Published by Alex Roderick, EE Power – Technical Articles: AC and DC Circuit Breakers for Overcurrent Protection, March 05, 2021.
This article highlights circuit breaker that is an overcurrent protection device (OCPD) designed to protect electrical devices and individuals from overcurrent conditions.
A circuit breaker is an overcurrent protection device (OCPD) designed to protect electrical devices and individuals from overcurrent conditions. Unlike most fuses, circuit breakers can be reset, which makes them a popular choice for overcurrent protection. Circuit breakers use an electromagnet and/or a bimetallic switch to detect an overcurrent condition.
Circuit Breaker Types and Characteristics
A circuit breaker may be reset by moving the trip lever handle to the full OFF position and then returning the handle to the ON position. Individuals must ensure the source of an overload is cleared before attempting to reset a breaker. There are three types of circuit breakers differentiated by their internal mechanisms for tripping:
• Magnetic • Thermal • Thermal-magnetic
Regardless of which internal mechanism a circuit breaker uses, most circuit breakers look the same externally, with the exception of the circuit breaker fuse. A circuit breaker fuse is a screw-in OCPD that has the operating characteristics of a circuit breaker.
The advantage of a circuit breaker fuse is that the fuse can be reset after an overload. Circuit breakers are available in a variety of amperages, but the voltage is typically rated as 110 V for single-pole residential breakers or 220 V for double-pole residential breakers.
Figure 1. Circuit breakers are available in a number of configurations, including single-pole and double-pole breakers.
To gain access to the circuit breaker connections in a service panel, the cover of the panel must be removed.
Magnetic
A magnetic circuit breaker is an OCPD that operates by using miniature electromagnets to open and close contacts. The basic idea is shown below.
Figure 2. Electromagnetic solenoids are an example of using electromagnetism to do work.
As you can see, an iron plunger is surrounded by an encased coil of wire and a set of contacts are attached to the iron plunger. With an electric current passed through the coil, the contacts attached to the iron core are pulled toward the coil. In this way, we can open or close the solenoid contacts. Note the figure shows both normally-open and normally-closed contacts.
As illustrated in Figure 3, the produced magnetic field can be strengthened by increasing the applied current and the number of turns per unit length as well as inserting an iron core through the coil.
Figure 3. An electromagnet can be strengthened by increasing the amount of current, increasing the number of turns in the coil, and inserting an iron core through the coil.
A solenoid in a magnetic circuit breaker opens the circuit based on the current limit of the breaker.
When the current through the coil exceeds the rated value of the breaker, the magnetic attraction becomes strong enough to activate the trip lever handle and open the circuit. See Figure 4.
Figure 4. In a magnetic circuit breaker, passing an electric current through the coil causes the contacts attached to the iron core to be pulled toward the coil. The solenoid in a magnetic circuit breaker opens and closes the contacts based on the current level.
Once the overload is removed, the trip lever handle can be reset to the original position, reactivating the circuit.
Thermal
Thermal circuit breakers use a bimetallic strip attached to a latch mechanism. The bimetallic strip is made of two dissimilar metals that expand at different rates when heated. The bimetallic strip bends when heated and opens the contacts. See Figure 5. The bimetallic strip may be heated directly by circuit current or indirectly by the rise in temperature caused by an increase in the circuit current.
Figure 5. Thermal circuit breakers use a bimetallic strip attached to a latch mechanism to open the circuit when a short circuit or overload occurs.
Thermal circuit breakers are designed so that the bimetallic strip bends to release the contact under spring tension based on the amount of continuous current flowing through it. The bimetallic strip must cool and return to its normal condition (size) at room temperature before the circuit breaker can be reset.
Thermal protection of a circuit is not instantaneous. It requires time to heat the strip and for the strip to bend far enough to cause the contacts to snap open. A magnetic circuit breaker is used in applications where this delay can cause damage to a circuit. Thermal circuit breakers can be reset by pressing the pushbutton only after the bimetallic strip has cooled.
Thermal-Magnetic
Thermal-magnetic circuit breakers include both a magnetic-tripping function for short-circuit protection and a thermal-tripping function for overload protection, as illustrated in Figure 6.
Figure 6. Thermal-magnetic circuit breaker.
Thermal-magnetic circuit breakers are also called inverse-time circuit breakers. As the alternative name inverse-time indicates, the higher the overload, the shorter the time it takes the circuit breaker to open.
When an overload condition occurs, the excess current generates heat, which is sensed by the bimetallic heat-sensing element. After a short period, depending on the breaker’s rating and amount of overload, the breaker will trip, disconnecting the voltage source from the load. If a short circuit occurs, the electromagnetic sensor responds immediately to the fault current and disconnects the circuit.
DC Circuit Breakers
A DC circuit breaker is an OCPD that protects electrical devices operating with DC and contains additional arc-extinguishing measures.
DC circuit breakers are a relatively new technology to most homeowners since most devices used in a house work with AC and AC circuit breakers. General AC circuit breakers for the home are rated to interrupt above 6 kA. Some manufacturers produce circuit breakers that are dual-rated for both AC/DC from 48 VDC to 125 VDC. DC circuit breakers are used with 24 VDC to 48 VDC programmable logic controllers (PLCs) and in wind power applications.
Though AC and DC breakers appear similar in form and function, internally they operate very differently. During an overload, the internal contacts of both AC and DC circuit breakers separate to protect the circuit. However, as the contacts pull apart from each other, an arc will form as the current jumps across the air gap created. Contact arcing is an electrical arc that occurs when opening and closing circuit breakers. See Figure 7. As the arc continues to jump across the air gap, the current will continue to flow through the circuit. These arcs must be extinguished quickly.
Figure 7. Contact arcing is an electrical arc that occurs when opening and closing circuit breakers.
The ways in which AC and DC breakers are designed to extinguish the arc are very different and this is why AC and DC breakers are not interchangeable. Only breakers that are labeled as DC rated should be used for DC applications.
An AC-rated breaker should never be used in a DC circuit. AC circuit breakers are not designed to handle the problems of arcing associated with DC. DC circuit breakers include additional arc-extinguishing measures to dissipate the electrical arc when opening and closing and elongate the device lifetime.
DC Arc Suppression
DC arcs are considered the most difficult to extinguish because the continuous DC supply causes current to flow constantly and with great stability across a much wider gap than an AC supply of equal voltage, often shown in metrics such as peak value and RMS.
To reduce arcing in DC circuits, the switching mechanism must be such that the contacts separate rapidly and with enough of an air gap to extinguish the arc as soon as possible when opening. When DC contacts are being closed, it is necessary that the contacts move together as quickly as possible to prevent some of the same problems encountered in opening them. If a circuit breaker is DC rated, it will be indicated on the breaker by the manufacturers.
Figure 8. Some circuit breakers are rated AC/DC. This information will be made clear on the manufacturer’s label.
It is worthwhile to mention that when a short circuit occurs across the terminals of a DC circuit, the current increases from the operating current to the short-circuit current depending on the resistance and the inductance of the short-circuited loop.
Some types of circuit breakers are rated AC/DC for use with either type of application. This information will be stated on the manufacturer’s label.
AC Arc Suppression
An AC arc self-extinguishes when the set of contacts opens. An AC supply has a voltage that reverses its polarity 120 times a second when operated on a 60 Hz line frequency. The alternation allows the arc to have a maximum duration of no more than a half-cycle.
The AC current reaches zero 60 times each second. See Figure 8. When AC reaches zero, no current flows, and therefore the arc is extinguished.
Figure 9. When AC current reaches zero, no current flows, and therefore the arc is extinguished.
Circuit Breakers as OCPDs
A circuit breaker is an overcurrent protection device with a mechanical mechanism that can automatically open a circuit when a short circuit or overload occurs. Circuit breakers use two principles of operation to protect the circuit: thermal and magnetic.
Thermal circuit breakers consist of a heating element and a mechanical latching mechanism. The heating element is usually a bimetallic strip that heats up when current flows through it.
Magnetic circuit breakers use an electromagnet to detect an overcurrent condition. Most magnetic circuit breakers contain both thermal and magnetic components. While the magnetic components protect the circuit against high overload current or short-circuit currents, the thermal components protect the circuit against a constant overload current that is not of sufficient level to activate the magnetic components.
A DC circuit breaker is used to protect electrical devices that operate with direct current (DC) and contains additional arc-extinguishing measures. DC circuit breakers are a relatively new technology and used in EV charging stations, photovoltaics, and battery storage systems, as well as industrial DC distribution networks.
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 Robert KOWALAK, Stanislaw CZAPP, Krzysztof DOBRZYNSKI, Jacek KLUCZNIK, Zbigniew LUBOSNY Gdansk, University of Technology, Faculty of Electrical and Control Engineering
Abstract. Voltage and current harmonics have a detrimental effect on the components of a power system. Current harmonics may result in the overload and damage to power transformers. Voltage harmonics may result in, for example, damage to capacitor banks used to compensate reactive power. Devices which contribute to both current and voltage distortion include traction rectifiers. This paper presents results of the computer investigation of the effect of these rectifiers on the power supply network. The results of the computer investigation have been compared with the result of experimental study.
Streszczenie. Wyższe harmoniczne napięcia i prądu oddziałują niekorzystnie na elementy sieci elektroenergetycznej. Z powodu wyższych harmonicznych prądu może dość do przeciążenia cieplnego i uszkodzenia transformatorów elektroenergetycznych. Wyższe harmoniczne napięcia mogą być przyczyną m.in. uszkodzenia baterii kondensatorów do kompensacji mocy biernej. Urządzeniami, które wywołują odkształcenia zarówno prądu, jak i napięcia są np. prostowniki trakcyjne. W artykule przedstawiono wyniki badań modelowych wpływu tych prostowników na elektroenergetyczną sieć zasilającą. Wyniki badań modelowych porównano z wynikami badań eksperymentalnych. (Harmoniczne powodowane pracą podstacji trakcyjnych – modelowanie i badania eksperymentalne).
Keywords: harmonics, modelling of power systems, power quality, power supply of electric traction. Słowa kluczowe: harmoniczne, modelowanie systemu elektroenergetycznego, jakość energii, zasilanie trakcji elektrycznej.
Introduction
The voltage and current waveforms in a power grid sometimes significantly diverge from a sinusoid. Current distortions most often result from a non-linearity of loads. Distortions in voltage waveforms result from a distorted current flowing through the supply network, from switching processes and resonance phenomena. An increased content of higher harmonics in the voltage is also affected by the fact that “distorting” (non-linear) loads are supplied with already distorted voltage. This also leads to an additional (secondary) distortion in their current waveforms and, in consequence in the voltages in the supply network. An increased content of harmonics is also affected by the asymmetry of the supply voltage.
The distorted waveforms of voltages and currents can be described by means of the Fourier series:
.
where:
.
T – time period of the function f(t),
.
t0 – any value of the time t.
The greatest influence on power quality in a distribution network is displayed by high-power loads, such as arc furnaces or power electronics devices. The latter group includes, for example, traction rectifiers.
A significant level of harmonics in the supply voltage may lead, for example, to damage to the capacitor banks used to compensate reactive power. Damage to capacitor banks has been reported in several 110 kV/15 kV substations which supply medium voltage networks in Poland’s Pomorskie Region (Voivodship). These reports, and the need to determine the level of voltage distortion on the buses of power substations which supply disturbing loads (here: traction substations) and other consumers, have become the basis for supply network modelling and measurements in the extent described in this paper. The problem of insufficient power quality due to operation of traction substations is considered for various power systems [1, 2, 3, 4].
Based on studies available in the literature, it can be ascertained that there is a need to design computer models for traction substations to enable evaluating their impact on the power supply network, in particular on the distortion of current and voltage waveforms. As part of the research, computer models of four sample traction substations have been designed and their impact on the supply network has been analysed through simulation. DIgSILENT PowerFactory® software was used in the simulation tests. Next, voltages and currents were measured in those four sample facilities, and compared with the results of harmonic simulations. The comparison made it possible to assess the precision of the computer model.
Traction substation characteristics
There are more than 11,000 km of electrified railway lines DC 3 kV in Poland. The traction network is supplied by means of ca 450 traction substations equipped with 6-pulse and 12-pulse rectifiers. Most of these substations are supplied from medium voltage networks (usually 15 kV). Only the more recent facilities are supplied from 110 kV networks [5].
Traction substations are regarded as some of the larger loads by the power system operator. They are characterised by significant rated powers of rectifier sets. Moreover, they are considered to be so-called disturbing loads and their load current has significant dynamics of value change.
Reflected in the model, the structure of the electric traction supply systems is presented as a diagram in Fig. 1.
Fig.1. Traction supply system diagram
Among the harmonics introduced to the supply network by rectifiers, the highest values are achieved by so-called characteristic harmonics, with their order determined by the following dependence:
.
where: m – an integer, i.e. m = 1,2,3,…, p – ripple factor (number of pulses during one time period of alternating voltage).
Four typical traction substations operating in Poland were taken as the basis for designing the computer models of traction substations. All of the analyzed tractions substations are supplied from a 15 kV network through power cable lines. In three (designated I, II and III) out of four tested substations, PK-17/3,3 6-pulse rectifiers [6] are installed, supplied via TZE3-4402 rectifier transformers [7]. These substations are supplied from a 110 kV network via TORb-25000/110 transformers [8]. In one case (substation designated as IV), PD-12/3,3 rectifiers [9] are installed, supplied via TMOS3AA-4400/15PN rectifier transformers [10]. This medium voltage network is supplied via a TONRb-10000/115 transformer [11]. More precise information about the facilities cannot be provided as their administrators did not give their consent.
The model
The designed model (Fig. 2) takes into consideration the short-circuit power on the 110 kV side in the substation, a 110 kV/15 kV transformer installed in it and a 15 kV power line. The traction substation model takes into consideration a rectifier transformer, a rectifier and voltage smoothing devices (filters). In the analysed facilities, a traction unit constitutes an electrical load, but from the point of view of the supply network, the traction substation itself is considered to be such a load.
Fig.2. Structure of the analysed network’s computer model: I – point of current measurement, V – point of voltage measurement
DIgSILENT PowerFactory® has built-in pre-existing model structures based on which one can create one’s own models of power grid components. In order to reflect “one’s own” facility, it is sufficient to input the relevant data to these models. This has been used when designing the components of the presented model, such as models of power system transformers and traction transformers, supply power lines and traction rectifiers. The pre-existing model structures were used because they were deemed appropriate for the assumed research – therefore, it was unnecessary to design our own models from scratch.
A traction vehicle is a traction substation’s load. Having reviewed the structures of traction vehicle drive units, it was decided that their precise modelling was not necessary. Based on the collected information, it was assumed that their influence on the level of voltage harmonics in the power supply network will be small. Therefore, the vehicle was modelled in the simplest way, as an impedance load.
Filters, a choke and a overvoltage protection system on the rectifier’s output have been included in the modelled traction substation. The facilities were modelled using RLC components. Because they influence the commutation processes in the rectifier, to omit them would affect the waveform of the substation’s supplying current.
A model built into the simulation software was used to model a 6-pulse traction rectifier. The model is described in detail in [12]. The diodes are shown in the model as ideal keys for which the forward and reverse resistances have been determined. The key is shunted by an RC overvoltage protection system. The parameters required for the model were determined based on [6]. The 12-pulse rectifier, in turn, was modelled using two 6-pulse rectifiers connected in series. The connection type and parameters corresponded to the data contained in [9].
Transformers were also modelled based on the models built into DIgSILENT. It is sufficient to input transformer specifications to such a model. A detailed description of a two-winding transformer is provided in [13], and of a three-winding transformer – in [14]. The data input method is illustrated in Fig 3.
Fig.3. Entering the parameters of the modelled transformer
The power line model used in the tests is also the model built into the simulation software. The power line is modelled as a four-terminal π network. The line parameters were determined on the basis of the data from the substation’s power supply systems. The power line model is described in detail in [15].
The power system is reflected as an ideal source of voltage and the impedance determined based on the known value of the short-circuit power.
Test results
At the first stage of the research, models of substation power supply systems were designed, based on the available information, and simulations were run. Then, measurements were performed, based on which the facility’s model was verified. The last step was to compare the harmonic content from the simulations and the measurements. This provided the basis for evaluating the simulation results and whether the computer model was correct.
A. Requirements for power quality
In a power grid with a voltage exceeding 1 kV, the accepted basic power quality parameters are frequency and voltage [16]. From the point of view of the impact of disturbing loads on the supply network, the content of higher harmonics in the voltage is the most important power quality parameter. The requirements for the quality of supply voltage in the analysed medium voltage network are described in the recommendations in [16, 17]. They are listed in Table 1.
Table 1. Permissible level of harmonics in voltage at the supply terminals (in percent of nominal voltage)
.
No specific requirements are determined for distortions in the load currents for the loads connected directly to the network. Such requirements are, however, determined for loads in an low voltage network.
B. Substation currents
A comparison of the current waveforms and their higher harmonics content obtained through measurements and simulations was used to verify the models designed for traction substation power supply systems. Current distortion assessment (from the point of view of power quality) was not performed due to the lack of formal requirements in this regard.
The following figures present selected current waveforms and the content of higher harmonics in the current in the analysed substations. The results correspond to each substation’s highest load recorded during the measurements.
Fig. 4 presents the current waveforms obtained for Substation I, and Fig. 5 – the content of the higher harmonics. Fig. 6 and 7 present the content of the higher harmonics in the load currents of Substations II and III. Fig. 8 presents the current waveforms, and Fig. 9 – the content of the higher harmonics obtained for Substation IV.
Fig.4. Waveforms of load currents of Traction Substation I: a) measured, b) simulated; (red) – phase L1, (green) – phase L2, (blue) – phase L3
Fig.5. Content of odd higher harmonics in the load current of Traction Substation I: (violet colour) – measured, (plum colour) – simulated
Fig.6. Content of odd higher harmonics in the load current of Traction Substation II: (violet colour) – measured, (plum colour) – simulated
Fig.7. Content of odd higher harmonics in the load current of Traction Substation III: (violet colour) – measured, (plum colour) – simulated
The greatest compliance between the computer model and the measurement data was obtained for Substations I and II. Greater discrepancies were found in Substations III and IV. There can be two reasons for this.
The first reason: the analysed power supply systems may not have been fully reflected. Unfortunately, not all the details of the modelled systems were obtained with success (the information on the 15 kV supply line and the equipment of the substation itself was incomplete). The incomplete data concerned the supply systems of Substations III and IV.
The second reason may be the supply voltage distortions in the real system. The analysed power substations supplied also other loads from the same 15 kV buses as the traction substations. Therefore, the voltage waveforms at the point of measurement was affected by other loads as well. The fact that a traction substation is supplied with a distorted voltage results in an additional (secondary) distortion of the current in the traction rectifier. As a result, the values of current harmonics characteristic for the rectifier may change; moreover, other harmonics that are not characteristic may appear.
Fig.8. Waveforms of load currents of Traction Substation IV: a) measured, b) simulated; (red) – phase L1, (green) – phase L2, (blue) – phase L3
Fig.9. Content of odd higher harmonics in the load current of Traction Substation IV: (violet colour) – measured, (plum colour) – simulated
An analysis of the presented results made it possible to ascertain that the designed models are satisfactorily precise in representing the load currents of the traction substations.
C. Supply voltage
The highest level of voltage distortion on the 15 kV buses of the power substation is expected when the value of the load current of a traction substation is the highest. The analyses took into consideration both the rated load of the traction rectifier and the permissible short-term overload (150% of rated load for up to 2 minutes).
It was assumed that the other loads in the 15 kV network, supplied from the same power substation’s buses, consumed the power equal to 10% of the 110 kV/15 kV transformer’s rated power. The results for the content of higher harmonics in the voltage on 15 kV power substation’s buses are presented in Fig. 10 to 13.
Fig.10. Content of odd higher harmonics in the voltage on 15 kV buses of power substation supplying Traction Substation I: (violet colour) – rated load, (plum colour) – short-term permissible overload
Fig.11. Content of odd higher harmonics in the voltage on 15 kV buses of power substation supplying Traction Substation II: (violet colour) – rated load, (plum colour) – short-term permissible overload
Fig.12. Content of odd higher harmonics in the voltage on 15 kV buses of power substation supplying Traction Substation III: (violet colour) – rated load, (plum colour) – short-term permissible overload
The simulations did not demonstrate that the substations operating in the analysed systems contributed to exceeding the permissible THD levels (see Table 1) in the voltage on the buses of the power substations which supplied them. They showed, however, that in Substation I, while at the permissible overload level, the 19th harmonic may increase above the permissible value. In the Substation IV supply system, in turn, the permissible level may also be exceeded for the 13th harmonic. As regards the other harmonics, the permissible levels were not found to be exceeded in any of the analysed systems.
Fig.13. Content of odd higher harmonics in the voltage on 15 kV buses of power substation supplying Traction Substation IV: (violet colour) – rated load, (plum colour) – short-term permissible overload
The last element of the analysis was to compare the voltage distortions on 15 kV buses of the power substation obtained, during the modelling and measurements, for the same values of traction substations’ currents. The results were obtained at the highest recorded load current values of the traction substations, because the higher the value of the load current, the higher the level of voltage distortion that can be expected on the buses of the power substation. Examples of selected voltage waveforms obtained through measurement and simulation are presented in Fig. 14. Fig. 15, 16, 17 and 18, in turn, show the harmonic levels in the voltages on 15 kV buses of the power substation in the analysed traction substation power supply systems.
Fig.14. Waveforms of voltage on 15 kV buses of power substation supplying Traction Substation II: a) measured, b) simulated, (red) – phase L1, (green) – phase L2, (blue) – phase L3
In the figures (Fig. 15 to 18), one can notice lower distortions in the waveforms obtained through simulation. This is fully justified. In the real system, many loads operated in the network, some of which introduced their own voltage distortions, whereas the simulations only analysed the operation of a traction rectifier. The presence of other loads in the 15 kV network was reflected by means of a single cumulative load as a set of RLC components, with its power and power factor specified on the basis of the information collected during the 110 kV/15 kV transformer’s load measurements.
In most cases, other harmonics were also measured on the buses of the power substation, next to the harmonics characteristic for the operation of traction rectifiers. Their presence is related to the operation of other loads in this network, as well as to the influence of the components of the network itself (for example transformers).
Fig.15. Content of odd higher harmonics in the voltage on 15 kV buses of power substation supplying Traction Substation I: (violet colour) – measured, (plum colour) – simulated
Fig.16. Content of odd higher harmonics in the voltage on 15 kV buses of power substation supplying Traction Substation II: (violet colour) – measured, (plum colour) – simulated
Fig.17. Content of odd higher harmonics in the voltage on 15 kV buses of power substation supplying Traction Substation III: (violet colour) – measured, (plum colour) – simulated
It is somewhat difficult to assess a traction substation’s influence on the overall level of harmonics in the power substation based on the performed simulations and measurements. It has been observed that in some cases the characteristic harmonics obtained in simulations reach higher values than those obtained in measurements. One possible explanation is that there is a greater ability to suppress higher frequencies in a real system than in a model. The second factor may be that in a network, currents are summed geometrically. As a result, certain harmonics can decrease in a real system compared to the impact of a single facility analysed in simulations.
Fig.18. Content of odd higher harmonics in the voltage on 15 kV buses of power substation supplying Traction Substation IV: (violet colour) – measured, (plum colour) – simulated
Nevertheless, the traction substations’ share in the overall distortion level, at least as regards characteristic harmonics, needs to be considered significant.
Neither the measurements nor their corresponding simulations demonstrated the presence in the voltage of the impermissible values of higher harmonics, either the odd ones, or the even ones which have not been presented in this paper.
Conclusion
When modelling the operation of a traction substation as a distorting load (harmonics), one can represent a real-life facility quite well. The main indicator of the degree to which a facility is represented in a model, in this case, is the waveform of the load current and the harmonics it contains. The more precise the system data, the greater the result compliance that has been achieved.
Unfortunately, this does not translate directly onto the possibility of assessing harmonics in the supply voltage on the 15 kV buses of a power substation, as it results from the interactions of all the loads connected to such a substation, along with the impact of the whole network. An additional difficulty comes in the fact that the harmonics contained in load currents are not summed algebraically. In extreme cases, the result may be a significant increase in the value of some harmonic in the voltage, or conversely, a marked decrease compared to what would follow from an analysis of a single load.
The simulation results may provide information about how much a given load (here: a traction substation) can influence the power quality at the point of connection. If a significant impact is discovered, one can try to persuade the facility’s owner to make them introduce, for example, specific higher harmonics filters to eliminate them from the load current. Simulation results can be very useful when assessing the influence of a traction substation’s modernization on power quality. They also allow one to assess the impact on the power supply network of a new traction substation that is only at the planning stage.
REFERENCES
[1] Yu-quan L., Guo-pei W., Huang-sheng H., Li W., Research for the effects of high-speed electrified railway traction load on power quality, 4th Int. Conf. on Electric Utility Deregulation and Restructuring and Power Technologies (DRPT), 2011 [2] Župan A., Tomasov i ć Tekl i ć A., Fi l i pov i ć-Grč i ć B. , Modeling of 25 kV electric railway system for power quality studies, EuroCon, 1-4 July 2013, Zagreb, Croatia [3] Djeghader Y., Zellouma L., Labar H., Toufout i R ., Chel l i Z. , Study and filtering of harmonics in a DC electrified railway system, 7th International Conference on Modelling, Identification and Control (ICMIC), Sousse, Tunisia – December 18-20, 2015 [4] Pawelek R. , Analysis of current and voltage higher harmonics measurements performed in selected traction substation, Przeglad Elektrotechniczny, 90 (2014), nr 7, 234- 238 [5] Report RAILWAY BUSINESS FORUM, Electrical power railway, Warsaw, February 2011 [6] Technical documentation of PK-17/3,3 rectifier, Elta, Lodz 1971 [7] Catalogue SWW 1113: Transformers and special reactors, Edition III, WEMA, Warsaw 1975 [8] Catalogue Power transformers, EV Zychlinskie Transformatory, 2006 [9] Technical documentation of PD-12/3,3 rectifier, Elta, Lodz 1988 [10] Technical documentation of TMOOS3AA-4400/15N transformer, Elta, Lodz 1987 [11] Catalogue SWW 1113-2: Oil-filled power transformers and autotransformers, WEMA, Warsaw 1975 [12] DIgSILENT Technical Documentation, 6-Pulse Bridge, 2006 [13] DIgSILENT Technical Documentation, Two-Winding Transformer (3-Phase), 2007 [14] DIgSILENT Technical Documentation, Three-Winding Transformer, 2007 [15] DIgSILENT Technical Documentation, Overhead Line Models, 2007 [16] Operation and Maintenance Instruction of Distribution Network, ENERGA-OPERATOR joint-stock company, 2016 [17] Standard EN 50160: Voltage Characteristics of Public Distribution Systems, 2010
Authors: dr inż. Robert Kowalak, E-mail: robert.kowalak@pg.gda.pl; dr hab. inż. Stanisław Czapp, E-mail: stanislaw.czapp@pg.gda.pl; dr inż. Krzysztof Dobrzyński, E-mail: krzysztof.dobrzynski@pg.gda.pl; dr inż. Jacek Klucznik, E-mail: jacek.klucznik@pg.gda.pl; prof. dr hab. inż. Zbigniew Lubośny, E-mail: zbigniew.lubosny@pg.gda.pl; Gdansk University of Technology, Faculty of Electrical and Control Engineering, ul. Gabriela Narutowicza 11/12, 80-233 Gdańsk, Poland.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 93 NR 6/2017. doi:10.15199/48.2017.06.04
Published by Petro D. LEZHNIUK1, Iryna O. HUNKO1, Sergiy V. KRAVCHUK1, Paweł KOMADA2, Konrad GROMASZEK2, Assel MUSSABEKOVA2, Nursanat ASKAROVA3, Abenar ARMAN2 Vinnytsia National Technical University, Chair of Electric Stations and Systems (1), Lublin University of Technology, Faculty of Electrical Engineering and Computer Science (2), Kazakh National Research Technical University after K.I. Satpaeva (3)
Abstract. One of methods of active power loss decrease in the electric microgrids of electrical power system that is based on the mutually agreed use of polytypic sources of the distributed generation is presented in this article. This paper presents and solves the problem of mode optimization of the microgrid according to the criteria of active power loss decrease of in the branches of equivalent network in the microgrid by the way of electric power regulation, generated by the hydropowerplant and choice of available hydropowerplant with the help of which control of the mode should be done in conformity with the current parameters of the nodal loads and generation of the distributed sources of the electric power.
Streszczenie. W artykule zaproponowano jedną z metod zmniejszenia strat mocy czynnej w sieciach microgrid systemu elektroenergetycznego, opartego na uzgodnionym wykorzystaniu zróżnicowanych źródeł rozproszonych. W pracy przedstawiono i rozwiązano problem optymalizacji microgrid zgodnie z kryterium strat mocy czynnej w gałęziach równoważnej sieci microgrid, metodą elektrycznej regulacji mocy, Źródłem generowanej mocy kompensayjnej jest szczytowa elektrownia wodna, natomiast algorytm dokonuje wyboru trybu sterowania, celem wykonania go zgodnie z obowiązującymi parametrami obciążeń w węzłach i generacji rozproszonej źródeł energii elektrycznej. (Wpływ rozproszonych źródeł energii na straty mocy czynnej w sieciach microgrid).
Słowa kluczowe: sieci microgrid, rozproszone źródła energii, elektrownie słoneczne, elektrownie wodne, straty mocy czynnej. Keywords: microgrids, distributed sources of energy, solar power plant, hydropowerplants, loss of active power.
Introduction
The intensive rate of the automation human fields of activity leads to the significant increasing of power consumption. In the conditions of fast rise of cost of traditional source of energy (as coal, oil, gas), the task of growth of alternative power is of current interest now. Most developed countries of the world implementing different programs for distributed generation sources capacity on the base of alternative power. Use of the alternative power is expedient in the case of question of ecological safety. Distributed generation is ecologically accepted and is able to solve the problem of power software of new consumers. System of distributed generation consists of electric stations with moderate capacity those are dispersed along the whole energetic system. They supply power to the closest consumers, and in the case of surpluses power appearing they are able to transfer it in the net of centralized power supply.
Therefore, companies and power providers have to get involved to the alternative methods and ways of generations to provide the increasing demand and to meet the consumers. Recently, because of the increasing interest according the ecological safety, the demand on the search of ecological power sources is increasing also. Distributed generation (DG) is played an important part in this context that is ecologically accepted and is able to solve the problem of increasing power demand maintenance. The system of distributed generation consists of moderate generators with power from 10 kW to 20 MW (even to 50 MW) situated in different places along the whole distributed electric power system territory. Such distributed power sources (DPS) provide power needed to the consumers. Thanks to this, demand to build additional local distributed power lines or to improve the existing ones, is removed. Also functional capacity of the whole system is increasing. Distributed generation (DG) is the system that consists of generating capacity from 10 to 250 kWt, which are connected to the distributed grid with the power to 11, 35 or 110 kV. In addition such system provides possibility to the consumer, that produces power for his own demands, to give the power excess in the grid of centralized power. So, a very important questions rises in this context, is how to unite the system of distributed generation with the distributed grid.
Nowadays thanks to the complicated condition in which are the distributed power grids in the conditions of exploitation of old equipment, absence of sufficient quantity of means of control and automatic management of its modes, operation of process optimization methods: regulation of flow of active and reactive power and regulation of nodal power became one of the most important tasks in the planning, exploitation and managing of distribution systems. Distributed generation may considerably influences the power, the supplying with necessary quantity of the power of the increasing charge [1, 4] that is important in the conditions of commissioning of new agro recycling companies and agro industrial complexes in Ukraine, may influences the power loss, economic and reliable work indicators of the power companies and distribution grids. Introducing of the distributed generation to the distributive electric system often causes the reverse (reversible) cross flow of power and overtime rejection of key power [1]. At certain terms, for providing of quality and reliability of power supply operatively-controller’s management of microgrids is provided, that enter in the complement of the distributive electric systems comes true the modes, by imposition of limits on the generation of DPS or consumption in these networks and on the possible level of tension in them, especially during realization of repair works in the distributive electric limits of Ukraine. And as regulation of active-power and key power is directly related to quality of electric power in the distributive electric system and influences on the losses of electric power, then such regulation becomes the main theme of research in this article.
Research of existent methods of electric power losses reduction in microgrids
If a level of tension in the distributive electric system (DES) is within the limits of standard of ANSI C84.1-2006 then, as Masters [1] marks, tension does not need the superfluous attention, but if this level exceeds the set limits, then it is needed to apply facilities of power regulation, to bring tension to the set limits [1-4].
Fig.1. Chart of the microgrid of 0.4/10 kV
Many methods of tension regulation are known. So Tae – Eung Kim and Jae – Eon Kim [5] considered the method of co-ordination of tension regulation in the nodes of joining of the distributed generation (DG) in a distributive network, which is taken to adjusting of reactive-power, and O.O. Kovalchuk [6] – the uses of distributed hydro generators and their influence on the DPS mode. Also Gonen [7] illustrated the method of adjusting of tension, well-known as a method of LDC (method of indemnification of power failure). Tae – Eung Kim and Jae – Eon Kim [7] prospected intercommunication between adjusting of tension and cross flows of electric power of the transformer with the adjusting of tension on-loading (LCT), by the scray of power failure on a line (LDC) and initial power of the distributive generators (DG).
Fig.2. Chart of computer model of the microgrid of 0,4/10 kV
А.V. Kylymchuk in the article [8] prospected the use of transformer with the adjusting of tension on-loading for a management the cross flows of electric power in the networks of energy procurement companies with the aim of reduction of losses of active-power. Joon – Ho Choi and Jae – Chul Kim [9-10] presented the method of adjusting of tension for indemnification of power failure on a few lines (multiple line drop compensation voltage regulation method) taking into account the distributive stores of energy (dispersed storage generation systems) and charts of loading and generating unbalanced in time in microgrids.
Nigel C. Scott [11] offered method of adjusting of loading that must remove the considerable brief changes of nodal tension, caused by the «built-in (distributed) generation of electric power. The task of adjusting of nodal tensions and powers, generating DPS is an optimization task. T.Niknam and other [12] suggested to regulate tension and active-power in a distributive network with the distributed generation (DG) by means of genetic algorithm (GA). One objective function was examined in this work only, and the main lack of this approach is a necessity of adjustment of parameters. Batrinu and other coauthors [13] considered how loading changes in the course of time, and used the evolutional programming (nested evolutionary programming) for the solving of problem of adjusting of tension in a distributive network. However, such approach requires the large charges of calculable resources even for a small distributive network. J. Enslin, P. Heskes [14] pay attention to the origin of cross-coupling in the case of large quantity of the distributed inverters of DPS in DES, and J. Jung, An One Arghandeh, Broadwater [15] ground the necessity of the coordinated management of the automated devices and photo-electric generators for the reduction of negative influence of consequences of increase of tension in the circles of DES by the sources of DPS. Also in the microgrid with the relatively unmanaged DPS the stabilizing of tension is an important task. So Jaesung Jung offers an algorithm on reduction of losses to power and stabilizing of tension by the use of voltage regulators and devices of indemnification of reactive-power in a microgrid [15]. L. Minchala – Avila, L. Garza – Castanon, A. Vargas – Martıof nez, Y. Zhang [16] conducted the review of literary sources through the question of optimal methods of management an energy consumption (EMS) and of control of the modes of MM and offered the hierarchical architecture of management of the energy consumption, that envisages the necessity of telecommunication infrastructure for connection of the distributed management at the level of DPS with the top level of management of energy procurement company, on which optimization of work of MM is provided. Despite to many existent criteria of optimization of the modes of MM, the basic requirement to the MM management is optimization on the criterion of providing of necessary balance between generating and consumed electric power in DES. It provides firmness of work of DES.
P. Hrisheekesha and J. Sharma confirm that losses in MM or weight coefficient of their components depend on importance of disbursement functions and limits. The meaning of disbursement functions and limits in different grids are different. It means that weight coefficients in different grids are different also. That’s why weight indicators for each grid have to be adjusted. To overcome the problems above [17-19], method of minimizing losses of power and overtime tension deviation in MM with the distributed generation is offered. This method provides the use of results of math modeling according the genetic algorithm of non-dominant sorting (NSGA).
DPS can substantially influence on quality of electric energy, namely on supply tension, on the coefficient of harmonious distortions of current on tension and on the losses of active-power, on what D. Galzina [20] and P. Lezhniuk, О. Rubanenko, І. Hunko in [21] pay attention.
Research of P. D. Lezhniuk and І. О. Hunko testify that the overtime increase of coefficient of accordions in a microgrid tension results in the damage of muffs of cable busses of electricity transmissions and measuring transformers of tension, and pulls out additional requirements in relation to the concerted management a few SES with the aim of reduction to duration of transients during their switching on. Therefore, in [22] it becomes firmly established that during optimal (after the losses of electric power) controller’s management of the electro energy systems the modes it costs to take into account damaged of high-voltage equipment and its remaining resource. Also, a problem of water deficit is actual today, and this problem becomes sharper in Ukraine every year. According to UNO information 1,2 milliards of people live under conditions of permanent deficit of water, about 2 milliards suffer from it regularly. For the last 40 years the amount of water for a person on Earth diminished on 60%, and in the nearest 25 years its amount will diminish twice [22]. It limits the possibilities of the use of the hydroelectric power stations (HPS). It is necessary to take this fact into account while developing of algorithms of optimal management generating power of HPS in a microgrid. In the networks where SЕS and HPS work under the unchanging SES power, it is possible to influence with the power of generating of HPS on the losses of active-power and tension in a microgrid.
The tasks of adjusting of nodal voltage generating with DPS and consumed in the microgrids of electric powers is:
а) reduction to the amount of damages of equipment of microgrid;
b) providing of quality levels of nodal voltage is due to minimization of rejections of their current values from rationed;
c) optimization of losses of active-power in a microgrid: the distributed generation (DG) results in emerging of reverse (reversible) cross flow of electric energy in a grid and changes the streams of electric power, influencing on the losses of electric distributive grid and others.
It is known that DPS is divided into guided (for example, diesel electric stations, which cost of generating electric energy is large), conditionally guided (for example, hydroelectric power stations, the amount of water for that not always satisfies to the necessities and that is why limits, at certain terms, the protracted generating) and not guided (for example, PV systems generating of which depends on whether terms and has probabilistic character).
Therefore the method of determination of the hydroelectric power station and electric power is offered in this paper in accordance with the chart of the daily allowance loading in the nodes of microgrid, that provide the minimum losses of active-power.
Formulation of the task
It is necessary to investigate influence of generating of HPS and places of its joining to the distributive electric microgrid on the losses of active-power in a radial distributive grid with the distributed generation and on to the nodal voltage. The article considers next control parameters: place of joining of HPS to the distributive electric microgrid and its generated power in accordance with the chart of the daily allowance loading and generation of other DPS (for example, SЕS) and nodal voltage.
A computer design in this article is provided with the aim of researches of conditions, which answer the objective function of F1 of minimization of losses of active-power in a microgrid:
.
where: m is an amount of areas of radial line (in future branches of chart) in a microgrid, ΔРj is losses of active power in j- branch of chart. These losses answer:
– to the condition of minimization of rejections of tension from optimal, after the losses of active-power, values
.
where where k is an amount of nods in a microgrid, Ucur. and Uopt. is a current and optimal value of tension in a і- that node, that answer the investigated mode;
– to the condition of being of nodal voltage [22] in the great number of possible values limit minimum and maximal legitimate values.
.
where Ucur. and current value of tension in і-that node for the investigated mode, Urt and basic value of tension. For distributive electric grids minimum and maximal legitimate values answer to 0.95 – Urt (-10% deviation from the basic value of tension) and 1.1 – Urt (+10% deviation is from the basic value of tension).
At variable places of point of section of the flow, the crossflow of power in the microgrid changes. For determination of losses of power in a branch, at first find power at the beginning of the branch after term (3)
.
where m = 1,2.b is an amount of branches, n=m+1, Um is tension at the beginning of branch, Im is a current in m- in the branch of chart.
Power at the end of branch find after term (4), taking into account direction of the power flow here. For positive direction take motion from the center of supply, and reverse (from HPS to the center of supply) “-” for negative
.
where m = 1,2.b is an amount of branches, n=m+1, Un is tension at the end of the branch, Im is a current in m- in the branch of chart.
Losses in the branches are determined as a sum of algebra of powers at the beginning and at the end of the branch, after the term (5)
.
where Рmn is power at the beginning of the branch, Рnm is power at the end of the branch. The sum of losses in the branches of the chart ΔР is determined after the term (6):
.
Microgrid work and its parameters analysis
The aim of the researches is an analysis of losses of active-power in electric microgrids by the choice of optimal HPS and its generating power for providing of minimum losses of active-power and implementation of limitations after voltage in the nodes of MM, set power and places of exploitation of existent HPS, generating power of SЕS. For research of change of losses of active-power ΔР in a radial distributive grid from DPS (Fig.1) the computer model of grid of 10/0,4 kV was built in a programmatic complex for the design of power system of PS CAD.
In the computer model of the grid (Fig. 2) as electric energy sources used: center of supply, PV systems of SES1 and SES2, and hydroelectric power stations of HPS1, HPS2, HPS3. SЕS1 and SЕS2 is connected to the microgrid through the increase transformers of tension of 0,4/10 kV.
Calculation of losses of active-power of grid which parameters of that are indicated in a tabl.1, at the beginning and at the end of each line multimeters that represent the value of tension and power are set. Total power of loading for this mode is – 2,8 MWt, set power of generating of SЕS1 and SЕS2 for 0,2 MWt. Power of generating of HPS1, HPS2 and HPS3 were changing during the experiment as shown in the table 1.
Table 1. Parameters of the chart of the microgrid
.
Table 2. Parameters of the branches of the chart of the microgrid
.
The experimental part
During the computer design of the modes of the microgrid, power of generating of SЕS (0,4 МWt) did not change, but changed power of every HPS separately. That’s why three experiments were held. Power of HPS1 changed in the first experiment, while HPS2 and HPS3 were turned off.
In the second experiment the management were held by HPS2, at turned off HPS1 and HPS3. Power of generating of HPS3 changed in the third experiment, at turned off HPS1 and HPS2. For three variants of connection of HPS (tires of substation 8, 9, 10), were determined the losses of active-power ΔР that concur the power of generating of HPS. Results are given in the table 3.
Table 3. Losses of active-power ΔР and powers of generating of HPS in the microgrid
.
The changes of tensions were controlled on the tires of substations and electric stations without the use of HPS and with the extra use of HPS1, HPS2, HPS3 and the change of powers generating by them, from 0 to 2,9 МWt (table4). Analysis of these table 4 testifies that deviation of nodal voltage from the rationed values does not exceed 10 %.
Table 4. Tension on the tires of substations and electric stations in the microgrid
.
Approximating data of table 3. by a quadratic polynomial, terms (7) – (9) are got, that allow to determine the losses of active-power in a microgrid depending on generating power and place of HPS joining. The error of approximation does not exceed 0,04%.
For HPS 1:
.
For HPS 2:
.
For HPS 3:
.
where ΔРHPS1, ΔРHPS2, ΔРHPS3 are losses of active-power at a robot HPS1, HPS2, HPS3 accordingly, РHPS1, РHPS2, РHPS3 are generating power of HPS1, HPS2, HPS3 accordingly.
Analysis of the charts of dependences of losses of active-power from power of generating HPS in the MM (Fig.3) built from the use of terms (7) – (9) testifies to that the use of HPS1, allows maximally decrease these losses.
Equating the first derivative of terms (7) – (9) to zero (10) – (12), and solving these equalizations relatively,,, accordingly, will get the optimal values of generating HPS1, HPS2, HPS3 of powers РHPS1,OPT=1,184 MWt, РHPS2,OPT=1,104 MWt, РHPS3,OPT=1,003 MWt and the corresponding values of active power losses ΔРmin.HPS1= 81,279 kWt, ΔРmin.HPS2= 87,034 kWt, ΔРmin.HPS3= 102,929 kWt that concur to РHPS1min, РHPS2min, РHPS3min.
.
where РHPS1, РHPS2, РHPS3 – generating power HPS1, HPS2, HPS3 accordingly. Determine the generating power of HPS and losses of active-power in the microgrid depending on the power of generating and place of HPS joining. Generating power of HPS in relative units:
.
where PHPSi– generating power і-that HPS, і – serial number of HPS, і=1…3, PHPSimin – generated HPS power under which the minimum loss in MM.
Losses of active-power in the microgrid during work of і-that HPS in relation to the minimum losses of power during the work of this HPS.
.
where ΔPHPSi is the power loss in MM during the work of i– th HPS, ΔPHPSimin – minimum losses of active-power in MM during the work of i – th HPS.
Fig.3. Dependences of power losses on generating power of HPS
The losses of active-power without working HPS in relative units in the branches of the chart of MM are determined as a relation of losses of power without working HPS to the minimum losses of power during work of і-that HPS.
.
where ΔРwithout HPS – power loss in MM without the HPS generating. The results of calculations are in the table 5.
Table 5. The Relative values of losses of active-power in MM and generating powers of HPS
.
In accordance with table 5, the graphics of losses of active-power dependences in MM from power of HPS generating (Fig.4).
Fig.4. Influence of the greatest HPS to the losses of active-power in the grid
So the most influential to the changes of losses of active-power in the grid is HPS1.
Thus, use of HPS1, will allow to attain the minimum value of losses of active-power at minimum power of HPS generating and charges of water.
Conclusions
An offer method of reduction of losses of active-power electric microgrids of the electro energy systems is based on the mutually concerted use of different typed sources of the distributed generation of SЕS and HPS.
Put and decided task of optimization of the mode of the microgrid on the criteria of reduction of losses of active power in the branches of equivalent chart of the microgrid by adjusting of electric power generating by the hydroelectric power station and reasonable choice of that of present HPS, by which is needed to carry out a management of the mode in accordance to the current values of parameters of the nodal voltage and generating of the distributive electric energy sources.
REFERENCES
[1] Masters C.L., Voltage rise: the big issue when connecting embedded generation to long 11kV overhead lines, Power Engineering Journal, 1 (2002), n.14, 5-12 [2] IEEE standard for interconnecting distributed resources with electric power systems, IEEE standard 1547TM -2003. [3] Tran K., Vaziri M., Effects of dispersed generation (DG) on distribution systems, Proc. of IEEE Power Engineering Society General Meeting, 3 (2005), 2173-2178 [4] Kiprakis A. E., Wallace A. R., Maximizing energy capture from distributed generators in weak networks, IEEE Proceedings on Generation Transmission and Distribution, 151 (2004), n.5, 611-618 [5] Kim T. E., Kim J., Voltage regulation coordination of distributed generation system in distribution system, Proc. of IEEE Power Engineering Society Summer Meeting, 1 (2001), 480-484 [6] Kovalchuk О., Nikitorovych О., Lezhniyk P., Kulyk V., HPS in the local electric systems with dispersed generating, Hydroenergetics of Ukraine, 1 (2011), 54-58 [7] Gonen T., Electric power distribution system engineering, 2nd Edition, CRC Press, (2007), 127-129 [8] Lezhniyk P. D., Rubanenko О. E., Kulymchuk А. V., Reduction of additional losses of electric power in the electric grids caused by their interaction, Vysnyk Vinnitsa Polytechnic Institute, 5 (2013), 48–52 [9] Kim T. E., Kim J., Considerations for the feasible operating range of distributed generation interconnected to power distribution system, Proc. of IEEE Power Engineering Society Summer Meeting, 1 (2002), 42-48 [10] Choi J-Ho, Kim J., Advanced voltage regulation method at the power distribution systems interconnected with dispersed storage and generation systems, IEEE Trans. Power Delivery, 15(2000), n.2, 691-696 [11] Scott N.C., Atkinson D.J., Morrell J. E.,Use of load control to regulate voltage on distribution networks with embedded generation”, IEEE Trans. Power Systems, 17 (2002), no.2, 510-515 [12] Niknam T., Ranjbar A.M., Shirani A.R., Impact of distributed generation on Volt/ Var control in distribution Networks, Proc. IEEE Bologna Power Tech Conference, 3 (2003), 23-26 [13] Batrinu F., Carpaneto E., Chicco G. et all, New nested evolutionary programming approach for voltage control optimization with distributed generation, Proc. IEEE Mediterranean Electrotechnical Conference, 3 (2004), 1007-1010 [14] Enslin J., Heskes P., Harmonic Interaction Between a Large Number of Distributed Power Inverters and the Distribution Network, IEEE Transaction on power electronics, 19 (2004), n.6, 1586-1593 [15] Jung J., Onen A., Arghandeh R., Broadwater R., Coordinated control of automated devices and photovoltaic generators for voltage rise mitigation in power distribution circuits, Renewable Energy, 66 (2014), 532-540 [16] Minchala-Avila L., Garza-Castanon L., Vargas-Martınez A., Zhang Y., A review of optimal control techniques applied to the energy management and control of microgrids, Proc. of the 5th International Conference on Sustainable Energy Information Technology (SEIT), 52 (2015), 780-787 [17] Hrisheekesha P., Sharma J., Evolutionary Algorithm Based Optimal Control in Distribution System with Dispersed Generation, International Journal of Computer Applications, 27 (2010), n.14, 31-37 [18] Pukach A., Teslyuk V., Ivantsiv R. A., Komada P., Method and means of measuring small quantities of electrical resistance, Informatyka, Automatyka, Pomiary w Gospodarce i Ochronie Środowiska, 4 (2012), 14-16 [19] Zyska T., Lozbin V., Analiza możliwości stosowania efektów termoelektrycznych do badań termoogniw, PRZEGLĄD ELEKTROTECHNICZNY 3(2008), 32-34 [18]Galzina D., Voltage Quality Improvement Using Solar Photovoltaic System, Journal of Sustainable Development of Energy, Water and Environment Systems, 3 (2015), n.2, 140-150 [19] Lezhniyk P., Rubanenko О., Gunko І., Techical Sciences, Vysnyk Khmelnytsky National Technical University. Series, n.2, pp.134-139, 2015. [20] Danylov-Danylian V., Global problem of fresh water deficits, Age of globalization, 1 (2008), 45-56 [21] Lezhniyk P., Rubnenko О., Nikitorovych О., The operative diagnosticating of high-voltage equipment is in the tasks of optimal management of the modes of electroenergy systems, Technical electrodynamics, (2012), n.3, 35-36 [22] National standard 29322-2014 (IEC 60038: 2009), the voltage standard, Moscow, Russian Federation, (2015), 16-17
Autorzy: Doctor of Eng. Sciences, Prof. Petro D. Lezhniuk, Chair of the Department of Electric Stations and Systems, M.Sc. Iryna O. Hunko, M.Sc. Sergiy V. Kravchuk, Vinnytsia National Technical University, Khmelnytske Shose 95, 21021 Vinnytsia, Ukraine, Email: ira_rubanenko@bk.ru; Ph.D. Paweł Komada, Ph.D. Konrad Gromaszek, M.Sc. Assel Mussabekova, M.Sc. Abenar Arman, Faculty of Electrical Engineering and Computer Science, Lublin University of Technology, ul. Nadbystrzycka 38A, 20-618 Lublin, Poland, E-mail: k.gromaszek@pollub.pl; M.Sc. Nursanat Askarova, Kazakh National Research Technical University after K.I. Satpayeva, 22 Satbaev Street, 050013, Almaty City, Kazakhstan.
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 93 NR 3/2017. doi:10.15199/48.2017.03.25
Published by Alex Roderick, EE Power – Technical Articles: Harmonic Mitigation Using Phase-shifting Transformers and Harmonic Filters, May 21, 2021.
Learn about how minimise the presence of harmonics in an electrical system with harmonic transformers and filters.
Three-phase loads do not generate triplen harmonics. Therefore, harmonic problems in situations where 3-phase loads dominate are primarily from currents flowing at the 5th, 7th, 17th, 19th, or higher harmonics. A harmonic mitigating transformer (HMT) can use dual secondary windings or pairs of transformers to reduce these harmonics. A bank of two or more transformers with a 30° phase shift between them can be used to treat these harmonics. This degree of phase shift is chosen to ensure that the harmonic components of one secondary are out of phase with those of another.
In the case where a transformer is supplying both single-phase and 3-phase loads, a combination approach is needed. Pairs of delta zigzag transformers with a 30° phase shift are often used as part of a separate transformer bank. The 30° phase shift between the transformers reduces the 5th, 7th, 17th, and 19th harmonics. The secondary zigzag windings greatly reduce the triplen harmonics.
Note: Voltage sags during startup can be recorded using a power quality meter.
To achieve the best results, the single-phase, line-to-neutral, nonlinear load must be balanced between two panels fed by two separate HMTs. One of the HMTs should be a delta-zigzag with a 0° phase shift. The second HMT could be either a delta-wye or a wye-zigzag with a phase change of 30°. The use of the two transformers would help eliminate the 5th, 7th, 17th, and 19th harmonics. Also, the harmonic attenuation will be more effective when the loads are balanced.
For example, if a main power panel feeding single-phase nonlinear loads requires 200 A, it is better to use two separate panels of 100 A each. See Figure 5. Two transformers are used to feed the separate panels. One transformer is wired in a delta-zigzag configuration, and the other transformer is wired in a delta-wye or a wye-zigzag configuration. The two transformers are 30° out of phase with each other. The computer loads draw current in pulses, and the harmonics move back through the transformers to the main power panel. The harmonics add together so that the overall system draws current in a waveform with very low THD.
Figure 5. Banks of transformers with a phase shift between them are used to cancel out harmonics.
HMT Impedance
The two HMTs should have the same impedance values, be located close to the source bus, and have the same load harmonic profiles. With a zigzag secondary, the impedance is less than the transformer nameplate impedance rating. In a delta-wye or delta-delta transformer, the single-phase impedance is the same as the positive and negative sequence impedance. This is the impedance on the nameplate.
With a delta-zigzag or a wye-zigzag transformer, the phase to neutral impedance is approximately 75% to 85% of the positive and negative sequence impedance. This results in a higher fault current in the event of a single-phase fault to neutral or ground. See Figure 6. This may require an overcurrent protection device with a higher rating. The impedance value given on the nameplate of the transformer is the positive/negative sequence impedance. Therefore, it is best to assume that any fault current is about 133% of a calculated fault current. This is very important when conducting a coordination study for arc flash protection.
Figure 6. The single-phase impedance of a zigzag transformer is about 75% to 85% of the nameplate impedance.
Harmonic Filters
A harmonic filter is a device used to reduce harmonic components and THD. A single-phase harmonic filter is used to reduce the harmonics from nonlinear single-phase loads by minimizing the third and other triplen harmonics. Three-phase harmonic filters, also called trap filters, are used to reduce harmonics produced by single-phase nonlinear loads connected to a 3-phase system or 3-phase loads such as AC variable-speed motor drives connected to the system. A 3-phase harmonic filter’s primary purpose is to reduce the fifth and seventh harmonic currents produced by six-pulse (six-diode) converters that convert AC to DC. The filter is usually tuned to just below the fifth harmonic and offers a low-impedance path that traps the fifth and most of the seventh harmonic. Harmonic filters should be installed as close as possible to the nonlinear load. With 3-phase drives, they are typically installed at the service equipment.
Harmonic filters may include different types of circuits or components designed to reduce harmonic currents, such as combinations of capacitors, inductors, and other components. Harmonic filters are typically classified as passive harmonic filters and active harmonic filters.
Passive Harmonic Filters
A passive harmonic filter uses capacitors and inductors that are tuned to remove particular harmonic frequencies. See Figure 7. The passive harmonic filter works like a band-pass or low-pass filter in an electronic circuit. It allows low frequencies (60 Hz) to pass through unchanged while removing higher frequencies at 180 Hz and above. Passive harmonic filters can be difficult to use because they often cause other problems like ringing, unwanted resonances, and overcompensation. Single-phase harmonics sources like SMPSs generally do not generate very much phase shift between current and voltage. Therefore, a passive filter can easily cause a circuit to switch from lagging to leading. In addition, passive harmonic filters tend to be fairly large and can be somewhat expensive.
Figure 7. A passive harmonic filter uses a set of resistors, capacitors, and inductors tuned to remove harmonic frequencies. Image courtesy PSCAD
Active Harmonic Filters
An active harmonic filter uses electronics to provide a variable impedance to remove harmonics from the circuit or to generate an adaptive current waveform that is 180° out of phase with the harmonics.
See Figure 8. Active filters have typically been very expensive and not widely available. However, advances in electronics are making these types of devices more available and cost-effective.
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 Prof. Silviu Darie, PhD (EE), Technical University Cluj Napoca, Romania, Honorary Member of the Romanian Technical Sciences Academy. Email: silviu.darie@enm.utcluj.ro.
Abstract: Based on the author’s experience in designing power systems for critical loads, the paper presents the ways of supplying essential loads, the distribution system layout configuration, the uninterruptible power supply sources (UPS), the UPS modeling and operation, the power system studies by using PTW/SKM professional industrial software.
Key Words: Electrical critical loads, UPS structure and operation, power system modeling procedures, power system studies using professional software.
Abbreviations: UPS – Uninterruptible Power Supply; FDR – Feeder / Cable; CB – Circuit Breaker; FTS – Fast Transfer Switch; PTW/SKM Professional industrial software.
Contributions: A guide for power systems studies with critical loads. Critical loads power system layout; UPS modeling procedures and simulation software settings; power system studies using PTW/SKM professional software.
1. Introduction
The Critical Loads are those to which the power supply has to be maintained under any circumstances and never be interrupted. Such electrical loads are Data Centers, Hospitals, Control Rooms and Control Rooms for Airports, Industrial Control Systems, and oil Shore Platforms. However, there is a need to maintain the power supply to the critical loads during the effective transition to an alternate power supply. The alternate power supply can be the uninterruptible power supply – UPS. The UPS was initially designed for computers. The UPS has a DC battery as an energy storage device. Several UPS are on the market and depend on how they are connected to the critical loads and the UPS configuration [1].
2. System Layout Configuration
Some configurations and structures depend on the size and load profile of the Critical Loads and the application.
One may have the following structure:
• UPS supply from different power sources such as two different Utility Power Supply; • UPS supply from a Utility Power Supply and a Diesel Generator Set.
One presents the typical system layout modeling configuration while industrial software is employed: SKM professional software [1].
Figure 1. Supply from 2 Commercial Sources: Power Utility and Local Generator (SKM Modelling)
Figure 2. Supply from 2 AC Commercial Sources: power system details
3. UPS Description
A UPS is a power supply system with energy storage that ensures the load continues to be supplied even if the primary supply voltage fails (EN 50091-1). The UPSs systems protect against data loss and system damage due to power failures, voltage dips, voltage spikes, under voltage, overvoltage, switching, interference voltages, frequency changes, and harmonic distortion.
A. UPS components: The UPS modules/components shall consist of the following main components:
• Rectifier/charger; • Static inverter; • Fast Transfer Switch (FTS); • Output isolation transformer; • Control panel; • Monitor panel; • Communication panel.
B. The main types of UPS
There are three major types of UPS as follows [Eaton]:
The UPS module shall operate online, fully automatically, in the following modes:
Normal Mode: The inverter shall continuously supply the critical load. The rectifier/charger shall derive power from the commercial AC source and shall supply DC power to the inverter while simultaneously float-charging the battery;
Emergency Mode / Battery Operation: When the commercial AC power is outside a –15/+10% window around nominal voltage, the critical load shall continue to be supplied by the inverter, which shall obtain energy from the batteries without any operator intervention. It’s an automatic operation. There shall be no interruption to the critical load upon failure or restoration of the commercial AC source;
Recharge Mode: Upon restoration of the AC source, the rectifier/charger shall recharge the batteries and simultaneously provide power to the inverter. This shall be an automatic function and shall cause no interruption to the critical load;
Bypass Mode: If the UPS module must be removed from the normal mode for overload, load fault, or internal failures, the Fast Transfer Switch (FTS) shall automatically transfer the critical load to the commercial AC power (usually in less than 0.2 seconds). Return from bypass mode to normal mode of operation shall be automatic. Bypass mode shall be capable of being initiated manually without the operation of the static switch from the front control panel.
Generally, a UPS supply system offers uninterrupted power to the AC load by converting DC into AC. UPS differs from an emergency power supply system or a standby generator, as it can protect devices from power outages by one or more connected batteries. The battery run time is relatively short, typically 5 to 15 minutes, but it is long enough to bring the auxiliary power supply online or protect devices from shutting down.
4. UPS modeling for power systems studies
The UPS unit is modeled as two parts: the primary part is connected to the AC power supply, and it is considered as the power system load. The second part is a power source that supplies the critical loads. While PTW/SKM professional software is employed, two UPS pages shall be developed.
Figure 4.1 The UPS input data is:
• UPS name and manufacturer; • UPS status: in-service or off; • UPS ratings on the load side; • UPS power factor and efficiency; • UPS connection on the line side and load side; • Rated voltages on the line side and load side; • UPS phases.
Figure 4.2 The UPS input data shall provide the following:
• UPS short circuit contribution as a percent of the UPS rating; • Short circuit X/R; • Battery charging as a percent of the UPS rating; • Bypass mode provides the technical information for the Fast Transfer Switch (FTS): UPS Zin percent and X/R ratio.
Figure 5.2 UPS Mode of Operation PTW/SKM professional software. Typical, Recharging, Emergency.
Figure 5.3 UPS Recharging Mode of Operation
Figure 5.4 UPS Emergency Mode of Operation and Bypass Fast Switch
Note: While UPS is in emergency mode, the FTS is closed in less than 0.2 seconds, consequently protecting the unit.
6. Modeling the Power System with Critical Loads:
The distribution system model shall be developed to be fully integrated and meet the performance specifications requested by the Project Scope of Work.
One recommends that the system model be laid out in multiple drawings/views and in a manner that provides for easy viewing of all analysis results. The one-drawing/view requirement ensures that problem areas found and highlighted by the program are easily seen and not hidden or buried; All one-line symbols shall be adequately spaced to facilitate viewing results on the one-line;
Equipment names used in the modeling software shall be identical to the equipment and naming convention shown on the existing facility drawings and equipment unless conflicts exist; The Consultant Engineer shall discuss facility operation with the designated Facility to determine the possible operating modes of the system and the UPS units. Each system operating mode shall be documented and modeled in the software as “Scenarios” to determine the electrical equipment’s worst-case and associated parameters.
One suggests that the lumped motor groups for MCCs shall be modeled per IEEE standards using groups >50 Hp and <50 Hp. Where motor list data is not available, single lumped groups may be modeled per IEEE-141 “Red Book”;
Medium voltage motors greater than 1.0 kV shall be modeled individually on their respective buses, including all protective phase and ground overcurrent relays and fuses. All substation low voltage power circuit breakers (LVPCB) shall be modeled.
All relay data shall be modeled based on the nameplate data, including manufacturer, type, style, trip device, and settings. Generic substitutions or assumptions shall not be allowed unless data cannot be field verified. All assumptions shall be documented in the report and discussed with the client. All equipment modeling must have a corresponding one-line diagram symbol, meaning there can be no hidden database models. The purpose is for the facility to see all equipment quickly and its associated data, to be able to link documents to the kit as a data repository, etc., and to see problems right on one line. All system modeling shall conform to accept modeling practices as outlined in IEEE-399 “Brown Book.” The Consultant may provide more advanced modeling techniques where compliance with the specification is maintained.
The following guidelines are offered as an aid to determine which technique may be the most appropriate for a particular system operation condition:
7. Power Systems Studies
The power systems simulation generates a TWIN that creates a mirror between a digital model replica and the real world. With all the features integrated into the TWIN, including the requested model date base, one may test power system performance under several conditions.
7.1 Power Flow Analysis
Power Flow / Load Flow is a critical task. The convergence of Power Flow demonstrates that the power system model is feasible and the input data are consistent.
Several methods are employed for Power Flow. The most typical are:
• Seidel Gauss; • Newton Raphson; • Fast Decoupled
Some experimentation is recommended to determine the best methods for each power system model.
The following guidelines are proposed as an aid to determining both the UPS modeling layout and the UPS input data:
• Check the critical loads with the designer; if the distribution system already exists, use the SCADA and get the loads as measured data (system as built);
• Pay attention to the UPS mode of operation: Normal, Recharging, Emergency, or By-Pass;
• The By-Pass mode is during a fault at the critical bus;
• Pay attention while setting the UPS input data: UPS ratings (P&Q), critical loads P and Q, UPS losses (P&Q); the total load on UPS = critical loads + UPS losses (P&Q) + UPS charging (P&Q);
• UPS input power factor – is not a problem in a modern UPS. The UPS rectifier has reactive and capacitive components, so it will also have a power factor, which must be accounted for when making the upstream electrical connection. The UPS input power factor is a design characteristic usually declared by the manufacturer in the technical specification. With modern IGBT (insulated-gate bipolar transistor) front-end rectifier technology, the input power factor is typically close to unity, 0.99 at 100% nominal load. However, the actual metered input power factor may be slightly different as, for example, highly nonlinear loads can cause the input power factor to decrease slightly. Typically, though, a UPS input power factor will still be in the range of 0.97 – 0.99 and not of great concern. With older technology, using six- or 12-pulse rectifiers, the THDI (total harmonic distortion of current) and power factor require more attention;
• UPS-rated output power factor is a UPS design factor. The rated output power factor describes the maximum active and apparent loading the UPS can tolerate by design. For example, a 100 kVA UPS with a rated output power factor 1.0 can handle loads up to 100 kW active power and 100 kVA apparent power. If the power factor is 0.8, these loads become 80 kW and 100 kV, respectively. The load’s active and prominent power must be known to select and size the UPS correctly. A UPS with a rated power factor of example, 0.8 can handle loads of higher power factor as well – and vice versa;
• A load analysis at the UPS output bus is a good approach for determining the total output load (P&Q) and the UPS power factor [1].
7.2 UPS estimated size
Depending on the possible level of damage in case of a data loss/production stoppage, critical applications require exclusively online UPSs, classification following IEC 62040-3 (double conversion UPS).
One has to calculate the possible damage by conducting a risk analysis with the customer. All other aspects, such as low purchase and operating costs (efficiency), are secondary and must take second place to damage avoidance.
However, the author presents the following simple approach [1]:
a. Get the Critical Input Data from the plant engineer:
.
b. Compute the UPS unit ratings as follows:
.
7.3 Power Flow Analysis Using Industrial Professional Software:
The power system model is generated using the SKM professional software as an exercise. The model is consistent with the requirements of the IEC and IEEE Standards. The model will also be helpful for future power system model upgrades and improve facility operations. The load flow model should be continuously updated as changes are made to the electrical power distribution system. One, in general, recommends that the facility team using this model keep the load flow model updated as changes are made in the system so it is an “as built” model.
PTW/SKM industrial, professional software is employed for system modeling. The PTW/SKM is a powerful industrial power software for designing and analyzing power systems. It is utilized worldwide by consultant engineers, designers, and utility engineers. The PTW/SKM has been on the market for over 38 years. It has a powerful Graphical User Interface (GUI) with several calculations, an extensive power system database, and intuitive display information. The PTW/SKM is used by over 35,000 engineers worldwide, offering specialized Power Tools design and robust modeling and documentation capabilities. Professional training is provided for PTW/SKM users.
• The load flow shows the bus voltages at all buses and the power flows in all branches: power transformers and lines. The results may be provided as text output results or may be given in the model drawings as Power Flow visualization;
• The convergence of the Power Flow demonstrates that the system is feasible and the system input data is consistent.
A summary of the model Input Data is shown in Figure 7 below:
Figure 7.1 Model Input Data (overview)
Figure 7.1 Model Input Data (continued)
Figure 7.1 Model Input Data (continued)
Figure 7.1 Model Input Data
For example, the Power Flow Results for the model shown in Figure 1, the Scenario Utility supply, and the Generator supply for each mode of operation are provided in Figure 7.2 below. The Critical Load is supplied from the UPS unit only briefly. The power flow results are listed on the one-line SKM model.
Figure 7.2 Comparative Power Flow Results
Comparative power flow results, Scenario UTIL-Supply, showing the results for each UPS mode of operation. For this example, during UPS recharging mode, the UPS1-FDR1 is heavily overloaded (140.15%0). Consequently, the size of this feeder would need to be increased.
Figure 7.3 Comparative power flow results, Scenario UTIL-Supply; UPS in recharging mode
Comparative power flow results, Scenario GEN-Supply, showing the results for each UPS mode of operation. For this example, during UPS recharging mode, the UPS1-FDR1 is heavily overloaded (140.15%0). Consequently, the size of this feeder would need to be increased.
Figure 7.4 Comparative power flow results, Scenario UTIL-Supply; UPS in recharging mode
UPS Overloads and Support Time
For UPSs, the levels and ranges of overloads and support time are [2]:
103% overload 10 minutes to continuous; 125% overload between 30 sec and 10 minutes; 150% overload between 10 sec and 60 sec; 200% overload 10 to 20 cycles (current limit).
Notes: Since the above UPS specifications contain many possible overload and support times, designers must determine if a longer overload time limit provides valuable protection for the critical load [1]. Data Centers with redundant UPS systems features have a significant UPS over-capacity.
The “Power Flow Results” while the UPS unit is in emergency mode are listed below, Figure 7.3. The Fast Transfer Switch (FTS) bypasses the UPS unit, and the current flow on the upper and lower side is the same.
Figure 7.5 Power Flow Results: UPS in Emergency Mode FTS bypasses the UPS Unit
Figure 7.6 Power Flow Results: Supply from Utility or Generator; UPS in Emergency Mode FTS bypasses the UPS Unit
7.4 Short Circuit Analysis:
Short Circuit Analysis is one of the major tasks related to analyzing and planning electric power systems. The scope of a Short Circuit Analysis is as follows:
• Verify the circuit current path against short-circuit current stress (electrical, mechanical, and thermal); • Evaluate and verify the interrupting capacity of existing switching devices; • Calculate and set up adequate system protective device settings; • Evaluate the system-wide post-fault voltage profile during a fault at a particular point; • Improve the system layout design to minimize the effect of system faults.
Short Circuit Current at CRT-LOAD1-TERM (critical LOAD 1 terminal bus) is presented in Figure 7.7: the Scenario Normal; UPS Bypass Operation Mode.
In the bypass mode, the UPS units are protected against the faults. The Fast Transfer Switch (FTS) moves the UPS unit into bypass mode in less than 0.2 seconds. The short branch circuit current is listed in the figure capture listed below, Figure 7:
Figure 7.7 Short Circuit at Bus CRT-LOAD1-TERM
Note: The fast transfer switch (FTS) bypasses the UPS in less than 0.2 seconds and consequently protects the UPS unit. It is represented by the bypass function on page 2 of the SKM UPS editor.
7.5 Protective Device Coordination
Any electrical distribution system has only one purpose: to provide a continuous energy supply to utilize equipment at a reasonable cost. When a fault occurs in a system, it is necessary to clear the spot to provide safety to personnel, protect the circuit elements, and prevent unnecessary power outages. This feature is achieved by using appropriate and proper protections. We apply protective equipment such as lightning arresters, surge capacitors, reactors, and circuit interrupting devices to accomplish this protection function. Any protection project requires two steps
• The selection of the proper device to do the task;
• Select the correct settings for the devices so they will operate selectively with other devices to disconnect that portion of the system in trouble and with as little effect on the rest of the system as possible.
A power system protection project requires not only proper device selection but also wants to achieve the best coordination possible with the equipment we decided to buy. The statement “Coordination and Selectivity” are, in a sense, complementary terms and are used to describe the relative speeds at which two protective devices operate for the same fault current.
A power system protection project requires not only proper device selection but also wants to achieve the best coordination possible with the equipment we decided to buy. The statement “Coordination and Selectivity” are, in a sense, complementary terms and are used to describe the relative speeds at which two protective devices operate for the same fault current.
Coordination Studies:
A coordination study involves selecting and setting all the protective devices from the load upstream to the power supply. In selecting or developing these protective devices, a comparison is made of the operating times of all the devices in response to various levels of overcurrent. The objective, of course, is to design a selectively coordinated electrical power system.
Coordination procedure [1]:
The following procedure should be followed when conducting a coordination study:
• Start the coordination process from the bottom of the circuit and select a convenient voltage base. Usually, the lowest system voltage will be chosen. The Time-current graphical interface is automatically associated with the specified path;
• Specify protection points. These include the motor starting curve with the current and starting times, magnetizing inrush points, and the limits for specific protective paths the user selects. Do not select more than five protective devices for one protective course;
• Using the overlay principle, trace the curves for all protective devices on a composite graph, selecting ratings or settings that will provide overcurrent protection and ensure no overlapping of curves.
Notes [9]:
1. When coordinating IDMTL relays, the interval is usually 0.3-0.4 seconds; The interval consists of the following components: Circuit breaker opening 0.08 seconds (5 cycles). Relay over travel 0.10 seconds. Safety factor for CT saturation, settings errors, etc.: 0.22 seconds;
2. When coordinating relays with downstream fuses, the circuit opening time does not exist for the fuse, and the interval may be reduced accordingly; the time margin between the fuse total clearing curve and the up-stream relay curve could be as low as 0.1 seconds where clearing times below 1 second are involved;
3. When low-voltage circuit breakers equipped with direct-acting trip units are coordinated with relayed circuit breakers, the coordination time interval is usually regarded as 0.3 seconds;
4. When coordinating CBs equipped with direct-acting trip units, the characteristics curves should not overlap.
Generally, only a slight separation is planned between the different characteristic curves. If CBs are in series, the overlaps are accepted.
One starts by selecting the protective paths from the bottom part of the power system. A few PDC paths are presented in Figures 7.8, 7.9, and 7.10, while SKM professional software is employed.
UPS must transfer to bypass operation mode to provide enough fault-clearing current to trip the load circuit breaker in a load-side fault.
• Long duration inverter overload capability does not help – inverter does not have sufficient fault current to clear the fault;
• Trying to sustain a fault on the inverter (waiting for the O/L timer) will stress or damage the inverter and result in an output voltage drop;
• If bypass is unavailable – UPS will eventually trip off if the fault isn’t cleared, or the load will fail due to under voltage.
Additional considerations required to determine the robustness and suitability of a UPS system are:
• Maximum continuous temperature rating of UPS running at full load; • Cooling system employed within UPS; • Placement of fans for increased life and elimination of hot spots in the event of fan failure; • Redundancy of fans and fan failure alarms; • Proper handling of fault currents to reduce stress on UPS and loads.
Figure 7.8 PDC Path: From CRITICAL-LOAD1 to UPS-1-CB
Figure 7.9 PDC Path: Motor M1 to BUS1
Figure 7.10 PDC Path: XFMR-1 Protection
Figure 7.11 PDC Path: From M4 to BUS-1
Table 7.1 – LV CBs Types, Frame, and Settings
.
8. Conclusions
The paper presents how to supply the critical electrical loads, the power system layout configuration, uninterruptible power supply (UPS) structure, modeling, and operation. The procedure for the power system studies Power Flow, Short Circuit, and Protective Devices Coordination using industrial, professional software – SKM is documented. Uninterruptible power supply (UPS) systems ensure continuous supply to critical loads and quality power. The UPS system design requires the proper technique, installation, and maintenance.
References
[1] Darie, S.: Modeling UPS for Critical Loads: Training Manuals (2005 to 2015), Power Analytics Corporation, San Diego, USA. [2] SKM Power Software: UPS Units Modeling, 2016. [3] CENELEC – EN 50091-1 Uninterruptible Power Supply. Part 1. General and Safety Requirements. [4] IEC 62040-3 Uninterruptible Power Supply (UPS); Part 3. Method of Specifying the Performance and test requirements. [5] ISO / IEC 9003,2018. Software Engineering Guidelines for applying ISO 9001, 2008 to Computer Software. [6] IEEE Brown Book, IEEE Std. 399, 2015. [7] IEEE Red Book, IEEE Std. 141, 2014. [8] IEEE Std. 142-1991, “Recommended Practice for Grounding of Industrial and Commercial Power Systems” (IEEE Green Book). [9] IEEE Std. 242-1986, “Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems” (IEEE Buff Book). [10] IEEE Std. 1100-1992, “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment” (IEEE Emerald Book). [11] APS, Schneider Electric: Selection of the UPS Configuration, APS Schneider Electric, 2012. Kevin McCarthy, Victor Avelar: Comparing UPS System Design Configuration. White Paper 75, Revision 3, Schneider Electric, 2016. [12] Piller Power Systems: Isolated-Parallel UPS Configuration. [13] Powerware 9315 (200 to 500 kVA) Static Uninterruptible Power Supply, Guide; Specification Models 225, 250, 300. [14] 400 500 kVA, Power Ware 2003; [15] IEC 62040-1, 2008. TEST REPORT Uninterruptible Power Systems (UPS) – Part 1: General and Safety requirements for UPS.
Author:Prof. Silviu Darie, PhD (EE)
Author: Prof. Silviu Darie, PhD (EE), Technical University Cluj Napoca, Honorary Member of Romanian Technical Sciences Academy, Former VP Power Analytics Corporation, USA.
Prof. Dr. Daries has more than 20 years’ work experience with Power Analytics products, and nearly 40 years of university-level electrical engineering instruction and industry consultancy in power system analysis computer applications, electrical power quality, transmission pricing, embedded generation, computer aided power system analysis and design. In addition to earning both his doctorate and master’s degrees in electrical engineering, he has authored or co-authored hundreds of technical books, student manuals, technical papers, and research projects.
Dr. Darie is a former professor of power systems and electrical engineering in Technical University of Cluj Napoca, Romania, and University of Cape Town, South Africa, as well as a former visiting professor in École polytechnique fédérale de Lausanne, Switzerland. He has received several awards and recognitions throughout his years of expertise including the Award Professor for Life of Faculty of Engineering, University of Cape Town 1993, Romanian National Research Award. Since 2005 he is the Vice President of Consulting and Engineering for Power Analytics Corporation.
Dr. Darie led nearly 180 electrical power projects worldwide; he constructed 18 prototypes designed for mass production, holds three patents, and is experienced in most leading software programs for electrical engineering. He has provided services to clients worldwide, and is a registered professional engineer in Romania, South Africa, and New Zealand.
Contact address: Prof. Silviu Darie, Ph.D., P.E., Romania: Bd. 21 Decembrie 1989, No. 104 Bl. L1, Sc. 1, Ap. 8 Cluj Napoca, 400124 Romania Mobile: +40728312222 Email: silviu.darie@gmail.com, Silviu.darie@enm.utcluj.ro
Published by U.S. Department of Energy – Energy Efficiency and Renewable Energy Alternative Fuels Data Center
Consumers and fleets considering electric vehicles (EVs)—which include all-electric vehicles and plug-in hybrid electric vehicles (PHEVs)—need access to charging stations. For most drivers, this starts with charging at home or at fleet facilities. Charging stations at workplaces and public destinations may help bolster market acceptance by offering more flexible charging opportunities at commonly visited locations. Community leaders can find out more through PEV readiness planning, including case studies of ongoing successes. The EVI-Pro Lite tool is also available to estimate the quantity and type of charging infrastructure necessary to support regional adoption of EVs by state or city/urban area and to determine how EV charging will impact electricity demand.
Charging the growing number of EVs in use requires a robust network of stations for both consumers and fleets. The Alternative Fueling Station Locator allows users to search for public and private charging stations. Quarterly reports on electric vehicle charging station trends show the growth of public and private charging and assess the current state of charging infrastructure in the United States. Suggest new charging stations for inclusion in the Station Locator using the Submit New Station form. Suggest updates to existing charging stations by selecting “Report a change” on the station details page.
Photo: Charging ports. Image used courtesy of afdc.energy.gov
The SAE J1772 charge port (right) on a vehicle can be used to accept charge with Level 1 or 2 charging equipment. The DC fast charge port (left) uses a different type of connector. In this photo, it is a CHAdeMO.
Charging Infrastructure Terminology
The charging infrastructure industry has aligned with a common standard called the Open Charge Point Interface (OCPI) protocol with this hierarchy for charging stations: location, electric vehicle supply equipment (EVSE) port, and connector. The Alternative Fuels Data Center and the Station Locator use the following charging infrastructure definitions:
• Station Location: A station location is a site with one or more EVSE ports at the same address. Examples include a parking garage or a mall parking lot.
• EVSE Port: An EVSE port provides power to charge only one vehicle at a time even though it may have multiple connectors. The unit that houses EVSE ports is sometimes called a charging post, which can have one or more EVSE ports.
• Connector: A connector is what is plugged into a vehicle to charge it. Multiple connectors and connector types (such as CHAdeMO and CCS) can be available on one EVSE port, but only one vehicle will charge at a time. Connectors are sometimes called plugs.
Charging Infrastructure – 1 Station Location. Image used courtesy of afdc.energy.gov
Charging Equipment
Charging equipment for EVs is classified by the rate at which the batteries are charged. Charging times vary based on how depleted the battery is, how much energy it holds, the type of battery, and the type of charging equipment (e.g., charging level, charger power output, and electrical service specifications). The charging time can range from less than 20 minutes to 20 hours or more, depending on these factors. When choosing equipment for a specific application, many factors, such as networking, payment capabilities, and operation and maintenance, should be considered.
Level 1 Charging
Approximately 5 miles of range per 1 hour of charging*
J1772 connector
Alternating Current (AC) Level 1 equipment (often referred to simply as Level 1) provides charging through a 120 volt (V) AC plug. Most, if not all, EVs will come with a portable Level 1 cordset, so no additional charging equipment is required. On one end of the cord is a standard NEMA connector (for example, a NEMA 5-15, which is a common three-prong household plug), and on the other end is an SAE J1772 standard connector (often referred to simply as J1772, shown in the above image). The J1772 connector plugs into the car’s J1772 charge port, and the NEMA connector plugs into a standard NEMA wall outlet. Note that Tesla vehicles have a unique connector. All Tesla vehicles come with a J1772 adapter, which allows them to use non-Tesla charging equipment.
Level 1 charging is typically used when there is only a 120 V outlet available, such as while charging at home, but can easily provide charging for most of a driver’s needs. For example, 8 hours of charging at 120 V can replenish about 40 miles of electric range for a mid-size EV. As of 2021, less than 2% of public EVSE ports in the United States were Level 1.
* Assumes 1.9 kW charging power
Level 2 Charging
Approximately 25 miles of range per 1 hour of charging†
J1772 connector
Tesla connector
AC Level 2 equipment (often referred to simply as Level 2) offers charging through 240 V (typical in residential applications) or 208 V (typical in commercial applications) electrical service. Most homes have 240 V service available, and because Level 2 equipment can charge a typical EV battery overnight, EV owners commonly install it for home charging. Level 2 equipment is also commonly used for public and workplace charging. This charging option can operate at up to 80 amperes (Amp) and 19.2 kW. However, most residential Level 2 equipment operates at lower power. Many of these units operate at up to 30 Amps, delivering 7.2 kW of power. These units require a dedicated 40-Amp circuit to comply with the National Electric Code requirements in Article 625. As of 2021, over 80% of public EVSE ports in the United States were Level 2.
Level 2 charging equipment uses the same J1772 connector that Level 1 equipment uses. All commercially available EVs in the United States have the ability to charge using Level 1 and Level 2 charging equipment.
Tesla vehicles have a unique connector that works for all their charging options, including their Level 2 Destination Chargers and chargers for home. All Tesla vehicles come with a J1772 adapter, which allows them to use non-Tesla charging equipment.
† Assumes 6.6 kW charging power
DC Fast Charging
Approximately 100 to 200+ miles of range per 30 minutes of charging‡
CCS connector
CHAdeMO connector
Tesla connector
Direct-current (DC) fast charging equipment (typically a three-phase AC input) enables rapid charging along heavy traffic corridors at installed stations. As of 2021, over 15% of public EVSE ports in the United States were DC fast chargers. DC fast charging is projected to increase due to fleets adopting medium- and heavy-duty EVs (e.g., commercial trucks and vans and transit), as well as the installation of fast charging hubs for transportation network companies (e.g., Uber and Lyft) and other applications.
There are three types of DC fast charging systems, depending on the type of charge port on the vehicle: SAE Combined Charging System (CCS), CHAdeMO, and Tesla.
The CCS connector (also known as SAE J1772 combo) is unique because a driver can use the same charge port when charging with AC Level 1, Level 2, or DC fast charging equipment. The only difference is that the DC fast charging connector has two additional bottom pins. Most EV models entering the market today can charge using the CCS connector.
The CHAdeMO connector is another common DC fast connector type.
Tesla vehicles have a unique connector that works for all their charging levels including their fast charging option, called a Supercharger. Although Tesla vehicles do not have a CHAdeMO charge port and do not come with a CHAdeMO adapter, Tesla does sell an adapter.
‡ Charging power varies by vehicle and battery state of charge.
Published by Józef LORENC, Krzysztof ŁOWCZOWSKI, Bogdan STASZAK Politechnika Poznańska, Instytut Elektroenergetyki
Abstract. In this paper possibilities for improvement of earth fault protection by adjustment of protective relay settings due to change of neutral point impedance in medium voltage networks are presented.
Streszczenie. W artykule przedstawiono zagadnienia dotyczące możliwości poprawy skuteczności działania zabezpieczeń ziemnozwarciowych typu YY0 poprzez dostosowanie wartości nastawczych do zmian spowodowanych modyfikacją sposobu pracy punktu neutralnego w sieci średniego napięcia. (Zabezpieczenia ziemnozwarciowe wspierane funkcjami adaptacyjnymi).
Słowa kluczowe: punkt neutralny, zwarcie doziemne, admitancja, elektroenergetyczna automatyka zabezpieczeniowa Keywords: neutral point, earth fault, admittance, earth fault protection systems
Introduction
Admittance relay was developed in Poland in Institute of Electrical Power Engineering of Poznan University of Technology [1, 2, 3, 4, 5, 6]. Principle of operation is explained in [7] and [8]. First relays were implemented in distribution system networks at the end of XX century as an analogue construction. Nowadays admittance criterion is implemented in digital protection relays, which are installed in bays of 110/15 kV or medium voltage substations as an decision-making algorithm. Moreover some distribution system operators install admittance based fault passage indicators [9]. In Poland admittance relays are typically installed in compensated medium voltage networks. Another admittance based criteria for protection relays are in the development stage: i.e. Cumulative Phasor Summing [10 ,11] proposes centralized earth-fault protection based on measurements of zero sequence current and voltage. Despite of admittance criteria for detection of high impedance faults, another criteria are being analyzed i.e. [12] proposes wavelet based criteria. New fault feeder detection methods for a resonant grounding system are also presented in [13, 14, 15, 16].
YY0 relay presented in the paper is improvement of original admittance relay. Principle of YY0 relay operation is based on zero sequence admittance growth – ΔY0 during single phase to ground fault and after reconfiguration of an neutral point impedance. Start-up value is given by formula (1) and (2).
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Where: SI1, SI3, SI4 – signals measured during earth fault before neutral point impedance reconfiguration, ΔYY0n –admittance growth setting, U0n– zero sequence voltage setting value.
Signals “S” are functions of zero sequence current and zero sequence voltage of lines during phase-to-ground fault. Signals S are described by the following formulas (3), (4), (5) and (6).
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Signal S is connected to the zero sequence voltage input of the relay and is described by formula:
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Coefficients ku, ki, ky and kn are used to convert zero sequence input signals. Coefficients ku, ki, and kn are dimensionless and describe a transformation ratio of instrument transformers (current and voltage transformers) and input divider, whereas ky represents admittance of additional voltage circuit.
Sensitivity of the relay depends on impedance of neutral point and does not depends on zero sequence impedance of line.
Application of active zero sequence current forcing arrangement (ACF) in compensated networks results in growth of measured admittance ΔY0 observed in faulted line, particularly in growth of conductance component – ΔG0, which is proportional to additional resistance connected in neutral point in parallel to Petersen coil.
Setting value of YY0 relay is typically in range of 50% of additional resistor conductance. According to operational experience from Poland relays operate effectively up to 2000 Ω of fault resistance. A reason for limited level of detected fault resistance is mostly due to loss of sensitivity of zero sequence voltage component presented in (2) and explained further in (8).
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where: U0p – zero sequence measured voltage, EF – phase voltage of a network, RF – fault resistance, C0s – earth fault zero sequence capacitance, ω – angular frequency, d0 – damping coefficient of zero sequence impedance described as a ratio of zero sequence conductance and zero sequence susceptance (9), s – detuning coefficient of Petersen coil (10).
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where: Ld – inductance of Petersen coil.
Analysis of formulas (7) and (8) clearly shows that sensitivity of zero sequence voltage component of YY0 can be improved by changing d0 and s or by decreasing U0n setting value during certain earth fault conditions. However reduction of U0n setting value can only be made during earth fault and after analysis of measured values therefore relay has to be additionally equipped with adaptive algorithm. Two methods of adaptive algorithms are presented in next paragraphs.
Decision making algorithm for shunt impedance connected to neutral point
Fig.1. Characteristic of YY0 protective relay
Based on analysis of formula (7) it is clear that zero sequence voltage level for a given C0sand RF values strongly depends on damping coefficient – d0 and detuning factor – s. During operation of ACF value of d0 coefficient is increased. Alternatively instead of forcing resistor it is possible to connect forcing reactor (reactive zero sequence current forcing arrangement – RCF). In that case value of coefficient of earth fault current compensation detuning s is changed. In practice not only value of detuning factor s can be changed but also its sign.
Another possibility to change s is to disconnect coil, which is normally connected to neutral point of a grounding transformer. If RCF is used an additional criterion which analyses zero sequence susceptance growth of line is required. Therefore universal admittance characteristic presented in the figure 1 should be used. In order to include susceptance region in the characteristic, additional criterion has to be included in decision making algorithm. Criterion is analogical to (1), but signals in I0 current circuit are additionally shifted 90°. As a result characteristic presented in the Figure 1 is created. Area inside a rectangle characteristic (square) is non-operational area. ΔB0 states for susceptance growth necessary for relay operation and ΔG0 is conductance growth necessary for operation.
As is previously described RCF is similar to ACF, however operational experiences shows that in many situations RCF devices can be more effective than ACF. Especially in case of high impedance earth faults.
Fig. 2 presents RF = f(s) curves, which describe maximum earth fault resistance seen by YY0 relay in 15 kV compensated network with total ground fault current equals to 120 A. Analysis of curves allows us to conclude that in typical compensated polish networks with detuning factor lower than 0,1 region of detected fault can be significantly bigger if RCF device is used (curve 2) comparing to ACF (curve 1). A phenomena is explained by the fact that negative influence of RCF reactance on zero sequence voltage is lower than in case of ACF.
Fig.2. Maximum fault resistances detected by YY0 relay after operation of active/reactive zero sequence current forcing arrangements
Curves 1 and 2 presents the effectiveness of YY0 relay for earth fault detection after operation of RCF or ACF which enforces 20% rise of a total earth fault current. It is assumed that U0n set value is 15% of phase voltage of a network. Moreover it is assumed that phase-to-ground capacitance is equal in all phases (symmetrical network) and natural damping factor d0 is smaller than 0,04.
In practice effectiveness of YY0 during operation of RCF device is limited by curve 3. Maximal effectiveness (4000 Ω) can be achieved when detuning coefficient after operation of RCF is reduced to 0,1. Following conditions can occur only when:
– network is undercompensated (detuning is no bigger than – 0,1; s = – 0,1) and after operation of RCF network becomes overcompensated (detuning factor is no bigger than 0,1),
– network is overcompensated (detuning factor is no bigger than 0,1; s = 0,1) and after operation of RCF network becomes undercompensated (detuning factor is no bigger than – 0,1; s = – 0,1).
Effectiveness of an earth fault detection is reduced for all other detuning factors. When operation of RCF results in relatively big detuning factor, an effectiveness of earth fault detection will be reduced to 2000 Ω so it will be lower than effectiveness after operation of ACF.
Adaptive algorithm of ACF and RCF compares different variants and choose a better one –ACF or ACF and optimal control of shunt impedance. It is recommended to reduce reactance of coil if network is undercompensated and to increase reactance when network is overcompensated. System for shunt impedance control is presented in the Fig. 3. Control algorithm, which is responsible for measuring voltage level and tuning of Petersen coil plays an important role in the system. RCF increases an earth fault current by connection of additional reactance LNW or reduce a ground fault current (reduce inductive current) by disconnecting a reactance, which is normally connected between neutral point of grounding transformer and a ground. In order to operate properly the system needs to measure detuning factor continuously. Commonly used systems for active compensation and passive systems for control of an earth fault parameters ensure access to necessary parameters.
Fig.3. Control system of shunt, neutral point impedance
Connection of additional resistance to neutral point is justified only when fault resistance is low (relatively big value of U0p – i.e. above 50% of phase voltage) or when detuning factor (absolute) is too big. It is also possible to make a decision about ACF activation based on level of natural asymmetry of a network.
Adaptive settings
As is explained in previous paragraphs an effectiveness of YY0 operation is limited by sensitivity of zero sequence voltage component. In typical polish networks start-up values of voltage are in range of 0,15-0,2 of phase voltage. Specific value is determined by natural phase-to-ground asymmetry of a network and resonance effect during normal operating conditions of a network. As a result of the phenomena voltage is increased permanently during normal operation conditions. Voltage rise is described by formula (11).
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where: Uasn – voltage resulting from zero-sequence leakage and natural asymmetry, U0rez – voltage resulting from resonance effect between phase-to-ground capacitance of a network and Petersen coil.
One can clearly observe that an amplitude of the voltage could be easily reduced by increasing d0 and s factors. The detuning is however not recommended since an earth fault current extinguishing capabilities of a Petersen fault are reduced. The best condition to extinguish an electric arc can be observed when coefficient of detuning of earth fault current compensation equals 0 – coil current fully compensates a capacitive current of a network. Consequently to reduce negative aspect of resonance effect it is only possible to apply devices, which increase phase-to-ground damping factor. In typical compensated medium voltage networks in Poland and typical ACF systems damping can be raised to approximately 0,2, in these way a voltage resulting from resonance effect is reduced a few times. As a result less restricted requirements could be used during selection of U0n starting values. Reduction of U0nsettings usually improves effectiveness of YY0 relay and increases the range of detected high impedance faults.
Voltage effects resulting from operation of ACF became the foundation for development of conductance protection decision algorithm making in Institute of Electrical Power Engineering of Poznan University of Technology. Adaptive functions are included in this algorithm [8]. Similar functions can be implemented in YY0 relay, which operates according to following rules:
– adaptive function is activated only during resistance fault and when U0p is below Uonafter operation of ACF device,
– setting value is changed only after additional resistive component of a current is detected (effect of ACF operation), – when adaptive function is activated a set value of voltage criterion is reduced and setting of conduction rise is increased,
– reduction of U0p is between 15% to 5% of phase voltage,
– a value of conductance rise depends on ratio of U0p measured before and after operation of PFR and typically is lower than 150% of base value
After taking into account defined network parameters a performance analysis of YY0 with adaptive function is performed. Partial results of the analysis are presented in the table 1.
Table 1. Values of fault resistance detected by YY0 relay with adaptive function
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One can easily notice that thanks to adaptive settings during harsh conditions – high resistance faults (above a few thousands ohms), an effectiveness of earth fault detection is significantly increased. Possibility of reduction U0n setting to 0,05 in well compensated network allows for detection of earth faults with fault resistance above 7000 Ω. In case of networks with a big share of overhead lines and capacitive current in range of 80 – 90 A the effectiveness can be even higher – up to 10000 Ω.
REFERENCES
[1] Lorenc J., Marszałkiewicz K., Andruszkiewicz J., Admittance Criteria for Earth Fault Detection in Substation Automation Systems in Polish Distribution Power Networks. CIRED, Birmingham (1997), Publication IEEE, No. 438 (1997) [2] Florkowski W., Lorenc J., Maćkowiak M., Musierowicz K., Sposób i układ wybiorczego zabezpieczenia od jednofazowych zwarć z ziemią w sieci o małym prądzie ziemnozwarciowym, Patent PL Nr 116699 [3] Lorenc J., Rakowska A., Staszak B., Limitation of Earth-Fault Disturbances and their Effects in Medium Voltage Overhead Lines. Przegląd Elektrotechniczny, no. 4 (2007), ss. 75-79 [4] Lorenc J., Torbus M., Staszak B., Automatyczna sterowanie kompensacją ziemnozwarciową w sieciach SN przy wykorzystaniu miernika parametrów ziemnozwarciowych, Wiadomości Elektrotechniczne, no. 12 (2013) [5] Lorenc J., Musierowicz K., Sposób i układ do pomiaru stopnia skompensowania prądu ziemnozwarciowego w sieciach kompensowanych średniego napięcia, Patent PL Nr 150320 [6] Lorenc J., Staszak B., Wiśniewski A., Sposób i układ do wykrywania zwarć wysokooporowych w liniach pracujących w kompensowanej sieci średniego napięcia, Patent PL Nr 226282. [7] Lorenc J., Admitancyjne zabezpieczenia ziemnozwarciowe. Wydawnictwo Politechniki Poznańskiej,j (2007) [8] Wahlroos A., Altonen J., Compensated networks and admittance based earth fault protection, ABB library, (2011) [9] Altonen J., Wahlroos A., Performance of Modern Fault Passage Indicator Concept in Compensated MV-Networks, CIRED Workshop – Helsinki, (2016) [10] Wahlroos A., Altonen J., Application of Novel Multi-frequency Neutral Admittance Method into Earth-Fault Protection in Compensated MV-networks, 12th IET International Conference on Developments in Power System Protection, (2014) [11] Balcerek P., Fulczyk M., Rosołowski E., Iżykowski J., Pierz P., New algorithm for determination of faulty feeder in distribution network, 11th IET International Conference on Developments in Power Systems Protection, (2012) [12] Michalik M., Rebizant W., Łukowicz M., Lee S.-J. Kang S.H., Wavelet Transform Approach to High Impedance Fault Detection in MV Networks, IEEE Russia Power Tech, (2005) [13] Mou-Fa Guo, Nien-Che Yang, Features-clustering-based earth fault detection using singular-value decomposition and fuzzy c-means in resonant grounding distribution systems, Electrical Power and Energy Systems 93 (2017), ss. 97–108 [14] Xiangning Lin, Shuohao Ke, Yan Gao, Bing Wang, Pei Liu, A selective single-phase-to-ground fault protection for neutral uneffectively grounded systems, Electrical Power and Energy Systems, 33 (2011), ss. 1012–1017 [15] Wahlroos A., Altonen J., Pekkala H-M., Post-fault oscillation phenomenon in compensated MV-networks challenges earth-fault protection, 23rd International Conference on Electricity Distribution, Lyon, (2015) [16] Linčiks J., Baranovskis D., Single Phase Earth Fault Location in the Medium Voltage Distribution Networks, Scientific proceedings of Riga Technical University, The 50th International Scientific Conference Power and electrical engineering, (2009)
Authors: prof. dr hab. inż. Józef Lorenc, Politechnika Poznańska, Instytut Elektroenergetyki, ul. Piotrowo 3a, 60-965 Poznań, E-mail: jozef.lorenc@put.poznan.pl; mgr inż. Krzysztof Łowczowski, Politechnika Poznańska, Instytut Elektroenergetyki, ul. Piotrowo 3a, 60-965 Poznań, E-mail: krzysztof.lowczowski@put.poznan.pl; dr inż. Bogdan Staszak, Politechnika Poznańska, Instytut Elektroenergetyki, ul. Piotrowo 3a, 60-965 Poznań, E-mail: bogdan.staszak @put.poznan.pl;
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 94 NR 8/2018. doi:10.15199/48.2018.08.31
Published by Ryan Hsu, Bright Toward Industrial Co., Ltd., EE Power – Industry Article: Insulation Failure Detection in EV Batteries, May 15, 2023.
One of the issues with electric vehicle batteries is insulation failure. A proven approach to detecting and correcting this failure lies in ground-fault detection. However, as higher voltages become increasingly common in electric vehicle battery systems, finding the right MOSFET to handle these voltages is vital.
One of the issues with electric vehicle (EV) batteries is insulation failure, and the ability to detect and correct it is critical. A proven approach lies in ground-fault detection, requiring solid-state MOSFET relays. However, as higher voltages become increasingly common in EV battery systems, finding the right type of MOSFET to handle these voltages reliably is vital.
As electric vehicles become more powerful and require more voltage, MOSFET relays with higher operating voltages are necessary. Image used courtesy of Pixabay
Insulation Failure in EV Batteries
When insulation materials lose their insulation ability, it can cause resistance to decrease, leading to hazards for the battery management system (BMS) that can lead to shorter battery life or a risk of fire.
Therefore, in the design and use of a BMS, it is essential to be aware of these factors that can cause insulation breakdown and take appropriate measures to prevent it. Such factors include:
•Overload: Applying too much voltage to insulation materials can cause a breakdown.
•Contamination: Dust, moisture, or other pollutants can weaken insulation materials, causing breakdown.
•Aging: Over time, insulation materials may break down due to thermal, chemical, or mechanical degradation.
•Physical damage: Scratches, cuts, or other physical damage to insulation materials can create a current path, causing breakdown.
•Voltage stress: Over time, the application of AC electric fields can cause insulation materials to break down.
•Temperature: High temperatures may make insulation materials brittle and prone to breakdown.
Early EVs had issues with slow charging times and short ranges, which led to engineers increasing the total voltage and current rating to improve these characteristics. However, because of higher currents and voltages, there was the potential for shorter battery life and overheating to the point of fire. Engineers began developing insulation monitoring functions for EV BMSes to address this issue.
Preventing Insulation Failure
The most common method for preventing insulation failure is measuring the resistance of the dielectric by detecting the ground-fault current.
When the insulation of a battery cell fails, the energized conductor will come into contact with metal that is not intended to carry current. That metal is usually bonded to part of the equipment-grounding conductor and becomes a path of least resistance to electrical currents, constituting a ground fault. The presence of a ground fault can be used to activate an alarm signal using a MOSFET relay between the current sensors and the ground. This insulation monitor/detection function in BMS ensures that the battery insulation is healthy and no leakage occurs. The insulation detection system aims to identify and isolate faults, ensuring the safety and reliability of the battery system and protecting the batteries from premature failure.
In the ground fault detection approach, the MOSFET is switching high voltage from the BMS through a non-contact relay and a set of series/parallel resistors, as shown in Figure 1. The MCU (microcontroller unit) then measures the voltage drop to calculate the insulation resistance of the BMS. The insulation resistance value must comply with safety regulations: AC 500 ohms/V and DC 100 ohms/V; if it is too low, an alarm signal is activated to provide immediate protection against potential hazards.
Figure 1. A typical circuit for ground fault detection. Image used courtesy of Bright Toward
Furthermore, it is necessary to promptly check and repair equipment or systems to restore them to normal operation. Maintenance checks can also be performed on the insulation detection system to ensure its proper functioning and provide accurate data.
However, when selecting the MOS relay, it must withstand a higher voltage than the battery pack’s nominal voltage. For example, a battery pack with an 800 V nominal voltage typically requires a relay with a load voltage greater than 1600 V.
SiC MOSFETs Versus Si MOSFETs
SiC (silicon carbide) MOSFETs provide some definitive benefits compared to the Silicon-based equivalents. SiC-based Opto-MOSFET relays, in particular, offer greater load voltages, excellent switching speeds, and more energy-efficient performance. And while they are used in a range of applications such as industrial robotics, security, and telecommunications, they have been extremely useful for insulation failure detection in electric vehicles–especially those involving higher voltages.
Why Opto-SiC MOSFET Relays are a Better Solution
The relay used in BMS insulation detection has changed over the years, as illustrated in Figure 2, beginning with reed relays when the nominal voltage was 380 V. As the nominal voltage for battery systems increased, the operating voltage for the relays also increased. That increase required MOSFETs to switch to much higher voltage levels.
Note that when the nominal voltage was 400 V, Si MOS Relays were sufficient. However, a different semiconductor material was needed to handle greater voltages efficiently.
Figure 2. How BMS insulation detection system relays have evolved through the years. Image used courtesy of Bright Toward
A Si-based Opto-MOSFET relay’s physical limit is around 1500 V, which is not high enough for newer, higher-voltage battery systems that demand 1800 V operating voltages. Hence, the move to SiC MOSFETs.
Opto-SiC MOSFET Relays
Bright Toward’s Opto-SiC MOSFET relays for automotive applications are ideal for EV insulation failure detection, including some rated for 3300 V. The two main series are the 58 Series and the 53 Series.
58 Series is rated for a peak load voltage of 1800 V, with the 53 Series rated at 3300 V. Also, note that 6600 V Opto-SiC MOSFET Relays will be released soon. The AA58 series is AEC-Q101 certified and rated for a peak load voltage of 1800 V. They are used not only for EV BMS but also for energy storage systems and automatic test equipment — and represent the most innovative and highest voltage MOS relay in the market.
The AS58F series has similar ratings and applications as the AA58 series but also includes creepage clearances of ≥ 8 mm for input-output and ≥ 8 mm between drain pins of MOSFETs for safety certification requirements. Major automotive companies have already validated the 1800 V Opto-SiC MOSFET Relays (AA58, AS58) with ongoing mass production.
As the demand for higher load voltage solid state relays increases, Bright Toward has developed SiC-based Opto-MOSFET Relays to improve and increase load voltage for applications, including EV battery insulation fault detection and BMS battery balancing and other applications in industries as diverse as telecommunications and aviation.
Author: Ryan Hsu, is the Marketing Manager at Bright Toward Industrial Co., Ltd, a company based in Taiwan with over 30 years of experience in manufacturing reed relays and solid state relays for various industries, with a strong focus on BMS and IC testing. The company’s dedication to providing innovative and reliable solutions in these industries has been a key driver of their success. The company’s annual revenue of 55 million USD is a testament to its extensive experience and expertise in these industries.
Published by G. A. Mendonça1, H. A. Pereira1,2 and S. R. Silva1, 1 Graduate Program in Electrical Engineering – Universidade Federal de Minas Gerais Av. Antônio Carlos 6627, 31270-901, Belo Horizonte, MG, Brazil. Phone: +55-31-3409-4842, e-mail: gforti@gmail.com, heverton.pereira@ufv.com.br, selenios@dee.ufmg.br 2 Department of Electrical Engineering, Universidade Federal de Viçosa Av. P.H.Rolfs S/N,36570-000, Viçosa, MG, Brazil
Abstract. Wind power plants are playing an important role in renewable energy generation in this decade. With the enhancements in its technology, mainly based on the aggregation of power electronics, many studies have been carried for evaluating their impact in power quality. Although the Brazilian Electrical System National Operator, who is responsible for transmission and generation system management, has suggested a procedure for such studies, there are many aspects related to the simulation algorithm and the electrical components modelling that are left aside from the problem, without an adequate reasoning of its impacts on power quality simulation results.
This paper presents a detailed analysis of a wind farm impact on the Brazilian distribution system power quality. Both internal (wind park elements) and external (power grid) components modelling effects on harmonic propagation are considered, and these effects are evaluated
Keywords: Wind Farm, Harmonic Analysis, Frequency Domain Simulation, Power System Modelling, Simulation Software;
1. Introduction
Adequate modelling of electrical components has always been a concern when analysing harmonic penetration. Several works had been carried out in order to investigate the problems incurred from evaluating harmonic studies with a comprehensive analysis of the used approach.
Many of these studies, [1]-[4], discuss the methods which can be used to evaluate harmonic distortion: single and three phase system representation, time and frequency domain, etc. These works have lead to what is considered to be the most important aspect in harmonic studies, a sense of how these methods can affect the quality of the results. But it does not state the difference in improvement from one to another in a quantity matter.
Another important subject when dealing with harmonic propagation is the electrical components modelling. The difficulty in finding the proper equivalent to represent the system’s main equipment without letting the problem become neither too complex, impracticable to simulate, nor too simple, with inaccurate results.
Using a wind power plant as the study case, a sensibility analysis can evaluate the impact of the assumptions that one can make in this sort of study. For that, two computer simulation programs will be used. The Alternate Transient Program – ATP, which has several available equipments models for applications ranging from steady state to very complex transient analysis, will be used for comparing component equivalents with different complexity degree. On the other hand, DIgSILENT PowerFactory, an engineering tool specifically designed for Power System Analysis, will be used to compare the effects of systems degrees of representation.
2. System Description
The studied wind is similar to one of the 54 wind parks connected to the Brazilian grid. It consists of a collector substation and three feeders with a total of 28 wind generators. The wind farm system, which operates with a voltage of 34.5 kV, is connected to the grid through a step-up transformer, 34.5/69 kV, and a transmission line with 21 km. Then, a substation elevates the voltage to 230 kV in order to connect wind farm with the primary transmission system.
In that wind farm, the wind turbine generators – WTG are connected in three parallel groups, one composed of ten and two of nine units. Figure 1 illustrates the first group with ten wind generators. The other two groups are constructed analogously. Each wind generator consists of a permanent magnet synchronous generator – PMSG, with a full converter, each unit been capable of generating up to 1.5 MVA. In order to meet power quality requirements, the converter is followed by a second-order low-pass filter that helps smoothing the voltage waveform. This LC filter is composed of a series 0.15mH inductor and a shunt 500μF capacitor per phase. A scheme illustrating the PMSG is pictured in Figure 2.
Fig.1. One-line diagram for Group I WTG of the simulated wind park
Fig.2. Wind turbine with PMSG, back-to-back PWM converter and output filter
Although the wind generator technology affects substantially the harmonic analysis, this paper will consider one technology, modelled as harmonic current source. It will help concentrate in how the system modelling affects the overall result, considering only one technology and its harmonic current spectrum, listed in Table I.
The electrical components parameters of the simulated wind park were obtained from commercial wind farms, manufacturer engineering catalogue and electrical standards. Electrical cable and transformer’s characteristics are presented in Tables II and III, respectively.
3. Electrical Equipment Modelling
For the modelling of the electrical system, the following assumptions are considered:
1) All electrical supplies are balanced 2) The system components are symmetrical 3) All wind turbines are equal
Having considered that, the wind turbines were modelled as harmonic current sources without specifying the harmonic phase angle spectrum. As recommended by [5], the lack of diversity presented in most wind parks could be interpreted as a high probability of the harmonics to be in phase. Also, since all electrical components are symmetrical, the system will be treated as single-phase one.
Table I. – WTG Harmonic Spectrum
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Table II. – Conductor Electrical Parameters
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Table III. – Transformer Specification and Parameters
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Focusing in the degree of system representation and the equipment modelling, the next sections will discuss them in detail.
A. Cable Modelling
The distribution system of the wind farm is composed by single-core XLPE cables with the conductor size ranging from 70 mm2 to 185 mm2. The parameters were obtained from manufacture catalogues and used to feed the models.
The electrical parameters listed in these catalogues, which are usually gathered from measured data, were evaluated against a simulated model. In [6], the author discusses the modelling of these cables in EMTP-type programs, e.g. ATP, delineating a procedure which helps to overcome all inaccuracies that the model might present. This procedure was concerned mainly with transient analysis. Thus, it concentrates on providing common materials properties, on representing semiconductor screens properly, on analysing the significance of grounding condition of sheath, etc. Therefore, the electrical parameters calculated according with this procedure can be used to assess the accuracy of catalogue data.
A XLPE single-core 300 mm2 cable was modelled according with [6], at ATP’s Line/Cable Constant – LCC routine, and the distributed line component. The maximum error found for both amplitude and phase angle was 4.02% and 1.47%, respectively.
Therefore, the LCC model present in ATP gives a better response for transient phenomena, but for limited frequency range simulations, such as harmonic analysis, the simpler distributed parameter model is sufficient.
B. Transformer Modelling
Usually, in steady-state studies, e.g. short-circuit and load flow, transformers are modelled simply by a series impedance. Considerations relating winding stray capacitance are generally made for higher frequency studies, where some authors state that its effects are only noticeable for frequencies higher than 4 kHz [1].
This assumption can be validated using ATP. The program has several transformer models available, but the most complete one is the hybrid transformer, which represents stray capacitance. Typical values for this parameter can be found in [7].
For the analysis up to 3 kHz, a simple voltage divider circuit simulation showed that the maximum difference between the results with and without the capacitance effect was 1.67% for the amplitude and 3.86% for the phase angle.
C. Distribution and Transmission System Modelling
What degree of representation should be considered as accurately sufficient? This question always bothered when investigating broader frequency spectrum problems. If representing the entire network is impractical, estimating its behaviour from point of common coupling – PCC, short-circuit impedance, as used sometimes, is unrealistic [4].
In [4], the author suggests a system equivalent which is based on the prominence of low order resonances. It represents the system impedance with an L-C-L equivalent circuit, estimated from short-circuit impedance and the first two resonant frequencies, i.e. parallel and series.
The Electric System National Operator (ONS in Portuguese) is the entity responsible for coordinating and controlling the operation of generation and transmission facilities in the National Interconnected Power System. It offers information on Brazilian’s system, including electrical parameters of transmission line, transformer, capacitor bank, etc. It also provides the system data base built in the programs developed by CEPEL. The data base can be converted for harmonic analysis program, HarmZs, to find the frequency response at any bus compounding the Brazilian grid.
For this study, a few considerations were made in order to simplify the analysis. All transmission lines are modelled as a single equivalent nominal π-model, all machines impedances were neglected and all loads were modelled as parallel loads. The frequency response observed in the PCC is pictured in Figure 3, which also shows the frequency response obtained from the short-circuit parameters and with the L-C-L equivalent circuit calculated according to [4]
Fig.3. System frequency response at the PCC
With the system data collected from the ONS database, the primary transmission was modelled in PowerFactory with eight, fifteen and nineteen buses. In each case, the frequency response was obtained with two types of representations of the part of system not explicitly modelled: short-circuit impedance and L-C-L circuit equivalent at each boundary bus. Table IV illustrates the maximum absolute error observed for the resonance amplitude and frequency in each approach when compared with the entire system response.
Table IV. – System representation error
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The prior analysis also considered secondary distribution system, representing lower voltage equipment. Thus, for a sensibility assessment, a second simulation was carried out. Starting with the representation which incurred the best result, with nineteen EHV buses, the system was simulated without any equipment rated lower than 230 kV. Then, the system complexity was slightly increased by representing the components connected to the point of common coupling. The comparison of these two results is summarized in Table V.
Table V. – System representation error
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As the high voltage transmission system has lower losses, its impedance dominates the frequency response. But, it’s not necessary to model accurately the entire primary transmission network. The reduced system frequency response converged to the expected result with nineteen buses. As for the secondary distribution system, the model should represent at least those equipments which are connected to the PCC.
D. Full Converter PMSG Turbine Modelling
For the present harmonic propagation study, the wind turbines are modelled as harmonic current source in parallel with a fundamental frequency source [8]. The synchronous machine parameters were set in order to give the fundamental power, 1.5 MW, without affecting the impedance frequency spectrum. Figure 6 illustrates the model used in PowerFactory.
The harmonic current listed on Table I was collected through measurements made according to IEC 61400-21. Since it is concerned the WTG power quality impact, all quantities were collected after the converter filter, before the transformer’s low voltage side. Thus, the harmonic spectrum obtained from turbine manufactures already considers the filter effect, but not its influence over the system’s impedance. Neglecting this effect could under or overestimate some dangerous resonant frequencies.
Therefore, the harmonic current spectre will be recalculated according to equation (1). The harmonic current injection, modelled as illustrated in Figure 4, will be modified according to Figure 5. Zgrid is the short circuit impedance at the 34.5 kV bus.
.
Fig.4. WTG model
Fig.5. WTG model
Fig.6. WTG model
Fig.7. Modified WTG model
The filter gain is illustrated in Figure 8. For this study, two values of short-circuit power were used: 150 MVA and 1500 MVA. The first value used is the short-circuit impedance at the 34.5kV bus for the study case system. The second value was used to find the impact of this parameter in the overall result, estimating the situation during IEC testing to obtain the current spectrum.
From (1), it can be seen that the filter inductor has no effect on the output current. The modified WTG model that will be simulated in PowerFactory is illustrated Figure 7.
E. Aggregate Model for Large Wind Power Plants
Finding an aggregated model for wind power plant to simplify the analysis has become a powerful tool when investigating large wind farms. IEC [5] proposes an aggregation method based on the assumption that the wind turbines and their converters are unequal. However, most wind farms turbines have equal technologies.
Fig.8. Calculated filter gain
Fig.9. Harmonic current spectrum
The limits that this method implies are very well discussed in [9], where the author proposes a method of aggregation using two-port network theorem, the equivalent ABCD parameters are obtained from individual cable parameter, which also modify the harmonic current injection. The wind park is composed with n cable sections, as illustrated Figure 10, and does not considers the generator transformer. The wind turbine current emissions are reflected to the high-voltage side of the transformer.
Fig.10. Wind turbine group for aggregation method
A second circuit simplification is discussed in [10], where the derivation is based on apparent power losses. In this approach the author scales the transformer impedance by the number of units. The results obtained from both methods are discussed in the next section, where the wind farm system is studied.
4. Study Case
The Wind Power System studied in the present paper is partially illustrated in Figure 1. First, it was simulated so that the two aggregation methods discussed previously could be evaluated.
As stated in [9], the first method is based on two-port network theory where the medium-voltage network is modelled by its linear passive elements. The harmonic current injection is considered on the high side of the WTG. Therefore, generator transformers are not considered. For the second method, all transformers were scaled according with the number of parallel units.
The impedance frequency response seen from the 34.5 kV bus was compared for the three cases: the system with 10 WTG and the aggregated equivalent using both methods. The maximum error found in each case was 8.32% and 21.74%, respectively. Secondly, the total harmonic distortions – THD, observed in the 34.5 kV and the 69 kV buses are listed in Table VI.
Considering both methods with and without the filter explicitly modelled, the results were compared through the 34.5 kV and the 69 kV bus THD values. Table VI gives the results.
The harmonic current injection was modified and the filter’s capacitor was explicitly modelling in each case, except for the first method of aggregation, which only uses the current seen from the high-voltage side of the transformers. The THD results for this simulation are also presented in Table VI.
Table VI. – Total Harmonic Distortion
.
Despite of having a better result when comparing the impedance frequency response, the voltage distortion obtained from the first aggregation method did not meet with the expected.
The study case was also simulated focusing on the impact that the distribution system modelling has on the harmonic results. The wind farm was simulated with its internal system aggregated by the second method, which showed better results.
The first two cases simulated the wind farm with the transmission system seen from the PCC represented with the short-circuit impedance and the L-C-L circuit equivalent. The last two results were obtained based on a more detailed system modelling, where 19 buses from the EHV system was explicitly represented. This simulation permitted the analysis of the transmission line impedance effect, where it can be evaluated using the distributed parameter or the concentrated parameter model. The results obtained for these cases are summarized in Table VIII.
The harmonic current was recalculated according with (1) considering two different cases: Zgrid obtained from a short-circuit power of 150 MVA and 1500 MVA. The different current spectrums, illustrated in Figure 9, were simulated to compare the effect of this parameter on the result.
Table VII. – Transmission system modelled as an equivalent
.
Considering the last case, with the transmission system represented with 19 EHV busses and the transmission lines modelled with distributed parameters, as a more accurate result, it can be seen that assumptions made for the equipment modelling play an important role in harmonic penetration analysis.
Simulating the system without accounting with the WTG second-order low pass filter presented very severe results. Also, from the point of view of the grid, the difference in the filter gain by altering the short-circuit power, Zgrid in (1), showed very little difference from each other. When comparing the first two results, the limitations of simpler representation are evident. Considering the THD at the 230 kV bus, when the system is modelled as a short-circuit impedance, the error presented is 53.4%. With the L-C-L equivalent, the error for was as high as 37.6%. The transmission line modelling affected very little the harmonic distortion results. The highest error observed for the THD at the 230 kV bus was 12%.
5. Conclusions
There are several ways to model an electrical system when analysing harmonic propagation. Although several studies propose a rule of thumb when dealing with electrical equipment representation, it always lacks a quantitative comparison against each possibility.
This paper simulates a wind power system with parameters estimated from commercial systems and analysed it, examining the overall impact of system modelling and the degree of representation.
For harmonic studies, cable and transformers models presented in power system analysis engineering software, e.g. DIgSILENT PowerFactory, are very accurate. The degree of representation of external system is important, representing it with the short-circuit impedance at the PCC is very poor. For the presented study case, the result converged for the expected result with the system modelled with 19 busses and considering only secondary distribution system connected to the PCC. Further studies must be carried out in order to verify the accuracy of the reduced representation in other parts of the Brazilian grid. The harmonic source modelling must be analysed beyond the current spectrum measured. Filters used to mitigate the harmonic propagation have an important effect on the system impedance, altering the study results. Another study using time-domain simulation could enhance the analysis of the filter impact.
References
[1] S.J. Ranade and W. Xu, “An Overview of Harmonic Modelling and Simulation”, Tutorial Harmonics Modeling and Simulation, IEEE Power Engineering Society, 1998. [2] J. Arrillaga and N.R. Watson, Power System Harmonics, John Willey and Sons, London (2003), pp. 261-348. [3] CIGRE JTF 36.05.02/14.03.03, “AC System Modelling for AC Filter Design – An Overview of Impedance Modelling”, ELECTRA No. 164, 1996. [4] P.F. Ribeiro, “Guidelines on Distribution System and Load Representation for Harmonic Studies”, in ICHPS V, pp. 272-280, Atlanta, 1992. [5] IEC 61400-21-2008, “Wind Turbine Generator Systems—Part 21: Measurement and Assessment of Power Quality Characteristics of Grid Connected Wind Turbines”. [6] B. Gustavsen, J. Martinez, and D. Durbak, “Parameter Determination for Modeling System Transients – part II: Insulated Cables”, in IEEE Transactions on Power Delivery, vol. 20, no. 3, pp. 2045–2050, 2005. [7] IEEE Std C37.011-2005, “IEEE Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Bases”, pp. 49-53. [8] J. Li, N. Samaan, S. Willians, “Modeling of Large Wind Farms Systems for Dynamic and Harmonic Analysis”, in Transmission and Distribution Conference and Exposition, pp. 1-7, 2008. [9] F. Ghassemi and K. Kah-Leong, “Equivalent Network for Wind Farm Harmonic Assessments”, in IEEE Transactions on Power Delivery, vol. 25, no. 3, pp. 1808-1815, 2010. [10] E. Muljadi, et al, “Equivalencing the Collector System of a Large Wind Power Plant”, in IEEE Power Engineering Society General Meeting, 2006.
Source: European Association for the Development of Renewable Energies, Environment and Power Quality (EA4EPQ). International Conference on Renewable Energies and Power Quality (ICREPQ’12) Santiago de Compostela (Spain), 28th to 30th March, 2012
Published by Joanna KOZIEŁ, Lublin University of Technology, Institute of Electrical Engineering and Electrotechnologies
Abstract. The paper presents the classification of superconducting fault current limiters. In particular the principle of construction and operations of superconducting current limiter of transformer type is presented. Functional physical model of such a limiter, designed, manufactured in the Laboratory of Superconducting Technologies of Institute of Electrical Engineering. The paper presents the results of experimental research and the analysis of the impact of the secondary winding limiter on the performance of superconducting current limiters of a transformer type and the conclusions from the analysis are introduced.
Streszczenie. W artykule zawarto klasyfikację nadprzewodnikowych ograniczników prądu zwarcia. W szczególności przedstawiono zasadę budowy i działania nadprzewodnikowego ogranicznika prądu typu transformatorowego. Omówiono funkcjonalny model fizyczny takiego ogranicznika zaprojektowany wykonany w Pracowni Technologii Nadprzewodnikowych Instytutu Elektrotechniki. Przedstawiono wyniki badań eksperymentalnych, analizę wpływu uzwojenia wtórnego ogranicznika na parametry nadprzewodnikowych ograniczników prądu typu transformatorowego i przedstawiono wynikające z analizy wnioski. (Analiza wpływu impedancji uzwojenia wtórnego na parametry nadprzewodnikowych ograniczników prądu typu transformatorowego).
Słowa kluczowe: nadprzewodnictwo, ograniczenie prądu zwarcia, nadprzewodnikowy ogranicznik prądu typu transformatorowego, impedancja uzwojenia wtórnego. Keywords: superconductivity, limiting of short circuit current, transformer type superconducting fault current limiter, the impact of secondary winding impedance.
Introduction
Superconducting fault current limiters – SFCL are composed of superconducting elements of alternating impedance, being connected in series in an electrical circuit [1], [2]. They show a low impedance while operating in rated conditions of a protected electrical circuit, and high impedance in short circuit conditions in a protected circuit [3].
Fig.1. The state in the superconductivity – the idea of phenomena [4], [5].
The rapid return of capabilities to limit the current after the disappearance of short circuit and long life together with low operating costs are the main advantages of superconducting fault current limiters.
The SFCL superconducting elements work in both the superconducting and in the resistive state. The requirements for SFCL superconducting materials are different than in the case of other superconducting devices intended to operate only in the superconducting state (Fig. 1) [1],[6].
The classification of superconducting fault current
limiters Literature distinguishes between the following superconducting fault current limiter types:
• resistive limiter, • inductive limiter [2],[7-10], • a) systems with an open magnetic core [11], b) systems with a closed magnetic core, • transformer type [12],[13].
You can accept the idea that the transformer type superconducting fault current limiter is a variation of the inductive type superconducting fault current limiter with a magnetic core. The transformer type superconducting fault current limiters have many advantages over resistance and inductive ones because they do not require current culverts, as it is the case of resistance limiters, and do not require secondary superconducting winding either, as it is the case of inductive limiters. In the transformer type superconducting fault current limiters the secondary winding impedance value of a limiter will increase the short circuit impedance during the short circuit, consequently the short circuit current will be limited to the value resulting from the parameters of the superconducting element used. The degree of reduction of the current in the transformer type superconducting fault current limiters is sufficient to limit the short circuit current with very large values.
The principle of the construction and operation of transformer type superconducting fault current limiter
Fig. 2 shows the idea of the construction and operation of transformer type superconducting fault current limiter. The limiter in question is composed of a conventional transformer with copper winding and of a superconducting element R2, shorting the secondary winding of a conventional transformer. The superconducting element is usually an inductor or a bifilar coil, wound with the HTS superconducting tape. The primary copper transformer winding is connected in series with the protected circuit of power grid and the secondary winding is shorted with the HTS superconducting coil, with the critical current value equal to the admissible value of the current of the protected circuit. When the current in the secondary winding of the conventional transformer exceeds, as a result of a short circuit, the value of the critical current of the superconducting winding, the winding loses superconductivity and transits into the resistive state. The HTS coil transition to a resistive state occurs rapidly. Within a few microseconds, the resistance of the secondary side of the transformer and the transformer type superconducting current limiter “changes” in the reactor limiting the current in the protected circuit. With such an activity of the limiter, the short circuit/fault current does not achieve its primary maximum, which protects electrical equipment, especially transformers, from the effects of mechanical forces that can damage the device mechanically.
Fig.2. The construction and operation of transformer type superconducting fault current limiter [4], [14]:
I1 – current of the primary side of the transformer, I2 – current of the secondary side of the transformer, U1 – voltage of the primary side of the transformer, U2 – voltage of the secondary side of the transformer, L1 – self-inductance of the primary side, L2 – self-inductance of the secondary side, US – mains voltage, M – mutual inductance of windings, ZL– load, R2– resistance of the superconducting limiting element.
If the value of the current of the power transmission line I1, which is equal to the current value of the primary side of the serial transformer, is small then the current limiter has a very low impedance (in the superconducting state), because RHTS = 0.
During the short circuit, as a result of the increase in the value of the I1 current the limiter has a high impedance (a resistive state), because the resistance of the superconducting element is significantly higher than zero RHTS> 0 [15].
Fig.3. The model of the cupper transformer Cu with a capacity of 10 kVA [16]
Design and construction of transformer type superconducting fault current limiter model
In the Laboratory of Superconducting Technologies of Institute of Electrical Engineering a functional model of a single-phase transformer type superconducting fault current limiter model was designed and constructed [16].
The limiter consists of a conventional 10 kVA transformer with copper secondary winding shorted with a superconducting element (Fig.3). Table 1 shows the Cu conventional transformer model parameters.
Table 1. Parameters for a transformer model with conventional Cu windings [16].
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The superconducting element is a superconducting coil composed of two independent windings w1, w2 wound on a common bobbin. This structure allows the configuration of the superconducting element to operate with different values of resistance and inductance of windings: winding w1or w2 windings connected in series or in parallel – either compatibly or contrarily.
Fig.4. The branched characteristics of tapes in the second generation [17]
This allows you to determine the effect of the parameters of the superconducting element on the process of limiting the current by a limiter. Superconducting windings are cooled in a bath of liquid nitrogen. The parameters of the superconducting windings are given in Table 2.
Table 2. Parameters for superconducting coils composed of two independent windings w1and w2made of HTS 2G SCS4050 [16]
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Both windings are made of HTS 2G SCS4050 superconducting tape produced by SuperPower. This is a tape with a width of 4 mm and a thickness of 0.055 mm [16],[18-20] laminated on both sides with copper, with a critical current Ic = 150 A. Maximum rated current of the superconducting windings is equal to the effective value of the critical current of the superconductor amounting to 82 A.
Fig.5. Windings made of superconducting tapes 2G HTS SCS 4050 [16].
Laboratory research for transformer type superconducting fault current limiter model
Experimental research was conducted in order to verify the possibility to limit the short circuit current by the transformer type current limiter and to determine the level of the current limit with respect to the parameters of the superconducting element (HTS windings configuration) [12]. The research was conducted in Laboratory of Superconducting Technologies in the measurement system shown in Fig. 6.
Fig.6. Short circuit measurement system for transformer type superconducting fault current limiter [16]
The superconducting fault current limiter model is powered by a voltage regulator connected to the power network in separate transformer. The shunts used to perform the current measurement have a value of 1 mV/1 A. The measurements were performed with the use of a measuring PC DAQ Card and LabView software. The short circuit was initiated by the short circuit system. The time of short circuit is 0.05 s.
The analysis was performed for the following superconducting winding configurations w1and w2:
Configuration I – the secondary winding of the Cu transformer shorted with w1 coil, Configuration II – the secondary winding of the Cu transformer shorted with w2 coil; Configuration III – the secondary winding of the Cu transformer shorted with w1 and w2 coils connected in parallel.
The electrical parameters of windings for each configuration of HTS coils are shown in Table 3.
Table 3. The parameters of superconducting windings for the three configurations of windings w1 and w2 [16].
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Fig. 7 shows the current waveforms on the primary and secondary side of the limiter for the selected configuration of the windings. After the current in the secondary winding of the Cu transformer has crossed the critical current value of the superconducting winding, the surge current (the first impulse of the short circuit current) is limited to the value resulting from the value of the short circuit impedance of the limiter – ZZW (ISC).
Fig.7. The waveforms of primary and secondary currents for II configuration of the superconducting winding [16].
The comparison of the primary current waveforms obtained for all the HTS configurations of the windings shown in Figure 8 compares the number of times the limited surge current exceeds the value of the critical current of the superconducting winding.
Fig.8. The comparison of the primary current waveforms for the three configurations of HTS windings [16].
The short circuit impedance of the limiter Zzw (ISC), is the sum of Cu transformer impedance – ZzwCu (ISCCu) and superconducting winding impedance ZHTS. If we assume for simplicity that ZzwCu (ISCCu) has a constant value, then the impedance value Zzw (ISC) depends on the reactance value XHTS and the resistance RHTS of the HTS winding, thus on the configuration of the winding and on the resistance of the superconducting tape used at the temperature of 77 K. The higher the RHTS and XHTS value, thus the value of the superconducting winding impedance the greater the limitation of the short circuit current. The time after which the limited surge current reaches the expected value of the short circuit current set, for configuration II of the HTS winding, amounts to about 6 ms.
Conclusions
The analyses conducted and experimental research results show that it is possible to build a transformer type superconducting fault current limiter, using the existing conventional transformer with the secondary winding shorted by the superconducting winding made of HTS tape.
The level of limiting the short circuit current, especially the first impulse of the surge current, depends on the value of the short-circuit impedance of the limiter, being the sum of the Cu transformer impedance and HTS winding impedance. HTS winding impedance depends on the winding configuration (inductors or bifilar coils, or many coils connected in series or in parallel), and also on the resistance of the superconducting tape. Selecting the proper HTS tape as well as the appropriate configuration of the superconducting winding, on the assumption of the constant value of the Cu transformer impedance, it is possible to build a transformer type superconducting fault current limiter with a desired level of limiting the short circuit current.
Acknowledgments. I sincerely thank my the promoter prof. Tadeusz Janowski you for any help, patience, kindness during the implementation of my doctoral thesis and my colleagues from the Laboratory of Superconducting Technologies of Institute of Electrical Engineering.
Autor: dr inż. Joanna Kozieł, Lublin University of Technology, Electrical Engineering and Computer Science Faculty, Institute of Electrical Engineering and Electrotechnologies, Lublin ul. Nadbystrzycka 38A, 20-618 Lublin, E-mail: j.koziel@pollub.pl
Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 92 NR 12/2016. doi:10.15199/48.2016.12.20
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