Power Quality Blog

Hybrid Power System of Public Lighting in Smaller Villages

Published by Stanislav MIŠÁK, Jaroslav ŠNOBL, František DOSTÁL, Daniel DIVIŠ,
VSB – Technical University of Ostrava

Abstract. This article deals with the possibility of public lighting power from renewable resources, it means of a hybrid system consisting of solar panels and wind power. The factual data for dimensioning the system was obtainedby extensive exploration of the state and consumption of public lighting in the villages of the Czech Republic. The article contains an economic reasoning and analysis of investment costs compared to cable distribution.

Streszczenie. Artykuł ten traktuje o możliwościach zasilania oświetlenia publicznego z odnawialnych źródeł energii, czyli systemu hybrydowego skladającego się z paneli słonecznych i elektrowni wiatrowej. Szczegółowe dane użyte do wszelkich wyliczeń uzyskano na podstawie szerokich badań dotyczączch stanu i zużycia energii przez oświetlenie publiczne w gmianch Republiki Czeskiej. Artykuł zawiera w sobe także uwagę ekonomiczną i analizę wydatków inwestycyjnych w porównaniu z siecią kablową. (Hybrydowy system zasilania oświetlenia publicznego w mniejszych gminach).

Keywords: public lighting, solar power station, wind power station, hybrid power source, energy balance.
Słowa kluczowe: oświetlenie publiczne, elektrownia słoneczna, elektyrownia wiatrowa, system hybrydowy, bilans energetyczny.

Introduction

During the realization of the SGS Project SP/201073 we have solved this year on the VSB-TU Ostrava, we implement a hybrid system using renewable energy sources (solar and wind power). These resources are applied to households with a defined power consumption. System consists of wind energy power 12kW and solar panels with capacity about 2kWp. Batteries are charging by the photovoltaic panels through the regulator.

Output of wind power is through the converter rectified into batteries, from which is continuously supplied simulated household during the time, according to its consumption. Based on this concept, we will try to apply the power of public lighting (hereafter PL) in small villages. The purpose is to modify the system so that can supply electricity public lighting in small villages or remote areas without power.

Statistical data on consumption

The data about electrical energy consumed by public lighting were obtained by extensive exploration, in which were subpoenaed cities and towns of the Czech Republic with a query about consumption in their city. The date we received was statistically processed and evaluated and results for small and medium-sized villages are listed in Table 1. Details of which we will determine are the installed power of one light point (hereafter SM) and the average number of lighting points per hundred inhabitants.

Fig.1. The concept of the proposed hybrid power system for public lighting

Table 1. Statistical data on electricity consumption of public lighting for small and medium-sized villages

Of the values listed in Table 1. is expressed the average number of lighting points and the average installed power for public lighting. In Table 2. is then calculated the average installed power of village public lighting. For the smallest intended village is power 6,63 kW, and this value will be dimensioned to hybrid system.

Table 2. Calculated data for public lighting

Table 3. Parameters and the price of a hybrid system

Proposal of a hybrid system to power public lighting the village

For using this system year round it must be designed to operate in winter months when the public lighting consumes electricity 16 hours a day. With regard to the possible realization and investment costs are for the calculation considered villages in which there lives 500 inhabitants or less. For this category of villages, the average installed power on light point is 78W, as we count 85 light points. So we obtain the installed power of public village lighting 6,63 kW. Power take-off from the rechargeable battery is designed for two nights during the winter without any charge from one or other renewable sources. The consumption for these two nights is 212kWh. During the summer demands for electricity are lower indeed, we consider the operation time only eight hours and a half consumption.

This system is used for illustrating the price costs of this kind of prototype and has function to explore whether is preferable to use a cable connection from the remote grid or need to build such a local island hybrid system. In both cases we expect that public lighting is newly built or will complete its reconstruction. The price of public lighting is not considered. Further there is the calculation and mutual comparation with a variant of remote connection and consumption of electrical energy to power 20 years, a life time of our hybrid system.

For making an idea there are compared different lengths of power connections with price of a hybrid system. The following table shows that the hybrid system is repayable under the conditions where the length of the power supply cable connections exceed 4 kms. The calculation considers a standard rates of electricity for public lighting for the year 2010, which are fixed during all 20 years.

Table 4. Parameters and prices of electrical supply

Table 5. Prices of electrical connections for various distances

Conclusion

This article shows theoretical possibility of solution of a separate power supply system for public lighting. Compares two different ways to power the locality and supply it by electrical energy. It demonstrate rough estimate of the cost for construction of electrical supply and costs of building the island hybrid system. The result shows that if the connection length exceeds 4 kms, the investment costs of these projects are equal. But if we take the life of the hybrid system maximum 20 years and the life of cable connection maximum 40 years then we have to submit that building connection with a length of 4 kms is much better from the economic point of view. For connections with a distance longer than 8 kms is preferable to install a hybrid system of the island power system. This application can be used especially for inaccessible areas with such a terrain, which would be considerably more expensive excavation works with regard to soil type and very remote areas, it means remote parkings and highway rest areas, a mountain cottages, villages or farms.

Acknowledgement

This article was created under project SP/201073, “Využití hybridních obnovitelných zdrojů elektrické energie”

REFERENCES

[1] Mišák, S., Prokop, L.: Analýza technických a ekonomických parametrů hybridních systémů. In 11th International Scientific Conference Electric Power Engineering 2010; (EPE 2010), 2010.
[2] Novák T., Mišák S., Sokanský, K.: Využití obnovitelných zdrojů energie k napájení svítidel veřejného osvětlení. In 11th International Scientific Conference Electric Power Engineering 2010; (EPE 2010), 2010.

Authors: VŠB-TU Ostrava, Fakulta elektrotechniky a informatiky, katedra Elektroenergetiky, 17.listopadu 15, 708 33, Ostrava-Poruba, http://www.fei.vsb.cz;
doc.Ing. Mišák Stanislav, Ph.D., tel: 597329308, E-mail: stanislav.misak@vsb.cz
Ing. Šnobl Jaroslav, tel: 597329309, E-mail: jaroslav.snobl@vsb.cz
Ing. Dostál František, tel: 597324198, E-mail: frantisek.dostal@vsb.cz
Ing. Diviš Daniel, tel: 597323468, E-mail: daniel.divis@vsb.cz.

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 4/2011

Impact of Protective Relays on Voltage Sag Index

Published by Xinke GAO¹,² , Yapeng LIU³ , Congying WANG⁴,
School of Electronic, Information and Electrical Engineering, Shanghai Jiao Tong University (1),
Institute of Information Technology Luoyang Normal College (2),
Power Supply of Qingdao Company, Shandong Electric Power Corporation (3),
School of foreign language, Shanghai Jiao Tong University (4)

Abstract. This paper provides the probability-assessment analysis on the characteristic value of the voltage sag by using Monte Carlo stochastic modelling method to stimulate the randomness of the short circuit fault. Furthermore, this article simulates the influence of the protection devices on the voltage sag to ensure the authenticity and the referential reliability. A system with inverse-time protection devices equipped on each lines which could coordinate together are designed to cut off the short-circuit fault. The voltage sag of the designed system is evaluated by the pre-and post system average RMS variation frequency index, and the voltage sag index is compared with the ITIC curves. The simulation results demonstrated that the inverse-curve relay protection equipments are well-coordinated, and the severity and the range of the voltage sag are influenced by the cooperation of the equipped inverse time protection devices.

Streszczenie. W artykule przedstawiono metodę szacowania prawdopodobieństwa wystąpienia zapadu napięcia na podstawie analizy jego charakterystycznych parametrów zamodelowanych metodą Monte Carlo. Ponad to, w celu weryfikacji skuteczności, dokonano symulacji wpływu urządzeń ochronnych na zapady napięcia. Zaprojektowano także system z urządzeniami umożliwiającymi odizolowanie zwarcia w obwodzie od reszty sieci. Wyznaczono współczynnik częstotliwościowy zmienności wartości średniej RMS zapadów napięcia w proponowanym układzie, który następnie porównano z krzywymi ITIC. Przeprowadzone badania symulacyjne potwierdziły skuteczność i szybkość działania systemu. (Wpływ przekaźników ochronnych na współczynnik zapadu napięcia).

Keywords: Voltage sag, Protective relay, Monte Carlo algorithm, simulation.
Słowa kluczowe: zapad napięcia, przekaźnik ochronny, algorytm Monte Carlo, symulacja.

1.Introduction

Owing to the rapid technology proliferation in industrial control processes, as well as the large implementation of sophisticated electrical apparatus, the high power quality is required by manufacturing factories and commercial electrical consumers. The major power quality problems that interested industries are the voltage sag and swell. The existence of voltage sag can cause damaged product, lost production, restarting expenses and danger of breakdown, but voltage swells can cause over heating tripping or even destruction of industrial equipment such as motor drives [1]. Nowadays, most of the equipments used in the industries are mainly based on semiconductor devices and microprocessors and hence these devices are very sensitive to voltage disturbances. Among power disturbances, voltage sags are considered as the most frequent types of disturbances in the field and their impacts on sensitive loads are severe. Voltage sags have attracted many researchers to perform assessment and mitigation related to such power quality disturbances [2].

The current statistical methods to analyse the influence of voltage sag can be divided into stochastic prediction and electromagnetic transient analysis. For the stochastic prediction there are the fault location and the critical distance methods, paper [3] gives a brief comparison: the critical distance method is more suitable for manual project calculation of lower computational accuracy; The fault location method is more precise for programming, and this method can assure a more precise result with enough fault locations. But for both the fault location and the critical distance method, the fault occurrences are manually set, without considering the randomness of the locations and the types of actual faults, the papers [4-9] use the Monte Carlo algorithm and the electromagnetic transient analysis, only taking the definite time delay protection equipment influence into consideration.

At the moment both the mid voltage and low distributive networks apply the three sectional over-current protections, whose shortcomings are that these will generate the unnecessary loss to ensure the selectivity needed to cut down the fault. Nevertheless the inverse-time protection referred in papers [10-18] hold the advantages of self-adaptive functions and less affected by the way of operation. With the development of the digital protection technology, CIGRE and IEEE both establish the standards for the time-inverse relay protection, which are being applied in national low-voltage distributive networks step by step.

To sum up, this paper uses the method which combines the electromagnetic transient simulation and Monte Carlo methods to analyze the low-voltage distributive networks with the inverse-time protection relays installed. This article mainly discusses the influence of the inverse-time protection relays including designing protection relays which effectively coordinate together to cut off the short-circuit fault, and gives an analysis based on the voltage sag criteria such as SARFI (System Average RMS Variation Frequency Index) parameter and ITIC curve. These analysis results could be used for the further studying the impacts of the protective devices for the voltage sag.

2.Setting coordination of inverse-time over-current relays

2.1. Introduction of inverse-time relay

For the moment, there are two criteria for inverse-time relays, which are IEC255-03 [11] and IEEE STD C37.112-1996 [12] with their time-current equations as follows.

Referring to IEC255-03(1989-05) the inverse-time standard formulas are classified into three kinds： inverse, very inverse, and extremely inverse:

INVERSE (FSXTX=1.0):
(1) t = 0.14 x TDS / ((I / I_pu)^0.02 -1)

VERY INVERSE (FSXTX=2.0):
(2) t = 13.5 x TDS / ((I / I_pu) -1)

EXTREMELY INVERSE (FSXTX=3.0):
(3) t = 80 x TDS / ((I / I_pu)² -1)

where I is the current value going into relays, t is time to trigger, TDS is a factor to distinguish each member of a family, I_puis the pickup current (the smallest value that will trigger the breaker).

Referring to IEEE Std C37.112-1996 the standard formula representing type CO and IAC relays, considering overtravel and resetting characteristics and the relay coordination are as follows:

(4) t_trip(I) = TDS(A / (M^P +1) +B) +K

(5) t_reset(I) = TDS(t_p / (1-M^q))

Where t_tripis the operating time to trip in seconds, t_reset is the operating time to reset in seconds. M is the multiple of pickup current, M = I / I_pu. TDS is time dial operation, and p and q are exponent constant to stand for various characteristics.

2.2. Design of inverse-time relay

The simulation module in this paper is designed as the low-voltage distributive network with the arc-suppression coils. When the power system is under the normal operation, there is no current flowing through the arc-suppression coils. While the network is under the thunder attack or single phase short circuit, the voltage at the neutral point reaches to the value as large as the phase voltage. At the same time, the inductive current which flows through the arc-suppression coils and the capacitive fault current caused by the single phase-to-earth fault are compensated with each other to small amount of residual current. The residual current is not so large enough that cause the arc to extinguish without arousing the overvoltage. The lower fault current makes the longer delay for the inverse-time relay protection operation.

The inverse-time protection relay equipment applies the module built-in PSCAD, and the parameter design is based on extreme inverse-time parameter designed as the following equation by the IEEE Std C37.112-1996 thoroughly explained in paper [12], using very inverse-time standard without considering resetting characteristics here.

(6) t_trip(I) = TDS(3.922 / (M^0.02 +1) +0.098) +K

Take example of the simulation analysis of the fault occurred triply. The single phase-to-earth needs the longest time delay for operating the inverse-time relay protection equipments. The theoretical delay time could be got according to the time-current curve in the inverse-time relay module designed in PSCAD.

The PSCAD functions in terms of time sequence, the actual tripping moment ( T_r ) lags behind the theoretical time ( T_t ) at the same current peak ( I_p ). The beginning time and the ending time are put in T_start and T_end . All these parameters are shown in table 1.

The table 1 and the Fig.1 and Fig.2 prove the excellent cooperations and operations among the inverse-time modules. All the seven lines are equipped with inverse-time relay protection, and the voltage-sag characteristics variations are to be explained later.

Table 1. Protective devices acting time table

Fig. 1. Fault occurrence current graph with successful reclosing

Fig. 2. Fault occurrence voltage graph with successful reclosing

3.Simulation analysis considering inverse-time relays

3.1. Introduction of the simulation model

The simulation system structure and the parameter are explained as follows: the voltage grade with 110/25/0.4 kV, Y_n / Y_n wiring in transformer T₁ with a voltage ratio 110/10.5kV; the transformer T₂ 、T₃ 、T₄ 、T₅ all configured as Δ – Y₀ wiring with a voltage ratio in 25/0.4kV. As shown in Fig.3, the system is of seven lines all with the inverse-time relay protection equipments owning the same characteristic curves, and they are respectively the L₁ ~ L₃ of 500m length, L₄ ~ L₇ of 250m length.

Fig. 3. System simulation module

3.2. Probabilities assessment for the simulation results

3.2.1 Assessment based on the ITIC curve

The ITIC curve shown in Fig.4 based on large amount of experiment data features the equipment endurance capability standard developed from the CBEMA curve describing the vulnerability level of the information industry equipments to the transient power quality (mainly the voltage sag, rise, short interruption). The curve currently recognized as IEEE446 standard to evaluate the influence of the transient power disturbances explains the capability for the loads to bear the voltage sag.

• Without relay protective devices
In order to summarize the characteristics of the single phase-to-earth fault, take the A phase as an example shown in the Fig.5 and Fig.6.

As the results shown in Fig.5 and Fig.6, the influence of the transformer Δ – Y that the fault voltage caused by the single phase-to-ground transformed from the TB₁ type to the normal type as the N type leads to the significantly lowered dangerous voltage sags at the LV side with excessive voltage conditions disappearing;

According to the historical statistical data, the percentage of the single phase-to-earth is 75%, i.e. the excessive voltage is 75%. Hence the excessive voltage on medium voltage of the B and the C phases caused by the single phase-to-earth fault holds the highest occurrence probability.

• With relay protective devices

Here presents the simulation results considering the protection configuration between circuit breaker using the time-inverse characteristics and reclosers (with 100ms reclosing interval) with different mean fault time duration at 100ms, 600ms and 1s.

From Fig.7 and Fig.8, the trips whose numbers are almost the same with one of the interruptions do not intensively increase because most trips are on the faulted feeder generated from interruptions caused by three-phase faults.

Fig .7. Voltage sag and ITIC curve-mean fault duration=100ms

Fig.8. Voltage sag and ITIC curve-mean fault duration=600ms

Fig.9. Voltage sag and ITIC curve-mean fault duration=1000ms

According to Fig.8, compared with Fig.9, the protective devices work more precisely as the fault duration gets enough longer because the value of the short circuit current is lower with the arc-suppression coils equipped. With shorter mean fault duration time, most interruptions caused by three-phase faults rather than single-phase ones will cause an equipment trip only to loads located on the faulted feeder while with longer mean time duration most trips will be caused by single-phase faults.

3.2.2 Calculation based on the SARFI index

The characteristic measures for the voltage sag are the RMS offset and the voltage sag duration time, hence the most common index is the SARFI (System Average RMS Variation Frequency Index). One of the two common forms is the statistic index number—SARFIx used to explain a specific threshold voltage x which is meant to get the probability of the voltage RMS below the voltage threshold x. For a certain node the SARFIx could be calculated by the following expression:

The SARFIx of the whole system could be obtained by the following expression:

Where N_i is the number of customers whose voltage RMS under threshold voltage; N_T is the number of the entire assessed customer; n_n is the number of nodes in the whole system; N_j is the number of the customers belonging to the node; and SARFI_(j) is the SARFI value of the specific node.

Calculate all the values of SARFI1.1_MV, SARFI0.9_MV, SARFI0.8_LV, SARFI0.6_LV (SARFI1.1_MV means the RMS is over 110%). The results of SARFI refer to table 2. The longer duration time makes the SARFI1.1_MV value lower because it generates more chances for the protective devices to trip when the single-phase-to-ground fault happens in the system with the arc-suppression coils equipped. For the LV-level side, the lowest SARFI values with a threshold voltage below 90% are achieved when protective devices reject to operate, because most of the voltage sags generated by single-phase-to-ground faults are higher than the nominal voltage by 60%.

Table 2. SARFI calculation results

4.Conclusion

The comparison and contrast between the ITIC curve and the voltage sag index before and after the protective devices equipped show that the longer the fault duration lasts, the higher probability for the protective devices to operate, the more times for the loads on the low voltage to trip, i.e. the longer time for the sensitive loads to shut down. Therefore, the extent of the voltage sag become more severe, the main reason of which is the longer time demand for the protective devices to operate when single phase-to-ground fault happens in the arc-suppression coils grounding mode.

Here only presents the configuration of reclosers and circuits breakers. Further studies are expected to be analysed with other protective devices (e.g. fuses and sectionalizers) added and other reclosing cooperating patterns.

5. Acknowledgements

This work was supported by the Shanghai Jiao Tong University Innovation Fund for Postgraduates under Grant No.AE030202, Henan Tackle Key Problem of Science and Technology under Grant No.102102210454, the Foundation of Education Committee of Henan Province under Grant No.2011B520028, the Cultivated Funded Project of Luoyang Normal College under Grant No.10000859.

REFERENCES

[1] Dehini R., Bassou A.,Chellali B., Generation of voltage references using Multilayer Feed Forward Neural Network, Przeglad Elektrotechniczny, 88(2012), No. 4A, 289-292.
[2] Ibrahim A. A., Mohamed A., Shareef H., et al., A new approach for optimal power quality monitor placement in power system considering system topology, Przeglad Elektrotechniczny, 88(2012), No. 9A, 272-276.
[3] Won D.J., Ahn S.J.,Moon S.I., A modified sag characterization using voltage tolerance curve for power quality diagnosis, IEEE Trans. On Power Delivery, 20(2005), No. 4, 2638-2643.
[4] Martinez J.A.,Martin-Arnedo J., Voltage sag stochastic prediction using an electromagnetic transients program, IEEE Trans. On Power Delivery, 19(2004), No. 4, 1975-1982.
[5] Yunting S., Yongji G.,Ruihua Z., Probabilistic Assessment of Voltage Sags and Momentary Interruption Based on MONTECARLO Simulation, Automation of Electric Power Systems, 27(2003), No. 18, 47-51.
[6] Martinez J.A.,Martin-Arnedo J., Voltage sag analysis using an electromagnetic transients program, in Power Engineering Society Winter Meeting,(2002). 1135-1140.
[7] Martinez J.A.,Martin-Arnedo J., Voltage sag studies in distribution Networks-part I: system modeling, IEEE Trans. On Power Delivery, 21(2006), No. 3, 1670-1678.
[8] Martinez J.A.,Martin-Arnedo J., Voltage sag studies in distribution networks-part II: voltage sag assessment, IEEE Trans. On Power Delivery, 21(2006), No. 3, 1679-1688.
[9] Martinez J.A.,Martin-Arnedo J., Voltage sag studies in distribution Networks-part III: Voltage sag index calculation, IEEE Trans. On Power Delivery, 21(2006), No. 3, 1689-1697.
[10] Benmouyal G., Meisinger M., Burnworth J., et al., IEEE standard inverse-time characteristic equations for overcurrent relays, IEEE Trans. On Power Delivery, 14(1999), No. 3, 868-872.
[11] Tan JC, McLaren PG, Jayasinghe RP, et al., Software model for inverse time overcurrent relays incorporating IEC and IEEE standard curves,(2002). 37-41 vol. 1.
[12] Standard B., Electromagnetic compatibility (EMC) Testing and measurement techniques, voltage dips, short Interruptions and voltage variations immunity tests, BS END, 61(000-4.
[13] Urdaneta A.J., Restrepo H., Marquez S., et al., Coordination Of directional overcurrent relay timing using linear programming, IEEE Trans. On Power Delivery, 11(1996), No. 1, 122-129.
[14] Wang J., Chen S.,Lie TT, System voltage sag performance estimation, IEEE Trans. On Power Delivery, 20(2005), No. 2, 1738-1747.
[15] Taylor C.W., Concepts of undervoltage load shedding for voltage stability, IEEE Trans. On Power Delivery, 7(1992), No.2, 480-488.
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[17] Aung M.T.,Milanovic J.V., Stochastic prediction of voltage sags by considering the probability of the failure of the protection system, IEEE Trans. On Power Delivery, 21(2006), No. 1, 322-329.
[18] Han W., Chenzhao YU,Jianwen ZHANG X.C.A.I., Control of Voltage Source Inverter with an LCL Filter without Voltage Sensors, Przeglad Elektrotechniczny, 88(2012), No. 5b, 119-122.

Authors: Xinke GAO, Ph.D.Candidate of Department of Instrument Science and Engineering, School of Electronic, Information and Electrical Engineering, Shanghai Jiao Tong University, No. 800 of Dongchuan Road, Minhang District, Shanghai, CHINA. He is also an associate Professor of Institute of Information Technology, Luoyang Normal College,CHINA. gxk622@163.com

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 5/2013

General Reference – Utility Capacitor Switching – Common Waveforms

Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Utility Capacitor Switching – Common Waveforms, Document ID: PQS0707, Date: January 1, 2007.

Abstract: The application of utility capacitor banks has long been accepted as a necessary step in the efficient design of utility power systems. Also, capacitor switching is generally considered a normal operation for a utility system and the transients associated with these operations are generally not a problem for utility equipment. These low frequency transients, however, can be magnified in a customer facility (if the customer has low voltage power factor correction capacitors) or result in nuisance tripping of power electronic based devices, such as adjustable-speed drives.

Capacitor energizing is just one of the many switching events that can cause transients on a utility system. However, due to their regularity and impact on power system equipment, they quite often receive special consideration.

INTRODUCTION

The application of utility capacitor banks has long been accepted as a necessary step in the efficient design of utility power systems. Also, capacitor switching is generally considered a normal operation for a utility system and the transients associated with these operations are generally not a problem for utility equipment. These low frequency transients, however, can be magnified in a customer facility (if the customer has low voltage power factor correction capacitors) or result in nuisance tripping of power electronic based devices, such as adjustable-speed drives (ASDs). Capacitor energizing is just one of the many switching events that can cause transients on a utility system. However, due to their regularity and impact on power system equipment, they quite often receive special consideration.

Transient overvoltages and overcurrents related to capacitor switching are classified by peak magnitude, frequency, and duration. These parameters are useful indices for evaluating potential impacts of these transients on power system equipment. The absolute peak voltage, which is dependent on the transient magnitude and the point on the fundamental frequency voltage waveform at which the event occurs, is important for dielectric breakdown evaluation. Some equipment and types of insulation, however, may also be sensitive to rates of change in voltage or current. The transient frequency, combined with the peak magnitude, can be used to estimate the rate of change.

There are a number of transient related concerns that are generally evaluated when transmission and distribution shunt capacitor banks are applied to the power system. These concerns include insulation withstand levels, switchgear ratings and capabilities, energy duties of protective devices, and system harmonic considerations. In addition, these considerations need to be extended to include customer facilities due to the increased use of power electronic based end-user equipment. Applications concerns often evaluated include:

− overvoltages associated with normal capacitor energization.
− open line/cable end transient overvoltages.
− phase-to-phase transients at transformer terminations.
− voltage magnification at lower voltage capacitor banks (including customer systems).
− arrester duties during restrike events.
− current-limiting reactor requirements.
− system frequency response and harmonic injection.
− impact on sensitive customer power electronic loads.
− ferroresonance and dynamic overvoltage conditions.

Power quality symptoms related to utility capacitor switching include customer equipment damage or failure, nuisance tripping of ASDs or other process equipment, transient voltage surge suppressors (TVSS) failure, and computer network problems.

CAPACITOR BANK ENERGIZATION – COMMON WAVEFORMS

This section includes a number of representative transient power quality waveforms that deal with utility capacitor bank applications. Relevant information and waveform characteristics are also provided.

Figure 1 shows a measured 4.16kV distribution bus voltage waveform during a utility capacitor bank energizing event. The resulting transient voltage was 1.35 per-unit (135%) and steady-state voltage rise was approximately 1.2%.

Figure 1 – Distribution substation capacitor bank energizing voltage waveform

Figure 2 shows a measured 13.8kV distribution feeder current waveform before-and-after energization of a pole-mounted 900 kVAr capacitor bank. Insertion of the bank creates a resonance that results in higher levels (13% THD) of current distortion.

Figure 2 – Distribution feeder capacitor bank energizing current waveform

Figure 3 shows a measured 23kV distribution feeder current waveform during back-to-back switching of two 1.8 MVAr capacitor banks. The conductor between the capacitor banks is approximately 4200 feet of 336 MCM aluminum tree wire.

Figure 3 – Back-to-back capacitor bank switching current waveform

Figure 4 shows a measured distribution substation bus voltage waveform during a multiple restrike event on a 34.5kV capacitor bank switch. The worst-case transient overvoltage was approximately 1.55 per-unit (155%). MOV arresters were installed on the substation transformer.

Figure 4 – Capacitor bank switch multiple restrike voltage waveform

Figure 5 shows a measured voltage waveform during the energization of a 300 kVAr, 15kV distribution feeder capacitor bank. The long pole span (time between phases closing) is because the capacitor bank is switched with three single-phase oil switches.

Figure 5 – Distribution feeder pole-mounted capacitor bank energizing voltage waveform

Figure 6 shows a simulated customer secondary bus voltage waveform (≈3.0 per-unit) during utility distribution substation capacitor bank switching. The customer has power factor capacitors (no arresters) and voltage magnification occurs.

Figure 6 – Customer secondary voltage waveform during utility capacitor bank energizing

Figure 7 shows a measured distribution bus voltage waveform during a multiple restrike event on a 34.5kV capacitor bank. The bank is protected with MOV arresters and the worst-case transient voltage was approximately 1.98 per-unit (198%).

Figure 7 – Distribution capacitor bank restrike voltage waveform

Figure 8 shows a measured distribution substation transformer secondary current waveform during a multiple restrike event (see corresponding voltage waveforms in Error! Reference source not found.) on a 34.5kV capacitor bank. The bank is protected with MOV arresters.

Figure 8 – Distribution capacitor bank restrike current waveform

Figure 9 shows a distribution substation transformer secondary current waveform during the energization of two distribution capacitor banks. The peak current on the CT secondary circuit was 401 A.

Figure 9 – Distribution substation capacitor bank energizing current waveform

Figure 10 shows a measured distribution substation transformer secondary current waveform during the energization of a distribution capacitor bank. The harmonic distortion of the steady-state current after the capacitor bank switching is approximately 11%.

Figure 10 – Distribution feeder capacitor bank energizing/resonance current waveform

Figure 11 shows a distribution bus voltage waveform during energization of a utility 34.5kV capacitor bank with another bank on the same bus. The resulting transient voltage and voltage rise were 1.5 per-unit (150%) and 1% respectively.

Figure 11 – Distribution substation capacitor bank back-to-back switching voltage waveform

Figure 12 shows a measured distribution feeder current waveform for an arcing capacitor bank switch during energizing of a 300 kVAr pole mounted capacitor bank on a 4.16kV distribution feeder.

Figure 12 – Distribution feeder arcing capacitor bank switch current waveform

Figure 13 shows a measured distribution feeder current waveform during energization of a distribution capacitor bank. The resulting peak transient current was 561 A and the full load capacitor bank current was approximately 65 amps rms.

Figure 13 – Distribution feeder current waveform during capacitor bank switching

Figure 14 shows a measured distribution feeder voltage during energization of a 4.16kV distribution capacitor bank. The resulting transient voltage was 1.48 per-unit (148%) and the steady-state voltage rise was approximately 0.4%.

Figure 14 – Distribution feeder capacitor bank energizing voltage waveform

SUMMARY

There are many events that can cause a power quality problem. Analysis of these events is often difficult due to the fact that the cause of the event may be related to a switching operation within the facility or to a power system fault hundreds of miles away. This document summarizes several of the more common power quality transient waveforms associated with the application of utility system capacitor banks. The frequent switching of utility capacitor banks coupled with the increasing application of sensitive customer equipment has led to a heightened awareness of several important events, including voltage magnification and nuisance tripping of ASDs.

These concerns have become particularly important as utilities institute higher power factor penalties, thereby encouraging customers to install power factor correction capacitors. In addition, nontraditional customer loads, such as ASDs, are being applied in increasing numbers due to the improved efficiencies and flexibility that can be achieved. This type of load can be very sensitive to the transient voltages produced during capacitor switching.

REFERENCES

G. Hensley, T. Singh, M. Samotyj, M. McGranaghan, and T. Grebe, Impact of Utility Switched Capacitors on Customer Systems Part II – Adjustable Speed Drive Concerns, IEEE Transactions PWRD, pp. 1623-1628, October, 1991.

G. Hensley, T. Singh, M. Samotyj, M. McGranaghan, and R. Zavadil, Impact of Utility Switched Capacitors on Customer Systems – Magnification at Low Voltage Capacitors, IEEE Transactions PWRD, pp. 862-868, April, 1992.

T.E. Grebe, Application of Distribution System Capacitor Banks and Their Impact on Power Quality, 1995 Rural Electric Power Conference, Nashville, Tennessee, April 30-May 2, 1995.

M. McGranaghan, W.E. Reid, S. Law, and D. Gresham, Overvoltage Protection of Shunt Capacitor Banks Using MOV Arresters, IEEE Transactions PAS, Vol. 104, No. 8, pp. 2326-2336, August, 1984.

S. Mikhail and M. McGranaghan, Evaluation of Switching Concerns Associated with 345 kV Shunt Capacitor Applications, IEEE Transactions PAS, Vol. 106, No. 4, pp. 221-230, April, 1986.

T.E. Grebe, Technologies for Transient Voltage Control During Switching of Transmission and Distribution Capacitor Banks, 1995 International Conference on Power Systems Transients, September 3-7, 1995, Lisbon, Portugal.

Electrotek Concepts, Inc., An Assessment of Distribution System Power Quality – Volume 2: Statistical Summary Report, Final Report, EPRI TR-106294-V2, EPRI RP 3098-01, May 1996.

Electrotek Concepts, Inc., Evaluation of Distribution Capacitor Switching Concerns, Final Report, EPRI TR-107332, October 1997.

RELATED STANDARDS
IEEE Standard 18-1992, IEEE Standard 1036-1992
ANSI/IEEE Standard C37.012-1979, ANS/IEEE C37.99-1990

GLOSSARY AND ACRONYMS
ASD: Adjustable-Speed Drive
PWM: Pulse Width Modulation
MOV: Metal Oxide Varistor
TVSS: Transient Voltage Surge Suppressors

Electrical Energy Storage Systems

Published by Zdeněk HRADÍLEK, Petr MOLDŘÍK, Roman CHVÁLEK
VŠB – Technical University of Ostrava, Department of Electrical Power Engineering

Abstract. Attention is paid to the electrical energy storage systems that are already used in the framework of electrical power system, and further to the systems that are studied and developed for this purpose. The described storage systems should work especially in synergy with unreliable renewable energy sources, such as wind and photovoltaic power plants. Those use the renewable source that gains ground quickly not only in the Czech Republic. Both the systems that make it possible to ensure high charging and discharging rates for a short time and the systems of great storage capacity that are able to store and transmit electrical energy for more hours are described.

Streszczenie. Skupiono się na systemach magazynowania energii elektrycznej obecnie stosowanych w ramach funkcjonowania systemu elektroenergetycznego, a w następnej kolejności na systemach będących w fazie badań i projektów. Opisane systemy magazynowania powinny współdziałać w szczególności z niestabilnymi źródłami odnawialnymi, jak wiatr i elektrownie fotowoltaiczne. Taki sposób użytkowania źródeł odnawialnych szybko ugruntowuje się nie tylko w Republice Czeskiej. Opisano zarówno systemy zapewniające wysoki wskaźnik ładowania i rozładowania w krótkim czasie, jak i systemy o wielkich zdolnościach magazynowych, które są w stanie przechowywać i przekazywać energię elektryczną przez wiele godzin. (Systemy magazynowania energii elektrycznej).

Keywords: energy storage, renewable source, storage system.
Słowa kluczowe: magazynowanie energii, źródło odnawialne, system magazynowania.

Introduction

There is a rising number of applications of renewable energy sources (RES) within electric power systems (EPS). Speaking of the Czech Republic, those are especially photovoltaic and wind power plants that are characteristic for their electric energy supply capacity variable and unreliable over time, fully dependant on weather conditions. These characteristics imply the need to establish a certain power backups to cater for blackout conditions and this backup concerns their full installed capacity. Problems associated with operation of these plants can be solved by means of storage of the electrical energy they produce. There are numerous different storage technologies.

Energy Storage Systems

Systems for storage of electrical energy can be divided into two basic categories. The first one includes those systems to enable high charging and discharging performance for a short period of time. There are Supercapacitors, Superconductive magnetic accumulators (SMES) and rotary Flywheel accumulators. The second category then includes systems with great storage capacity able to store and transmit electrical energy for several hours. This category comprises Batteries, Compressed air accumulators (CAES), Redox flow batteries, Pumped storage hydro plants, and the last but not the least are the Hydrogen storage systems – the system of electrolyzer and a fuel cells.

Battery Energy Storage (Batteries)

These accumulators transform the chemical energy to electric power and vice versa. They comprise secondary cells, which are similar to the primary units (single discharge only) with the limited amount of reactants. However, the reaction products developed during cell discharge can be transformed back into initial active reactants, with the external electric power feed, which will re-instate the cell´s charged condition. The battery´s ability to supply power is then limited by its internal resistance, which increases with the cell ageing. There is a similar limit on the charging current, which implies the long period needed for charging.

The most common batteries currently are the lead-acid, NiCd, NiMH and Li-ion cells, different with the electrolyte and electrode materials used. The life span of most batteries ranges within hundreds of charge/discharge cycles. Apart from the charging method, the life span is also strongly affected by the operational temperature. Batteries will not be very suitable for environments requiring strong impulse current, these would rather work under constant loads. These are mainly used in vehicles, consumer electronics and UPS units. [1]

Sodium-Sulfur (NAS) Batteries

NAS batteries are high capacity battery systems developed for electric power applications. This battery consists of liquid (molten) sulfur at the positive electrode and liquid (molten) sodium at the negative electrode as active materials separated by a solid beta alumina ceramic electrolyte. The electrolyte allows only the positive sodium ions to go through it and combine with the sulfur to form sodium polysulfides (2Na + 4S = Na₂S₄). [2]

During discharge, as positive Na+ ions flow through the electrolyte and electrons flow in the external circuit of the battery producing about 2 volts. This process is reversible as charging causes sodium polysulfides to release the positive sodium ions back through the electrolyte to recombine as elemental sodium. This hermetically sealed battery is kept at approximately 300^°C and is operated under conditions such that the active materials at both electrodes are liquid and the electrolyte is solid. At this temperature, since both active materials react rapidly and because the internal resistance is low, the NAS battery performs well. Because of reversible charging and discharging the NAS battery can be used continuously. Efficiency of NAS battery cells is about 89%. [2]

Redox Flow Batteries

The reduction-oxidation (Redox) flow batteries are considered highly prospective, as these are able to store large amounts of electric power. For this purpose, the ability of some chemical elements (e.g. vanadium) to have more valence states is utilized. The core of this system comprises a reversible reduction-oxidation cell to accommodate the transformation of electrical energy into chemical energy, bound within the electrolyte. These Redox batteries allow for continuous exchange of electrolyte. This ensures their continuous operation until the electrolyte supply has been used up. Electrolytes circulate within two circuits separated by the ion exchanger membrane inside the very cell. The cell interior provides for oxidizing of one form of the electrolyte upon electrochemical process, with the other one being subject to reduction due to the electric current fed or drained into the external electric circuit with the use of electrodes (see Figure 2).

Fig.2 Diagram of Redox flow battery working principle [4]

The storage capacity is determined by the amount of electrolyte in store tanks, while the actually achievable volume energetic density of electrolyte for a full charging (discharging) cycle is listed within the range of 15 to 25 kWh/m³. The unit does not show any performance or capacity reduction after more than 12 thousand charging cycles, the estimated life span of membranes is approximately 15 years. [3]

Supercapacitors

Also referred to as super condensers, are actually electrolytic capacitors with high capacity of thousands of F (Farad) and the ability for rapid charging and discharging. The electrodes are made from special materials as the micro-porous active carbon, which features extreme surface properties of up to 2000 m²/g and the gap of several nanometres between particular charged layers. These electrodes are separated with the polypropylene foil, the space within is filled with liquid electrolyte. The operation voltage is approx. 2,5V. Any higher voltage can be achieved by series setup of basic cells. The low internal resistance value enables for rapid discharge, the excellent performance provided by super capacitor achieves the values of several kW per 1kg of weight. Their electric parameters are maintained even under low temperatures down to -40°C. Supercapacitors represent ideal units to be used in applications with the need for peak current supply for a limited period of time. Their commercial utilisation has been launched recently only. [3]

High-Speed Flywheels

There are still the more common low-speed flywheels (rotating by up to approx. 7.000 rpm) provided with steel rotors. The very strong composite materials allow for development of light high-speed flywheels with the maximum speed up to approx. 50.000 rpm. To reduce the friction produced, the rotor will be housed within vacuum with magnetic uplift. The rotor also comprises permanent magnets, which help with its initial roll-up or generate current within coils.

These units also feature sophisticated electronics to ensure the safe and maintenance free operation. The contemporary flywheels can provide the performance from several kW up to approx. 1 MW. Their advantage lies in the optional operation of several units in parallel. A steel flywheel can provide approx. 200 kW of backup power within a 3-phase 400V network with the revolutions range of 7.700 – 4.000 rpm. Any such unit can be also operated separately as a short-term backup or a component within a larger system. [5]

Superconducting Magnetic Energy Storage (SMES)

The superconducting magnetic electrical energy storage units enable the rapid absorption and supply of power without any limitations or losses. Energy is stored within the magnetic field of coil bearing current, this coil being housed within a cryostat. Such coil is made from a superconductor in order to eliminate any resistance losses. However, there are power losses incurred during operation of the cooling unit, which maintains the coil superconductor below the critical temperature level.

The SMES system helps to store up to thousands of MWh and it can be used to balance peaks of electric power take-off. The very superconducting coil, depending on the size and method of use, can be made of a solenoid toroid. The superconductor energy accumulator might be the alternate solution for electrical energy storage in forthcoming years. However the need for cooling to preserve its operational temperature below critical level makes this unit costly compared to other technologies. [5]

CAES (Compressed Air Energy Storage) System

This is a compressed air accumulator linked with a turbine to supply power during take-off peaks within the network as needed. It is a peaking gas turbine power plant that consumes less than 40% of the gas used in conventional gas turbine to produce the same amount of electric output power. This is because, unlike conventional gas turbines that consume about 2/3 of their input fuel to compress air at the time of generation, CAES pre-compresses air using the low cost electricity from the electrical power system at off-peak times and utilizes that energy later along with some gas fuel to generate electricity as needed. The compressed air is often stored in appropriate underground mines or caverns created inside salt rock (see Figure 3). The CAES technology is still used very rarely in global commercial projects. [6]

Hydrogen Storage System

The main parts of this system comprise the hydrogen generator, the so called “electrolyzer”, the magazine to hold the hydrogen produced, the demineralised water magazine and the hydrogen fuel cells. Further components necessary include the semiconductor invertors, compressors and vents. The cooperation between the hydrogen system and the non-controlled RES is based on the principle that the power produced by RES will be used to produce hydrogen within the period of lower EPS workload and this hydrogen will be stored. The hydrogen in storage will be later used in fuel cells, consumed during production of electric energy in the period associated with demand for electric energy by EPS. Figure 4 shows the diagram of hydrogen system.

Pumped-Storage Hydro Plant

A pumped-storage hydro plant (PSHP) represents a system able to store large amounts of electric energy. PSHP units are used to cater for peak demands for electric energy within EPS. Apart from the latter, these units also play their unique role within the control of output of the national energetic system in terms of emergency reserve. Their installed capacity within the territory of Czech Republic amounts to 1.175 MW. The PSHP serves for storage of electric energy using the potential gravity energy of water. The unit comprises of two reservoirs, whereas one of them is placed below the other. These reservoirs are linked with gravity piping of large diameter. During the period of power surplus within the EPS (at night), this power is used to pump the water from the bottom reservoir up into the top one. Once the ESP has developed a demand for large amount of peak power, this water will be subject to controlled discharge from the top reservoir down into the bottom one via the hydro plant turbine. This storage system is costly and requires significant landscape adaptations. The rate of efficiency of this pumping cycle is approx. 75%.

Storage Systems Benchmark

The storage systems described above have been grouped within the Table 1 to allow for proper comparison of basic parameters. Figure 5 then shows a graphic comparison of these systems, in order corresponding to their power rating and the discharge time.

Laboratory Research into Hydrogen Storage System

The storage system based on the hydrogen technology seems to be very prospective with regard to cooperation with the non-controlled RES. There is currently a research focused on this technology in progress, together with examination of its practical application. This research also involves our laboratory at the Department of Electrical Power Engineering, VŠB – TU Ostrava, concerned with sophisticated experimental laboratory implementation of the model hydrogen system for storage of electric energy produced by photovoltaic panels. Further detailed information about these panels is contained within other paper by authors dealing with the issue of photovoltaic power plant operation within the territory of the Czech Republic.

Our research concerns testing and measuring of particular components of the hydrogen system designed in order to optimise their operation to provide reliable, safe and highly efficient units. See the block diagram in Figure 4.

Table 1. Basic parameters of storage systems [7]

Fig.4. The hydrogen storage system diagram

Fig.5. Graphic comparison of energy storage systems [7]

Electrolyzer

Electrolyzer producing hydrogen is the first one of the most important parts of the entire hydrogen system. The process of electrolysis helps use the energy to decompose water to the very elements: hydrogen and oxygen. The electrolyzer comprises a series of cells equipped with positive and negative electrodes respectively, these are dipped in water. The level of conductivity is achieved by addition of hydrogen or hydroxyl ions (hydroxides), the most common form is the alkali potassium hydroxide (KOH). The amount of hydrogen produced depends on the flow density. The current electrolyzers feature energetic efficiency between 65 and 80 %. We use the Hogen GC600 electrolyzer comprising of electrodes and the gas separator to provide for separation of hydrogen produced from oxygen.

Electrolyzers of this kind use the sulhponated tetrafluorethylene (Nafion) to substitute the liquid electrolyte. The input of this electrolyzer is 1.100W, its operation temperature ranges around 85°C, the volume of hydrogen produced equals to 0,6 l/min (high purity: 99,999%), the output pressure of hydrogen reaches up to 13,8 bar. The source of energy used for decom-position of water is represented by photovoltaic panels.

Despite the low specific density, hydrogen has the highest ratio of energy to weight among all the fuels. As far as gases are concerned, its density is the lowest possible and it also features the second lowest boiling point of all the known substances. These properties then determine the options for its storage. [8]

It can be stored as a highly compressed gas, as a liquid in cryogenic magazines or as a bonded gas (e.g. in metal-hydrides). The storage of hydrogen in gaseous form requires large magazine volumes and high compression. High pressure hydrogen storage units represent the most common method used. If liquid, the hydrogen can be stored below its boiling point only, which is equal to 20 K (-253 °C). Owing to the latter, the liquefying of hydrogen is a strongly energy demanding process. The storage systems with metal-hydrides are based on the principle of easy absorption of gas by certain materials under high pressure and moderate temperatures. These substances would then release hydrogen, when beating heated under low pressure and relatively high temperatures. [8]

We use metal-hydride bottles and pressure cylinders for laboratory research.

PEM Fuel Cell

A fuel cell is a device, which uses the electrochemical reaction to transform chemical energy held by the fuel (hydrogen), aided by the oxidizing agent, to electric power, water and heat. This transformation occurs within catalytic reactions on electrodes and it is mainly based on reversed principle of water electrolysis. This efficiency of this energy production process is up to 60 % (under laboratory conditions), the real value would then be between 35 and 50 % (depending on the load and fuel cell type). This efficiency is mainly ensured by the method of energy transformation being direct with no intermediate levels (heat and mechanic energy), unlike in case of steam power plants, combustion engines or turbines, for example. [9]

There are currently six types of fuel cells under development and these differ by electrolyte composition, operation temperatures and the type of fuel used. Proton Exchange Membrane (PEM) fuel cells are cells with polymeric electrolyte membrane known for their high conductivity, which allows for their design of light weight and reasonable dimensions. These cells use the electrolyte able to conduct H⁺ ions from anode towards cathode. The electrolyte used will usually be perfluorinated polymer of the sulphonic acid inserted between two electrodes impregnated with catalyser. PEM usually work under temperatures between 50 and 100 °C and the pressure of 1 to 2 bar. Figure 6 shows PEM fuel cell principle. These fuel cells provide for the chemical reaction listed below:

Anode: 2 H₂ ⇒ 4 H⁺ + 4 e^–
Cathode: O₂ + 4 e^– + 4 H⁺ ⇒ 2 H₂O

The H⁺ions pass through the electrolyte, from the anode towards cathode, whereas electrons are forced to pass from the anode towards cathode through the external electric circuit. Water produces by the cell, aggregated on the cathode, must be drained out of the cell continuously. [9]

The output of both described devices is directly dependant on the surface of electrode with main impact on their total cost. The total amount of the accumulated energy depends on the size of hydrogen container and the output from RES, i.e. the photovoltaic system.

The stage of testing of specific parts of the storage system built will be followed by their linkage to form a working system. The main components of this system comprise the renewable energy source based on the photovoltaic technology, the hydrogen production block with electrolyzer and the container for storage of hydrogen produced and the production electro-block with fuel cell modules and semi-conductor invertors, current inverters respectively.

Measurements on Fuel Cell System

Our research laboratory is equipped with two low-temperature fuel cells Nexa Power Module (Ballard Power Systems Inc.). Rated power output of one Nexa Module is 1200 W. They are being tested in various operating conditions. The measuring procedure consists of cyclic measurements of the load characteristics of NEXA, whereas the system is connected to the distribution network via inverter, in between these measurements, to supply the electric energy. The load characteristics you can see on Figure 7. That is demonstrated in the fuel consumption graph (see Figure 8). This fuel consumption has been determined from the flow meter after the so called purge of cells. This purge deprives cells of impurities and water on regular basis, as those are accumulated on electrode surfaces to intercept the electrochemical reaction.

Fig.9. Stack current and voltage during normal operation

The frequency of purges rises with the increase of cell power output. The NEXA power system measures the voltage over two cells within a stack (fuel cells connected in series), the so called purge cells, to conduct the purging of fuel cells with hydrogen once the voltage has dropped below a certain level to restore the higher voltage in cells again (see Figure 9).

The fuel used for cleaning is drained out of the system unused for the reaction within fuel cells. Yet it shall be included into the overall consumption. That was the reason why we conducted accurate measurements within the system under electronic load at the particular nominal power output, one hour for each. The consumption was determined using a flow meter with an integration member. The Figure 9 shows the normal curve of stack values during the operation.

Wind-Hydrogen Power Plant

This is a complex combining a wind power plant and the hydrogen storage system based on the Utsira island at the Western coast of Norway. The entire complex is fully autonomous. The insular power distribution network supplies electric power to every household on the island, whose annual consumption amounts to approx. 200 MWh. Figure 11 shows the photograph of the entire complex with the evident tube of wind power plant with the installed capacity of 600kW, with the container and electrolyzer providing the installed capacity of 48kW situated below, with the hydrogen production capacity of 10m³/h, and the compressor with the output of 5,5kW. The photograph shows the container for 2.400 m³ of hydrogen compressed at the ratio of 200 bar. There are containers housing he fuel cell providing the output of 55kW on the right hand side. Both the electrolyzer and the fuel cell are of PEM type. [10]

When the wind turbine at Utsira is running at optimum level, it will produce more energy than the consumption is.

Fig.10 Photograph of power plant at Utsira [10]

Fig.11. Operation of Utsira power plant – Low wind mode [10]

The surplus energy is used to produce hydrogen through water electrolysis. The hydrogen produced is compressed and stored in a gas storage vessel and is available when needed. Under circumstances when the wind turbine is not in operation (i.e. when there is too little or too much wind) the hydrogen is used in a fuel cell and a combustion engine-generator unit to produce power. Example of operation of power plant at Utsira is illustrated on Figure 11. On this graph you can see that the wind power decreases and cannot supply the demand. In this period the fuel cell and engine-generator are started and are balancing the load. [10]

Conclusion

The accumulation of electric power, especially the power gained from the photovoltaic and wind power plants, is of significant importance with respect to their operation in relation with the electric power system. Those are sources providing variable and unreliable supply of electric power over time, which has negative impact on the operation of the electric power system. The accumulation of electric power produced by those units can contribute towards substantial reduction of the control power, which shall be maintained within the electric power system. There is a whole range of suitable accumulation technologies available. However, their practical application in relation with the RES mentioned is still subject to research in progress.

Redox flow batteries are seen as a prospective technology as their accumulation capacity is limited by the amount of liquid electrolyte within only, i.e. the size of magazines used. These can be used to build large accumulation facilities within electric power networks. Construction of a CAES will be mainly dependant on the local geological conditions. This system is very complex from the implementation point of view, same as the pumped-storage hydro plant. There are very few CAES pilot projects in operation only.

The hydrogen system can be used to provide for accumulation of electric power in large amounts as well.

The higher cost of implementation might still represent certain problems as those are determined by the cost of materials used. However, this technology is subject to continuous dynamic development, whose future lies not only in stationary applications yet even in mobile projects. The accumulation system mentioned above can be supplemented with flywheels or super capacitors able to cater for short-term fluctuations in supply of electric power.

This work is supported by The Ministry of Education, Youth and Sports of the Czech Republic, project CEZ MSM6198910007″.

REFERENCES

[1] Cenek, M., Accumulators: from principle to practise. FCC Public, Praha, Czech Republic, 2003
[2] NAS batteries [online],
[3] Hradílek, Z., Moldřík, P., Šebesta, R., Storage of Electric Energy Gained from Renewable Sources. Proceedings of AsiaPES, 2009, Beijing, China
[4] About Flow battery [online],
[5] Moldřík, P., Hradílek, Z., Chválek, R., Research of Energy Storage Gained from Renewable Sources. Proceedings of Elnet, 2009, Ostrava, Czech Republic
[6] CAES system [online],
[7] De Boer, P., Raadschelders, J., Flow batteries. Leonardo Energy website [online],
[8] Hradílek, Z., Chválek, R., Research accumulation of energy from renewable energy sources in fuel cells. Proceedings of EPE, 2009, Kouty nad Desnou, Czech Republic
[9] Larminie, J., Fuel Cell Systems Explained. John Wiley and Sons Inc., 2003, Chichester, United Kingdom
[10] Nakken, T., The Utsira wind-hydrogen system: operational experience. Proceedings of EWEC, 2006, Athens, Greece

Authors: prof. Ing. Zdeněk Hradílek, DrSc., Ing. Petr Moldřík, Ph.D., Ing. Roman Chválek, Technical University of Ostrava, Faculty of Electrical Engineering and Computer Science, Department of Electrical Power Engineering, ul. 17. listopadu 15, 708 33 Ostrava – Poruba, Czech Republic, E-mail: zdenek.hradilek@vsb.cz; petr.moldrik@vsb.cz; roman.chvalek@vsb.cz

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 87 NR 2/2011

Prospective Short Circuit Test and Prospective Fault Current Test (PSC & PFC TEST)

Published by Carelabs, Carelabs (Carelabz) is authorized provider of Electrical Installation’s Study, Analysis, Inspection, and Certification services in UAE. Website: carelabz.com

Prospective  Short  Circuit (PSC) and Prospective Fault  Current (PFC) are both intended to calculate the highest current that will stream within a fault loop path during the occurrence of an electrical flaw as needed by rules.

The  Prospective Short Circuit Current (PSC) is the utmost current that could flow between Line and Neutral conductors on a single-phase supply or between Line conductors on a three-phase supply. A PSC test calculates the current that will flow in the event of a short circuit fault between the live conductors. That is, Line and Neutral on a single-phase installation or Line to Line/ Line to Neutral on a three-phase installation.

Prospective Fault Current (PFC) is the common term used for the highest amount of current that will stream under fault conditions. The PFC will continuously be the highest at the source of the installation as the impedance/resistance is always the lowest there. So as a regulation, if it’s not too extreme at the mains it will be fine everywhere else. This is because it will decrease due to the increase in resistance as we move further away from the origin. (ohms law I = V/R).

Because of the type of dissimilar supplies, you would assume to find a PSC value greater than a PFC value on both TT and TN-S systems, yet on a TNC-S system both the PFC and PSC readings should be same.

What is done During PFC and PSC Tests?

PSC is decided by the voltage and impedance of the supply system. It’s far of the order of some thousand amperes for a well-known domestic mains electrical set up, but can be as little as a few milliamperes in a separated extra-low voltage (SELV) device or as excessive as hundreds of heaps of amps in huge industrial strength systems

PFC is conducted at the source of the installation, like the main switch or at other switchgear connected straight to the tail from the electricity distributor’s metering device. Where a calculation is made at a point in the installation other than the source, such as a piece of switchgear served by a distribution circuit, it would not be the highest value for the installation.

Particular guardianship should be exercised during the testing summons, as flaw conditions are most severe at the origin of an installation, where this test is performed. The earthing conductor, main protective bonding conductors and circuit protective conductors should all be connected as for normal operation during these trials, because the presence of these and any other latitude ways to earth may reduce the impedance of the earth flaw loop and so increment the level of prospective fault current.

PSC will be greater than the PFC. Prospective fault current and short circuit current of a circuit is automatically calculated when making a loop impedance test. The calculation uses a nominal circuit voltage, not the actual circuit voltage.

Why PSC and PFC are Done?

PFC and PSC test is necessary for choosing the correct protective device for the circuit because it’s going to carry the maximum fault current flowing in a circuit. Regulation 612.11 of BS 7671 requires that the prospective fault current under both short circuit and earth fault conditions be determined for every relevant point of the installation.

How is PSC and PFC Performed?

The minor supply voltage used in the calculation is automatically chose depending on the real circuit voltage. The instrument uses the following voltage values:

Prospective Short Circuit Test Procedure

• PFC tester or the Prospective short circuit function of a multifunctional tester such as the Megger 1553 is chosen, and we make sure that the supply is ON, but the Main Switch is in OFF position.

• The test leads are joined on the incoming side of the Main Switch, one test lead on Line and another on the Neutral terminals of the Main Switch.

• TEST switch is pushed and a note of the value (kA) is made.

• For three phase installations each phase is tested separately and the measured reading (test between Line 1 and Neutral, then Line 2 and Neutral and last Line 3 and Neutral) is doubled.

• Some test meters need that the third (usually green) lead to be connected on the Neutral during this test. Please refer to the test meter manufacturer’s instruction.

Prospective Fault Current Test Procedure

• We use PFC tester or select the PFC function of a multifunctional tester such as the Megger 1553, and it has to be made sure that the supply is ON, but the Main Switch is in OFF position.

• Next, we connect the test leads on the Line and Neutral terminals of the Main Switch, as well as on the Earth terminal.

• The TEST switch is pushed and we make note of the reading (kA).
For three phase installations each phase is checked separately and the measured reading is doubled (L1 – N – CPC, L2 – N – CPC, L3 – N – CPC).

• Having obtained these values by the measurements described above, we will select the highest value and write it down on the Electrical Installation Certificate as the value of PFC.

• The value of PFC obtained is compared with the breaking capacity of all the protective devices within the installation. The breaking capacity of the protective devices should be greater than the value of PFC.

Benefits of PSC and PFC Tests

They give accurate results as its live testing.
The testing is simple and not much calculations are needed.
Increased safety for employees and third parties.
Decreased Insurance Premiums.
Asset Data management and tracking systems.
Small repairs of equipment made on-site to reduce down time.

Source URL: https://carelabz.com/prospective-short-circuit-test-prospective-fault-current-test/#:~:text=Prospective%20Fault%20Current%20(PFC)%20is,is%20always%20the%20lowest%20there.

General Reference – Utility Capacitor Switching

Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Utility Capacitor Switching, Document ID: PQS0302, Date: January 10, 2003.

INTRODUCTION

CAPACITOR BANK ENERGIZATION

Transient characteristics are dependent on the combination of the initiating mechanism and the electric circuit characteristics at the source of the transient. Circuit inductances and capacitances – either discrete components such as shunt capacitance of power factor correction banks or inductances in transformer windings, or stray inductance or capacitance because of proximity to other current carrying conductors or voltages – are responsible for the oscillatory nature of transients. Natural frequencies within the power system depend on the system voltage level, line lengths, cable lengths, system short circuit capacity, and the application of shunt capacitors.

Characteristics of Energizing an Isolated Capacitor Bank

Energizing a shunt capacitor bank from a predominantly inductive source results in an oscillatory transient that can approach twice the normal system peak voltage (V_pk). Figure 1 illustrates the simplified equivalent system for the energizing transient. The characteristic frequency (fs) of this transient is given by:

and the peak inrush current (I_pk) is determined using:

where:

[example]
f_s = characteristic frequency (Hz) = [379 Hz]
L_s = positive sequence source inductance (H) = [17.53mH]
C = capacitance of bank (F) = [10.03μF]
f_system = system frequency (50 or 60 Hz) = [60 Hz]
X_s = positive sequence source impedance (Ω) = [6.61 Ω]
X_c = capacitive reactance of bank (Ω) = [264.50 Ω]
MVA_sc = three-phase short circuit capacity (MVA) = [2000 MVA]
MVA_r = three-phase capacitor bank rating (MVAr) = [50 MVAr]
ΔV = steady-state voltage rise (per-unit) = [2.5%]
V_pk = peak line-to-ground bus voltage (V) = [93897.11 V]
Z_s = surge impedance (Ω) = [39.35 Ω]

Relating the characteristic frequency of the capacitor energizing transient (f_s) to a steady-state voltage rise (ΔV) design range provides a simple way of quickly determining the expected frequency range for utility capacitor switching. For example, a 60 Hz system with a design range of 1.0% to 2.5% would correspond to characteristic frequency range of 380 to 600 Hz. For a shunt capacitor bank on a high voltage bus, transmission line capacitance and other nearby capacitor banks cause the energizing transient to have more than one natural frequency. However, for the first order approximation, the equation above [1] can still be used to determine the dominant frequency.

Figure 1 – Equivalent Circuit for Capacitor Energizing

Because capacitor voltage cannot change instantaneously (remembering that i(t)=Cdv/dt), energization of a capacitor bank results in an immediate drop in system voltage toward zero, followed by an oscillating transient voltage superimposed on the 60 Hz fundamental waveform. The peak voltage magnitude depends on the instantaneous system voltage at the instant of energization, and can reach 2.0 times the normal system voltage (V_pk – in per-unit) under worst-case conditions. The voltage surge is at the same frequency as the inrush current (I_pk) and rapidly decays to the system voltage.

For a practical capacitor energization without trapped charge, system losses, loads, and other system capacitances cause the transient magnitude to be less than the theoretical 2.0 per-unit. Typical magnitude levels range from 1.2 to 1.8 per-unit and typical transient frequencies generally fall in the range from 300 to 1000 Hz. Figure 2 illustrates an example (measured) distribution system capacitor energizing transient.

Figure 2 – Typical Distribution Bus Voltage during Capacitor Energizing

Energizing an ungrounded-wye capacitor bank can result in slightly higher transient overvoltages because of unequal pole closing. In general, the transient overvoltages associated with normal closing are similar to those for grounded banks.

Characteristics of Energizing Back-to-Back Capacitor Banks

Energizing a shunt capacitor bank with an adjacent capacitor bank already in service is known as “back-to-back” switching. High magnitude and frequency currents, illustrated in Figure 3, will flow between the banks when the second bank is energized. This current must be limited to acceptable levels for switching devices and current transformer burdens. Generally, series reactors are used with each bank to limit the current magnitude and frequency, although pre-insertion resistors/inductors may be used with some types of switches.

The frequency and magnitude of the inrush current during back-to-back switching depends upon the size of the discharging capacitor bank, the impedance of the discharging loop, and the instantaneous capacitor bank terminal voltage at the time of contact closure. The impedance of the discharging loop is determined by the inductance between the banks rather than the system inductance (L_s). The magnitude of the inrush current is therefore much higher than for the isolated bank energization (I_pk). Typically, the inrush current lasts for only a fraction a power frequency cycle.

This high-frequency inrush current may exceed the transient frequency momentary capability of the switching device (e.g. ANSI C37.06-1987) as well as the I²t withstand of the capacitor fuses. It may also cause false operation of protective relays and excessive voltages for current transformers (CTs) in the neutral or phase of grounded-wye capacitor banks. The current must be evaluated with respect to the transient frequency momentary capability (close and latch) rating of the switch, as well as the I²t withstand of the capacitor fuses. Switch manufacturers should be consulted for the appropriate current (I_pk) and frequency (f) ratings of the device. High frequency substation ground mat currents may be controlled by connecting the two neutral points together and grounding with a single connection to the grid.

Figure 3 – Example Distribution Feeder Current during Back-to-Back Switching

Solutions to excessive inrush currents usually involves:

− adding current-limiting reactors to decrease the peak current and frequency of the oscillatory inrush current.
− adding pre-insertion resistors or inductors to the switching device.
− adding synchronous closing control to the switching device.
− selecting component ratings (e.g. breaker, CT burdens, etc.) to withstand the inrush current characteristics.

OVERVOLTAGE MITIGATION

Devices for capacitor switching transient control either attempt to minimize the overvoltage (or overcurrent) at the point of application, or limit (clip) the overvoltage at local and remote locations. These devices include:

− a) synchronous closing control (also known as zero voltage closing)
− b) pre-insertion devices (resistors and/or inductors)
− c) fixed inductors
− d) MOV arresters

Previous research has suggested that the effectiveness of these control methods is system dependent, and that detailed analysis is required to select the optimum control scheme. While often justifiable for large transmission applications, transient analysis of distribution capacitor applications is rarely performed, and in general, banks are installed without transient overvoltage control. Each of these methods has various advantages and disadvantages in terms of transient overvoltage reduction, cost, installation requirements, operating/maintenance requirements, and reliability.

Timing Control

Synchronous closing is independent contact closing of each phase near a voltage zero, as illustrated in Figure 4. To accomplish closing at or near a voltage zero (avoiding high prestrike voltages), it is necessary to apply a switching device that maintains a dielectric strength sufficient to withstand system voltages until its contacts touch. Although this level of precision is difficult to achieve, closing consistency of ±0.5 milliseconds should be possible. Previous research has indicated that a closing consistency of ±1.0 millisecond provides overvoltage control comparable to properly sized pre-insertion resistors. The success of a synchronous closing scheme is often determined by the ability to repeat the process under various (system and climate) conditions. Adaptive, microprocessor-based control schemes that have the ability to “learn” from previous events address this concern. The primary benefits of this capability are the control’s ability to compensate for environmental factors and the increased reliability (less maintenance) that can be achieved

Grounded capacitor banks are controlled by closing the three phases at three successive phase-to-ground voltage zeros (60° separation). Ungrounded banks are controlled by closing the first two phases at a phase-to-phase voltage zero and then delaying the third phase 90 degrees (phase-to-ground zero).

Figure 4 – Concept of Synchronous Closing Control

Pre-insertion Devices

A pre-insertion impedance (resistor or inductor) provides a means for reducing the transient currents and voltages associated with the energization of a shunt capacitor bank. The impedance is “shorted-out” (bypassed) shortly after the initial transient dissipates, thereby producing a second transient event. The insertion transient typically lasts for less than one cycle of the system frequency. The performance of pre-insertion impedance is evaluated using both the insertion and bypass transient magnitudes, as well as the capability to dissipate the energy associated with the event, and repeat the event on a regular basis. The optimum resistor value for controlling capacitor energizing transients depends primarily on the capacitor size and the source strength.

Fixed Inductors

Fixed inductors have been used successfully to limit inrush currents during back-to-back switching. Typically the value of these inductors is on the order of several hundred microhenries. In addition, inductors provided for outrush (into a nearby fault) current control may be applied, and are typically 0.5 – 2.0 millihenries. Previous research indicates that these fixed reactors do not provide any appreciable transient overvoltage reduction.

MOV Arresters

Metal oxide varistors (MOVs) can limit the transient voltages to the arrester’s protective level (maximum switching surge protective level, typically 1.8 – 2.5 per-unit) at the point of application. The primary concern associated with MOV application is the energy duty during a restrike event. Although a rare occurrence, a switch restrike generally results in the highest arrester duty for arresters located at the switched capacitor. In addition, remote arresters (including low voltage customer applications) may be subjected to severe energy duties if voltage magnification occurs. This condition could be especially troublesome for distribution systems if SiC arresters remain in service.

POWER QUALITY CONSIDERATIONS

Voltage Magnification:

Voltage magnification occurs when a transient oscillation, initiated by the energization of a utility (transmission or distribution) capacitor bank, excites a series resonance formed by a lower voltage system. The result is a higher overvoltage at the lower voltage bus. Previous analysis has indicated that the worst magnified transient occurs when the following conditions are met (refer to Figure 5):

− The size of the switched capacitor bank is significantly larger (>10) than the lower voltage (often customer power factor correction) bank (i.e. 50MVAr versus 1.8MVAr ≈ 28).

− The energizing frequency (f_s) is close to the series resonant frequency formed by the step-down transformer and the power factor correction capacitor bank (f₂) (i.e. 465Hz @ 230kV bus vs. 440Hz @ 13.2kV bus).

− There is relatively little damping (resistive) provided by the low voltage load (typical industrial plant configuration – primarily motor load).

Distribution system overvoltages, resulting from transmission capacitor bank energization, may be sufficient to spark-over SiC arresters. MOV arresters should be capable of withstanding the event, however this should verified using computer simulations and/or TNA analysis. Low voltage customer systems may be exposed to transient overvoltages (illustrated in Figure 6) between 2.0 and 4.0 per-unit, (previously determined by computer simulations and in-plant measurements) and these overvoltages may occur over a wide range of low voltage capacitor sizes (note that the low voltage capacitor must exist for magnification to occur). Typically, the transient overvoltages will simply damage low-energy protective devices (MOVs) or cause a nuisance trip of a power electronic-based device. However, there have been several cases when complete failure of customer equipment (single process device) has occurred.

Important system variables to consider when analyzing this phenomenon include:

− Switched capacitor bank size
− Lower voltage capacitor bank size and location
− System loading
− Transformer characteristics
− Circuit breaker characteristics (closing resistors/inductors, closing control, etc.).
− Arrester size(s), rating(s), and location(s)

Figure 5 – System Diagram for Voltage Magnification Condition

Figure 6 – Transient Overvoltages during Voltage Magnification

A number of utilities and their customers have evaluated and tested several possible solutions to the voltage magnification problem, including:

− Detuning the circuit by changing capacitor bank sizes, moving banks, and/or removing banks from service (utility and/or customer).
− Switching large banks in more than one section.
− Using one of the presently available overvoltage control methods, such as:
• pre-insertion resistors or inductors
• synchronous closing control
− Applying surge arresters (MOVs) at the remote location(s).
− Detuning the circuit by converting low voltage power factor correction banks into harmonic filters.

Each of these methods has been utilized in the field with varying degrees of success. Typically, the optimum approach considers the economics of the solution in conjunction with the engineering analysis. Quite often the economic evaluation is incomplete due to the fact that it may be very difficult to determine the cost of a particular power quality event for an individual customer. A cooperative approach between utility and customer(s) will generally lead to a mutually agreeable, cost-effective engineering solution.

Nuisance Tripping of ASDs

Nuisance tripping refers to the undesired shutdown of an ASD (or other power-electronic-based process device) due to the transient overvoltage on the device’s dc bus. Very often, this overvoltage is caused by transmission and/or distribution capacitor bank energization. Considering the fact that many distribution banks are time clock controlled, it is easy to see how this event can occur on a regular basis, thereby causing numerous process interruptions for the customer.

An ASD system consists of three basic components and a control system as previously illustrated in Figure 7. The rectifier converts the three-phase ac input to a dc voltage, and an inverter circuit utilizes the dc signal to produce a variable magnitude, variable frequency ac voltage, that is used to control the speed of an ac motor. A dc motor drive differs from this configuration in that the rectifier is used to control the motor directly.

Figure 7 – Simplified Diagram of the ASD Circuit

The nuisance tripping event consists of an overvoltage trip due to a dc bus overvoltage on voltage-source inverter drives (i.e. pulse-width modulated – PWM). Typically, for the protection of the dc capacitor and inverter components, the dc bus voltage is monitored and the drive tripped when it exceeds a preset level. This level is typically around 780 volts (for 480 V applications), which is only 120% of the nominal dc voltage. The potential for nuisance tripping is primarily dependent on the switched capacitor bank size, overvoltage controls for the switched bank, the dc bus capacitor size, and the inductance between the two capacitors. It is important to note that nuisance tripping can occur even if the customer does not have power factor correction capacitors.

The most effective methods for eliminating nuisance tripping are to significantly reduce the energizing transient overvoltage, or to “isolate” the drives from the power system through the use of series inductors, often referred to as “chokes”. The additional series inductance of the choke will reduce the transient magnitude at the input to the ASD and the associated current surge into the dc link filter capacitor, thereby limiting the dc overvoltage.

While determining the precise inductor size for a particular application may require a fairly detailed computer simulation study, a more common approach involves the wide-spread application of a standard “3%” value. The 3% size is based upon the drive kVA rating and is usually sufficient for most applications where voltage magnification isn’t also a concern. Figure 8 illustrates an example (simulation) dc overvoltage transient before and after the application of a 3% ac choke.

Figure 8 – Illustration of Impact of ac Choke on dc Overvoltage Level

Note: The reader is warned that should the application of a choke in the 3-5% range not solve the nuisance tripping problem, the customer should not arbitrarily increase the size (i.e. 10%) in hopes of eventually solving the problem. It is likely that the drive would fail to function properly. The utility and drive manufacturer should be contacted.

SUMMARY

There are many events that can cause a power quality problem. Analysis of these events is often difficult due to the fact that the cause of the event may be related to a switching operation within the facility or to a power system fault hundreds of miles away. This document summarizes several of the more common power quality problems associated with the application of utility system capacitor banks. The frequent switching of utility capacitor banks coupled with the increasing application of sensitive customer equipment has led to a heightened awareness of several important events, including voltage magnification and nuisance tripping of ASDs.

REFERENCES

T.E. Grebe, Application of Distribution System Capacitor Banks and Their Impact on Power Quality, 1995 Rural Electric Power Conference, Nashville, Tennessee, April 30-May 2, 1995.

M. McGranaghan, W.E. Reid, S. Law, and D. Gresham, Overvoltage Protection of Shunt Capacitor Banks Using MOV Arresters, IEEE Transactions PAS, Vol. 104, No. 8, pp. 2326-2336, August, 1984.

S. Mikhail and M. McGranaghan, Evaluation of Switching Concerns Associated with 345 kV Shunt Capacitor Applications, IEEE Transactions PAS, Vol. 106, No. 4, pp. 221-230, April, 1986.

Electrotek Concepts, Inc., An Assessment of Distribution System Power Quality – Volume 2: Statistical Summary Report, Final Report, EPRI TR-106294-V2, EPRI RP 3098-01, May 1996.

Electrotek Concepts, Inc., Evaluation of Distribution Capacitor Switching Concerns, Final Report, EPRI TR-107332, October 1997.

RELATED STANDARDS
IEEE Standard 18-1992, IEEE Standard 1036-1992
ANSI/IEEE Standard C37.012-1979, ANS/IEEE C37.99-1990

GLOSSARY AND ACRONYMS
AS: Adjustable-Speed Drive
PWM: Pulse Width Modulation
MOV: Metal Oxide Varistor
TVSS: Transient Voltage Surge Suppressors

Study of Power Quality Disturbance for Restructuring of Power Systems

Published by Rimjhim Tiwari, Dilip Kumar, International Journal of Engineering and Technical Research (IJETR), ISSN: 2321-0869, Volume-2, Issue-12, December 2014.

Abstract— In recent years, the traditional power systems’ structures have been changed, and the concern over power quality has increased due to the new generation of load equipments. This equipments has been fully automated electronically, so it can be highly sensitive to any power quality disturbances. Indeed, power quality disturbances may cause malfunctions in the equipment, which leads to higher production costs due to decreased production efficiency. Moreover, the electronic converters in these loads produce harmonic currents that increase current distortion. Eventually, the impact of electronic converters on power quality will be increased proportional to the converters lifetime; therefore, maintaining power quality levels above specific baselines will be an essential requirement in future decades.

Index Terms— production efficiency, power quality, harmonic currents.

I. INTRODUCTION

Many of the loads installed in present-day power systems are harmonic current generators. Combined with the impedance of the electrical system, the loads also produce harmonic voltages. The nonlinear loads may be viewed as both harmonic current generators and harmonic voltage generators. Until 1970s, speed control of AC motors was primarily achieved using belts and pulleys.

Now, adjustable speed drives (ASDs) perform speed control functions very efficiently. ASDs are generators of large harmonic currents. Fluorescent lights uses less electrical energy for the same light output as incandescent lighting but produce substantial harmonic currents in the process. Due to increase of personal computer use it has resulted in harmonic current in commercial buildings. Harmonic distortion is no longer a phenomenon confined to industrial equipment and processes, where the first power quality concerns developed. Uninterruptible power supplies (UPSs), personal computers (PCs), and electronic and entertaining devices proliferate nowadays in commercial and residential installations. These special kinds of loads represent formidable sources of harmonic currents and they increase with the expanding use of video recorders, digital clocks, and other sensitive electronic equipment.

II. BACKGROUND

The first demonstration of electric light in Calcutta was conducted on 24 July 1879 by P W Fleury & Co. On 7 January 1897, Kilburn & Co secured the Calcutta electric lighting licence as agents of the Indian Electric Co, which was registered in London on 15 January 1897. A month later, the company was renamed the Calcutta Electric Supply Corporation. The control of the company was transferred from London to Calcutta only in 1970.

Enthused by the success of electricity in Calcutta, power was thereafter introduced in Bombay. Mumbai saw electric lighting demonstration for the first time in 1882 at Crawford Market, and Bombay Electric Supply & Tramways Company (B.E.S.T.) set up a generating station in 1905 to provide electricity for the tramway. The first hydroelectric installation in India was installed near a tea estate at Sidrapong for the Darjeeling Municipality in 1897. The first electric train ran between Bombay’s Victoria Terminus and Kurla along the Harbour Line, in 1925. In 1931, electrification of the meter gauge track between Madras Beach and Tambaram was started.

III. LITERATURE SURVEY

A new recursive algorithm for autoregressive (AR) spectral estimation was introduced by Marple (1980) based on the least squares solution for the AR parameters using forward and backward linear prediction. The algorithm had computational complexity proportional to the process order squared, comparable to that of the popular Burg algorithm. The computational efficiency was obtained by exploiting the structure of the least squares normal matrix equation, which may be decomposed into products of Toeplitz matrices. AR spectra generated by the new algorithm had improved performance over AR spectra generated by the Burg algorithm. These improvements include less bias in the frequency estimate of spectral components, reduced variance in frequency estimates over an ensemble of spectra, and absence of observed spectral line splitting.

Prony analysis by Hauer,et al.(1990), extends Fourier analysis by directly estimating the frequency, damping, strength, and relative phase of modal components present in a given signal. The ability to extract such information from transient stability program simulations and from large-scale system tests of disturbances would be quite valuable to power system engineers.

Moo et al.(1995)presents an enhanced measurement scheme on the harmonics in power system voltages and currents which was not limited to stationary waveforms, but can also estimate harmonics in waveforms with time-varying amplitudes. It starts with a review of the common techniques for harmonics measurement based on the Fast Fourier transform (FFT).The major pitfalls in the common FFT application techniques are described and the concepts of a new scheme for reducing the picket-fence effect are introduced. The proposed scheme was based on Parseval’s relation and the energy concept which defines a “group harmonic” identification algorithm for the estimation of the energy distribution in the harmonics of time-varying waveforms.

Prony’s method has been proposed in order to improve the monitoring of electrical machines. Costa et al. (2005) showed that the method was efficient to track frequency deviations. The proposed method was advantageous over the method proposed in Costa et al. (2004) because the former does not rely on the previous knowledge of fundamental frequency. It has also been showed that the quantization can deeply affect the spectrum of the analyzed signals. Feilat (2006) presents an efficient method for the detection of the instantaneous flicker level. The technique was based on extracting the magnitudes, frequencies, and phase angles of all frequency components of the voltage envelope using Prony analysis. By reconstructing the voltage waveform as linear combination of sinusoids, Hilbert transform can be applied to the predicted signal to develop the envelope of the voltage waveform.

Two cases of flicker with single low frequency and wide band frequency interharmonics are investigated using simulated voltage signals.

Costa et al.(2007)proposes a technique for harmonic analysis in electrical power systems. At the end, a frequency estimator, the Prony’s method, has been matched to a Kalman filter. In the proposed technique, the sinusoid amplitudes of electrical power signals are estimated by the Kalman filter. The Kalman filter regressors are built up using the frequencies estimated by the Prony’s method. The technique have been tested to both synthetical and experimental signals.

Many algorithms have been proposed for harmonic estimation in a power system. Most of them deal with this estimation as a totally nonlinear problem. Consequently, these methods either converge slowly, like GA algorithm or need accurate parameter adjustment to track dynamic and abrupt changes of harmonics amplitudes, like adaptive Kalman filter (KF). A novel hybrid approach, based on the decomposition of the problem into a linear and a nonlinear problem, was proposed by Joorabian et al. ( 2009),a linear estimator, i.e., Least Squares (LS), which is simple, fast and does not need any parameter tuning to follow harmonics amplitude changes, is used for amplitude estimation and an adaptive linear combiner called ‘Adaline’, which was very fast and very simple and was used to estimate phases of harmonics.

An improvement in convergence and processing time is achieved using this algorithm. The one-dimension frequency analysis based on DFT (Discrete FT ) is sufficient in many cases in detecting power disturbances and evaluating power quality (PQ). The character of the signal, using time-frequency analyses are performed by Szmajda et al. (2010). The most common known time-frequency representations (TFR) are spectrogram (SPEC) and Gabor Transform (GT). However, the method has a relatively low time-frequency resolution. The other TFR: Discreet Dyadic Wavelet Transform (DDWT), Smoothed Pseudo Wigner-Ville Distribution (SPWVD) and new Gabor-Wigner Transform (GWT) are described. The main features of the transforms, on the basis of testing signals, was presented.

IV. POWER QUALITY

Many sources in the literature have addressed the importance of power quality; however, there is no single agreed definition for the term “power quality”, and various sources have different and sometimes inconsistent definitions for it. “power quality” is sometimes used loosely to express different meanings: “supply reliability”, “service quality”, “voltage quality”, and “current quality”. The multiple meanings of power quality are the result of defining power quality from different perspectives. Power quality, in generation, relates to the ability to generate electric power at a specific frequency, 50 or 60 Hz, with very little variation; while power quality in transmission can be referred to as the voltage quality. At the distribution level, power quality can be a combination of voltage quality and current quality. From the marketing point of view, electricity is a product and the power quality is the index of the product quality.

The Institute of Electrical and Electronics Engineers (IEEE) defines power quality in the IEEE standard 1159-1995 as: “power quality is the concept of powering and grounding sensitive equipment in a matter that is suitable to the operation of that equipment.” This definition limits the term power quality to only sensitive equipment, and this definition narrows down the impact of harmonic currents to consider it as affecting only that equipment. The International Electro-technical Commission (IEC) states in IEC 61000-4-30 that “Characteristics of the electricity at a given point on an electrical system, evaluated against a set of reference technical parameters.” The definition evaluate power quality as depending on its measurement and quantity from a power system point of view. Heydt, in Electric Power Quality (1994), defines power quality as ―power quality is the measure, analysis, and improvement of bus voltage, usually a load bus voltage, to maintained that voltage to be a sinusoid at rated voltage and frequency.‖ It is cleared that Heydt defined power quality from the utility’s point of view; the definition confines the eaning of power quality only to voltage quality. Indeed, before deregulation took place, the electrical systems structure was vertical, and the electrical utility was the only entity taking care of power quality problems. The electrical utility can only control the voltage and the frequency; however, it has no control over the current that particular loads might draw. Thus, voltage quality problems were the focus at that time, or in other words, power quality problems were handled as voltage quality problems.

The increasing of nonlinear and sensitive loads in the distribution system causes noticeable current deviations that lead to power quality disturbances; therefore, power quality problems are no longer considered as only voltage quality problems. Dugan et al. define power quality problems in as “any power problem manifested in voltage, current, or frequency deviations that results in failure or disoperation of customer equipments.” This definition covers the possible reasons that can cause power quality disturbances; however, power quality disturbances can result from more than one source. Because of the close relationship between voltage and current in any practical power system, any deviation in the current will affect the voltage and vice versa. Bollen defines power quality in his book Understanding Power Quality Problems as “power quality is the combination of voltage quality and current quality. Thus power quality is concerned with deviations of voltage and/or current from the ideal.” So, any deviations of voltage or current from the ideal is a power
quality disturbance.

It is hard to distinguish between voltage disturbances and current disturbances due to the close relationship between the two, and there is no common reference point that the disturbance can be seen from. For instance, starting a large induction motor leads to an over current; this is a current disturbance from the network perspective. However, the neighboring loads can suffer from a voltage dip, which is considered a voltage disturbance from another perspective. This action, starting an induction motor, leads to a disturbance that can be looked at from different perspectives: as a voltage disturbance from one point and a current disturbance from the other. The distinguishing complexity makes using the term “power quality disturbance” more preferable in general; however, the underlying cause of a disturbance is still either a voltage deviation or a current deviation.

However, the typical power quality disturbance classification is usually based on voltage magnitude and frequency variation for different time durations. The typical classification has been specified by many sources, such as IEEE and IEC. The classification of power quality disturbances can help in understanding power quality phenomena, and it is considered the base for monitoring and mitigating power quality problems.

V. POWER QUALITY DISTURBANCES CLASSIFICATION

In order to be able to classify different types of power quality disturbances, the characteristics of each type must be known. In general, power quality disturbances are classified into two types: steady state and non-steady state. This classification is done in terms of the frequency components which appear in the voltage signals during the disturbance, the duration of the disturbance, and the typical voltage magnitude. These disturbances are mainly caused by :

• External factors to the power system: for example, lightning strikes cause impulsive transients of large magnitude.
• Switching actions in the system: a typical example is capacitor switching, which causes oscillatory transients.
• Faults which can be caused, for example, by lightning (on overhead lines) or insulation failure (in cables). Voltage dips and interruptions are disturbances related to faults.
• Loads which use power electronics and introduce harmonics to the network.

Waveform Distortion

• This is a steady-state deviation from an ideal sine wave of power frequency, principally characterized by the spectral content of the deviation. There are five types of waveform distortion:

• DC Offset

DC Offset is defined as the presence of a DC voltage or current in an AC power system. This phenomenon can occur as the result of a geomagnetic disturbance or be due to the effect of half-wave rectification. Incandescent light bulb life extenders, for example, may consist of diodes that reduce the RMS voltage supplied to the light bulb by half-wave rectification. Direct current in alternating current networks can be detrimental due to an increase in transformer saturation, additional stressing of insulation, and other adverse effects.

• Harmonics

Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate. Harmonics combined with the fundamental voltage or current can produce waveform distortion. Harmonic distortion exists due to nonlinear characteristics of devices and loads on the power system. Voltage distortion results as these currents cause nonlinear voltage drops across the system impedance. Harmonic distortion is a growing concern for many customers and for the overall power system due to increasing application of power electronics equipment. Harmonic distortion levels can be found throughout the complete harmonic spectrum, with the magnitudes of each individual harmonic component varying inversely with their position in the spectrum. Furthermore, the phase angle of each component is unique unto itself. It is also common to use a single quantity, the total harmonic distortion (THD), as a measure of the magnitude of harmonic distortion.

• Inter-harmonics

Inter-harmonics are defined as voltages or currents having frequency components that are not multiples of the frequency at which the supply system is designed to operate. Interharmonics can be found in networks of all voltage classes. They can appear as discrete frequencies or as a wide-band spectrum. The main sources of inter-harmonic waveform distortion are static frequency converters, cyclo-converters, induction motors, and arcing devices.

VI. NOTCHING

Notching is a periodic voltage disturbance caused by the normal operation of power electronics devices when current is commutated from one phase to another. Voltage notching represents a special case that falls between transients and harmonic distortion. Three-phase converters that produce continuous DC current are the most common cause of voltage notching.

VII. NOISE

Noise is unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines. Noises in power systems can be caused by power electronics devices, control circuits, arcing equipments, loads with solid-state rectifiers, and switching power supplies. Noises problem are often exacerbated by improper grounding. The problem can be mitigated by using filters, isolation transformers, and certain line conditioners.

CONCLUSION

The restructuring of power systems raises the concerns over power quality problems resulting from harmonics distortion. Electrical power organizations have proposed some standards in order to protect their electrical power systems from the consequences of harmonics pollution. Due to the highly complex interconnected networks in the distribution systems, identifying the harmonics pollution can be achieved, The problem of precise estimation of fundamental harmonics frequency is very important used in power quality monitoring systems. Poor quality can be defined as any event related to the electrical network that results in a financial losses. For precision short time analysis of power waveform fluctuations the least square (LS) Prony’s method can be used , which enables in estimation of fundamental harmonics frequency. The LS Prony’s method is based on the representation of a signal as a linear combination of exponential functions.

REFERENCES

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[9] Zygarlicki, J., Zygarlicka, M., Mroczka, J. 2009. Prony’s metod in power quality analysis. Energy Spectrum, 4(2), 26-30.
[10] Joorabian, M., Mortazavia, S.S., Khayyami, A.A. 2009. Harmonic estimation in a power system using a novel hybrid Least Squares-Adaline algorithm. Electric Power Systems Research, 79(1),107-116.
[11] Szmajda, M., Górecki, K., Mroczka, J. 2010. Gabor transform, SPWVD, Gabor-Wigner transform and wavelet transform – tools for power quality monitoring. Metro. and Meas. Syst., 16(3), 383-396.
[12] Zygarlicki, J., Zygarlicka, M., Mroczka, J., Latawiec, K. 2010. A reduced Prony’s method in power quality analysis – parameters selection. IEEE Transactions on Power Delivery, 25(2), 979- 986.
[13] “IEEE Recommended Practice for Monitoring Electric Power Quality,” IEEE Std. 1159-1995.
[14] A. P. J. Rens and P. H. Swart,2001. “On techniques for the localization of multiple distortion sources in three-phase networks: Time-domain verification,” ETEP, vol. 11, no. 5, pp. 317–322.
[15] El-Saadany ,E. 1998 Power Quality Improvement for Distribution Systems under Non-linear Conditions., University of Waterloo, Waterloo, Canada.
[16] W. Xu, X. Liu, and Y. Liu, 2002. An investigation on the validity of power direction method for harmonic source determination. IEEE Power Engineering Review, 22(7):62-62.
[17] “IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis,” IEEE Std. 399 1997.
[18] M.H.J. Bollen., 2000.Understanding power quality problems: voltage sags and interruptions. Wiley-IEEE Press.

Manuscript received December 23, 2014.

RIMJHIM TIWARI, Research Scholar(Power Electronics), Saroj Institute Of Technology And Management,Lucknow- 226002, U.P., India.

DILIP KUMAR, Assistant Professor,EN Department , Saroj Institute Of Technology And Management, Lucknow- 226002, U.P., India.

Website: www.erpublication.org

2017 Opportunities and Challenges… A View From PJM Interconnection

Published by Craig Glazer, Vice President-Federal Government Policy, PJM Interconnection.

WIRES University, Congressional Briefing, Date: February 16, 2017.

Presented by WIRES – a national coalition of entities dedicated to investment in a strong, well-planned and environmentally beneficial electricity high voltage transmission system in the US.

Distribution Transformers Protection Against High Frequency Switching Transients

Published by Dariusz SMUGAŁA¹, Wojciech PIASECKI¹, Magdalenia OSTROGORSKA¹,
Marek FLORKOWSKI¹, Marek FULCZYK¹, Paweł KŁYS²
ABB Sp.z.o.o., Corporate Research Center, (1), ABB Sp.z.o.o., ABB Distribution Transformers, (2)

Abstract. Distribution transformers protection method against high frequency transients generated during Vacuum Circuit Breaker (VCB) operations is presented in this paper. ATP/EMTP simulations of transients generated during the VCB operation and protection method effectiveness is described. In addition prototype windmill installation is presented. For simulations realistic VCB model was used enabling one to study both prestrikes, generated during the contact making process as well as re-ignitions generated during the contact breaking.

Streszczenie. W artykule przedstawiono metodę ochrony transformatorów dystrybucyjnych przed wysokoczęstotliwościowymi przepięciami generowanymi podczas operacji łączeniowych z użyciem wyłączników próżniowych. W artykule przedstawiono symulacje komputerowe przepięć generowanych zarówno podczas załączania jak i wyłączania transformatorów. Symulacji dokonano przy użyciu pakietu ATP/EMTP. Zaprezentowano również przykładową istniejącą instalację opisanych w artykule urządzeń. (Ochrona transformatorów dystrybucyjnych przed wysokoczęstotliwościowymi przepięciami łączeniowymi).

Keywords: przepięcia łączeniowe, transformatory, ochrona.
Słowa kluczowe: switching transients, transformers, protection.

Introduction

Transformers operating in power network are exposed to various types of surges. High frequency transients and high rate overvoltages pose a hazards to the connected equipment [2] and can create local overstressing of the insulation system. One of the potential sources of high du/dt transients are Vacuum Circuit Breakers (VCB). According to [1], of special concern are:

transformers connected to cables of moderate length,
transformers connected to GIS,
transformers connected through a VCB,
dry type transformers connected through the cables,
transformers exposed to lightning,
transformers exposed to frequent switching operations.

Complex internal structure of the transformer results in multiple internal resonances which may cause non uniform voltage distribution at high frequencies and local resonant amplification of voltage.

The phenomena resulting from the VCB–cable–transformer interaction may generate VFT overvoltages overstressing the insulation system of transformers which can negatively affect the equipment lifetime and may lead to an internal short-circuit.

VCB Switching transients – problem description

Connecting and disconnecting a transformer using a VCB involving the interaction between the cable and transformer capacitance and transformer inductance is well described in the literature [3÷5,8]. It results in dangerous HF stresses on transformer windings insulation.

Cable capacitance combined with the inductive character of the transformer impedance results in oscillatory escalation of the Transient Recovery Voltage (TRV) across the breaker contacts. In consequence the fast TRV built-up during switching-off may lead to multiple re-ignitions (Fig.3-4). This process depends strongly on the system parameters (mainly on the inductive current value and on the phase-to ground capacitance), e.g for:

a. Unloaded small transformer with large L and very low inductive current ( ~0.1A),
b. Unloaded large transformer with rather large L and low inductive current (~1A),
c. Inductively loaded large transformer with low L and large inductive current (~50A).

Fig.1. Voltage across the transformer terminals for low inductive current value

Fig.2.Voltage across the transformer terminals for large inductive current value

Fig.3. Voltage across the transformer terminals for large inductive current value

It is clearly seen that for the predefined value of C, the value of the inductive current has a critical importance on the re-ignitions generation process. The process of generating re-ignitions in the VCB can be briefly described as follows:

During contact breaking, after physical separation of the contacts arc conducts the current until it drops below the chopping current value (typically 2-5A). When the current is chopped, the energy is trapped in the oscillatory circuit (L,C) and the voltage at the transformer starts to oscillate. When the TRV across the contacts exceeds the dielectric withstand, arc re-ignites, the C is re-charged and the current is chopped again. The process continues until the contacts separate so that the dielectric withstand exceeds the TRV.

During the contact making of the VCB high du/dt transients may also be generated, especially, when relatively short cables of small surge impedance between the VCB and the transformer exists. This type of a short, low surge impedance connection has a low du/dt limiting effectiveness. Therefore high value of overvoltages and high frequency transients are expected. The process of high du/dt transients generation during the contact making process is briefly summarized below.

When the distance between the contacts becomes small, arc ignites and the internal capacitance of the transformer is charged from the network. The charging time constant depends on the C and on the source impedance (network). Inductances of the connections and the capacitance results in oscillations and overshoots. Voltages at both sides of the VCB equalize and the arc is quenched. Voltage at transformer (L and C) oscillate and when the TRV exceeds the dielectric withstand arc ignites again. The process continues until the contacts mate. The reflections occurring in the short cables may additionally increase the high frequency overvoltages.

There are cases known in literature of the transformer failures when the VCBs are used for operation through the relatively short cables [7]. It is supposed that the HF transients occurring during the switching are the most likely the cause for that.

Due to complicated internal structures of the transformers comprising capacitances and inductances, high frequency components generated may additionally lead to a local amplification of voltage. These overvoltages may overstress the transformer insulation, and in consequence, reduce significantly the equipment lifetime due to internal short-circuit destroying the windings insulation. The Transient Overvoltages (TOV) problem is dangerous not only to the transformers but also to other equipment connected, such as cables and cable accessories.

VFTs preventing methods

There are various protection methods against high TOVs and VFTs. Applied protection method depends on character of the transient and on the application of the apparatus protected. The most popular protective method is the use of surge arresters connected to the transformer terminals. Surge arresters provide overvoltage protection only and do not limit high du/dt. Therefore, in many cases the high du/dt transients are not affected by the surge arresters as their amplitudes may be lower than the protection level. The surge arresters do not filter HF oscillations and do not eliminate wave reflections.

Different, commonly used in practice protective element are RC-filters with large value of the phase to ground capacitance. Usually typical value of this capacitance (C≤0.5 μF) is combined with resistance (R=5-25 W). This type of solution is characterized by large size and cost which typically limits its applicability only to the cases, when the equipment reliability is of primary concern (e.g. industrial applications). An interesting modification of the RC-snubber technology is the solution known as ZORC (by Strike Technologies). The ZORC surge suppressor is comprising of capacitors, resistors and Zinc Oxide (ZnO) surge arresters.

The solution presented by Strike Technology however very effective, has the same limitations as the RC-snubber solution.

A completely different protection character method and simultaneously most efficient solution of high value of du/dt and surges generated during switching operation elimination is the synchronized switching.

This solution requires significant modification of the breaker what result in high cost of implementation. Besides, this method does not provide protection for transformers working with conventional breakers.

Yet another possibility of mitigating the high du/dt transients resulting from the switching operations is the use of pre-inserted resistors. Considering costs of this method application and construction complication this solution is not commonly used.

New solution for VFTs suppression

The limitations of the mitigation methods described led to the development of a new concept of high du/dt mitigation using a series-connected R-L choke [6]. The main problem in avoiding the VFTs is a low value of the equivalent impedance of the surge source due to a low impedance of power cables. The amplitude built-up and the repetitive nature of transients is additionally a result of lack of appropriate termination of the end of the cable. The problem of very high du/dt is enhanced in the case of short connections to the surge source. Increasing the impedance of the surge source which may be utilized in the appropriate surge filtering may be achieved by introducing an additional series element upstream the equipment. Proposed method comprising a series impedance element (choke) installed upstream the protected device (transformer), as shown in Fig. 5. The use of series filter as protecting device is a common practice in many applications, mostly as common mode chokes in various low voltage systems comprising power electronics. In medium voltage systems however, the common-mode choke complicates significantly the design due to higher insulation system requirements between individual phases. Therefore in the present approach, single-phase chokes are proposed. The R-L choke of appropriately designed frequency characteristic allows one to significantly reduce the voltage wavefront rise time and, at the same time, minimize its influence on the equipment under normal operating conditions. This means that the choke impedance at power network 50/60Hz frequency must be close to zero. In some applications specific it might be advantageous when the series impedance element (choke) is complemented with a small surge capacitor connected phase-to-ground.

Fig.5. Idea of R-L choke placed prior to protected device

The use of appropriately designed series choke device can:

Limit the du/dt values at transformer terminals (Z_choke + optional C),
Limit transient overvoltage (filtering HF by Z_choke + optional C),
Eliminate wave reflections in cable and HF oscillations (when Z_choke = Zs).

Eliminate or reduce the number of re-ignitions (requires C in order to lower oscillation frequency.

ATP/EMTP simulation results – 1600 kVA, 22/0.69KV transformer simulation case study

In order to demonstrate the applicability of the series choke concept to mitigating high du/dt transients resulting from the VCB operation, ATP/EMTP simulations were performed for a realistic case of a distribution type 1600kVA transformer (22/0.69kV) transformer switching to a 22kV distribution network. The schematic diagram illustrating the case analyzed is shown in Fig. 6.

Fig.6. Case study 1600kVA, 22/0.69kV transformer

Transformer represented by ATP Hybrid model and surge capacitances was utilized with:

C_pg=2nF, C_ps=1.5nF, C_sg=3nF, C_pp=1nF.

The method of modeling a VCB behavior as a controllable switch, known from literature, was applied [10]. The connection between the transformer and the VCB was modeled as a short (5m) section of a transmission line of 50Ω surge impedance. Voltage waveforms at the transformer terminals for various schemes examined are shown in Fig. 8-Fig. 11.

Fig.7. Transformer surge capacitances model

a) Connecting unloaded transformer; no protection

Fig.8. Voltage waveforms at the transformer terminals

During contact making significant surges are observed at the transformer terminals. Overvoltages and pre-strikes are present within few milliseconds. Additionally, HF oscillations are present at frequency ~1.5MHz and high rate voltage (~200kV/μs) is observed.

b) Disconnecting unloaded transformer; no protection

Fig.9. Voltage waveforms at the transformer terminals

While contact breaking multiple re-ignitions are observed. HF overvoltages having peak values almost 60 kV are combined with low frequency, TRV oscillation. The corresponding du/dt reaches 250kV/μs. The HF oscillations (~1.5MHz) can also be seen.

c) Connecting unloaded transformer; protection with chokes only

Fig.10. Voltage waveforms at the transformer terminals

During the switching-on operation for the configuration with chokes implemented as a transformer protection there is observed number of pre-strikes reduction.

The high rate voltage is significally reduced (over 2x) and is <90kV/μs. High frequency oscillations for configuration with choke implemented are eliminated.

d) Disconnecting unloaded transformer; protection with chokes and 10 nF capacitors

Fig.11. Voltage waveforms at the transformer terminals

e) Connecting unloaded transformer; protection with chokes and 10 nF capacitors

Fig.12. Voltage waveforms at the transformer terminals

The optimal protection provides combination of the choke device with additional small capacitor. Especially for the case when the transformer is connected next to the circuit- breaker when connection between switchgear and transformer is relatively short and the cable surge impedance is small.

For this configuration single pre-strikes are present. Voltage rate reduction is very significant (more than 10x) and for this case is <20kV/μs. HF overvoltages and oscillations occurring for non protected transformer are eliminated.

f) Disconnecting unloaded transformer; protection with chokes and 10 nF capacitors

Fig.13. Voltage waveforms at the transformer terminals

In this case, when a small capacitor complements the protection with the choke the TRV build-up rate is reduced to a safe limit and nor re-ignitions are generated. Also, the amplitude of the low frequency overvoltage oscillation is significantly reduced.

VFTs suppression device concept practical implementation

Prototypes of chokes were experimentally tested as a protection of a small, dry-type transformer. Some of typical experimental results are shown in figure 14 and 15.

Fig.14. High frequency transients occurring in the power network during switching – on: a) without protection b) with choke and small capacitor protection

Fig.15. Voltage rate reduction of a single pre-strike

The experimental results confirmed the applicability of the series-choke protection concept to mitigating high du/dt transients resulting from the VCB switching operations.

In cases when the transformer internal capacitance is low, which is the case especially for dry-type transformers, additional small surge capacitor plays an important role in the transients suppression. It has to be pointed out, that the value of the capacitance used was more than an order of magnitude smaller, than typical the value of the typical snubber capacitor.

Figure 16 shows one of the first pilot installations of the series choke-based protection for a small transformer in the wind farm in Poland.

Fig. 16 VFTs suppression device pilot installation

Conclusions

The problem of potential VFT-related hazard to transformer and other power equipment resulting from switching operations was demonstrated on a practical example.

A new mitigation method against these hazards in a form of a series-connected choke element was shown. It was demonstrated that the use of the choke significally reduces voltage steepness and number of re-ignitions generated during transformer operated through the VCBs. Additionally there is observed noticeable overvoltage reduction. The number of pre-strikes during contact making was reduced and high frequency oscillations were practically eliminated. The practical case analysis using ATP/EMTP simulations demonstrated that in some cases (especially when a short connection between the transformer and the VCB exist), the voltage steepness as high as ~250kV/μs was simulated. This du/dt was over 2 times reduced with the use of the chokes only. Further reduction was achieved when a small (10nF) surge capacitors were used. In this case the du/dt was reduced below 20kV/μs. Additionally, the small surge capacitor significantly reduces LF overvoltages (45kV) and helps to eliminate the re-ignitions.

Prototypes of chokes were experimentally tested and confirmed the applicability of the series-choke protection concept to mitigating high du/dt transients resulting from the VCB switching operations.

LITERATURE

[1] CIGRE working group A2-A3-B3.21, Electrical Environment of Transformers; Impact of fast transients”, ELECTRA 208, (2005)
[2] Lopez–Roldan J., De Herdt H., Min J., Van Velthove R., Decklerq J., Sels T., Karas J., Van Dommelen D., Popow P., Van der Sluis L., Aquado M., Study of interaction between distribution transformer and vacuum circuit breaker, Proceedings of 13th ISH (2003), pp. 62÷64
[3] Morched A. S., Marti L., Brierly R. H., Lackey J. G., Analysis of Internal Winding Stresses in EHV Generator Set-Up Transformer Failures, IEEE Trans. on Power Delivery, Vol. 11, No. 2, (1996), pp. 888÷894
[4] Popov M., Acha E., Overvoltages due to switching off an unloaded transformer with a vacuum circuit breaker, IEEE Trans. on Power Delivery, Vol. 14, No. 4, (1999), pp. 1317÷1322
[5] Burrage L. M., Shaw J. H., McConnell B. W., Distribution transformer performance when subjected to steep front impulses, IEEE Trans. on Power Delivery, Vol. 5, No. 2, (1990)
[6] Piasecki W., Bywalec G., Florkowski M., Fulczyk M., Furgal J., New approach towards Very Fast Transients suppression, Proceedings of IPST’2007
[7] Paul D., Failure Analysis of Dry-Type Power Transformer, IEEE Transaction on Industry Applications, Vol. 37, No. 3, (2001)
[8] Wong S. M., Snider L. A., Lo E. W. C., Overvoltages and reignition behavior of vacuum circuit breaker, Proceedings of IPST’2003

Autorzy:
Dariusz Smugała, Ph.D. Eng., E-mail: dariusz.smugala@pl.abb.com
Wojciech Piasecki, Ph.D.Eng., E-mail: wojciech.piasecki@pl.abb.com
Magdalena Ostrogórska, Ms.C.Eng. E-mail: magdalena.ostrogorska@pl.abb.com
Marek Florkowski, Ph.D.Eng., E-mail: marek.florkowski@pl.abb.com
Marek Fulczyk, Ph.D.Eng., E-mail: marek.fulczyk@pl.abb.com
ABB Corporate Research Center, Starowiślna 13 A Str., 31-038 Cracow, Poland, Paweł Kłys, Ms.C.Eng., ABB Transformers, Aleksandrowska 67/93 Str., 91-205 Lodz, Poland, E-mail: pawel.klys@pl.abb.com

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY (Electrical Review), ISSN 0033-2097, R. 88 NR 5a/2012

MISO Planning Overview

Published by Clair Moeller, Executive VP, Mid-Continent ISO, Date: February 16, 2017.

Presented by WIRES – a national coalition of entities dedicated to investment in a strong, well-planned and environmentally beneficial electricity high voltage transmission system in the US.