush current
Table III. Parameters of the power circuit
.
Fig.4. Comparison of the first impulses of the inrush current when increasing the resistance of the supply network by 4 mΩ and 3 Ω
The pulse of the first impulse of the inrush current recorded in the measurements is shown in Figure 4. With a power supply line resistance of 15 mΩ, the current impulse exceeds by 170 A the critical value of the primary winding current of 87 A. After increasing the resistance of the power line to 3.1 Ω, the first current pulse reaches 81 A and does not exceed the critical current value for the winding. At this resistance value, the inrush current disappears completely for 10th pulse (Fig. 5), i.e. after 0.18 ms, while for resistance 15 mΩ, this is only done after 4 seconds.
Fig.5. Comparison of inrush currents impulses in the interval of 0.18 ÷ 0.2 ms
Numerical analysis
The high stochasticity of the results of the measurement of characteristic parameters of the inrush current wave significantly impedes the analysis of the impact of impedance changes in the power supply line. The equations describing the waveform of the inrush current of the HTS transformer have been derived. The starting point was the general equation (2) of the circuit from Figure 3, analyzed in the intervals I, II and III given in Figure 1, taking the boundary conditions into account.
.
In the numerical analysis, good compliance with the measurement results has been obtained (Fig. 6). The relative error between the maximum value measured and the calculated value is 1.3% for the first impulse and 8.2% and 0.4% for the subsequent ones, respectively. The relative error of the duration of the first impulse is 7.6%, followed by 8.1% and 8.5% for the subsequent ones.
Fig.6. Comparison of the first three impulses of the inrush currents obtained in measurements and calculations
Fig.7. Comparison of the courses of curves describing the rising edge of the first impulse of the inrush current
Differences in the inrush current impulses obtained from the calculations, especially visible in the non-current breaks (Fig. 6) but also in the shape of pulses (Fig. 7), result from omitting the determined component of the current, i.e. the idling current of the transformer and from the omission of the influence of the shape of the magnetic hysteresis loop of the core.
Numerical analysis of impedance changes of the power supply line of the HTS transformer with the power of 13.8 kVA on the waveform of the inrush current and its individual impulses was carried out. The change in the resistance of the line has the greatest influence on the waveform of the inrush current and its individual impulses (Fig. 8 and Fig. 10). The change in the inductance of the network has a smaller influence (Fig. 9 and Fig. 11).
A change of the line resistance by +50 mΩ, -100 mΩ (at constant inductance value) has a small influence on the maximum value of the first current impulse (Fig. 8) and its duration (Fig. 10). The maximum value of the first impulse changes accordingly by +23 A, -10 A and the duration of the impulse by +0.53 ms, -0.23 ms. For almost the entire duration of this impulse, the windings of the HTS transformer are in a resistive state and their resistance plays a decisive role in damping the inrush current.
A significant influence of the power supply line resistance occurs for the second and subsequent impulses, when the HTS transformer windings are in a superconducting state for a relatively short time, or when they maintain this state all the time. The reduction of the line resistance by 100 mΩ significantly increases the maximum value of the second and subsequent impulses of the inrush current and extends the time of the current wave decay. The first three impulses then exceed the critical value of the current (87 A) of the transformer’s primary winding.
Fig.8. Comparison of the inrush current course at the change of resistance of the power supply line by +50 mΩ and -100mΩ
Fig.9. Comparison of the inrush current course at the change of inductance of the line by +200 mH and -100mH
Changing the inductance of the power supply line (at a constant resistance value) has a lesser influence on the waveform of the inrush current than the change of its resistance. The greatest effect of the change in the inductance of the line occurs for the first impulse of the inrush current and decreases for subsequent pulses. The change of the network inductance by +200 mH, -100 mH causes, respectively, a change in the maximum value of the first current impulse by +25 A, -39 A and a change in its duration by +0.29 ms, -0.19 ms (Fig. 11). Increase in the maximum value of the first current impulse and decrease the second and subsequent ones while reducing the inductance is characteristic (Fig. 9). When increasing the inductance, the effect is reversed. The reduction of the inductance of the power supply line also entails a shortening of the duration of the inrush current impulses.
Fig.10. Comparison of the first impulse of the inrush current at the change of the resistance of the line by +50 mΩ and -100mΩ
Fig.11. Comparison of the first impulse of the inrush current at the change of the line inductance by +200 mH and -100mH
Fig. 12. Comparison of the inrush current course when the line resistance changes by +50 mΩ and inductance by +200 mH, as well as -100mΩ and -100mH
Figure 12 shows the course of the first five impulses of the inrush current with change of the impedance of the power supply line for two cases: 1) when the resistance of the line was increased by +50 mΩ and inductance was increased by +200 mH, 2) the resistance and inductance of the line were simultaneously decreased by -100mΩ and – 100mH, respectively.
Summary
The impedance of the power supply line is a parameter significantly influencing the maximum value and duration of the inrush current of the HTS transformers.
During the rising and falling edge of the inrush current impulse, when the HTS transformer windings are in a superconducting state, and, therefore, when their resistance is zero, the resistance of the power supply line is the only parameter that is responsible for damping the current impulses. After exceeding the critical current of the windings, i.e after their transition to a resistive state, the resistance of the line has less influence on the current attenuation. The transformer’s winding resistance, which can reach multiple times higher values than the network resistance, has a significant influence on the inrush current. In the case of current impulses that do not exceed the critical value of the HTS winding current, and, therefore, when the transformer windings are in the superconducting state, only the resistance of the line is responsible for attenuation of the inrush current wave. At low values of the line resistance and zero resistance of HTS windings, the inrush current of the superconductor transformer can reach very long durations.
While the increase in the resistance of the power supply line entails a reduction of the maximum inrush current and the reduction of its duration, the influence of changes in the inductance of the line is more complex. Increasing the inductance of the line causes the reduction of the maximum value of the first few impulses of the HTS transformer inrush current and the increase of the maximum value of the remaining impulses. This increases the number of current impulses with a maximum value exceeding the critical value of the HTS windings. The higher inductance of the power supply line translates into a longer duration of individual impulses of the inrush current and a longer duration of its wave.
The research was conducted in scope of the project “Analysis of inrush current phenomenon and the phenomena related in superconducting transformers.” The project was financed with means of National Science Center given with the decision no. DEC- 2012/05/D/ST8/02384.
REFERENCES
[1] V. Selvamanickam, Y. Xie, „Progress in scale-up of 2G HTS wire at SuperPower,” Dept. of Energy Annual Review, Superconductivity for Electrical Systems, Arlington, VA, July 29-31, 2008. [2] Y. Xie, M. Marchevsky, X. Zhang, K. Lenseth, Y. Chen, X. Xiong, Y. Qiao, A. Rar, B. Gogia, R. Schmidt, A. Knoll, V. Selvamanickam, G. Ganesan Pethuraja, P. Dutta, „Second-generation HTS conductor design and engineering for electrical power applications,” IEEE Transactions on Applied Superconductivity, vol. 19, no. 3, June 2009. [3] R. A. Turner, K. S. Smith, „Transformer inrush currents,” IEEE Industry Applications Magazine, vol. 16, no. 5, pp. 14–19, 2010. [4] L. Prikler, G. Bánfai, G. Bán, P. Becker, „Reducing the magnetizing inrush current by means of controlled energization and de-energization of large power transformers,” Electric Power Systems Research, vol. 76, no. 8, pp. 642-649, 2006. [5] M. Steurer, K. Frohlich, „The impact of inrush currents on the mechanical stress of high voltage power transformer coils,” IEEE Transactions on Power Delivery, vol. 17, no. 1, pp. 155-160, August 2002. [6] T. R. Specht, „Transformer inrush and rectifier transient currents,” IEEE Transactions on Power Apparatus and Systems, vol. PAS-88, iss. 4, pp. 269-276, April 1969. [7] G. Komarzyniec, „14 kVA superconducting transformer with (RE)BCO windings transformers,” 2017 International Conference on Electromagnetic Devices and Processes in Environment Protection with Seminar Applications of Superconductors (ELMECO & AoS), IEEE Conferences, s. 1–4, Nałęczów, 3–6 grudnia 2017.
Authors: Grzegorz Komarzyniec, PhD, e-mail: g.komarzyniec@pollub.pl, Lublin University of Technology, Institute of Electrical Engineering and Electrotechnologies, Nadbystrzycka 38a, 20-618 Lublin,
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 94 NR 12/2018. doi:10.15199/48.2018.12.55
Published by Piotr GAJEWSKI, Wrocław University of Science and Technology, Department of Electrical Machines, Drives and Measurements
Abstract. The paper presents the improved control strategy for Wind Energy Conversion System (WECS) during voltage sags. The considered back-to-back converter system includes: Ma-chine Side Converter (MSC) and Grid Side Converter (GSC) with control circuits. To the wind turbine the direct driven Permanent Magnet Synchronous Generator (PMSG) has been connected. In the control of MSC the modified Direct Torque Control (DTC) has been applied. The modified Direct Power Control (DPC) for control of GSC has been used. Under the voltage sags, the control circuits of MSC and GSC are forced to fulfil the Low-Voltage Ride Through (LVRT) requirements. The constant DC link voltage during grid faults is achieved by storing the surplus active power in the mechanical system inertia of the wind turbine. The effectiveness of considered control methods have been tested by simulation studies during unsymmetrical voltage sags. The obtained simulation studies confirmed good performance of applied control methods. The application of positive sequence components of grid voltage vector allows to reduce influence of unsymmetrical voltage sags.
Streszczenie. W artykule przedstawiono zmodyfikowaną strategie sterowania przekształtnikowym układem elektrowni wiatrowej podczas zapadu napięcia sieci AC. Przekształtnikowy układ elektrowni wiatrowej składa się z: Przekształtnika Maszynowego (PM), Przekształtnika Sieciowego (PS) oraz układów sterowania. Do turbiny wiatrowej został przyłączony bezprzekładniowy generator synchroniczny o magnesach trwałych (PMSG). Do sterowania PM zastosowano zmodyfikowaną metodę bezpośredniego sterowania momentem generatora (DTC). Do sterowania PS zastosowano zmodyfikowaną metodę bezpośredniego sterowania mocą (DPC). Podczas występowania zapadów napięcia sieci AC zmodyfikowano układy sterowania PM i PS w celu spełnienia wymagań LVRT. W celu utrzymania stałego napięcia w obwodzie pośredniczącym DC zastosowano metodę pozwalająca na zgromadzonej nadwyżki energii kinetycznej w łopatach turbiny wiatrowej. W celu potwierdzenia dużej skuteczności rozpatrywanych metod sterowania przeprowadzono badania symulacyjne. Uzyskane wyniki badań symulacyjnych potwierdzają dużą skuteczność zaproponowanych metod sterowania. (Zmodyfikowana strategia sterowania przekształtnikowym układem elektrowni wiatrowej z generatorem PMSG podczas zapadów napięcia sieci AC).
Keywords: wind turbine, PMSG, DTC, DPC, simulation studies. Słowa kluczowe: turbina wiatrowa, PMSG, DTC, DPC, badania symulacyjne.
Introduction
Wind energy has become one of the largest and the fastest growing renewable energy sources because of its large reserves and non-pollution effects [1, 2]. In this growing trend, the influence of WECS to the AC grid become very significant. Therefore, the Grid Connection Requirements (GCR) should be updated gradually for power systems operators [3, 4]. Nowadays, the requirements on the power quality are higher than before [4]. The power growing of installed Wind Energy Conversion System (WECS) and the increased requirements of GCR for connecting wind turbines to the distribution systems enforce the analysis for possible influences of faults of WECS during voltage sags [5, 6]. According to the standards, the voltage sags can be defined as the sudden temporary reductions of the RMS (Root Mean Square) voltage magnitude at a point of electrical system [7]. The voltage sags may arise from huge currents caused by many grid faults, including connection of large loads or grid shortcircuits [8, 9]. During the voltage sags, the shutdown of wind farm may have a significant effect on the operation of distribution system. For this reason, the new rules of GCR should be determined. This Low-Voltage Ride Through (LVRT) requirements determine the behaviour of WECS during the voltage sags. Therefore, it is important to investigate a suitable method to enhanced the LVRT capability of WECS with direct-driven permanent magnet synchronous generator (PMSG) [10, 11]. The configuration of WECS with PMSG and full-scale back-to-back converter system has been shown in Figure 1. The presented configuration consists of: wind turbine, PMSG generator, Machine Side Converter (MSC), Grid Side Converters (GSC), grid filter and control circuits. The AC side of MSC is connected to the stator of direct driven PMSG. As results of low speed of PMSG generator, the gearbox usually is not applied. In the recent years, in the WECS the gearbox is characterized as most a faulty element of the system [11, 12]. The AC side of GSC is connected through the L (Lowpass filter) to the AC grid [13, 14]. The main function of MSC control is to extract the maximum power of wind turbine and to control of PMSG. The MSC controls the torque and the reactive power of PMSG [14]. The GSC control the DC link voltage and control the active and reactive power delivered to the AC grid [15, 16]. During voltage sags, operation of control of the converters can be disturbed. The voltage sags have harmful influence of WECS.
Fig.1. Wind turbine with PMSG and back-to-back converter system
To avoid disturbed operation of WECS during voltage sags the LVRT requirements have been developed [17]. The LVRT requirements determine, that the WECS during voltage sags should be remain connected to the distribution system for specified time. The time of stay connected of wind turbine system can be illustrated in LVRT characteristic. The characteristic of LVRT requirements for WECS connected to the distribution system under voltage sags has been presented in Figure 2 [18, 19].
Fig.2. Characteristic of typical operation regions of WECS during LVRT
The presented characteristic defines two operating regions of WECS during voltage sags. The I region assumes of specific period, when WECS allows to stay connected to the system under voltage sags, therefore the II region concerns of specific period of disconnection of wind turbine system for distributed system. During voltage sags, when the voltage will not cross line below the minimal voltage dedicated by the line and this voltage will return to 90% on rated voltage within 1s, WECS must stay connected to the system. However, when voltage sags occurred in II region, the WECS should be disconnected from system. The disconnection of WECS during operation in II region, may have negative effect on the distribution system. The specification of periods of connection and disconnection of WECS during voltage sags can be different for individual countries [21].
The current LVRT rules also required, that the WECS beside the remain connected to the distribution system during voltage sag, should also support distribution system by delivery of the reactive power [17, 18].
The value of reactive power delivery to the AC grid is mostly determined by depth of voltage sags. In the Figure 3 the characteristic of Reactive Current Injection (RCI) by WECS under voltage sags has been shown [4, 7, 17].
Fig.3. Characteristic of demanded reactive current delivered to AC grid
The value of the reactive current delivered to the AC grid can be designated directly in agreement with on the base of the characteristic presented in Fig. 3. The slope of the injected current curve is determined by different operators on distribution power system [4, 17].
The aim of this article is to carry on study of the WECS behaviour during the voltage sags. The influence of unsymmetrical voltage sags for operation of WECS has been analysed. In the control scheme of WECS, the novelty control algorithms have been applied in order to meet LVRT requirements with high accuracy.
Wind turbine model
The mathematical relation for the amount mechanical power Pt extraction by wind turbine can be expressed as follows [5, 10]:
.
where: ρ – air density; A=πR2 – area swept by the rotor blades; R – radius of the turbine blade; Cp – power coefficient of the wind turbine; λ – tip speed ratio; β – blade pitch angle; vw – wind speed.
The wind turbine power coefficient Cp is a nonlinear function of tip speed ratio λ and blade pitch angle β. The tip speed ratio can be shown as [6]:
.
The Figure 4 shows the power coefficient Cp as the function of tip speed ratio λ and blade pitch angle β. As it can be noticed, for each value of angle β the optimal tip speed ratio λopt exists at which the power coefficient Cphas the maximum value Cpmax.
Fig.4. Power coefficient Cp as function of tip speed ratio λ and pitch angle β
The mechanical torque Ttof wind turbine can be described as:
.
The characteristics of the wind turbine operating at various wind speeds have been shown in Figure 5 [6]. The presented wind power characteristics represent the wind turbine power curves as function of rotor angular speed ωm at various wind speeds vw. According to Figure 5, it can be determined, that for each wind speed, the maximum power point can be achieved. This operation of Maximum Power Point Tracking (MPPT) is achieved, during wind turbine will be operating at optimal rotor angular speed ωopt.
Fig.5. Characteristic of output turbine power versus rotor angular speed at various wind speeds
When wind speed exceeds the rated wind speed, the power of wind turbine should be reduced by application of pitch angle control or stall and active stall control algorithm [10].
Permanent magnet synchronous generator model
In order to formulation developed the mathematical model of PMSG the following assumptions have been included [10]. The typical assumptions for modelling of PMSG have been presented in the literature [10, 14]. The mathematical model of PMSG is considered in synchronous rotating reference frame. The d axis is aligned with the direction of the rotor flux and the q axis is 90 ahead. The mathematical equations of the PMSG in dq frame can be described as follows [6, 10, 12]:
.
.
where: vsd, vsq – components of the stator voltage vector; isd, isq – components of the stator current vector; Ld, Lq– direct and quadrature stator inductances; Rs – stator resistance; ψPM – flux linkage established by the permanent magnets; np – number of pole pairs; ωe, ωm – electrical and mechanical angular speed of the PMSG rotor. The electromagnetic torque of PMSG can be expressed as:
.
When considering the assumption of equal inductances Ld=Lq=Ls, the torque equation can be presented in the form:
.
The dynamics equation of mechanical system with wind turbine and PMSG is formulated as:
.
where: J – the equivalent inertia, Kf – the coefficient of viscous friction.
Control of machine side converter
In the control scheme of MSC the Direct Torque Control (DTC) has been used. The operation of DTC is based on directly selecting the appropriate stator voltages vectors according to the differences between the reference and the actual values of magnitude of the stator flux vector and electromagnetic torque. In the regular DTC control is realize with three control loops. The outer control loop regulates the angular rotor speed of PMSG. Two inner control loops are responsible for control of the magnitude ψs of stator flux vector and the electromagnetic torque Te of PMSG. During voltage sags, the conventional DTC will not ensure the proper operation of WECS. Therefore, to achieve the LVRT condition, the operation of DTC should be modified.
In Figure 6 the modified control scheme of Direct Torque Control (DTC) of MSC has been presented. The operation of control scheme of MSC can be divided into: the normal operation and the operation during voltage sags.
Fig.6. Control scheme of Direct Torque Control of MSC
During the normal operation the MSC control scheme is focused to achieve the maximum power from the wind. For this reason, the Maximum Power Point Tracking (MPPT) algorithm should be applied. During the operation at voltage sags, the control scheme of MSC is focused to fulfil the LVRT conditions. When the voltage sag has been detected, the change in the control scheme of MSC is forced in order to meet requirements.
In normal operation of WECS, the control scheme of MSC consists of three control loops with PI controllers. The outer control loop regulates the generator speed to track the optimum speed of wind turbine. The optimum speed ωmoptof wind turbine is obtained according to MPPT algorithm [6, 10]:
.
The MPPT technique has been used in order to obtain the maximum wind turbine mechanical power. The reference speed ωmopt is compared with measured ωm of PMSG. The error signal is sent to the PI controller. The output signal of PI controller determines the reference electromagnetic torque Te* of PMSG.
The inner control loops regulate the magnitude of stator flux vector ψs and the electromagnetic torque Te of PMSG. For an estimation of the magnitude of stator flux vector and the value of electromagnetic torque, several techniques have been found in literature [12].
The magnitude of stator flux ψsand electromagnetic torque Te of PMSG are compared with their reference values and are sent to PI controllers.
The outputs of PI controllers determine the reference components vsd *, vsq * of the stator voltage vector. These both signals are transformed to the reference components vsα * and vsβ * of the stator voltage vector for SVM control of MSC. The SVM block generates the required switching signals for MSC.
During operation, when voltage sag occurs, the control objective of MSC and GSC is switched. According to the LVRT requirements, during grid faults the operation control of MSC and GSC is focused to reduce the delivered active power to the AC grid. The reduction of active power allows to avoid the sustaining grid faults [4, 7, 12]. For this reason, the delivered active power by GSC and MSC should be reduced to the zero. During to the grid voltage sags, the maximum active power injected to the AC grid is reduced in proportion to the terminal voltage reduction and also can be limited by LVRT requirements [12].
When the grid faults have been identified, the “LVRT signal” generated by voltage detector is sent to switching control block “SB1” of MSC control. The application of switching block SB1 allows to change the control priority of MSC. The control loop of PMSG generator speed is disconnected. Instead of speed control loop of PMSG generator, the value of reference power pg * calculated by GSC scheme is delivered to the control of MSC. During the switching control loop, the regulation of DC link voltage vdc is realized by MSC control scheme. During the activation of this control operation, the value of PMSG electromagnetic torque is forced to near zero. This condition ensures, that the DC link voltage will be regulated at reference value. The reduction of electromagnetic torque of PMSG generator allows to reduce the PMSG power delivered to the MSC converter. The reduction of PMSG power by MSC control scheme, will cause the increase of the kinetic energy of inertia of WECS mechanical system [2, 14]. This increase will cause the increase of mechanical angular speed of wind turbine and PMSG. The final value of the increased speed of PMSG and wind turbine can be established from the equation of dynamic power Pd in the mechanical system [4, 7]:
.
where: Pg – generator power; Pt – wind turbine power. The reduction of Pgpower without reducing the Pt power will cause the rise of surplus power in the system. This surplus power will cause the increase of the angular rotor speed of generator from ωm to ωmk which can be found as [7]:
.
where: ωmk– the angular rotor speed of PMSG at the final instant of time of voltage sag; Jz– total inertia of mechanical system of wind turbine; tf – duration of voltage sag.
The value of angular rotor speed ωmk of the PMSG at the final moment of duration of the voltage sag at the condition of constant power Pd, can be expressed as:
.
The values of total wind turbine inertias Jz in typical designs of WECS are very high. The speed changes caused by the switching of the control system will be slow and small.
Control of grid side converter
In Figure 7 the control scheme of GSC has been presented. The operation of GSC can be divided into two control modes: the mode of normal operation and the mode of operation during voltage sags.
During the normal operation, the main control objective of GSC is to control the delivered power to the AC grid and control DC link voltage vdc [20]. In the mode of operation during voltage sags, the control aim of GSC is to reduce the delivered power to the AC grid and support the AC grid by injection of reactive current [4, 17].
The improved Direct Power Control (DPC) for GSC has been applied. In the mode of normal operation, the control scheme of GSC consists of three control loops with PI controllers. The outer control loop regulates the DC link voltage. The reference voltage vdc * is compared with measured DC link voltage vdc. The signal error is sent to the PI controller. Grid Side
Fig.7. Control scheme of Direct Power Control of GSC
The signal value from PI controller designates the reference component idg * of the grid current vector. The value of idg* is multiplied by measured DC link voltage vdc in order to obtain the reference active power pg * of AC grid. Two inner control loops regulate the instantaneous active power pg and reactive power qg. The instantaneous active power pg and reactive power qg in the stationary αβ frame can be estimated as follows [10, 15]:
.
.
where: vgα, vgβ – components of the grid voltage vector; igα, igβ – components of the grid current vector. The reference active power pg * is compared with estimated power pg of AC grid. The error signal is sent to the PI controller which determines the reference voltage vgcd * of GSC. The second inner control loop regulates the instantaneous reactive power qgof AC grid. The output of PI controller determines the reference voltage vgcd * of GSC. The reference reactive power qg* is compared with estimated reactive power qg of AC grid. In the mode of normal operation, the reference instantaneous reactive power is forced as zero qg *=0 to achieve the operation at the unit power factor. The error signal is sent to PI controller. The output of PI controller determines the reference voltage vgcq* of GSC. The obtained converter reference voltages vgcd *, vgcq* are then transformed to the stationary αβ system. The obtained converter reference voltages vgcα *, vgcβ * are sent to the block of Space Vector Modulation (SVM). The SVM block determines the reference switching signals for GSC.
When the voltage sags occur, the voltage detector generates the “LVRT fault signal”. This LVRT fault signal is delivered to the switch blocks: “SB2” and “SB3”. The LVRT signal delivered to the switch blocks enforce the change of the control scheme of GSC. The main priority of GSC is to meet the LVRT requirements. It means, that during the voltage sags, the GSC should ensure the proper control of instantaneous active power pg and reactive qgpower according to LVRT requirements.
During the voltage sags, the control loop of DC link voltage is detached and then is attached to the MSC instead of speed control loop. The reference power pg* is sent to the control loop of MSC. In the literature the many different techniques can be described [7, 17]. In this article, according to LVRT demand it is assumed, that during voltage dips, the instantaneous active power should be enforced to be zero pgLIVRT*=0. It is also assumed, that the reactive power is delivered to the grid with accordance with RCI requirements [7].
Voltage dips occurred in AC grid can be divided into symmetrical and asymmetrical. During symmetrical and asymmetrical voltage sags, the positive, negative and zero sequence components occur in the system. In the literature, different control strategies for unsymmetrical voltage sags can be found [18]. During unsymmetrical voltage sags, it is necessary to use of symmetrical components in the control of GSC. The use of symmetrical components in the control of GSC, allows to avoid the consequence of unsymmetrical voltages sags, appearing with double oscillations in waveforms of instantaneous active and reactive power [7, 18]. In order to use of symmetrical components in the control scheme of GSC, the appropriate synchronization system should be applied. Typically, in the control scheme of DPC the angle of θg is determined by the Synchronous Reference Frame – Phase Locked Loop (SRF-PLL) block. However, the operation of SRF-PLL have only good properties during symmetrical grid voltage dips [15, 18]. When the unsymmetrical voltage occurs, the SRF-PLL it is not enough to obtain the proper grid angle of θg. In order to determine the proper grid angle θg it is necessary to use the improved SRF-PLL. In the literature different techniques are presented in order to independent harmful effect of unsymmetrical voltage dips. The one of the proposed solutions is the Double Decoupled Synchronous Reference Frame PLL (DDSRF-PLL). The DDSRF-PLL during unsymmetrical voltage sags, defines as unbalanced voltagevector, which is consisting of: positive and negative sequence components. The d axis of the synchronous reference frame has been aligned with the positive sequence vector components of the grid voltage (vqg +=0). This means, that the only positive-sequence current circulates through the L filter. The power controllers are implemented only for the positive sequence. The detailed description of applied DDSRF-PLL can be found in literature [16, 18].
In many works, it has been shown that an asymmetric voltage sags, may also have an influence on the waveforms of DC link voltage. For that reason, in the control scheme, the application of double harmonic oscillator filter for measured voltage is added. The use of this symmetrical components filter allows to avoid the double oscillation in waveforms of DC link voltages and also allows to reduce the electromagnetic torque ripple during switching control of MSC and GSC. In order to obtain better achievement of control circuit of GSC, the negative sequences of grid voltages vga -, vgb – are fed-forward to the reference positive sequence vgca +, vgcb+ of GSC voltages.
Simulation results
The simulation results were conducted for WECS with PMSG. The simulation model of WECS with considered control circuits has been developed in MATLAB/Simulink using SimPowerSystem. The data of WECS used in simulation are presented as follows: rated power of wind turbine: Pt=20 kW; blade radius R = 4.4 m; air density ρ=1.225 kg/m3 and for 3-phase PMSG data and parameters: rated power Pg=20 kW; stator rated phase current Isn=35.1 A; rated speed nn= 210 rpm; stator resistance Rs= 0.1764 Ω; stator dq-axis inductance Ld=Lq=4.48 mH. The chosen simulation results of considered WECS are presented in Figures 8-9. In simulation it is assumed, that the considered WECS has been tested during the voltage sags in fixed wind speed.
The simulation results of control of GSC during low voltage sag are presented in Figure 8 The simulation results of control of MSC during low voltage sag are presented in Figure 9. In the Figure 8 the three phase grid voltages vgabc have been presented. It was assumed, that the voltage drops took place only in one phase.
The voltage drop of the voltage is equal to 50% of the reference voltage before the voltage sag. The voltage sag occurred at 0.15s and then in the 0.3s the grid voltage is starting recovery. In the Figure 8b the three phase grid currents igabc during voltage sag have been shown. It can be determined, that during unsymmetrical voltage sags, the applied control strategy allows to keep sinusoidal waveforms of grid phase currents. The obtained waveforms of grid currents igabc confirm the good performance of applied positive sequence control methods of GSC.
The waveforms oscillating instantaneous active pg and reactive qg power control results have been presented in Figure 8c. During the normal operation of WECS, the instantaneous reactive power is forced to zero qg=0. In this strategy, only the instantaneous active power is delivered to the AC grid and the operation at unity power factor has been achieved. However, when voltage sags have been detected, the priority of control has been reversed. The control strategy of WECS should fulfil the LVRT requirements. It means, that during the low voltage sag, the delivery of instantaneous active power is limited and is set to zero pgLVRT *=0 and instead of this the instantaneous reactive power qgLVTR * is delivered to the AC grid. In the Figure 8d the grid current vector components igd, igq have been presented.
The oscillations in waveforms of grid current vector igd, igq components have been eliminated by application of control scheme with positive sequence components.
.
Fig.8. Waveforms of: a) grid phase voltages vgabc; b) grid phase currents igabc; c) instantaneous active and reactive power pg, qg; d) grid current vector components igd, igq; e) DC link voltages vdc; f) grid phase angle ϴg
The waveforms of DC link voltage vdc has been presented in Figure 8e. When the voltage sag is occurred, the DC link voltage is regulated by the MSC. In Figure 8f the waveforms of angle position θg of grid voltage vector, obtained from DDSRF have been illustrated. The application of DDSRF allows to determine the proper angle of grid voltage vector during unsymmetrical voltage sags.
In the Figure 9 the obtained results during voltage sags of MSC have been presented. In the Figure 9a the waveforms of reference speed ωopt and measured speed of generator ωm have been presented. During normal operation the measured speed tracks accurately the reference speed. In the control system, the reference speed is determined by the MPPT algorithm. However, during the voltage sags, the measured speed increased. The increasing speed of generator is caused by storage energy in inertia of wind turbine system [2, 3]. Due to reduction of electromagnetic torque to the zero, as consequence there is a torque mismatch in the mechanical system, which causes the speed to increase.
Fig.9. Waveforms of: a) reference speed ωopt and measured speed ωm of PMSG; b) electromagnetic torque Te of PMSG; c) trajectory of stator flux vector; d) magnitude of stator flux vector; e) real wind speed vw; f) tip speed ratio λ; g) power coefficient Cp of wind turbine
Figure 9b presents the responses of electromagnetic torque Te of PMSG. From this Figure, it can be noticed, that the electromagnetic torque Te during voltage sag is forced to zero, in order to reduce the PMSG power. This reduction of electromagnetic torque Te allows to keep the DC link voltage in the reference value and allows to keep balance the power between MSC and GSC.
The Figure 9c presents the waveforms of the magnitude ψsof stator flux vector and Figure 9d presents the trajectory of stator flux vector. From Figure 9c, it can be noticed that the stator flux vector rotates with a constant magnitude.
The reference wind speed trajectory is shown in Figure 9d. The waveforms of tip speed ratio λ and power coefficient Cp have been presented in Figure 9e-9f. From this Figure, it can be observed, that during normal operation of WECS the maximum power is obtained. During voltage sags, when the control of MSC and GSC are switched, it can be noticed, that the value of tip speed ratio is increasing while the power coefficient of wind turbine is decreasing.
After recovery voltage to the reference value, the MSC and GSC returns to the control of normal operation. The control objective of MSC is control of PMSG speed and obtain the maximum power. For GSC objective is control of DC link voltage and regulate the power.
Conclusions
In this paper the operation of WECS during the voltages sags has been considered. In the control of wind turbine system, the improved schemes of Direct Torque Control and Direct Power Control methods have been applied. The DTC with MPPT algorithm during normal operation of WECS allows to obtain the maximum power of wind turbine. The applied DPC control of GSC in normal operation ensures to regulates DC link voltage and adjust the active and reactive power of the system.
Under voltage sag, the priority of control methods of MSC and GSC has been changed. For regulation of PMSG power and DC link voltage is responsible MSC. The control method of GSC is focused to reduce the delivered power to AC grid and for injection of reactive power to the AC grid in order to meet the LVRT requirements. The application of switch control loops of DTC and DPC, during grid faults, allows to store the surplus energy in the rotor inertia of wind turbine.
The application of DDSRF-PLL allows to obtain the appropriate angular voltage vector position during the symmetrical and unsymmetrical voltage sags. To avoid oscillation in waveforms of DC link voltage and waveforms grid currents caused by the unsymmetrical voltage disturbances, the positive sequence components is only used in the control scheme of GSC. The proposed control methods have been verified by simulation studies.
REFERENCES
[1] Abdelrahem M., Mobarak M.H., Kennel R., Realization of low-voltage ride through requirements for PMSGs in wind turbines systems using generator-rotor inertia, International Conference on Electrical and Computer Engineering (ICECE), Dhaka, pp. 54-57 (2016). [2] Nasiri M., Milimonfared J., Fathi S.H., A review of low-voltage ride-through enhancement methods for permanent magnet synchronous generator based wind turbines, Renewable and Sustainable Energy Review, no. 47, pp. 399-415 (2015). [3] Ibrahim R. A., Hamad M. S., Dessouky Y. G., Williams B.W., A review on recent low voltage ride-through solutions for PMSG wind turbine, International Symposium on Power Electronics, Electrical Drives, Automation and Motion, Sorrento, pp. 265- 270 (2012). [4] Gajewski P., Pieńkowski K., Control of wind turbine system with PMSG for low voltage ride through, International Symposium on Electrical Machines, IEEE Xplore, SME, pp. 1-6 (2018). [5] Muyeen S.M., Takahashi R., Murata T., Tamura J., A variable speed wind turbine control strategy to meet wind farms grid code requirements, IEEE Transactions on Power Systems, vol.25, no. 1, pp. 331-340 (2010). [6] Ki-Hong K., Yoon-Cheul J., Dong-Choon L., Heung-Geun K., LVRT scheme of PMSG wind power systems based on feedback linearization, IEEE Transactions on Power Electronics, vol. 27, no. 5, pp. 2376-2384 (2012). [7] Alepuz S., Calle A., Busquets-Monge S., Kouro S., Wu B., Use of stored energy in PMSG rotor inertia for low-voltage ride-through in back-to-back NPC converter-based wind power systems, IEEE Transactions on Industrial Electronics, vol. 60, no. 5, pp. 1787-1796 (2013). [8] Zheng X., Liu Y., Liu Z., Li Y., Wang C., Feng Y., Coordinating control method to improve LVRT ability of PMSG, IEEE Conference on Industrial Electronics and Applications (ICIEA), Wuhan, pp. 1461-1465 (2018). [9] Yan Z., Mingliang C., Zhen X., Li Xu., Wang W., An experimental system for LVRT of direct-drive PMSG wind generation system, IEEE 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia), Hefei, pp. 1452-1456 (2016). [10] Gajewski P., Analysis of power converter system of wind turbine with permanent magnet synchronous generator, PhD thesis, Wrocław University of Science and Technology, (2018). [11] Muyeen S.M., Takahashi R., Murata T., Tamura J., A variable speed wind turbine control strategy to meet wind farms grid code requirements, IEEE Transactions on Power Systems, vol. 25, no. 1, pp. 331-340 (2010). [12] Dey P., Datta M., Fernando N., Senjyu T., Fuzzy-based Coordinated Control to Reduce DC-link Overvoltage of a PMSG based Wind Energy Systems during Grid Faults, International Conference on Electric Power and Energy Conversion Systems (EPECS), Kitakyushu, Japan, pp. 1-6 (2018). [13] Muyeen S.M., Takahashi R., Murata T., Tamura J., Low voltage ride through capability enhancement of fixed speed wind generator, IEEE Bucharest PowerTech, pp. 1-6 (2009). [14] Gajewski P., Pieńkowski K., Advanced control of direct-driven PMSG generator in wind turbine system, Archives of Electrical Engineering, vol. 65, no. 4, pp. 643-656 (2016). [15] Zielonka P., Jasiński M., Bobrowska-Rafał M., Sikorski A., Sterowanie przekształtnika sieciowego AC-DC podczas zapadów w sieci elektroenergetycznej, Przegląd Elektrotechniczny, R.87, nr 6/2011, pp. 79-84 (2011). [16] Rodriguez P., Pou J., Bergas J., Candela J. I., Burgos R. P., Boroyevich D., Decoupled Double Synchronous Reference Frame PLL for Power Converters Control, IEEE Transactions on Power Electronics, vol. 22, no. 2, pp. 584-592 (2007). [17] Jarzyna W., Lipnicki P., The comparison of Polish grid codes to certain European standards and resultant differences for WWP requirements, EPE JOINT Wind Energy and T&D Chapters Seminar, pp. 1-6 (2012). [18] Jarzyna W., Zielinski D., The impact of converter’s synchronization during FRT voltage recovery in two-phase short circuits, Selected Problems of Electrical Engineering and Electronics (WZEE), Kielce, pp. 1-6 (2015). [19] Rizo M., Rodríguez A., Bueno E., Rodríguez F. J., Girón C., Low voltage ride-through of wind turbine based on interior Permanent Magnet Synchronous Generators sensorless vector controlled, IEEE Energy Conversion Congress and Exposition, Atlanta, pp. 2507-2514 (2010). [20] Jasiński M., Kaźmierkowski M.P., Bobrowska M., Okoń P., Control of AC-DC-AC converter under unbalanced and distorted input conditions, Power Quality, Alternative Energy and Distributed System, pp. 139 – 145 (2009) [21] Nguyen T. H., Lee D., Song S., Kim E., Improvement of power quality for PMSG wind turbine systems, IEEE Energy Conversion Congress and Exposition, Atlanta, GA, pp. 2763- 2770 (2010).
Author: Piotr Gajewski Ph.D., Wrocław University of Science and Technology, Department of Electrical Machines, Drives and Measurements, ul. Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, piotr.gajewski@pwr.edu.pl
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 96 NR 4/2020. doi:10.15199/48.2020.04.26
Published by Alex Roderick, EE Power – Technical Articles: What are Overcurrent Protection Devices?, February 10, 2021.
Learn about the basics of overcurrent, how it occurs, and ways to protect against and prevent it in power circuits.
To have a properly operating circuit, current flow should be confined to a safe level. This safe level of current is determined by the current handling capability of the load, conductors, switches, and other system components. Under normal operating conditions, the current in a circuit should be equal to or less than the normal current level. However, at times an electrical circuit may have a higher-than-normal current flow (overcurrent).
What is Overcurrent?
An overcurrent is a condition that exists in an electrical circuit when the normal load current is exceeded. An overcurrent condition can be caused by a short circuit or overload situation.
Short Circuits
With a short circuit situation, the current takes a shortcut around the normal path of current flow.
Although a partial short can increase the current level, it may or may not cause damage depending on the ratings of the circuit components. However, with a dead short, the resistance of the load will be completely removed from the normal current path. This is illustrated in Figures 1a and 1b.
Figure 1a. A partial short circuit.
Figure 1b. A dead short circuit.
If the source has enough stored energy when a dead short occurs, circuit components can be damaged or explode. Switches can melt or vaporize, conductors can overheat, and the insulation can burn off. It can also damage the power source.
Fires that result in a loss of property and life can occur due to the temperatures generated by a partial or dead short. With so much at stake, all circuits must be protected against short circuit situations.
Overloads
An overcurrent condition can also be caused by an overload situation. For example, consider a situation where too many loads are connected to a given power source. Even with each of these individual loads drawing their normal current, the overall current can exceed the rated value of the source.
If an overload only lasts for a brief time, the temperature rise is minimal and has little or no effect on the equipment or conductors. Sustained overloads, however, are destructive and must be prevented.
Unlike short circuits, overloads do not cause a sudden arc and the system might survive an overload situation even if we do not remove it from the system immediately. Though, over an extended period of time, overloads may cause a fire by overheating the equipment and conductors.
Figure 2 depicts an overloaded circuit. In this case, the rated current capacity of the branch is 15 A; however, the sum of the currents drawn by the parallel loads is 17 A. The circuit is overloaded by 2 A and, as a result, the breaker trips.
Figure 2. Overloaded circuit.
Overcurrent Protection Circuit
The resistance of a fuse or circuit breaker is very low and usually an insignificant part of the total circuit resistance. Under normal circuit operation, it simply functions as a conductor.
Fuses and circuit breakers are both connected in series with the circuit they protect. In general, these overcurrent devices must be installed at the point where the conductor being protected receives its power; for example, at the beginning of a branch circuit, as illustrated in Figure 3.
Figure 3. Connection of overcurrent protection device.
In the event of an overcurrent situation, fuses will blow or circuit breakers will trip. Although these devices protect the circuit against overcurrent conditions, they only open the circuit and disconnect the supply of electricity. They are not normally capable of correcting the problem. For this reason, we’ll need to locate and correct the problem before replacing a fuse or resetting a circuit breaker.
Common Overcurrent Protection Devices (OCPDs)
An overcurrent protection device (OCPD) is a piece of electrical equipment used to protect service, feeder, and branch circuits and equipment from excess current by interrupting the flow of current.
Overcurrent protection simply means a fuse, breaker, or fusible link is used to protect the equipment, a circuit in the equipment, or the equipment’s wiring. These terms are often used interchangeably because they have some similarities. Breakers or fuses are normally used to protect the whole unit from excessive current, but they can be sized to protect one component in the unit. This provides overcurrent protection for the unit and offers optional protection for components like the transformer or circuit board.
Figure 4 shows two common fuses used in a control circuit board: the plug-in fuse and the glass (Buss) fuse. These types of fuses can also be found on the secondary side of a transformer.
Figure 4. Plug-in fuses are used to protect a circuit board from overcurrent conditions. A glass fuse can be used as a plug-in fuse or in a fuse holder. (Penny included for size reference.)
Figure 5 shows a circuit board with the plug-in U-type fuse.
Figure 5. This is a circuit board for an air handler with an option for electric heat strips. Notice the 3 A plug-in fuse located at the upper left side of the circuit board.
Breakers or fuses of the correct amperage and voltage rating should be within easy access of the heating system. Typically, the breaker is the same rating as the maximum amperage listed on the nameplate of the electrical heating unit.
The installing contractor may need to analyze the amperage values of an installation to apply the correct size breaker. In some instances, a breaker of 115% of the unit’s “minimum” amperage may be specified.
An excessively large breaker should not be used. A breaker is designed to protect the equipment and wire. A breaker of too much amperage will not turn off the electrical supply in the event of an overcurrent draw. A breaker that is too small will turn off the power before the maximum current is drawn by the unit.
Fusible Links
A fusible link (see Figure 6) is often wired in series with an electrical heating element. The purpose of the link is to open when either high amperage or high heat is encountered.
Figure 6. This common fusible link is found in series with a heating circuit.
The fusible link cannot be reset and must be replaced if open. The cylinder is silver and has manufacturer information printed on it. The information may include temperature and amperage ratings. The cylindrical device has one square end and one tapered end. The taper may be black or red, depending on the color of the material used in its manufacture. The link resistance can be checked to determine if it is open (it should exhibit a resistance of zero ohms).
OCPDs Ratings
Fuses and circuit breakers are rated for both current and voltage.
Continuous-Current Rating
The continuous-current rating marked on the fuse or circuit breaker represents the maximum amount of current the device will carry without blowing or tripping open. The current rating must match the full-load current of the circuit as closely as possible. For example, undersized fuses blow easily, while oversized fuses may not provide enough protection.
Voltage Rating
The voltage rating of a fuse or circuit breaker is the highest voltage at which it is designed to safely interrupt the current. Specifically, the voltage rating determines the ability of the device to suppress the internal arcing that occurs when a current is opened under overcurrent or short-circuit conditions. The voltage rating must be at least equal to or greater than the circuit voltage. It can be higher but never lower. Low-voltage circuit breakers protect circuits using less than 1000 V of electricity.
Interrupting-Current Rating
The interrupting-current rating (also known as short-circuit rating) of a fuse or circuit breaker is the maximum current it can safely interrupt. If a fault current exceeds a level beyond the interrupting capacity of the protective device, the device may actually rupture, causing additional damage.
The interrupting-current rating is many times greater than the continuous-current rating and should be far in excess of the maximum current the power source can deliver. Typical interrupt ratings are 10,000 A, 50,000 A, and 100,000 A.
Current-Limiting Ability
Current-limiting ability is a measure of how much current the overcurrent protection device can let through the system. Current-limiting protection devices operate within less than one-half cycle. For example, a current-limiting fuse delivering a short-circuit current will start to melt within one-fourth cycle of the AC wave and clear the circuit within a one-half cycle.
Time-Current Characteristics
The time-current characteristics or response time of a protection device refers to the length of time it takes for the device to operate under fault current or overload conditions.
Fast-acting-rated protection devices may respond to an overload in a fraction of a second, while standard types may take 1 to 30 seconds, depending on the amount of the overload current. Being very sensitive to increased current, fast-acting fuses are used to protect exceptionally delicate electronic circuits that have a steady flow of current through them.
The Vital Role of Circuit Overcurrent Protection
Circuit overcurrent protection is a vital part of every electric circuit. Electric circuits can be damaged or even destroyed if their voltage and current levels exceed the safe levels they are designed for. In general, fuses and circuit breakers are designed to protect personnel, conductors, and equipment. Both operate on the same principle: to interrupt or open the circuit as quickly as possible before damage can occur.
Author: Alex earned a master’s degree in electrical engineering with major emphasis in Power Systems from California State University, Sacramento, USA, with distinction. He is a seasoned Power Systems expert specializing in system protection, wide-area monitoring, and system stability. Currently, he is working as a Senior Electrical Engineer at a leading power transmission company.
Published by Henry, theengineeringknowledge.com, May 20, 2020
The basic difference between grounding and earthing is that in grounding the conductor through which the current is flowing is connected with the ground while in earthing the conductor through which the current is not flowing is attached with the ground.
In this post, we will have a detailed look at earthing and grounding than relate them to find their differences. So let’s get started with a Difference Between Grounding and Earthing.
Difference Between Grounding and Earthing
Earthing
• Earthing is a process that is used to link the part of the device called the dead portion that has zero value of current with the earth. • The frame of your fridge is a dead part and connects it of the earth • Earthing helps to make a reduction in the getting shock if there is current flowing in the device due to fault. • If earthing is done then current will flow to that part that has less resistance. • For earthing, there is green-colored wire is used • The main purpose of earthing is to save a person from getting shocked by a damaged device that has a current in its body • If someone gets a touch to the body can get shocked if decide is not properly earthed • In case of lighting bouls earthing also saves homes and other apartments • It rescues the fire probability in the different system
Importance of Earthing
The main component of electrical safety is earthing. It entails directly connecting electrical instruments metallic surfaces and exposed conductive portions to the ground. It is mostly used to minimize electric shock and lower the danger of fire from electrical failures.
Earthing in Electrical Systems
When earthing is used in electrical systems, fault currents are ensured to be dissipated and excess voltage is kept from building up on conductive surfaces. By enabling the safe discharge of fault currents into the ground, it helps to prevent electric shock in people.
Components of Earthing
Components like earthing pits, electrodes, conductors, and earthing terminals are used in earthing systems. These components are intended to make a dependable connection configuration with the ground and offer a low-resistance channel for fault currents.
Grounding
• In this process, the active portion of the device means through which the current is passing is linked to the earth. • Its main purpose is to save the device from getting damaged an example is the neutral of the transformer linked to the ground • The main purpose of transform neutral linked to the ground is that if lighting bouts are fallen on it then its grounding will give less resistance path to move to the current to ground • For grounding there is black color wire is used
Importance of Grounding
Electrical engineering’s foundational concept is grounding. It entails attaching electrical devices and systems to the ground or a conductor that acts as a standard for electrical potential. In the case of a failure or surge, grounding primarily serves to offer a safe path for electrical currents to follow.
Grounding in Electrical Systems
In electrical systems, grounding offers a point of reference for voltage levels and helps in system stabilization. By diverting fault currents to the ground and protecting people and instruments from potentially hazardous conditions, it helps in the prevention of electrical shock dangers. With that, grounding decreases electromagnetic interference and guarantees the correct operation of delicate electronic equipment.
Components of Grounding
Grounding systems come with different parts, like conductors, grounding terminals, and grounding connectors. Together, these components make a low-resistance channel for fault currents, helping them to efficiently dissipate into the earth.
Similarities Between Grounding and Earthing
Electrical Safety
By eliminating electrical shock dangers and minimizing the chance possibility of electrical fires, both earthing and grounding help to ensure electrical safety. They offer fault currents paths, providing the safe release of extra energy.
Equipment Protection
Electrical devices can be protected from harm by transient voltages, power surges, and lightning by being grounded and earthed. They decrease the possibility of damage by rerouting fault currents away from the equipment and creating a low-resistance channel to the earth.
Fault Current Management
For successful management of fault currents, both earthing and grounding are needed.. They enable the safe dissipation of fault currents, minimize electrical risks, and decrease the effects of faults on the system as a whole by offering low-impedance routes to the ground.
Grounding vs Earthing
Grounding
Earthing
1. Grounding us the connection if electrical systems to the ground.
1. Earthing is connecting conductive parts and surfaces of electrical equipment to the earth.
2. It provides a path for electrical faults and surges to safely dissipate.
2. It prevents electric shock and decreases the risk of fire.
3. Grounding protects against electrical malfunctions and damage to equipment.
3. Earthing makes sure the safety of individuals and equipment.
4. It stabilizes voltage levels and decreases electromagnetic interference.
4. It discharges fault currents and avoids the buildup of excess voltage.
5. Grounding is necessary for electrical system stability and proper functioning.
5. Earthing is important to make sure electrical safety and prevent accidents.
6. It is get through grounding conductors, rods, and grounding electrodes.
6. It is done through earthing conductors, mats, and earthing electrodes.
7. It is required by electrical codes and standards for safety compliance.
7. It is mandated by regulations to meet safety requirements.
8. It protects against lightning strikes, power surges, and voltage transients.
8. It protects against electric shock and equipment damage.
9. Grounding reduces the risk of electrical noise and interference.
9. Earthing minimizes the risk of electric shock hazards in various settings.
10. It is important in industrial, commercial, and residential electrical installations.
10. Earthing is compulsory in all types of electrical systems and environments.
11. It ensures proper grounding continuity and fault current path.
11. It provides a low-resistance path for fault currents to flow.
12. It prevents static discharge and potential differences in electrical circuits.
12. It prevents the buildup of static electricity and potential differences.
13. This system protects sensitive electronic devices from voltage fluctuations.
13. it protects individuals from electric shock in case of faults.
14. It is important for electrical system safety and the protection of personnel.
14. It decreases the risk of electrical accidents and injuries.
15. it comes with grounding busbars, grounding conductors, and ground fault protection.
15. it involves earthing conductors, grounding electrodes, and earth leakage protection.
16. It maintains electrical system integrity and prevents overvoltage conditions.
16. it maintains a reference potential and prevents electric shock incidents.
17. This process provides a path for fault currents to return to the source safely.
17. This method comes with a safe route for fault currents to flow into the ground.
18. It is important for equipment grounding and protection against electrical faults.
18. it is necessary for equipment safety and minimizing electrical hazards.
19. Grounding is part of a comprehensive electrical safety program and risk management.
19. it forms an integral part of electrical safety strategies and protocols.
20. It ensures the safe operation and longevity of electrical systems and equipment.
20. This techniques ensures the integrity and safety of electrical installations and appliances.
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Author: Henry, I am a professional engineer and graduate from a reputed engineering university also have experience of working as an engineer in different famous industries. I am also a technical content writer my hobby is to explore new things and share with the world. Through this platform, I am also sharing my professional and technical knowledge to engineering students.
Published by Zia Hameed1, Muhammad Rafay Khan Sial2, Adnan Yousaf3, Muhammad Usman Hashmi, Faculty of Engineering and Technology, Superior University, 17-km off Raiwind Road Lahore Pakistan Emails: zia.hameed@superior.edu.pk, rafay.khan@superior.edu.pk, adnan.yousaf@superior.edu.pk
Abstract—Power System Harmonics is a real point of concern for Electrical Engineers. In power systems, non-linear loads are permanently connected, unlike transients and other distortions are produced. Due to non-linear loads, distortions are produced in the sinusoidal waveform so active shunt filter is used in parallel with the load to minimize these distortions and in a result pure sinusoidal waveform is obtained. Active shunt filter is work as a current source, but opposite in phase sequence to the current which produces by non-linear loads. Harmonics is an all-time problem. Relay protection devices are not good enough to resolve this problem, so other techniques are to be studied to minimize their effects. In this paper my major concern is to identify the loads which causes harmonics, how to design a filter for removing harmonics their effects on power systems, how to design a filter for removing harmonics, proposition of useful filters for altered types of loads and their simulation on ETAP (Electrical Transients and Analysis Program).
Keywords — AC wave, Even Harmonics, Filters, Odd Harmonics, Linear and non-linear loads, ETAP.
I. INTRODUCTION
Service dependability and worth of power have become growing consternations for many capacity directors, especially with the increasing sensitivity of electrical equipment and programmed controls[4]-[6]. There are several types of voltage variations that can cause anomalies, including surges and spikes, sags, harmonic distortion, and temporary disruptions. Harmonics can cause sensitive equipment to failure and other problems, as well as overheating of transformers and wiring, irritation breaker trips, and a bridged power factor[2], [11].
The part of distribution of electric voltages to the power system is very important. This objective is difficult due to Harmonics currents that are produced [9]. They produce harmful effect on the system and disturb its continuity. So when harmonics are produced it is necessary to reduce it for better performance of the system. There are two concepts for which we can understand, how harmonics affect the power system [2], [10]. Firstly, the harmonics are produced due to non-linear loads and the second is that how harmonics current flow and produce harmonic voltage [6].
II. HISTORY
In 1888, Tesla familiarized the concept of poly-phase systems after that in 1890, at Portland, Ore a 1st power transmission line of length 13 miles at frequency of 132 Hz was setup [1], [2]. In the same year, Bedell studied the field of alternating current and also studied the effects of alternating current wave forms in power systems [2], [3]. In 1893, at Hartford engineers dealing with a heating problem of a motor had selected harmonics analysis as a technique to identify the causes of motor heating and tried to solve the problem[3], [4]. Steinmetz discouraged the use of high frequency in power systems because of the high transmission line resonance [6]. It was noticed that the voltage wave form having frequency 133 Hz or 125 Hz was plentiful in harmonics. Steinmetz suggested two solutions for the removal of higher harmonics. First was to reduce the system frequency of 133 Hz or 125 Hz to half i.e. 66.5 Hz or 62.5 Hz. The second suggestion was to refit the iron laminations in the motor which can bear higher in-service voltage [1], [2]. In 1895 generator manufacturing companies Westinghouse and GE presented such generators having distributed armature winding to make the waveform more sinusoidal. It was also noted that when two generators operate in parallel and solidly grounded excessive neutral current flows which causes harmonics. 3rd harmonic was reduced by changing armature winding pitch factor when the neutral of a machine was solidly grounded [3], [5]. In 1910 telephone interference factor was given great importance even in 1980s it was included in the standards due to the large usage of mercury arc rectifier which is a large source of this distortion. In 1960s, in instruction to progress power factor and to reduce the power system harmonics large number of shunt capacitors and filter banks were installed in industrial power systems. In 1812, Jean Fourier developed a mathematical way to analyze the complex functions [1], [2]. This technique expands the complex functions into sine and cosine functions. Harmonics analysis is the name given by Thomson and Tait [1]-[3]. Bernoulli, Euler and Maxwell also used this technique in 18th century. In 1966, J.W Coley and J.W Tuky suggested the Fast Fourier Transform (FFT) as a technique for computer code so that it can give results hurriedly. IEEE standard 519 is now the principle interface standard used by most engineers to judge harmonics issues [3], [4].
III. HARMONIC CONCEPTS
Due to distortion of voltage and current waveform harmonics are produced. Harmonics are mentioned to be a section of a waveform that is the integral multiple of the fundamental frequency. If the load is inserting normal power back to the source at harmonic frequencies, it can be called a Harmonic source.
Fig.1. Odd Harmonics
IV. LINEAR AND NON-LINEAR LOADS
In a power system, current waveform is same as voltage because current is proportional to voltage. Examples of linear loads are heaters and motors.
Fig.2. Linear loads waveform
But for Non-linear loads the current and voltage waveform are different. Examples of non-linear loads are UPS and DC motor drives.
Fig.3. Non-linear load wave form
The current waveform is not periodic but it remains same cycle to cycle. Due to sum of sinusoidal waves, periodic waves are generated.
V. VOLTAGE AND CURRENT HARMONICS
The expression ‘harmonics’ is often used by itself without further qualifications. However, the voltage and current harmonics are separate in their effects and are also mutually related. Non-linear loads at the consumer end appear to be injecting the harmonic currents in the power system. For this reason, they are normally treated as harmonic current sources. On the other hand, the harmonic voltages are the result of harmonic current times the linear impedances of the control system. The harmonic current passing through the system resistances causes the voltage drop across it which results in voltage harmonics. Thus, the voltage harmonics are the function of current harmonics and the linear impedances of the power system.
Fig.4 shows a voltage waveform of peak value equal to the secondary distribution level of Pakistan i.e. 220 V. Likewise, it also depicts the harmonics mechanisms with amplitudes of (1/3) to (1/5) and (1/5) to (1/7) of 220V and having the frequencies three, five and seven times the essential frequency correspondingly. Assuming the voltage harmonics are due to the passage of harmonic current through a system resistance.
Fig.4. Sinusoidal 50 Hz waveform with 3rd, 5th and 7th Harmonics
Fig.5. Sinusoidal 50 Hz waveform distorted by 3rd, 5th and 7th Harmonics
VI. COMPARISON BETWEEN TRANSIENTS AND HARMONICS
Transients and Harmonics often cause confusion and most of the times one is blamed instead of the other for a particular quality disturbance in power system. The main differences between harmonics and transients are shown in the Table 1.
Table 1: Comparison between Transients and Harmonics
.
VII. EVEN AND ODD HARMONICS
Harmonics are fundamentally the integral multiples of the fundamental frequency. Even harmonics are the even multiples and odd harmonics are odd multiples of the fundamental frequency.
Even harmonics: 2𝑓,4𝑓,6𝑓,…,2𝑛𝑓 Odd harmonics: 3𝑓,5𝑓,7𝑓,…,(2𝑛+1)𝑓 here, 𝑛 is a natural number.
Similar waves contain only odd harmonics but odd and even both harmonics are produced due to asymmetrical waves.
In odd harmonics both positive and negative parts of the wave are same but in asymmetrical waves both positive and negative parts are different. Asymmetrical wave is the result of half wave rectifier.
Control scheme produces only odd harmonics. This is due to same waves. Due to this reason, only odd harmonics will be discussed in upcoming sessions.
VIII. HARMONICS PHASE SEQUENCE
In order to describe a physical three phase system, Power Engineers have adopted a technique of balanced machineries which is based on Fortescue’s theorem. That is stated as:
“An unstable set of 𝑛 phasors may be resolute into (𝑛−1) stable n-phase systems of diverse phase sequence on one zero-phase sequence system.”
Phase sequence of the phasors is the order in which they pass through a positive maximum. A physical 3-phase system with phases A-B-C can be resolved into following three component sets of balanced phasors:
• Positive-sequence contains three sinusoids which are at 120𝑜 from each other. • Negative-sequence contains three sinusoids which are at 120𝑜 from each other and they are opposite to the positive sequence. • Zero-sequence contains three sinusoids that are in-phase with each other.
So for second harmonic, 𝑛=2 we get 2×(0𝑜,−120𝑜,120𝑜) 𝑜𝑟 2×(0𝑜,120𝑜,−120𝑜) It shows negative sequence.
For third harmonic, 𝑛=3 we get 3×(0𝑜,−120𝑜,120𝑜) 𝑜𝑟 (0𝑜,0𝑜,0𝑜), which is the zero sequence. Here is the detail of only odd harmonics:
Harmonics of order 𝑛=3,9,15 … are also named as Triplens. They deserve special consideration because of their critical nature and considerable effects on the behavior of the power system. They are of much importance while discussing grounded-star system containing the neutral current. The two main problems associated with triplens are:
• Overloading of the neutral • Telephone interference
Due to the triplen harmonics, an excessive current flows through the neutral conductor resulting in the overloading of neutral. Among the triplen harmonics, 3rd Harmonic got the much consideration of power engineers.
IX. MEASURING PARAMETERS OF HARMONICS
The result of harmonics is measured by the following methods.
a) Total Harmonics Distortion
Total Harmonic Distortion is identified by Harmonic Distortion Factor which is the most common technique to calculate harmonics distortion of current and voltage. For an ideal system, THD is equal to zero. THD is determined by:
.
𝑉𝑛 is the rms voltage at harmonic, 𝑁 is the maximum harmonic order and 𝑉1 is the line to neutral rms voltage.
b) Total Demand Distortion
THD can also be applied to study the current distortion stages but in the case of low fundamental load current, it can be deceiving. A small current may carries high THD which is danger for system. For example speed drives shows the high THD values at very light loads for any value of input current. The magnitude of harmonic current is low. This high THD value for input current is not considerable concern even though its comparative distortion to the fundamental frequency is high.
TDD is scientifically calculates:
.
Where 𝐼𝑅 is the peak hours demand load current at the fundamental frequency component determined at point of joint coupling (PCC). There are two ways of calculating 𝐼𝑅. With the load which is already in the system, it can be determined simply by averaging the peak demand current for the preceding 12 months.
X. SOURCES OF HARMONICS
Due to non-linear loads and switching processes harmonics distortion is produced. These sources of waveform can be found in engineering installation in thousands of KVA value. The main sources of harmonics in power system are:
• Due to windings in the transformer and magnetic capacity in stators and rotors of Electrical machines • In the transformer core due to magnetic saturation • Due to rectifiers and inverters • Due to nonlinear loads
A. Rotating Machines
Revolving machines become a source of harmonic distortion because of irregularities in stator and rotor slots or due to winding patterns. So these harmonics produce emf. But these harmonics are very less in quantity as compared to variable speed drives.
B. Transformer
An excessive magnetic flux is produced through the core when transformer operates near the saturation zone. Due to this excessive magnetic flux, linear rise of the magnetic flux density is limited. Core saturation of transformer is resulted when it operates either:
• Above then the rated power • Above then the rated voltage
At rated power harmonics are produced due to peak hour voltages. A Transformer is function on a saturation region so non-linear magnetizing current is found which produces odd harmonics and due to hysteresis losses distortion is produced. Distortion is characteristically due to triplen harmonics, but mostly due to the third harmonic. Delta connection is used to restrict the third harmonic current within the transformer. This helps in preserve a supply voltage with a sensible sinusoidal waveform.
C. Power Electronic Converters
There is a large use of Electronic converters in domestic and industrial purposes due to domestic uses. Single phase rectifier is very common converter which is used for domestic and industrial applications but three phase converter is more danger as compared to single phase converter because it produces 3rd order harmonics which are more dangerous for the power system.
D. Arcing Devices
The foremost harmonic sources in this group are the arc welder’s electric arc furnaces, and discharge type lighting (arc furnace, sodium vapor, florescent) for magnetic (rather than electronic) ballasts. Due to arc furnace in industries, harmonics are produced. So when the arc increases, voltage will decrease in the power system.
E. Future Sources of Harmonics
For Electrical system designer it is a challenge that to design such an instrument for domestic uses and industry that operate at harmonic level. Due to very large use of sensitive electrical and electronic devices harmonics are produced so it is very dangerous in the near future. Due to very large use of switching devices and instruments harmonics are produced which are very dangerous in the near future. Due to distributed generators harmonics are produced specially in peak hours.
XI. EFFECTS OF HARMONICS
Harmonics are very dangerous for the remaining power system and the equipment’s that are attached with the power system. The main effects of voltage and current harmonics within the power system are:
• The possibility of amplification of harmonic levels resulting from series and parallel resonances • Degradation of the power factor • Overheating of the phase and neutral conductors • Efficiency of the generators is reduced day by day due to harmonics • Eddy current and hysteresis losses in transformers • Overheating of the system components e.g. generators, motors and transformers etc. • Flow of additional current through power capacitors • Decrement in the useful lives of the incandescent lamps • Increase skin and proximity effects • Interference problem with telecommunication • Effects the relay protection system
For the adverse effects of harmonics on the power system, it is the major demand of the today’s power system that these harmonics should be mitigated by appropriate designing of the filters either active or passive.
XII. DESIGNING OF FILTERS
Distribution network is a network which is close to the consumers end. Non-Linear loads are attached at the consumer’s end, so mainly Harmonics are produced at the consumers end. So for the removing of Harmonics we used Active shunt filters at the distribution side. Due to non-linear loads reactive current is produced which causes Harmonics. So for removing of reactive current Hysteresis band control method is used to produce trigger signal to the inverter to produce reference current. Due to non-linear loads distortions are produced in the sinusoidal waveform so active shunt filter is used in parallel with the load to minimize these distortions and in a result pure sinusoidal waveform is obtained. Active shunt filter is work as a current source, but opposite in phase sequence to the current which produces by non-linear loads. Similarly filters are also used which work as a voltage source for the removing of Harmonics. When the Harmonics will be removed from the Electrical System then efficiency and life will be increases of the equipment’s. Harmonic current calculator is used for the calculation of Harmonic current, calculator sensed load current and multiply it with unit magnitude of sine and cosine wave, in this way we are able for identify harmonics in the Electrical Power Systems. So Hysteresis bases control circuit is used in the filters for the removing of Harmonics. There are three simulations which are used to filter the Harmonics 1- Simulation of shunt active filter, 2- Simulation of Harmonic current calculator, 3- Simulation of voltage source inverter. But in this paper simulation of shunt active filters is used by using ETAP (Electrical Transient Analyzer Program) software is used. Filters are tuned in such a way that at which frequency they are tuned resonance will be occur and that harmonic content will be filter from the wave.
XIII. ETAP AS A BRILLIANT TOOL FOR HARMONICS ANALYSIS
ETAP is a best tool for the study of harmonics in a power system. With the help of ETAP we can study the Harmonics Analysis of any type of circuit and with the help of ETAP we can also study the Harmonics spectrum. By load flow analysis we can study the harmonics analysis. First of all we study the load flow analysis at the fundamental frequency. With the help of load flow analysis we can study the power factor at different buses in the electrical power system and after that we can check the harmonics analysis and order of harmonic spectrum. By doing the harmonics analysis, low order frequencies are produced.
Here we study different cases of “Variable Frequency Drives” using ETAP and observe the effectiveness of this tool.
Fig.6. 6-pulse Harmonics Analysis with VFD
Fig.7. THD with VFD and different Loads.
Fig.8. Removing Harmonics of Non-linear by using Filters filters
CONCLUSION
Harmonic distortion is one of the major issues to maintain the power quality. From the results we shown that harmonics are removed by using active shunt filters. Harmonics not only effects the power quality but also cut down the useful life of the power apparatus. It is associated with the major power system components i.e. transformer, synchronous motors, power converter and electrical furnaces. Since these components are continuously connected to the power system, harmonics is all time concern present in the fundamental signal. It is therefore crucial to mitigate this distortion. Harmonics analysis is also very important to study all the effects and the losses which we have to bear. So ETAP (Electrical Transient and Analysis Program) is an important simulation software to check the systems losses and effects before its installation. We conclude that we can check Harmonics Analysis by using ETAP and can be removed by using active shunt filters.
REFERENCES
[1] Edward L. Owen, “A History of Harmonics in Power Systems”, IEEE Industry Application Magazine, Jan/Feb 1998, pp 6-12 [2] P. Berdell,” History of AC wave form, Its determination and standardization”, AIEE, Trans, vol.61, 1942, pp.864-68 [3] S.P. Thompson, “A new Method of Harmonic Analysis by selected ordinate”, Proc. Of the physical society [4] T. R. Bosela, Introduction to Electrical Power System Technology, New Jersey, Prentice Hall, 1997, pp. 458 462. [5] Angelo Baggini, Zbigniew Hanzelka, Handbook of Power Quality, John Wiley and Sons Ltd. England 2008, pp. 187 236. [6] Francisco C. De La Rosa, Harmonics and Power Systems, Distribution Control Systems, Inc. Hazelwood, Missouri, U.S.A. 2006, pp. 1-56. [7] Roger C. Dugan, Mark F. McGranaghan, Surya Santoso, H. Wayne Beaty, Electrical Power Systems Quality, Second Edition, pp. 167 223. [8] R. Rexte, “Power Electronic Polluting Effects”, IEEE Spectrum, May 1997, pp. 33 39. [9] M. Izhar, C. M. Hadzer, S. Masri, S. Idris, “A Study of the Fundamental Principles to Power System Harmonic”, National Power and Energy Conference (PECon) Proceedings, Bangi, Malaysia, 2003. [10] C. Sankaran, Power Quality, CRC Press LLC. [11] John J. Grainger, William D. Stevenson, Power System Analysis, McGraw-Hill Companies, Inc. New York, c1994. pp. 417 418. [12] S. P. Ghosh, A. K. Chakraborty, Network Analysis and Synthesis, McGraw-Hill Education, c2010, pp. 933. [13] S. L. Clark, P. Famouri, W. L. Cooley, “Elimination of Supply Harmonics”, IEEE Ind. Appl. Magazine, March 1997. [14] E. B. Makram, R. B. Haines, A. A. Girgis, “Effect of Harmonic Distortion in Reactive Power Measurement”, IEEE Trans. Ind. App., vol. 28, no. 4, July 1992. [15] Control of Harmonics in Electrical Power Systems, American Bureau of Shipping, May 2006, pp. 29 48. [16] L. Cividino, “Power Factor, Harmonic Distortion; Causes, Effects and Considerations”, IEEE Telecommunications Energy Conference INTELEC 92, 14th International, Oct. 1992, pp. 506 513. [17] Jos Arrillaga, Bruce C. Smith, Neville R. Watson, Alan R. Wood, Power System Harmonic Analysis, University of Canterbury, Christchurch, New Zealand, pp. 7 25. [18] W. M. Grady, R. J. Gilleskie, “Harmonics and how they relate to Power Factor”, IEEE San Diego, Nov. 1993, pp. 1 8. [19] IEEE Harmonics Modeling and Simulation Taskforce, “Modeling and simulation of the propagation of harmonics in electric power networks part I”, IEEE Trans on power delivery, vol.11, no. 1 , Jan 1996, pp.466-474. [20] A. Median, “Harmonic simulation techniques (Methods and Algorithms)” IEEE Power Engineering Society General Meeting , Vol.1 , June 2004, pp.762-765 [21] Pravin Chopade and Dr. Marwan Bikdash, “Minimizing Cost and Power loss by optimal placement of capacitor using ETAP”, IEEE 2011 pp.24-3
Author: Zia Hameed was born in 1991 in Bahawalpur, Pakistan. He is doing his Graduation in Electrical Power Engineering from The Islamia University of Bahawalpur (2010-14). He contributed his part in this project work especially in the sources and effects of Harmonics. He also played a major role in the study of Filters. He complete different Electrical courses from best worldwide universities like MIT, Delft institute, University of Toronto. Currently he is serving as a Lab Engineer at Electrical Engineering Department of Superior University, Lahore. Contact: +92-343-7177273 Email: zia.hameed@superior.edu.pk
Published by Jakub KELLNER1, Michal PRAZENICA2, Department of Mechatroncs and Electronics, Faculty of Electrical and Information Technology, University of Zilina, Slovakia
Abstract. This article deals with the problem of elimination inrush currents. The document proposes an active method for limiting the inrush currents during circuit switching when high inrush currents occur. The proposed system limits current in the circuit by means of a series connected Mosfet transistor. The Mosfet transistor is controlled in a linear resistive region. In the case of an inrush current in a circuit, the Mosfet transistor limits the magnitude of the current flowing into the circuit. The article also solves the problem of transistor power load. In the article there is a chapter that deals with the maximum magnitude of the limiting current that can flow through the transistor so as not to destroy it. In this new inrush current limiting configuration, the operator can directly define the amount of surge current that must not be exceeded after the circuit is closed. The proposed system is also complemented by other protective and control elements that are described in this article.
Streszczenie. Artykuł dotyczy problemu eliminacji prądów rozruchowych. Zaproponowano aktywną metodę ograniczania prądów rozruchowych podczas przełączania obwodu, gdy występują duże prądy rozruchowe. Proponowany system ogranicza prąd w obwodzie za pomocą szeregowo połączonego tranzystora Mosfet. Tranzystor Mosfet jest kontrolowany w liniowym obszarze rezystancyjnym. W przypadku prądu rozruchowego w obwodzie tranzystor Mosfet ogranicza wielkość prądu wpływającego do obwodu. Artykuł rozwiązuje również problem obciążenia mocy tranzystora. W artykule jest rozdział poświęcony maksymalnej wielkości prądu ograniczającego, który może przepływać przez tranzystor, aby go nie zniszczyć. W tej nowej konfiguracji ograniczania prądu rozruchowego operator może bezpośrednio określić wielkość prądu udarowego, której nie wolno przekroczyć po zamknięciu obwodu. Proponowany system uzupełniono również innymi elementami ochronnymi i kontrolnymi opisanymi w tym artykule. Eliminacja nadmiernego wzrostu prądu rozruchowego
With increasing demands on the power of electric motors for various technical sectors (e.g. electric vehicles for traction purposes), the power of power semiconductor converters for connecting these motors is also increasing. As the power of the inverters increases, there is a problem with the inrush current. The problem of inrush current is compounded when using traction battery power used in cars.
The inrush current is the instantaneous input current of a high amplitude circuit that occurs when the circuit is switched on as a result of charging capacitors, inductors and transformers. This inrush current has a large amplitude and can reach currents of up to several tens of kilo-ampere [1] – [4].
Fig. 1 Current curve when the circuit is switching on (inrush current)
Therefore, it is very undesirable in electrical circuits. In circuits where the inrush current occurs, the elimination of this undesirable phenomenon is solved by increasing the resistance in the circuit. In most cases, a resistor or an NTC thermistor is used to increase the resistance in the circuit. The duration of the inrush currents is of the order of milliseconds, the duration of this action being dependent on the size of the RC members in the circuit. Figure 1 shows an example of the current waveform when in the circuit the inrush current was generated [5] – [8].
The system designed by us solves the elimination of surge currents for the 9kW inverter, which is used to power supply the asynchronous motor. Input voltage for the inverter is realized by traction batteries with nominal voltage 300V, DC. The total charge capacity in the circuit is 5mF and the parasitic resistance in the circuit is estimated at 50mΩ. In this case, the inrush current would occur during the power on the Ipeak = 6kA circuit. This is confirmed by the simulation shown in Figure 2. Since the capacitor at the moment of switching on was a short circuit, the current was limited only by the resistance in the circuit:
.
Fig. 2 Simulation of inrush current in the circuit, without limitation
Due to the high inrush current, it is not suitable to use a resistor or an NTC thermistor for limitation. Therefore, we decided to design a system that uses a controlled Mosfet transistor. The advantage of this system is that we can adjust the magnitude of the inrush current. This system can also be used for other circuits that have lower voltage and current parameters for which this system was designed [9].
Inrush current limitation with Mosfet transistor
As mentioned above, the inrush current during the initial start-up of the inverter that feeds the asynchronous motor can be limited by the Mosfet transistor. A schematic diagram of this circuit is shown in Figure 3.
Fig. 3 A schematic diagram of the system
By controlling the transistor in its resistive area, we can reduce the magnitude of the inrush current generated by the charging of the capacitors of the inverter. This method is much more efficient and preferable than using a resistor or NTC thermistor. The advantages of this configuration are:
1.) Possibility to set maximum surge current. The user can set the amount of current which must not be exceeded. This feature allows the designed system to be used for other applications where inrush current limitation is required [10] – [12].
2.) System efficiency. Because it is a power semiconductor converter, we try to make the converter efficiency as high as possible. Therefore, the use of a resistor is not appropriate. We would reduce the efficiency of the inverter. But if we use a Mosfet transistor, we can effectively regulate the power supplied to the DC bus inverter [13].
3.) Power load of the limiting component. If we used a Resistor to limit the current, the power dissipation at the resistor would be too high.
Example of calculating instantaneous power on the limiting resistor (Pdissipation), for the proposed application: Supply voltage: UIN = 300V, DC; Circuit Capacity: C = 5mF; Limiting resistance: R = 20Ω. The effective value of the current in the circuit was: Irms = 5,9A. Power dissipation is [14]:
.
Power part design
Figure 4 shows a schematic of the power section of the inrush current limiting system. The power part of the proposed system consists of three main parts. These are active power semiconductor components. The main semiconductor component that provides the whole principle of inrush current limitation is Mosfet transistor. Its control ensures the limitation of the current flow in the circuit. The transistor is active in the case of over currents that occur during circuit switching on. Another element in the circuit is relay 2, which serves as a bypass member.
Fig. 4 Power part of the system
Relay 2 is connected in parallel to the transistor and at the moment the inrush current limitation is complete, relay 2 closes and the transistor is bypassed. The third active element in the power section is relay 1, which ensures the overall start and stop of the system. It also serves as a protective relay.
In the power section, an RC snubber is also contemplated to optimize the transistor to avoid failure during operation. The system can conduct current in both directions. Therefore, it is suitable for use in traction applications where we can measure and adjust the amount of current flow.
Simulation results of the proposed system
The proposed system was simulated in Matlab / Simulink environment. The simulation is based on current limitation during the initial inverter switching on. At this point, high capacity is charging. Figure 5 shows the voltage on the main circuit and the current flowing into the circuit. Using a current limiting transistor, we regulated the current in the circuit to I = 10A. Where we suppressed the current from 6kA to 10A. This, however, greatly increased the delay time τ.
Fig. 5 Simulation of inrush current limitation
Figure 6 shows the waveforms that determine the operation of the transistor and bypass relay.
Fig. 6 Conductivity waveforms of transistor and bypass relay
The black waveform shows the current on the transistor that limits the inrush current to 10A. The orange waveform represents the voltage across the transistor, which gradually decreases, while the voltage across the main circuit increases. The green curve represents the current through the bypass relay. We can see that if the voltage on the transistor drops to zero, the voltage on the main circuit will be equal to the supply voltage. Then the transistor turns off and the bypass relay is turned on, through which the load current flows into the circuit.
Simulation verification of transistor power load
This chapter shows the results of the transistor power load during the current limitation. In the simulation for the limitation, we detected the power losses on the Mosfet transistor at the time the transistor was in operation using the “Pe_getPowerLossSummary” function [15] – [17].
Table 1 shows the Mosfet current through the transistor, the time delay, the power dissipation on the transistor, and the energy on the transistor. The simulation input voltage is UIN = 300V. Main circuit capacity is C = 5mF.
Table 1. Transistor load simulation varication
.
For the measured loads and time delays, we created a graph of the magnitude of the current through the Mosfet transistor, which can be seen in Figure 7. From the waveform we can see that with increasing current the delay time decreased but the power load of the transistor increased periodically.
Fig. 7 A waveform to determine the maximum permissible current limiting through a transistor
Using this function, we can determine the maximum current limiting magnitude for a selected Mosfet transistor so that it does not exceed its maximum permissible power load. To prevent Mosfet transistor from straining.
Fig. 8 Real sample of proposed system
System verification on real sample
The proposed system was verified on a real sample. Measurements were performed at reduced parameters. The prototype was created for the experimental verification of system functionality. The thickness of roads and conductive connections of real sample does not correspond to the power load for which the system was designed. Therefore, all experimental measurements were verified at reduced voltage. Figure 8 shows a prototype of the proposed system. The illustration shows the description of each part of the system. The designed system was controlled by a C2000 microcontroller from Texas Instrument TMS 320F28069. Figure 9 shows the waveforms from the inrush current limitation measurement. The inrush current is limited to Ilim = 3,6A. The main circuit capacity is C = 4,2 mF. Supply voltage is UIN = 37V.
We can see from the waveform that the current in the circuit did not exceed the allowed current during charging.
Fig. 9 Inrush current limitation measurement
The yellow waveform represents the voltage on the main circuit. The blue waveform represents the current in the circuit. The main circuit was gradually charged to the supply circuit voltage. After the capacitor was charged, the current in the circuit remained at I = 4A. Because the main circuit was loaded with a resistance R = 9,2Ω. The time delay in the circuit was 120ms.
Figure 10 shows the waveforms of inrush current limitation of a main circuit with the following parameters: UIN = 70,5V; Rload = 39Ω; C = 2,2mF. The yellow waveform represents the voltage on the main circuit, which gradually increased to the value of the supply voltage. The blue waveform represents the voltage on the transistor. This, in turn, drops to zero. The purple waveform is the current in the circuit. The current was limited to Ilim = 1,8A. The load current in the circuit is Iload = 1,7A. The green curve represents the current through the bypass relay. From this we can see that the transistor led the current during the inrush current limitation. Then the transistor and the relay led simultaneously. And then only the bypass relay conducts current to the load.
Fig. 10 Measurement of main circuit inrush current limitation
Figure 11 shows a measurement workstation consisting of a power supply, current probes, oscilloscope, multimeter, voltage probes, load resistance, load inductance, auxiliary power supplies and a notebook for communication with the microprocessor.
Fig. 11 Measuring workplace
Conclusion
This article presents a new design of the inrush current limiting system. It is an effective way of limiting. In the proposed system there is the possibility to specify the magnitude of the maximum inrush current. Therefore, the system can be used for various applications. Current limitation is solved with Mosfet transistor, which provides considerable advantages over conventional methods of limiting inrush current. The proposed system was verified by simulation and on a real sample. Results from simulations and measurements are presented in this article. From the results we can see that the proposed system successfully limits the magnitude of the inrush current during the initial switching on of the main circuit.
Acknowledgement This work was supported by projects: Vega 1/119/18 Research of the methodology for optimization of EMC of WPT systems, ITMS 26210120021.
LITERATURA
[1] J-Ch. Wu, H-L. Jou, K-D Wu, N-T Shen, Hybrid Switch to Suppress the Inrush Current of AC Power Capacitor, IEEE Transaction on Power Delivery, vol. 20, n. 1, pp. 506-511. [2] T. Jiang, P. Cairoli, R. Rodrigues, Y. Du, (2017). Inrush current limiting for solid state devices using NTC resistor. SoutheastCon 2017. doi:10.1109/secon.2017.7925398. [3] Iuga, B., & Tirnovan, R. A. (2019). Step by step Limiting for Capacitors Inrush Current Used in Voltage Power Supplies. 2019 8th International Conference on Modern Power Systems (MPS). doi:10.1109/mps.2019.8759664. [4] H. Suryoatmojo, M. Ridwan, I. Izzatur Rahman, D. Candra Riawan, M. Ashari, Design of Bidirectional DC-DC Cuk Converter for Testing Characteristics of Lead-Acid Battery, Przeglad Elektrotechniczny, doi:10.15199/48.2020.03.26. [5] R. Araria, K. Negadi, M. Boudiaf, F. Marignetti, Non-Linear Control of DC-DC Converters for Batery Power Management in Electric Vehicle Application, Przeglad Elektrotechniczny, doi:10.15199/48.2020.03.20. [6] G. Mallesham, K. Anand, Inrush Current Control of a DC/DC Converter Using MOSFET, International Conference on Power Electronic, Drives and Energy Systems 2006, India. [7] K. Praveen, N. Kulshrestha, L. Srivani, D. Thirugnanamurthy, B. K. Panigrahi, Prognostics of Electrolytic Capacitors under Inrush Current Overstress, International Conference on Smart City and Emerging Technology (ICSCET) 2018, India. [8] Eun-Ju Lee, Jung-Hoon Ahn, Seung-Min Shin, Byoung-Kuk Lee, Comparative analysis of active inrush current limiter for high-voltage DC power supply system, 2012 IEEE Vehicle Power and Propulsion Conference. [9] M. Frivaldsky, P. Špánik, J, Morgos, M, Pridala, Control strategy proposal for modular architecture of power supply utilizing LCCT converter, Energies, Vol. 11, N. 12, 2018, Article Number: 3327. [10] Gonthier, L., & Renard, B. (2015). AC/DC reversible mixed inverter with built-in inrush-current limitation and cut-off standby losses. 2015 17th European Conference on Power Electronics and Applications (EPE’15 ECCE-Europe). doi:10.1109/epe.2015.7309066. [11] K. Dabala, M. P. Kazmierkowski, Converter-Fed Electric Vehicle (Car) Drives, Przeglad Elektrotechniczny, doi:10.15199/48.2019.09.01. [12] M. Frivaldsky, J, Morgos, B. Hanko, Start-up power supply for automotive applications, International Conference Elektro 2018, CZ. [13] Madani, S. M., Rostami, M., Gharehpetian, G. B., & Haghmaram, R. (2012). Inrush current limiter based on threephase diode bridge for Y-yg transformers. IET Electric Power Applications, 6(6), 345. doi:10.1049/iet-epa.2011.0317. [14] H. Hoshi, T. Tanaka, M. Noritake, T. Ushirokawa, K. Hirose, M. Mino, Consideration of Inrush Current on DC Distribution System, Proceedings of Intelec 2012, USA. [15] J.K. Kim, S. S. Lee, W-S. Oh, J-E. Kim, G-W. Moon, Ch-H. Gil, J-R. Cho, Start- up inrush current reduction technique of asymmetrical half-bridge DC/DC converterfor PC power supply, 7th Internatonal Conference on Power Electronics, 2007, South Korea. [16] J. Sedo, S Kascak, Control of single-phase grid connected inverter system, International Conference Elektro 2016, Slovakia. [17] B. Dominikowski, Inteligentne pomiary szybkozmiennego prądu akumulatora trakcyjnego pojazdu elektrycznego wykorzystujące interwałowe zbiory rozmyte typu-2 o wnioskowaniu Takagi-Sugeno-Kanga, Przeglad Elektrotechniczny, doi:10.15199/48.2019.11.12.
Authors: Ing. Jakub Kellner was born in 1995 in Kezmarok, Slovakia. He is graduated at the University of Zilina (2019) in Power Electronics. He is currently an internal PhD student at the University of Zilina – Department of Mechatronics and Electronics in the power electrical engineering study program. The main research interest is about power electronic systems.
Ing. Michal Prazenica, PhD was born in 1985 in Zilina, Slovakia. He is graduated at the University of Zilina (2009). He received the Ph.D. degree in Power Electronics from the same university in 2012. He is now Research worker at the Department of Mechatronics and Electronics at the Faculty of Electrical Engineering, University of Zilina. His research interest includes analysis and modelling of power electronic systems, electrical machines, electric drives, and control.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 96 NR 8/2020. doi:10.15199/48.2020.08.20
Published by Shivanand M N1, Y. Maruthi1 Phaneendra Babu Bobba2, Sandeep Vuddanti1 1GRIET, EEE Department, Hyderabad, Telangana, India 2CUK, Electrical Engineering Department, Kalburgi, Karnataka, India *Corresponding author: shivanandnaduvin@gmail.com
Abstract. India has taken major step in adopting the electric vehicle by means of FAME Scheme (Fast Adoption and Manufacturing of Electric Vehicles), a government initiative. ARAI (Automotive Research Authority of India) and DHI (Department of Heavy Industry) have published standardization protocol for both EV charging infrastructure. Many of those standards are derived from the SAE (Society of Automotive Engineers) Internationals and IEC (International Electrotechnical Commission). USA, Europe and China are also following the same standards to build the EV (Electric Vehicle) infrastructure. This paper provides the Indian standards to build EV charging infrastructure and comparing it with other countries. Glimpses on energy demand for electric vehicles in Indian market. It also provides the demanding wireless power transfer technology in EV’s. Status of Standards provided by the industry on wireless power transfer. Factors that are necessary to be considered before drafting the standards for WPT.
1 India’s Charging Infrastructure
India aims to have at least 15 percent of the vehicles on its roads to be electric in five years, signalling the government’s wish to join a long list of countries around the world that are already seeking to cut fossil fuels aggressively. While cumulative global sales of passenger electric vehicles likely surpassed 4 million in August, with China accounting for more than a third since 2011, India sold an estimated 2,000 EVs 2017. EVs may account for about 7 percent of sales in India by 2030 by [1]. The government of India (Department of Heavy Industries – DHI) is already providing incentives through FAME scheme (Faster Adoption and Manufacturing of Electric Vehicles) from the year 2015 in order to reduce the price of EV’s. The government has also approved pilot projects, charging infrastructure projects and technological development projects. Energy Efficiency Services Limited (EESL), under the administration of Ministry of Power, Government of India (GoI), has ordered 10,000 EVs [2]. With all the initiation that India has been taking, there is serious issue of charging infrastructure for both wired and wireless charging system. Standardization to the technical aspects of the EVs charging, range and price are the major barrier for deployment. Currently in India, there are very small scale of AC and DC charging stations available, which are installed by EV manufacturers. Even though the AC slow charging infrastructure at residences, workplaces and public places requires very low investment when compared to fuel stations. Still, the EV charging infrastructure growth in India is not up to the mark. To grow large scale of charging infrastructure in India it requires continuous support from the government, utility grid authorities and automobiles manufacturers.
1.1 Indian EV Charging Standards
Department of Heavy Industry (DHI) & Automotive Research Association of India (ARAI) drafted industry standard for Electric Vehicle Charging infrastructure on December 2015. The first draft of AC-charging configurations are Type-I and Type-II. Type-III standards are published in the second draft on May 2017. [3]
1.1.1 Type-I: AC001(3.3KW,15A,230V)
● Derived from IEC 60309 Standard ● AC Slow charging ● Each outlet will have up to three independent charging sockets. ● Input: 3 phase AC Supply, 5 wire (3 phase+ Neutral +PE- Protective Earth). Nominal Input Voltage is 415V (+6% and -10%) as per IS 12360 ● Frequency is 50Hz ± 1.5 Hz ● Output: Single phase, two wire system 230V (+6% or -10%) and 15 Amps as per IS12360 ● No Communication protocol is used between EV and EVSE (EV Supply Equipment)
1.1.2 Type-II: AC001(>3.3KW)
● Charger power is greater than 3.3KW. ● This AC Fast charging. ● Outlet is derived from the Standard IEC 62196 and IEC 61851. ● Charging with 415V, three phases, 63A. ● Control Pilot (communication protocol) extends to control EV charging Equipment system (EVSE).
1.1.3 Type-III: DC001(48V/2V,10KW/15KW)
● Input: 3 phase AC Supply, 5 wire (3phases+N+PE). Nominal Input Voltage is 415V & frequency is 50Hz ± 1.5 Hz (+6% and -10%) Maximum 200 Amps as per IS 12360. ● Output- 48V or 72V DC, based on suitable charger Configuration for vehicle battery. ● Charger Configuration Types ● Single Vehicle charging at 48V/72V with a maximum of 10kW, or a 2 Wheel vehicle charging at 48V with maximum 3.3kW. ● Single Vehicle charging at 48V with a maximum of 10KW, or 72V with maximum 15kW or a 2 Wheel vehicle charging at 48V with maximum 3.3kW.
1.2 Charging System Available in The World
1.2.1 The International Electrotechnical Commission (IEC 62196) modes definition:
Mode-1: Slow charging from a regular electrical socket (single or three phase) Mode-2: Slow charging from a regular socket but which equipped with some EV specific protection arrangement Mode-3: Slow or Fast charger using a specific EV multi-Pin Socket with control and protection functions Mode-4: Fast charging using some specific charger technology such as CHAdeMO. [4]
1.2.2 European EV Standards
● Normal power or Slow Charging with rated power inferior to 3.7kW is used for domestic application or for long-time EV parking. ● Medium power or quick charging with a rated power from 3.7-22kW is used for private and public EV ● High power or fast charging with a rated power superior to 22kW is used for public EV [4]
1.2.3 American EV Standards
TYPE-1: The charger is on-board and provides an AC voltage at 120 or 240 volts with a maximum current of 15A and a maximum power of 3.3kW
TYPE-2: The charger is on-board and provides an AC voltage at 240V with a maximum current of 60A and a maximum power of 14.4kW.
TYPE-3: The charger is off-board, so the charging station provides DC voltage directly to the battery via DC connector with maximum power of 240kW. [4]
Table 1. Electrical Rating of Different Charge Method in North-America
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1.2.4 EV charging based on Power and Usage Location
Charging ratings of different electric vehicles based on their usage are classified from normal power rating of 3.7kW to >22kW.
Table 2. Classification of EV charging based on Power and Usage Location.
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2 Electric Vehicles in India
2.1 Energy Demand for EV
Energy required by utility grid increases as demand in the electric vehicle market. The automobiles production in India is at a CAGR of 5.81% from 2007 to 2018. The vehicles include 80.9% of two wheelers, 13.2% of passenger cars, 2.5% of three wheelers & 3.4% of commercial vehicles [5]. Out of these, close to 99.9% are conventional ICE vehicles which are burning petrol & diesel for traction. The figure 1 shows the historical vehicle production data along with the estimated production volumes by 2030 which is indicated with blue line and also it shows the historical & estimated fuel (both petrol & diesel) consumption in MMT (Million Metric Tonnes) which is indicated with red line.
Fig. 1 Fuel Consumption & Vehicle Production Data
The total electricity generation in India during the year 2016-17 is 1160TWh which is used in domestic, commercial, industrial & agriculture sectors. The total electricity required, if all the existing ICE vehicles are converted into BEV’s during the year 2016-17 is 439TWh which is 34.26% additional to the existing electricity generation. By 2030, the electricity generation (Egen) in India will grow along with the demand in the domestic, commercial, industrial & agriculture sectors which is shown in the figure 3 (indicated with blue). Apart from this, there will be a growth in the production of automobiles in India by 2030 and if all these vehicles are 100% BEV’s the required electricity generation (Egen_tr) demand on the utility grid increases which is also shown in the figure 3 (indicated with red).
Fig. 2 Projected Electricity demand up to 2030
Hence by 2030 in India, 100% of BEV’s would require an energy of 929.3 TWh which is about 37% additional electricity to be produced apart from estimated electricity generation of 2500 TWh (includes domestic, commercial, agriculture & industrial sectors etc.) [6].
2.2 Indian Automobile Industry – An Overview
The Indian Automobile Industry is currently ranked 5th largest in the world and is set to be the 3rd largest by 2030.The requirement of mobility in India is set to change dramatically in the near future to cater to the requirement of 1.30 billion population While there is a vision for 100% electric vehicles by 2030, most industry experts indicate that around 40-45% EV conversion by 2030 is a realistic expectation. A major push towards EVs will be led by the public transportation requirements in India – Fleet cars, E-Buses, 3 wheelers and 2 wheelers. Personal vehicle options for EVs will still be a relatively smaller element in the whole pie. The Government plans to work towards creating a demand for EVs by buying in bulk, which could provide for large orders for automakers. A tender for 10,000 cars is already issued and now a major tender for electric buses in 11 cities is likely to be released soon. [7]
2.3 The outlook for 2018
– Passenger cars to see a higher increase in new model launches compared to utility vehicles.
-Production capacity will also be added at car makers to reduce waiting periods and to boost demand. (Passenger vehicle segment growing between 7-9%).
– In the two-wheeler segment, motorcycles are expected to grow moderately while scooters will continue to grow in double digits with two-wheelers growing between 9-11% in FY’18.
– Industry moving towards a March 2020 launch of BS-6 and most OEM/Auto component firms have made investments to meet this deadline.
2.4 EV Chargers
2.4.1 EV Chargers in India
Post FAME and the Niti Ayog Plan announcement, we have seen an increased trust on setting up EV chargers in India. Some names we could confirm, who have joined interest in EV Chargers manufacturing are Raychem RPG India, Analogic India, Deltron, EOS Power, AdorPowertron, Kraft Power Con, Elind etc.
2.4.2 Global EV Charger firms on Indian Market
More than six key global firms eyeing the EV Chargers market closely. Firms like ABB India, Delta India, Schneider India, Siemens India etc are looking at the Indian market closely. These firms have their global designs and products. They are studying the technical/specifications, business models and potential for their products. All these firms are only looking at the 4 Wheelers’ (Cars) EV Chargers.
Table 3. Total number of EV Charging Stations in India
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Table 4. Likely future market for EV Chargers in India
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3 EV Wireless Charging Standards
As the world is advancing towards the wireless charging from smaller biomedical application to the electric vehicle charging system. Standardization is the main barrier holding the commercialization of high-voltage and high-power WPT for EV charging. It includes safety criteria, efficiency, EM(Electromagnetic) limits, and interoperability targets, along with test setup for getting wireless charging. It should provide the compatible charging station to all the EV owners. IEC-61980-1 standard contains the total system of WPT from supply network to EVs charging the battery or any equipment of the same at the standard supply of 1000-Vac or 1500-Vdc. These all are addressed by SAE in its standard SAE TIR J2954 (TIR-Technical Information Report). This is the first standard developed by SAE in WPT for an EV charging application. This standard is developed specifically for SWC. The frequency band, interoperability, safety, coil definitions, and EMC/EMF limits from SAE TIR J2954 allow any attuned vehicle to charge wirelessly from its wireless home charger, office or a commercial charger with the same charging ability. Table 5 shows key standards for wireless charging. [8] In the near term, vehicles that are capable to be charged wirelessly under recommended practice should also be able to be charged by SAE J1772 plug in chargers. SAE recommended practice J2954 is intended to be used for stationary applications (charging while vehicle is not in motion). Dynamic applications may be considered in the future based on industry feedback. SAE Recommended Practice J2954 is meant to be used for interoperability, performance and emissions testing, where a single standard coil-set has been chosen for the WPT Power Class 1 and 2 to 7.7kW, per Z-Classes (1 through 3) which is circular topology. However, there are two reference options for WPT 3 to 11kW per Z-classes (1 through 3) with two topologies. The next revision of the Recommended Practice in 2018 is slated to have one standard coil set for WPT 3. SAE TIR J2954 establishes a common frequency band using 85 kHz (81.39 – 90 kHz) for all light duty vehicle systems. [9].
Table 5. Wireless Charging standards
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4 CONCLUSION AND FUTURE WORK
The major obstacle to the adaptation wireless EV’s charging is the standardization. The inductive coupled power transfer has given greater results than the other types of system. The factors that needs standardization are geometry of coils, volume and weight, position of coil, alignment, frequency, compensation topologies, distance and Efficiency. Including the human and environmental consideration due to electromagnetic radiation. DHI and ARAI has provided standards for conductive charging infrastructure. The standards provided are very limited to the charging equipment and its features. But lacks in charging time, driving range, lack of awareness and efficiency.
Published by Huimin Wang1, and Zhaojun Li2, 1School of mechanical and electrical engineering University of Electronic Science and Technology of China Chengdu, China. Email: hmwang1206@163.com 2Department of Industrial Engineering and Engineering Management Western New England University, Springfield, MA, 01106. Email: zhaojun.li@wne.edu
Abstract—The failure of power system transient stability is one of the main factors causing catastrophic accidents of power systems. Therefore, it is of great significance to evaluate the transient stability of a power system. This paper first introduces the evaluation methods of power system transient stability, including the assessment methods based on time domain simulation, direct method, artificial intelligence-based methods and the probabilistic assessment method. The key challenges in power system transient stability assessment are reviewed and analyzed, including the stability evaluation of power-electronized power system and the main elements of artificial intelligence method used in transient stability assessment. Last, the future research directions and conclusions are discussed.
Keywords – transient stability assessment; power system; power-electronized power system; probability assessment; artificial intelligence
I. INTRODUCTION
Power system transient stability is that the ability of generators to continue to operate synchronously after the system is disturbed [1]. The causes of failure of power system transient stability include short-circuit fault, sudden disconnection of lines or generator, etc. Accurate and fast transient stability assessment method is important to the security operation of power system. With the gradual advancement of smart grid construction, long-distance, huge capacity transmission mode and high-proportion power electronics, the new risks are introduced in power system [2]. Power shortage accidents and complex cascading failures further rise the difficulty of power system stability analysis and control.
The direct method, time domain simulation method and artificial intelligence (AI) method are commonly used for transient stability analysis of traditional power system [3].
The time domain simulation method is to solve the differential equations and algebraic equations, which describe the transient process of the system by various numerical integration methods. Then the stability is judged according to the change of the relative angle between the rotor of the generator. In each step interval, it is approximated that the rotor is in constant acceleration motion [4].
The direct method is mainly based on Lyapunov stability criterion, which is proposed by constructing a function directly in order to quantitatively measure the power system transient stability [5]. Traditional transient stability calculation is carried out under the condition that the topology, parameters, operation conditions and disturbance modes of power system are given. The structure of power system network model is presented in Figure 1.
Figure 1. The model structure of power system whole network
In Figure 1, Idi and Iqiare rotor current of direct-axis and quadrature-axis respectively; Ediand Eqi are rotor voltage of direct-axis and quadrature-axis; δi is power angle; Exi and Eyi are stator voltage of direct-axis and quadrature-axis; Ixi and Iyi are stator current of direct-axis and quadrature-axis; Efi is excitation voltage; Vtiis node voltage; Pmiis input power of prime mover; wiis angular speed of prime mover.
However, previous works focused on the application of different methods to establish Lyapunov functions for power systems. With the development of Lyapunov functions, it is recognized that another key problem is to accurately estimate the stability region after failure of power systems [6]. The calculations of energy function of flexible AC transmission system (FACTS) devices and revised transient energy function of FACTS are formulated in [7].
With the advantage of wide-area measurement system (WAMS) technology, the artificial intelligence prediction method based on WAMS can use real-time measurement data to train the transient stability classifier online, instead of using off-line model to simulate various disturbances to obtain data [8]. Artificial intelligence generates databases as input of established networks through a large number of off-line simulations, and uses intelligent algorithms to construct stable classifiers. Then the stability of the system is evaluated by training stable classifiers [9]. The method of Artificial intelligence is used to develop a load disaggregation approach for bulk supply points based on the substation rms measurement in [10]. The optimization problem is formulated, and the Cuckoo search algorithm is adopted for optimal designing of power system stabilizer in [11]. The analysis about artificial intelligence optimum plans and improving the functioning of the power systems economically are made in [12]. Compared with the time domain method, the artificial intelligence method does not need to establish the mathematical model of the power system. Artificial intelligence method uses the measured response information to extract the characteristics that can reflect the physical nature of the system transient stability. Then the transient stability assessment is carried out by establishing the mapping relationship between the characteristics and the system stability.
Usually, artificial intelligence takes an offline approach to obtain feature samples which can accurately characterize the inherent mechanism of power system operation under anticipated accidents. The acquired samples are trained and learnt repeatedly, and the method of predicting the transient stability of power system is constructing the classifier. Meanwhile, the feature samples are input into the classifier in real time. The comparison of the three methods for evaluating the power system transient stability is shown in Table I.
TABLE I. COMPARISONS OF THREE METHODS
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II. BASIC ANALYSIS STEPS OF DIFFERENT METHODS APPLIED TO TRANSIENT STABILITY ANALYSIS
A. Time Domain Simulation Method for Transient Stability
From the previous discussion, the calculation of traditional transient stability is carried out under the condition that the topology, parameters, operation conditions and disturbance modes of power system are given. The time domain simulation method is to solve the differential equations and algebraic equations, which describe the transient process of the system by various numerical integration methods. The equations are as follows:
.
where δ is angle of power energy; w is angular frequency; wN is rated angular frequency; TJis electromagnetic torque; PT is mechanical power and Pe is electromagnetic power.
B. Direct Method for Transient Stability
The direct method is mainly based on Lyapunov stability criterion. The direct method contains potential energy boundary surface (PEBS), relevant unstable equilibrium point (RUEP) and extended equal area criterion (EEAC) [13]. The EEAC method refers to that the regulation of excitation can also promote the transient stability of power systems [14]. A multi-objective optimization method is proposed, which is to model transient stability as an objective function rather than an inequality constraint in [15]. Only the excitation system with fast rise of excitation voltage and high peak voltage can have a significant effect on improving transient stability. The reason is that fast excitation reduces the acceleration area, increases the deceleration area and then improves the transient stability of the system. Good excitation control plays a more important role in increasing artificial damping and eliminating the second pendulum or multi-pendulum out-of-step [16]. Basic steps of direct method are shown in Figure 2.
Figure 2. Basic steps of direct method
C. Probabilistic Assessment for Transient Stability
Some parameters of power system are random due to errors in measurement, estimation or calculation [17]. The operating conditions and random disturbances are ever-changing, and deterministic analysis does not consider the possibility of various accidents. Transient stability probability analysis is different from deterministic analysis [18]. Different from deterministic analysis, the transient stability probability analysis determines the probability indicators according to the statistical characteristics of the main stochastic factors affecting power system transient stability.
Considering the power system, tcr and tcl are assumed to be the critical and actual clearing times of the fault respectively. The principle of calculating the probability indicators of transient instability is shown in Figure 3 [19,20].
Figure 3. The principle of calculating the probability indicators of transient instability
If and only if tcl > tcr, the system will be transient unstable. The fault clearing time tcl is a random variable, and the system transient stability assessment can be expressed by the probability of system instability when the fault occurs.
.
In the formula, I refer to the event leading to transient instability of the system and the probability density function of the fault clearing time is tcl. If tcr and probability density function of the fault are known, it is easy to obtain the probability indicators of transient instability of the system under this fault condition.
In summary, the method of probabilistic transient stability analysis of power system is divided into analytical method and Monte Carlo method [21]. In order to assessing the probability of power system stability, the conditional probability theory is used. The influence of probability distribution of random factors is mainly considered. Comparison of flow charts of deterministic and probabilistic of transient stability analysis of power system is shown in Figure 4.
The analytical method uses conditional probability theory in statistics to evaluate the stability probability of the system. The probability indicators of transient stability are determined according to the statistical characteristics of the factors that will affect the power system transient stability. Probabilistic transient stability analysis makes up for the limitation of deterministic method in transient stability analysis, which is an important breakthrough and supplement to deterministic method.
Figure 4. Comparison of flow charts of deterministic and probabilistic power system transient stability analysis
But the calculation of probabilistic transient stability analysis is usually very large. Probabilistic analysis of transient stability of power system involves the probability of the system state, which depends not only on the location and type of the fault, but also on the relay protection settings, and the system state before the fault. The probabilistic transient stability analysis procedures are shown in Figure 5.
Usually, disturbance accident simulation is that using probability model to simulate disturbance accident, disturbance includes location, type and other information.
In the process of Monte Carlo simulation, some uncertain parameter models can be obtained from historical data or assumed to be a probability distribution function. The probabilistic transient stability assessment includes state sampling, transient stability simulation and transient instability indicators calculation. Probability assessment of transient stability of power system based on Monte Carlo method is shown in Figure 6.
Figure 6. The assessment of transient stability of power system based on Monte Carlo method
D. Artificial Intelligence Method
Artificial intelligence method is applied to on-line transient stability analysis. Input feature selection and evaluation model are the key points in the research of assessment of transient stability of power system based on artificial intelligence.
In the construction of assessment of transient stability strategy based on artificial intelligence, the stable discriminant input characteristic is composed of some combined operational variables reflecting the dynamic response during system failure. Subsequently, artificial intelligent technologies are applied to establish the relationship of the input characteristics and the stable state of transient stability characteristics. In the process of modeling, choosing appropriate input features is the key to design, and the electrical value should be converted in center of inertial (COI) frame. A large number of studies have applied feature transformation algorithms such as injection principal component analysis to reduce the input feature dimension and improve learning efficiency [22]. The basic steps of obtaining input characteristic variables are shown in Figure 7.
Figure 7. The basic steps of obtaining input characteristic variables
III. TRANSIENT STABILITY ASSESSMENT OF POWER ELECTRONIC DOMINATED POWER SYSTEM
A. Power Electronics Dominated Power System
With the rapid development of new semiconductor materials and control technology, the penetration of power electronic converter in power supply side, transmission network and load side of power system is getting higher and higher, and the power level of power electronic converter is also rising. Since the 1950s, power semiconductor technology has made great progress. Converters are formed to connect to power systems as voltage or current sources [23]. The power-electronized power system is presented in Figure 8.
Compared with traditional AC power system, the characteristic of power-electronized power system is that the system topology changes with the switching action of power electronic devices. The whole system is time-varying (nonautonomous), and there is interaction of multi-time scale control. In addition, the power electronic converter itself has structural nonlinearity and complexity, and there are overmodulation and limiting phenomena when using pulse width modulation. So that the power electronics dominated power system has saturation nonlinearity. These characteristics bring difficulties to power system stability analysis [24]. The stability of power electronic dominated power system was determined by time domain method, impedance analysis method and generalized short circuit ratio previously. At present, the small signal stability analysis of power-electronized power system has achieved preliminary results. Small signal stability can ensure the asymptotic stability of the equilibrium point, which is a necessary step in device design. However, considering small signal stability alone, the boundary of the stability region can not to be determined and the stability margin of the equilibrium point can not to be judged. Therefore, it is needed to analyze the transient stability of power electronics dominated power system.
B. Artificial Intelligence Method for Transient Stability Assessment of Power Electronics Dominated Power System
Time domain simulation method is considered to be the most mature and reliable method, which is also applicable to power electronic power system. However, the simulation method has the disadvantages of large amount of calculation, long simulation time and impossible to simulate all operation states.
Another main method of transient stability analysis which can be used in power-electronized power systems is that direct method based on modern differential dynamic system. The direct method determines the transient stability of the system by comparing the transient energy with the critical energy of the power system at the time of fault clearing.
However, conventional algorithms of stability analysis and the stability control systems is unable to evaluate the power system operation under smaller and smaller stability margin. Thus, the AI method are complementary to the traditional transient stability analysis method. Artificial intelligence method is applied to on-line transient stability analysis.
Figure 8. Power electronics dominated power system
Artificial intelligence method is mainly used in combination with wide area monitoring system, which undertakes the functions of pretreatment and post-processing. Transient stability assessment is a mapping from characteristic information to stability category. Its basis is to determine the classification step of decision attribute. Eight attribute variables are listed in the Table II. The class variables are stable and unstable, which are expressed by 1 and 0 respectively.
TABLE II. COMPOSITION OF ATTRIBUTE VALUE OF INPUT VECTOR
.
There are many characteristics reflecting transient stability. One of the purposes of classifier design is to select representative conditional attributes variables, in order to provide as much information as possible by using conditional attributes. Some features related to transient stability can be extracted. Machine learning algorithm is applied to assessment of transient stability, the key problems including initial feature input, feature selection, model learning and training. The difficulty lies in obtaining the characteristic sample set which can represent the physical essential characteristics and designing the classifier with the lowest error rate for transient stability assessment. The transient stability assessment process of power electronics dominated power system is shown in Figure 9.
IV. DISCUSSION ON FUTURE TRENDS
Figure 9. The assessment of transient stability of power system
The artificial intelligence method can realize the identification and decision-making of fast determination of power system transient stability, which is based on the measured data rather than the power system model and parameters. However, this method does not take into account the physical mechanism of the dynamic response of the power system, and requires precise learning samples when calculating. However, it is difficult to provide a large number of valid samples matching the actual operation through off-line simulation. Therefore, artificial intelligence method applied to power system transient stability has the difficulties including deep mining of massive WAMS data, off-line simulation and errors in the use of measured data.
In addition to the traditional power system analysis method, a new analysis method of power system transient stability should be proposed according to the characteristics of power electronics dominated power system. The analysis method combined with bifurcation theory can be studied.
In the aspect of data acquisition, simulation method is used to obtain transient stability analysis data instead of relying on actual fault data. The main reason is that the probability of actual power system failure, especially transient instability, is low. Because the power system vary with time, the applicability of historical data decreases and it is difficult to provide high-quality training data for artificial intelligence algorithm. From this point of view, how to improve the consistency between simulation data and actual fault data is an urgent challenge to be solved.
V. CONCLUSION
This paper presents an overview of the power system transient stability assessment methods. A comprehensive analysis and comparison of the deterministic assessment and probabilistic assessment is presented. Compared with traditional AC power system, the characteristics of power electronics dominated power system have changed dramatically, but it is essentially still a time-varying and nonlinear complex system. The characteristics of power-electronized power system are analyzed, and the artificial intelligence methods of transient stability for power-electronized power system have been presented. In addition, the AI methods which have been used to analyze power system transient stability are reviewed, and data acquisition, feature extraction and algorithm application are discussed.
This research is partially supported by the National Science Foundation of China, No. 71771038.
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[1] P. Kundur, N. J. Balu, and M. G. Lauby, Power system stability and control, New York: McGraw-hill, 1994. pp.30-46. [2] F. Li, W. Qiao, H. Sun, et al. “Smart transmission grid: Vision and framework,” IEEE Trans. on Smart Grid, vol. 1, pp. 168-177, February 2010. [3] M. Pavella, “Power system transient stability assessment-traditional vs modern methods,” Control Engineering Practice, pp. 1233-1246, October 1998. [4] M. L. Crow, Computational methods for electric power systems, Crc Press, 2015. [5] J. Geeganage, U. D. Annakkage, T. Weekes, et al. “Application of energy-based power system features for dynamic security assessment,” IEEE Trans. on Power Systems, vol. 30, pp. 1957-1965, April 2015. [6] T. L. Vu, K. Turitsyn, “Lyapunov functions family approach to transient stability assessment,” IEEE Trans. on Power Systems, vol. 31, pp. 1269-1277, February 2016. [7] J. Aghaei, M. Zarei, M. Asban, et al. “Determining potential stability enhancements of flexible AC transmission system devices using corrected transient energy function,” IET Generation, Transmission & Distribution, vol. 10, pp. 470-476, February 2016. [8] S. M. Ashraf, A. Gupta, D. K. Choudhary, et al. “Voltage stability monitoring of power systems using reduced network and artificial neural network,” International Journal of Electrical Power & Energy Systems, vol. 87, pp. 43-51, 2017. [9] K. W. Kow, Y. W. Wong, R. K. Rajkumar, et al. “A review on performance of artificial intelligence and conventional method in mitigating PV grid-tied related power quality events,” Renewable and Sustainable Energy Reviews, vol. 56, pp. 334-346, 2016. [10] Y. Xu, J. V. Milanović, “Artificial-intelligence-based methodology for load disaggregation at bulk supply point,” IEEE Trans. on Power Systems, vol. 30, pp. 795-803, February 2015. [11] S. M. A. Elazim, E. S. Ali, “Optimal power system stabilizers design via cuckoo search algorithm,” International Journal of Electrical Power & Energy Systems, vol. 75, pp. 99-107, 2016. [12] S. M. Zahraee, M. K. Assadi, R. Saidur, “Application of artificial intelligence methods for hybrid energy system optimization,” Renewable and Sustainable Energy Reviews, vol. 66, pp. 617-630, 2016. [13] S. C. Savulescu, Real-time stability in power systems: techniques for early detection of the risk of blackout, New York: Springer, 2006. pp. 147-165. [14] B. Wang, B. Fang, Y. Wang, et al. “Power system transient stability assessment based on big data and the core vector machine,” IEEE Trans. on Smart Grid, vol. 7, pp. 2561-2570, May 2016. [15] C. J. Ye, M. X. Huang, “Multi-objective optimal power flow considering transient stability based on parallel NSGA-II,” IEEE Trans. on Power Systems, vol. 30, pp. 857-866, February 2015. [16] Y. Zhou, H. Huang, Z. Xu, et al. “Wide-area measurement system-based transient excitation boosting control to improve power system transient stability,” IET Generation, Transmission & Distribution, vol. 9, pp. 845-854, September 2015. [17] M. Panteli, D. N. Trakas, P. Mancarella, et al. “Boosting the power grid resilience to extreme weather events using defensive islanding,” IEEE Trans on Smart Grid, vol. 7, pp. 2913-2922, June 2016. [18] C. Peng, J. Zhang, “Delay-distribution-dependent load frequency control of power systems with probabilistic interval delays,” IEEE Trans. on Power Systems, vol. 31, pp. 3309-3317, April 2016. [19] M. Vatani, T. Amraee, A. M. Ranjbar, et al. “Relay logic for islanding detection in active distribution systems,” IET Generation, Transmission & Distribution, vol. 9, pp. 1254-1263, December 2015. [20] T. Gonen, Electrical power transmission system engineering: analysis and design, CRC press, 2015, pp. 300-328. [21] S. Liu, P. X. Liu, X. Wang, “Stochastic small-signal stability analysis of grid-connected photovoltaic systems,” IEEE Trans. on Industrial Electronics, vol. 63, pp. 1027-1038, February 2016. [22] K. P. Wong, “Artificial intelligence and neural network applications in power systems,” International Conference on Advances in Power System Control, pp. 37-46, 1993. [23] Q. C. Zhong, “Power-electronics-enabled autonomous power systems: Architecture and technical routes,” IEEE Trans. on Industrial Electronics, vol. 64, pp. 5907-5918, July 2017. [24] Y. Yang, H. Wang, A. Sangwongwanich, et al. Design for reliability of power electronic systems, Power Electronics Handbook. Butterworth-Heinemann, 2018, pp. 1423-1440.
Published by Sivaraman P, SMIEEE, PEng (India), Senior Power Systems Engineer Chennai, Tamil Nadu, India
We all know that Total Harmonic Distortion (THD) and Total Demand Distortion (TDD) are used to evaluate the presence of harmonics in the power system.
The “IEEE Std 1547-2018 IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces” and “IEEE Std 2800–2022 IEEE Standard for Interconnection and Interoperability of Inverter-Based Resources (IBRs) Interconnecting with Associated Transmission Electric Power Systems” provides another terminology called “Total Rated Harmonic Distortion (TRD)” for evaluating the presence of current harmonics in the Inverter Based Resources (IBR) plant or renewable power plants.
As per IEEE Std 1547-2018, TRD is defined as the total root-sum-square of the current distortion components (including harmonics and inter-harmonics) created by the DER unit expressed as a percentage of the DER rated current capacity (Irated).
As per IEEE Std 2800–2022, the non-fundamental frequency RMS current flowing (including harmonics, interharmonics, and noise) between the transmission system (TS) and the inverter-based resource (IBR) plant with respect to the rated RMS current capacity (Irated).
As per IEEE Std 1547-2018, the TRD can be calculated by using the below expression,
.
Where,
• I1 is the fundamental current as measured at the RPA • Irated is the DER rated current capacity (transformed to the RPA when a transformer exists between the DER unit and the RPA) • Irms is the root-mean-square of the DER current, inclusive of all frequency components, as measured at the RPA
As per IEEE Std 2800-2022, the TRD can be calculated by using the below expression,
.
Where,
• I1 is the fundamental-frequency current as measured at the RPA • Irated is the IBR plant rated current capacity based on IBR plant MVA rating at the RPA • Irms is the root-mean-square of the IBR plant current, inclusive of all frequency components up to 50th order, as measured at the RPA; measurement of harmonics to the 50th order requires meters compliant to IEC 61000-4-30 Class A
Note: RPA – Reference Point of Applicability. RPA shall be Point of Common Coupling (PCC) or any other location
Author: Sivaraman P, SMIEEE, PEng (India), Senior Power Systems Engineer Chennai, Tamil Nadu, India. LinkedIn Profile
Published by Somudeep Bhattacharjee1, Saima Batool2, Champa Nandi1 and Udsanee Pakdeetrakulwong3,* , 1Tripura University, 2School of Information Systems, Curtin Business School, Curtin University, 3Nakhon Pathom Rajabhat University *Corresponding author; e-mail: udsanee@webmail.npru.ac.th
Abstract. Selecting the location for installing electric vehicles charging stations is important to ensure EV adoption and also to address some of the inherent risks such as battery cost and degradation, economic risks, lack of charging infrastructure, risky maintenance of EVs, problems of its integration in smart grid, range anxiety, auxiliary loads and motorist attitude. In this article, we investigate these problems by studying three aspects – 1) three types of electrical vehicle charging stations (Level 1, Level 2 and DC), 2) different types of batteries and 3) different types of electric vehicles. We compared and contrasted the features of these charging stations, batteries and EV to identify the best choice for a given scenario. We applied the framework proposed in [1], and used Agartala, India as a case study to identify location for charging stations in and around Agartala suburbs.
Keywords: Electric vehicle, charging stations, electric vehicle battery, charging stations location conditions, infrastructure
1. Introduction
An electric car is actually an alternative-design automobile that basically uses an electric motor to provide power to the car, with the electricity being provided by a battery. On the other hand, a conventional car does have a lead-acid battery as part of its standard equipment but this battery is used for operating the starter and not providing power to the vehicle. This technology works in this way that the electric vehicle uses a motor just like conventional, internal combustion engine cars. The main difference is that the electric vehicle power supply is derived from its battery-stored electricity and not from the mechanical power derived from burning gasoline. The electric vehicle replaces the traditional gasoline or diesel engine and fuel tank with an electric motor, a battery pack and controllers. The vehicle uses a controller that provides power to the electric motor that uses rechargeable batteries as its energy source. The motor itself can be either AC or DC. The main advantage of electric vehicle is mainly the motor and battery configuration. This allows the vehicle to run more fuel efficiently. PHEV (plug in hybrid electrical vehicle) is a hybrid vehicle that can be plugged into the power grid for charging the battery. In this vehicle, a medium-capacity battery is available that helps the electrical vehicle in allowing it in all-electric modes , to achieve several kilometers , and acceleration rates and also it help to attain top speeds comparable to those of gasoline-powered vehicles. Examples: Chevrolet Volt (often classified as an EREV), Ford C-Max and Fusion Energi, Cadillac ELR and Toyota Prius PHEV. On the basis of different types of power trains (or drive trains), hybrid electric vehicles can be classified into three categories:
(1) Parallel hybrid, (2) Series hybrid, and (3) Power-split hybrid.
Among these, the parallel hybrid electric vehicle is commonly adopted. PHEVs are usually consists of an electric motor and an additional ICE for propulsion. This mixed propulsion system helps in enabling PHEVs to be driven in two modes: charge depleting (CD) mode and charge sustaining (CS) mode. When this type of electric vehicles operated in CD mode then it mainly drawn energy from on-board battery packs. If the battery state of charge (SOC) has been depleted to a pre- determined level, PHEVs will then switch to CS mode and utilize the ICE system for further propulsion. When it is operated in CS mode, PHEVs combine both power sources so that it can operate as efficiently as possible. Meanwhile, the controller can monitor the battery SOC level and then maintain it with in a pre- determined band.
2. Objectives
Global warming is becoming a major problem and the best way to combat it is to reduce air pollution. Electric vehicles (EVs) are considered a best option to reduce air pollution and making environment safe again. In order to operate, electric vehicles need charging stations at suitable places. If appropriate and recommended places are not chosen then it will decrease the utilization, visibility and effectiveness of a charging station, which results in adoption of traditional carbon- emitting gasoline vehicles and a decrease in EV sales. Hence, it is very essential to carefully select locations for EV charging stations for promotion of EVs and the cause of avoiding global warming.
The main objective of this study is to determine the best locations for installing EV public charging stations in Agartala, India. Our selection for EV charging station location will depend on the set of conditions that have to be met in order to qualify for a place to be established as an EV public charging station. In addition, we will also determine the best type of charging station based on the type and charging duration of the particular EV type. Finally, this research will provide a specific and thorough insight of establishing EV public charging station in growing cities like Agartala, India.
3. Research Methodology
The framework proposed by [1], is used as a guideline to assess the implementation of EV charging infrastructure for Agartala city. To achieve this, 3 areas are studied 1) Different types of charging stations, 2) EV types, 3) battery types. First of all, different types of charging stations are studied and compared. Next, various types of EVs are analyzed thoroughly. Moreover, the charging vehicle location selection conditions, infrastructure and the best suitable places in Agartala are selected based on the electric vehicle charging station location selection conditions and the map of Agartala.
Preliminary Concepts
We know that as the global benefits of a serious energy crisis, alternative energy for sustainable development is renewable energy .The generation of this energy is pollution free and so this is the first choice of many countries of the world like the United states, Japan and Europe and so the development of electric vehicles is a way to save nature and to resolve important issue of planning the national grid. An electrical vehicle requires charging station and so the locations of charging station have to be determined carefully. A charging station is a location where an electric vehicle can be plugged in to have electric charge deposited in to their batteries. They are not chargers, but can be considered as an electrical energy source.
Different types of charging stations:
There are mainly three types of charging stations which are categorized as Level 1, Level 2 and DC charging stations.
LEVEL 1 Charging Station (120 volts and up to 16 amps): In all electrical vehicles, an on-board Level 1 charger is equipped that can be plugged into any normal power outlet (C S A 5-15R*). It gives an advantage of not requiring any electrical work, or at least minimizes any installation costs. Table 1 shows the charging time using a Level 1 charger based on distance driven. 12-A charging cable and 120-V outlet is considered. Charging cable rated less than 12A require longer charging times [3].
LEVEL 2 Charging Station (240 volts and 12-80 amps): In this type, the charging time of Level 2 charging stations can be limited by the specifications of the on-board charger and the state of the battery, irrespective of the rated power of the charging station. It is believed that the charger capacity is going to increase in future, for example, Tesla already offers on-board 10 kW and 20 kW chargers. Table 1 shows that level 2 charging stations takes less time to charge as compared to the level 1 charging stations even though the distance traveled is similar. Level 2 charging stations have smart and timeless design. It is simple to use (plug the EV in and let it charge). It helps in reducing energy consumption. In addition, it offers Ethernet network for Radio Frequency Identification (RFI) authorization and vehicle ground monitoring circuit. The cord holder keeps the cord organized and out of the way of parking spaces, sidewalks and streets, etc. One example of this is Schneider EV link Indoor Charging Station, which has ground monitor and user friendly LEDs to display status like charging, detected fault, power etc. It has the capability for automatic recovery and restart after ground fault interrupt or main power loss [1].
DC Fast Charging Station (480 volts and up to 125 amps):
DC fast-charge stations generally support two standards: The North American SAEJ 1772 Combo standard and the Japanese JEVS G105-1993* standard. The configuration of the charging plug and the electrical vehicle socket follows the same basic principle compared to the communication protocol between the charging station and the electrical vehicle but have different standards [1]. Table 1 shows the time required to charge a battery with a 100-km range to 80% of its full capacity.
Analysis of Different Charging Stations
Table 1 shows the comparison of different charging stations which are categorized as Level 1, Level 2 and DC charging stations based on electric vehicle distance travelled in km, estimated energy consumption of electric vehicle in kWh, charging station power of electric vehicle in kW, approximate charging time in hour.
Table 1: The comparison between different charging stations levels (Adopted from [1]
.
Why Level 2 charging station is more suitable?
The answer to this question lies in this fact that the most important condition for selecting an electric vehicle charging station location is that how much time is spend by the consumer for charging his vehicle in the charging station. So the time spend is an important factor.
From these charts, it is clear that for the same amount of distance travelled in km, electric vehicle required different charging time in each type of charging station. Also it is shown that Level 2 charging station provides facility to consumer to charge his vehicle in a very short time as compared to other types of charging station. The charts that are shown above prove this.
Types of Electric Vehicles (EVs)
EVs can be divided into the following categories. First, on-road highway speed vehicle that is an electrical vehicle capable of driving on all public roads and highways. The performance of these electrical vehicles is similar to Internal Combustion Engine vehicles. Second, the city electric vehicles, normally, the city electric vehicles have been BEVs (Battery Electric Vehicle – these vehicles can be powered 100% by the battery energy storage system available on-board the vehicle) that are capable of driving on most public roads, but basically not driven on highways. The maximum speed is typically limited to 55 mph. Third category of EVs is also known as low speed vehicles (LSVs). Actually they are BEVs that are limited to 25 mph and are allowed in certain jurisdictions to operate on public streets posted at 35 mph or less. Commercial On-Road Highway Speed Vehicles is the last category of EVs. The commercial electric vehicles include commercial trucks and buses. These vehicles are available in both BEVs and PHEVs (Plug-in Hybrid Electric Vehicle – the vehicles utilizing a battery and an internal combustion engine (ICE) which is powered by either gasoline or diesel). Table 2 provides information on several different on-road highway speed electric vehicles, their battery pack size, and charge times at different power levels to replenish a depleted battery.
Table 2: Different types of EVs with battery pack size and charging times at different power levels (Adopted from [1])
.
Note: Power delivered to battery is calculated as follows: 110VAC x 12Amps x.85 eff.; 110VAC x 16Amps x .85 eff.; 220VAC x 32 Amps x.85 eff.; 480VAC x √3 x 85 Amps x .85 eff. From Table 2, it is clear that different electric vehicle configuration require different charging time for different battery size at different power levels to replenish a depleted battery. This helps to find out the charging time in minutes required by different electric vehicle configurations of different battery size at different power levels. Using Table 3, we study the charging time for 100km of BEV range with power supply, power (in kW), voltage (in V) and maximum current (in A). It helps to show the relation of charging time of fixed 100km distance with its power supply, power (in kW), voltage (in V) and maximum current (in A). Thus, the driver finds charging an electric vehicle as simple as connecting a normal electrical appliance. In addition, Table 12 provides comparison between different recharge times of BEV for 100km range. Consequently, it seems clear that charging through single phase takes longer time then 10 minutes of direct current charging, that is the reason for advocating for DC charging infrastructure for EVs.
Table 3: Charging time for BEV range of Electric Vehicles (Adopted from [1])
.
ELECTRIC VEHICLE BATTERY
The electric vehicle battery is the core component of an electric vehicle with one of the two propulsion sources of HEV and PHEV. Basically, the battery is the sole propulsion source for BEV. There are still some constraints on present EV battery technology, which works as a barrier for wider EV uptake. The current EV battery has relatively low energy density. This low energy density directly affects the maximum all-electric drive range of the EV. In addition, high battery cost of EV is also a big disadvantage as the purchase cost of EV is considerably higher than conventional internal combustion engine vehicle. Some concerns are also present about the battery life cycle and its safety features. However, EV battery goes through some tremendous improvements in the past decades. EV battery technology goes through a few development phases for inventing the battery with highenergy density, high power density, inexpensive, safe and durable. Lead-acid battery was the initial battery technology used in transportation and its name comes from the combination of lead electrodes and acid used to generate electricity. Lead-acid battery is a really a matured technology and also cheap. However, some apparent drawbacks of lead-acid battery are present, such as low energy density, heavy, require inspection of electrolyte level and are not environmentally friendly.
Table 4: Comparison of EV Battery Types and their specifications (Adopted from [3])
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Table 5: Comparison of EV Battery Types and their specifications (Adopted from [3])
.
Charging Vehicle Location Selection
The sites of the charging station have a very significant impact loads, at this point, charging station is very similar to traditional gas station, charging station requires a higher penetration of electric vehicles in areas surrounding the construction of a natural high, such as new urban planning to support key enter prices and so on. Our work contributes to identify suitable locations for construction of public charging stations. In this paper we have analyzed possibilities of establishing a public EV charging station in Agartala, India in particular. Charging stations located along the highways are also in high demand since high speed EVs usually requires fast charging.
Public Charging Stations
In this section we will list and describe the most suitable charging locations for the installation of public charging stations. These charging stations can be located at parking lots that serve train stations, shopping centres, restaurants, hotels and resorts. When selecting a potential charging station, the following criteria should be considered.
Traffic density
The first criterion is traffic density. Traffic density is a necessary factor because the size of the installation should be related to be expected number of users. If the charging station is located near a major road, with high traffic density, then maximum number of people may use it for charging their electrical vehicles. However, areas with high traffic density are in densely populated locations, where the land value is significantly higher. One way to address this concern is to use land that is already allocated for traditional parking lots and convert them to EVSPLs (Electric vehicles solar parking lots) [4]. Further, these lots can also be converted to multi-level parking’s where the EV can be on the top where they receive sunlight whereas the traditional vehicles can be underneath.
EV Charging Duration
The second criterion is EV charging duration; i.e. how long does it take to charge an electric vehicle. Electric vehicle need time to charge so it is necessary that the charging stations should be located near public places like shopping centres, work places, educational institutions so that people do not have to wait while their EVs are charging. The charging can happen while they are doing their usual activity such as being at work, weekly shopping etc. As (Nunes et al., 2016) suggests that public charging stations should be installed on worksites and public parks. This allows EV users to charge their EVs without having to wait [4].
Surrounding Vehicle Movement
The third criterion is the surrounding vehicle movement. This is important because charging vehicles must not hinder normal traffic flow, as it will become a hurdle, which may even cause accidents. Further, this location must not hinder pedestrian traffic or be subject to high pedestrian traffic because of the associated risk of vandalism. Public EV charging stations have numerous effects on its surrounding environment, transportation and energy needs and hence these implications have to be examined carefully [4]. One way of implementation would be along the street side parking bays. Electric vehicles (EV) have a very diverse characteristic, as it can act both as consumer and producer. In first case, EV’s act as consumer, it is depended on renewable energy resources, batteries, smart grid (G2V- Grid to vehicle) and electric chargers to recharge. In second case, it is producer as well, as EVs can be an essential part of the smart grid. It can act as an energy producer since it stores energy and can provide it back to smart grid when the demand is at peak, this process is known as discharge or (V2G- vehicle to grid). Whereas, there are some concerns about customers’ behaviour in participating in V2G programs that is uncertainty about their participation. Describing the solar panels on parking panels and its impacts on energy in the surrounding areas, it is evident that parking lots are a lot more visible and hence can attract potential customers for electric vehicles making EV adoption much more easier [4]. Consequently, EV adoption can have significant positive impacts on human health [4]. Another impact of solar parking lots is their benefit to local market. Since customers will choose a shopping centre with solar parking lots because it will charge their cars while they do their shopping. This will boost the local economy [4]. In addition, there will be lots of employment opportunities for the local technicians to install and maintain a solar parking lot [4]. Hence, installing a solar parking lot is beneficial in a number ways for a particular location like Agartala, India and its surrounding areas.
Winter Accessibility
The fourth criterion is winter accessibility. The location must be cleared and accessible during winter since some countries have severe winters [1]. The use of EVs should not depend on weather and hence EV public charging stations should be available at all times. Photovoltaic covered EV charging stations protect it from severe weather conditions like condensation, freezing rain and frost etc. [5].
Protection from Collisions
The fifth criterion is protection from collisions. The location must provide protection against collisions. It is necessary to provide protection for avoiding accidents and public property damage. Also for maintaining peace in the environment of the road by avoiding fights which may occur due to the collisions of vehicles [1].
Cellular network
The sixth criterion is cellular network access. Access to a cellular network is necessary if required by charging station [1]. Charging stations need to be in line of communication with smart grid since utilities like load management, peak demand and V2G programs depend on the communication that requires cellular network and Internet access as well. These two facilities can attract customers also since they cannot be out of coverage while present at a charging station. WIFI access can also help them connect to apps associated with their EVs and charging stations. In fact, public stations may provide telecommunications features, which will be different for different manufacturers. Many models contain transmitters compatible with cellular telephone networks and do not require additional infrastructure, while others will require a local wireless network, such as a ZigBee protocol network, which involves careful sitting of stations and transmitters. Also many stations communicate over a wired link, such as a twisted-pair or fiber-optic Ethernet network, which should be included in the design of the electrical installation.
Visibility
The seventh criterion is visibility of charging station. Visibility of the charging station to encourage its use by drivers is an important factor [1]. It helps to increase number of users. If users can see the station from far places then it will help them to locate the charging station that increases the use of charging station.
Feasibility of required excavation work and Proximity of distribution panel
The eight criterion is feasibility of required excavation work and the ninth criteria are proximity of distribution panel [1]. Where a distribution panel is the component of an electric panel, its function is is to divide the electricity feed to the “subsidiary” circuits [5]. Both of them are very important factor, which help to make the location more suitable for charging station. The proximity to the electrical service is an important factor in locating the public parking areas.
Table 6: Prime locations in Agartala and their mapping in different criteria
.
.
Location Feasibility Analysis
For fast charging station infrastructure requires a concrete base and their installation is similar to that of street side locations. For this station, the conditions are:
• The configuration of the station • The locations of any underground lines and tanks • The distance from the street(the charging cable must never extend over the sidewalk) • It required excavation work • The proximity of distribution panel • The planning of any underground conduits and excavation work. • It requires consultation with Info-Excavation before starting work. • The possibility of installing a concrete base • It requires contractor expertise (must have appropriate RBQ and CMEQ licenses).
For publicly available charging, the sitting requirements are involve many questions such as ownership, vandalism, payment for use and maintenance. Also we must take care that flood prone area restrictions must be considered as well as issues of standing water or high precipitation. The people will not be comfortable when operating with the EVSE (Electric Vehicle Supply Equipment – these equipment helps in the transfer of energy between the electric utility power and the electric vehicle.) in standing water. The area designated for Public use should be in a preferred parking area.
Installation Flowchart for Public Charging
The above flowchart summarizes the whole process of installing an EV public charging station. It starts with step one which is “consultation with utility” it includes utility consideration. The second step is “consultation with the governing authority”, it includes all the steps associated with public planning. Then, the constructors “consult with the EV enthusiasts”, these are the individuals or parties who want to promote and advertise EV and public charging stations. Subsequently, step four the builders consults the EV suppliers and EVSE suppliers that is determining the level of charging stations i.e level- 1, level-2 or fast DC charging stations. The step five of this charging station installation flow chart includes consultation with the local business owners for examples determining the quantity of solar energy for EVs. Step six, involves communication with electric contractors to assess the safety and accessibility measures for electric vehicle parking lots. Step seven, includes consultation with property owners and EV promoters. Step eight, involves the major step of developing the site plan development. It includes drawing the designs for electric vehicles parking lots. Step nine, includes obtaining required permits from government. Here all particular building rules should be satisfied. Step ten is the second last step of conducting installation. Step eleventh, in this step the construction of completed charging station is inspected and if every required is fulfilled then it is approved.
Figure 4. Installation flow chart for installing Public EV charging Station (Adopted from [6])
Proposed Locations for Charging Stations in Agartala
Based on the criteria discussed above, we have identified some places for placing an electrical vehicle charging station that is further divided into some categories:
Schools with parking place
Schools with parking places especially solar parking lots where EVs can recharge is one of the best scenarios. An EVSPL (electric vehicle solar parking lot) is suitable for schools since parents of the students can recharge their EVs while they come to school for any engagement. In the same way since schools have large parking lots specially so it can be an alternative place for recharging EVs when other solar parking lots are fully packed. In addition, number of schools are greater then rare EVSPLs so school locations with EVSPLs can be an effective of reducing “range anxiety” and can result in successful EV adoption. Keeping in view the earlier mentioned criteria for EVPLs we have identified some schools. These schools’ parking lots can be transformed in to EVSPLs. These schools are Holy cross school, Don Bosco School, the Agartala international school, Henry Derozio School.
Restaurant with parking places
Similarly the following places are suitable for EVSPLs. Momos n More, Raaste Cafe, Coffee Tea and Me, Hotel Sonari Tori, Hotel invitation, Royal Veg, Curry Club Restaurant.
College with parking place: Colleges that are suitable for constructing EVSPL are : Tripura Sundari College of nursing, Women’s College, Maharaja Bikram College, BBM College, Tripura Government College.
Government offices with parking places: Government offices with EV charging stations can be an effective solution as well for strengthening EV market. CBI Office , Office of the AG, Agartala municipal council office, Directorate of higher education office, Tripura Public Service commission office, Krishi Bhawan office.
Hotel with parking places
These hotels with EV charging stations is ideal since they are public and potential customers spend more hours there. Hotel Welcome Palace, Hotel City Center, Executive INN, Hotel Jaipur Palace, Rajdhani Hotel, Royal Guest House (Hotel), Ginger Hotel.
Hospital with parking place
Hospitals with EV charging stations can be count on in times of emergency as well. ILS Hospital, GB Hospital, GB Pant Hospital, Devlok Hospital, Apollo Gleneagles Hospital Information Center, GB Hospital Medical College, Tripura Medical College, Agartala Government Medical College.
Resort with parking place: Resorts are also a better place to install public charging stations. Since, not only visitors visit this place but hotel staff and general public can also come to resorts for festive seasons. Hence, it becomes a densely populated area with requirement for a electric vehicle public charging stations. Some are of the suitable places for this purpose in Agartala India are Green Touch Resort, Shyamali Tourist Resort, Hotel Woodland Park, Rose Valley Amusement Park.
Temple with parking place
Temples are best locations for installing public electric vehicles charging stations since this is one of the public places with good space. Some of the appropriate places for setting up EV charging stations in temples of Agartala are: Laxminarayan Bari Mandir , Jagannath Mandir, Iskcon Bari, Durga Bari , Ummaneshwar temple, Fourteen Gods Temple, Tripura Sundari temple.
Shopping center with parking place
In addition, shopping centers are one of the most suitable place for public charging stations due to its parking requirements and the frequency of potential EV customers’ visit. Some public charging stations can be installed in these shopping centres in Agartala i.e ML Plaza, Metro Baazar, Bag Bazar, Agartala City Center, Femme Zone/FEM Salon and spa, Saradamani Shopping mall.
Agartala airport parking place
Agartala airport parking place is another example of suitable place of installation of EV parking place due to the availability of parking space and public reach. Agartala airport can provide convenience for airport visitors, cab owners and staff of the airport. A public charging station installed at airport can also attract new EV customers due to its convenience.
Other public places for EV public charging stations installation
Subsequently, railway stations, petrol stations and cinema halls with parking spaces are ideal for constructing public EV charging stations. Due to high traffic density, visibility, availability of cellular network and the entire criterion based on above table we can suggest that the EV public charging stations should not only be installed here but it will also strengthen EV customer base in Agartala, India. Some places identified in this regard are Rupasi cinema hall, Balaka cinema hall and Tripura puppet theatre.
4. Future Work
It is better to visit each parking place then make a record of the number of users using these place, infrastructure is needed to make favorable electrical vehicle charging station or making a website showing locations of private and public charging stations in Agartala. It will increase more users and a website can be developed displaying the cost ratings and quality of charging stations in Agartala. We further check which type of charging stations are more suitable for the location based on the number of users utilising it.
References
[1] “ELECTRIC VEHICLE CHARGING STATIONS. Technical Installation Guide.” [Online]. Available: http://docplayer.net/9608482-Electric-vehicle-charging-stations-technical-installation-guide.html. [Accessed: 01-Feb-2017]. [2] J. Y. Yong, V. K. Ramachandaramurthy, K. M. Tan, and N. Mithulananthan, “A review on the state-of-the-art technologies of electric vehicle, its impacts and prospects,” Renew. Sustain. Energy Rev., vol. 49, pp. 365–385, Sep. 2015. [3] P. Nunes, R. Figueiredo, and M. C. Brito, “The use of parking lots to solar-charge electric vehicles,” Renew. Sustain. Energy Rev., vol. 66, pp. 679–693, Dec. 2016. [4] T. Lepley and P. Nath, “Photovoltaic covered-parking systems using lightweight, thin-film PV,” in Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference – 1997, 1997, pp. 1305-1308. [5] “Distribution board,” Wikipedia. 03-Mar-2017. [6] “Electric Vehicle Charging Infrastructure Deployment Guidelines,” Electric Transportation Engineering Corporation, Jul. 2009.
Source: The 2nd International Conference of Multidisciplinary Approaches on UN Sustainable Development Goals (UNSDGs) | Bangkok Thailand | 28-29 December 2017
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