Zero-Sequence Harmonics Current Minimization Using Zero-Blocking Reactor and Zig-Zag Transformer

Transcription

1 ero-sequence Harmonics Current Minimization Using ero-blocking Reactor and ig-ag Transformer Qipeng Song, hongdong Yin, Jinhui Xue and Lixia hou Abstract--In the distribution power system, the third harmonics of zero-sequence caused by nonlinear loads usually result in high-voltage distortion levels throughout the facility, neutral conductor overloading, motor heating, transformer heating, increased losses, and excessive harmonic injection onto the utility supply system. This paper presents a novel method for minimizing the zero-sequence harmonics by using zero-sequence blocking reactor (SBR) and ig-ag transformer. ig-ag transformer is a special connection of three-phase transformer s windings. The SBR is also a special connected transformer, whose three windings are wounded in the same core. The SBR has zero reactance for positive and negative-sequence components but giving three times of self-reactance for zerosequence reactance. The SBR placed in series with the source provides high zero-sequence impedance while the ig-ag transformer placed parallel with the load provides low zerosequence impedance. Thus, the zero-sequence harmonics currents tend to flow through the ig-ag transformer instead of the source, and the purposes of eliminating harmonic is gained. In this paper, an analysis is carried out; simulations and laboratory tests are used to evaluate the performance of the ig- ag transformer and SBR under ideal and non-ideal power conditions. The simulation and laboratory test results indicate that the combination of SBR and ig-ag transformer as filter is a better and effective way to attenuate the neutral current, which also provides an innovational way to improve power quality. Index Terms-- harmonic; neutral current; nonlinear loads; power quality; simulation; zero-sequence blocking reactor; ig- ag transformer w I. INTRODUCTION IDE use of non-linear loads such as personal computers, monitors, laser printers, variable speed drives, UPS systems and other electronic equipment have led to harmonics being a major issue in the electrical industry today. Commercial and industrial power distribution systems designed for the old, linear-style loads are simply no longer suitable for servicing these non-linear, harmonic generating loads - especially when found in high densities. Some This work was supported by electric railway project of North China Power Grid (SGKJ[2007]02 ) Qipeng Song, hongdong Yin, Jinhui Xue and Lixia hou are with Key Laboratory of Power System Protection and Dynamic Security Monitoring and Control under Ministry of Education (North China Electric Power University), Beijing, ( laoguai2002@sohu.com) common power system problems include [5][6][8][9]: ) Overloaded neutral conductors 2) Overheated distribution transformers 3) High neutral-to-ground voltage (Vn-g) 4) Poor power factor 5) Distortion of the voltage waveform supplying these loads. Third harmonic is most serious for nonlinear loads. The current of integer multiples 3rd are regarded as zero-sequence current. ero-sequence current flowing in the neutral conductor of the three-phase four-wire distribution power system is three times of the zero-sequence components of each phase current. It causes the main power quality problems, so it is very necessary to find ways to minimize the zero-sequence harmonic current. II. METHODS TO ATTENUATE THE NEUTRAL HARMONIC CURRENT There are two basic ways to attenuate the neutral current: passive filter and active filter. Passive filter as a small investment, high efficiency, simple structure, maintaining the advantages of convenience, is widely used in the main means of harmonic suppression [][3]. The traditional passive filter remove the neutral harmonic current by provided for a parallel low harmonic impedance pathway (series capacitor resonant inductor) in system, shown in Fig.. I S I NS Fig.. Passive filter remove the neutral harmonic current Its characteristic is decided by the ratio of filter impedance and system impedance. So it has the following drawbacks: - Be prone to be influenced by system parameters; -Can only remove several specific harmonics, and may amplify some of the harmonic; -When harmonic current increases, making the filter easy overload; I F I NL I h /08/ 2008 DRPT

2 2 -Capacitor parameters changes along with the dielectric of aging, and the filtering effect significantly decreased. Due to the above-mentioned shortcomings of passive filter, with the continuous development of power electronics technology, the application of APF has catch people s attention. It uses controllable power semiconductor devices to inject current to the network, the current is equal in amplitude and contrast in phase with original harmonic current, leading the total harmonic current to zero, realize the purposes of realtime compensation harmonic current[][2][2]. Its performance advantages: -with adaptive function; -can also achieve the harmonic and reactive power compensation; -Because it can track power grid s frequency changes, so it is not affected by the network impedance, it is not easy to resonate with power networks impedance; Although Active Filter s performance of compensation is better than passive filter, its circuit topology, control complexity, high cost, low reliability, limiting its application. With the objective to reduce or to eliminate the zero sequence currents circulation this paper uses an electromagnetic configuration with a three-phase three-leg core in which the windings are connected in zigzag. Under normal conditions of operation the zig-zag transformer requires only a small power loss in the windings and core. III. BASIC THEORY ig-ag transformer is a special connection of three singlephase transformer s windings or a three-phase transformer s windings [3][4]. The circuit connection is as shown in Fig. 2(a). In the three-phase four-wire distribution power system, the three-phase zero-sequence currents ( i a0, ib0 and i c0 ) have the same amplitude and the same phase, and they can be represented as ia0() t = ib0() t = ic0() t () The neutral current in () t is the sum of three-phase zerosequence currents, and it is represented as in() t = 3 ia0 () t (2) Because the turn ratio of the transformer s windings is : in Fig. 2, the input current flowing into the dot point of the primary winding is equal to the output current flowing out from the dot point of the secondary winding. So, we have i () t = i () t (3) za zb izb() t = izc () t (4) izc () t = iza () t (5) Equations (3) (5) indicate that three-phase currents flowing into three transformers must be equal. This means that the ig- ag transformer can supply the path for the zero-sequence current. Fig. 2(b) shows the phase diagram of Fig. 2(a). From Fig. 2(b), it can be found that the voltage across the transformer s winding is of the phase voltage of the threephase four-wire distribution power system. Fig.2. ig-ag transformer. (a) Circuit connection and (b) phase diagram Utility sn Voltage S z i za izb ig ag Transformer Fig.3.The system configuration of three-phase four wire distribution power system with the ig-ag transformer Fig.4. The zero-sequence equivalent circuit isn izn izc i Ln i La i Lb i Lc n Load IV. ANALYSIS OF IG-AG TRANSFORMER IN THETHREE- PHASE FOUR-WIRE SYSTEM Fig.3.shows the system configuration of the ig-ag transformer applied in the three-phase four-wire distribution power systems. In Fig. 3, Ln is the impedance of neutral conductor between the load and the ig-ag transformer, Sn is the impedance of neutral conductor between the utility and the ig-ag transformer and is the impedance between the S utility and the ig-ag transformer. The current flowing through the ig-ag transformer is only the zero-sequence

3 3 component, and the zero-sequence equivalent circuit of Fig. 3 is shown in Fig. 4. This consists of two zero-sequence sources, vs0() t and il0( t ). In the practical three-phase four-wire industry distribution power system, the unbalanced utility voltages may occur frequently due to the unequal load distribution of the upstream in each phase or the abnormal phase change even when the loads are balanced. The v () s0 t is a zero sequence voltage source caused by the unbalanced utility voltages. Assuming the thee-phase voltages ( van () t, v () bn t, vcn() t )are unbalanced, the zero-sequence voltage can be expressed as[7]: vs0( t) = ( van( t) + vbn( t) + vcn( t)) (6) 3 i () L0 t is the zero-sequence current source, and it contains the unbalanced fundamental load currents and zero-sequence of harmonic load currents, and it can be derived as il0() t = ( vla() t + vlb() t + vlc()) t (7) 3 In Fig.4, zn is zero-sequence impedance of the ig-ag transformer. The effects of vs 0 () t and il 0() t to the neutral current of the utility side after using the ig-ag transformer can be analyzed by using the superposition theory. For i t, v () 0 t should be assumed to be considering the effect of () L0 s a short circuit in Fig. 4. Then, the utility side neutral current i' sn( t) caused by i () L0 t can be expressed as zn i' sn( t) = il0( t) (8) ( Sn + S ) + zn Equation (8) indicates that the magnitude of the utility side neutral current caused byil 0 () t will be reduced after applying the ig-ag transformer. If is zn educed or is increased, S i' sn( t) in the utility side can be further attenuated. For considering the effect of vs 0 () t and i () L0 t should be assumed to be an open circuit in Fig. 4. The neutral current of the utility side caused by can be expressed as i'' sn () t = vs0() t (9) ( + ) + Sn S zn Equation (9) shows that the ig-ag transformer supplies a path for the zero-sequence current flowing between the utility and the ig-ag transformer. However, the impedance of the utility system, the ig-ag transformer and the neutral conductor are very small in most of the three-phase four-wire distribution power systems. This implies that a significant neutral current will be generated after applying the ig-ag transformer even if under a very small unbalanced utility voltages. This significant neutral current may result in the burn-down of the ig-ag transformer, the neutral conductor and the distribution power transformer. It violates original intention of the filter. Fig.5. The winding arrangement of SBR So this paper presents a novel method to avoid this problem, it is to insert a zero-sequence blocking rector (SBR) between the utility and the ig-ag transformer. The impedance of the SBR is z, shown in Fig. 3. The winding arrangement of SBR is shown in Fig.5.The SBR is also a special connected transformer, whose three windings are wounded in the same core [0]. Thus, the coupling coefficient can be assumed as unity and mutual reactance is equal to self reactance. It has zero reactance for positive and negative-sequence components but giving three times of self-reactance for zero-sequence component. In (9), the denominator can be changed to ( Sn + S + zn ) + z and z >> ( + + ), greatly Sn S zn reducing the zero-sequence current i'' sn( t) caused by unbalanced utility voltages v 0 () t. Thus, the zero-sequence s current can only flow between the load and the zig-zag transformer, but not the utility, perfectly realizing the function of zig-zag transformer to attenuate the zero-sequence current. V. SIMULATION AND ANALYSIS UNDER EMTDC/PSCAD Simulations based on EMTDC/PSCAD under different utility and load conditions are made to verify the performance of the ig-ag transformer and SBR in the application for attenuating the neutral current of the three-phase four-wire distribution power system. The parameters used in the computer simulation are shown in Table I. The load in the following computer simulation is single phase rectifier with a load of capacitor (C) and resistor (R) connected in parallel. In general, the input power stage of computer related equipment could be regarded as this kind of load. The current of singlephase rectifier contains rich harmonics, such as 3th, 5th, 7th, etc. orders. Because only the steady state is considered in this paper, the start time of computer simulation is 500 ms in the following simulation. Table MAJOR PARAMETERS USED IN THE SIMULATION Utility voltage sn Ln s zn z R C 220V 50H 0.05Ω 0.mH 0.0Ω 0.mH 0.0Ω 0.mH 0.002Ω 0.0mH 0.002Ω 0mH 000Ω 8000μf

4 4 A. ig-zag Transformer s Performance under balanced Utility Voltages The simulation results of the ig-ag transformer s performance under the balanced nonlinear loads is shown in Fig. 6. The current harmonic spectrums are shown in Fig.7. The loads are three same single-phase rectifier loads and the dominant harmonic current of single-phase rectifier is the zero-sequence current. In Fig. 7, the 3th harmonic current is decreased from 0.67A to 0.0A after applying the ig-ag transformer. The neutral current on the utility side is very small and only.8% of that on the load side. This result shows that the ig-ag transformer has the expected performance for attenuating the neutral current effectively under balanced utility conditions. Moreover, the THD (total harmonic distortion) of the utility current is reduced from 276.7% to 87.4% because the 3rd harmonic current is attenuated by the ig-ag transformer. B. ig-zag Transformer s Performance without SBR under Unbalanced Utility Voltages Fig.8. Simulate results of phase A under the unbalanced nonlinear loads Fig.6. Simulate results of phase A under the balanced nonlinear loads. (a) Load current (b) utility current (c) zig-zag transformer current (d) utility side neutral current (e) load side neutral current (same to the following fig.8, 0.). (Ordinate: I/A; Abscissa Ordinate: T/S) Fig.7. Current harmonic spectrums (a) Load current spectrums (b) Utility current spectrums (c) zig-zag transformer current spectrums (same to the following fig.9,.). Fig.9. Current harmonic spectrums In the practical three-phase four-wire industry distribution power system, the unbalanced utility voltages caused by the unequal load distribution in each phase or the abnormal phase change may occur frequently. Since the unbalanced threephase voltages contain a zero-sequence voltage, 2 volts zerosequence voltage was added to utility in the simulation. From the simulation results, we can see even if so small a zerosequence voltage, it can generate a significant fundamental component flows between the utility, the neutral conductor on the utility side and the ig-ag transformer. This coincides with the above analysis that the use of ig-ag transformer in an unbalanced three-phase four-wire distribution power system will induce a significant unexpected neutral current. As shown in Fig.8. (d)(e), the neutral current on the utility side is as high as 40 A, and that is 6 A on the load side. The neutral current of the utility side becomes larger, and that is more than six times of that on the load side. The above results show that the neutral current and phase current of three-phase four-wire distribution power system under the unbalanced utility voltages becomes larger after applying the ig-ag transformer. At the same time, the current flowing through the ig-ag transformer is also as high as 42 A. These results are very consistent to (9).

5 5 C. ig-zag Transformer s Performance with SBR under Unbalanced Utility Voltages From the analyses of above, we can known that the unbalanced utility voltages, which may cause the neutral current after applying the ig-ag transformer to become larger than that before applying the ig-ag transformer, is depended on the impedances of S, zn and Sn. In some papers, an inductor is suggested to be inserted into the neutral conductor on the utility side to reduce the neutral current on the utility side. However, this may cause following problems: -the creation of an impedance grounded 4-wire system prohibited by then National Electrical Code (NEC). -over- and under-voltages created by neutral reference shift when loads are unbalanced. So, here, we insert a SBR between the utility and the zig-zag transformer, it has only zero-sequence impedance z (shown in Fig.4 and Table). It takes off the impact of unbalance voltage and intensify the effect of the ig-ag transformer for attenuating the neutral current. the load side; the THD of the utility current is reduced from276.7% to 87.9%. That is to say, the zero-sequence current is mostly flowing between the zig-zag transformer and the load. VI. LABORATORY TESTS A series laboratory tests had carried out to verify the accuracy of the theoretical analysis and simulations. Topas 2000 Power Quality Analyzer was used to measure voltage and current waveforms. From it we can more clearly see the effect of the zigzag transformer in suppressing zero-sequence harmonic current. Three single-phase rectifiers were used in the experiment and the loads were light boxes and capacitors connected in parallel. The following is the wiring diagram and results of the experiment. Fig.0. Simulate results of phase A under the unbalanced nonlinear loads Fig.2.Experiment equipments installation and wiring diagram Fig.3. Phase current waveforms of the utility side before and after the zig-zag transformer and SBR were installed Fig..Current harmonic spectrums From Fig.0., We can see clearly that the performance of combination of zig-zag transformer and SBR under unbalanced voltage is as well as that of zig-zag transformer under balanced voltage. Under the same unbalanced voltage as B, the neutral current on the utility side is only.2% of that on Fig.4. Phase current harmonic spectrums of the utility side before and after the zig-zag transformer and SBR were installed.

6 6 filter for the neutral conductor, in Proc. IEEE IAS, vol., 2002, pp [3] P. P. Khera, Application of ig-ag transformers for reducing harmonics in the neutral conductor of low voltage distribution system, in Proc. IEEE IAS, vol. 2, 990, pp [4] Hurng-Liahng Jou; Jinn-Chang Wu; Kuen-Der Wu; Wen-Jung Chiang; Yi-Hsun Chen; Analysis of zig-zag transformer applying in the threephase four-wire distribution power system, Power Delivery, IEEE Transactions on, Volume 20, Issue 2, Part, April 2005 Page(s): Fig.5. Neutral current waveforms of the utility side before and after the zigzag transformer and SBR were installed. Fig.6. Neutral current harmonic spectrums of the utility side before and after the zig-zag transformer and SBR were installed. Fig.5, 6 shows the zero-sequence neutral current is remarkably decreased after the zig-zag transformer and SBR are installed. The results of the experiments are very consistent to the theoretical analysis and simulation. VII. CONCLUSIONS In today s three-phase four-wire distribution power systems, the over-load of the neutral conductor is a more and more serious problem. Although this problem can be solved effectively by using the three-phase four-wire active power filter, the use of three-phase four-wire active power filter is limited due to its high cost and control complexity. The combination of ig-ag transformer and SBR provide a popular solution for this problem due to its low cost, easy installation and free maintenance. The analysis, simulation and laboratory tests results in this paper show that: () The ig-ag transformer can effectively attenuate the neutral current and zero-sequence harmonic currents on the utility side under the balanced utility voltages; (2) The utility side neutral current becomes larger under the unbalanced utility voltages after applying the ig-ag transformer; (3) The insertion of SBR in the utility side can improve the undesired increasing of the neutral current and the zero-sequence harmonic currents of the utility side after applying the ig-ag transformer under the unbalanced utility voltages and the distorted utility voltages with zerosequence harmonic components; (4) The insertion of SBR in the utility side can increase the attenuated rate of the utility side neutral current. VIII. REFERENCES Periodicals: [] P. N. Enjeti, W. Shireen, P. Packebush, and I. J. Pitel, Analysis and design of a new active power filter to cancel neutral current harmonics in three-phase four-wire electric distribution systems, IEEE Trans. Ind. Applicat., vol. 30, pp , 994. [2] K. Wada and T. Shimizu, Mitigation method of 3rd-harmonic voltage for a three-phase four-wire distribution system based on a series active Books: [5] Wang hao an, Yang Jun, Liu Jinjun. Harmonic suppression and var compensation [M]. Beijing: China Machine PRESS.998 [6] Xiao Xiangneng. Analysis and Control of Power Quality. China power press [7] Li Guangqi. Transient stability analysis of power system. XI AN JIAOTONG UNIVERSITY. Water and electricity power Press.984 Papers from Conference Proceedings (Published): [8] Halasz, S.; Csonka, G.; Hassan, A.A.M., Sinusoidal PWM techniques with additional zero-sequence harmonics, Industrial Electronics, Control and Instrumentation, 994. IECON '94., 20th International Conference on, Volume, 5-9 Sept. 994 Page(s):85-90 vol. [9] P. A. Dahono, R. E. Widjaya, Syafrudin, and Qamaruzzaman, A practical approach to minimize the zero-sequence current harmonics in power distribution systems, in IEEE Proc. Power Conversion Conf., vol. 2, Aug. 997, pp [0] Syafrudin, M.; Hadzer, C.M.; Sutanto, J., ero-sequence harmonics current minimization using zero-blocking transformer and shunt LC passive filters, Power System Technology, Proceedings. PowerCon International Conference on, Volume, 3-7 Oct Page(s):6-20 vol.. [] I.Volkov Prof,Dr, Universal Harmonic Filter LINEATOR new approach to harmonic mitigation 0th International Conference on Harmonics and Quality of Power. Proceedings (Cat. No.02EX630), 2002, pt. 2, p vol.2. [2] C. A. Quinn, N. Mohan, and H. Mehta, A four-wire, current- Controlled converter provides harmonic neutralization in three-phase, four-wire systems, in Proc. IEEE APEC, 993, pp [3] Mahamad, N.; Hadzer, C.M.; Masri, S., Application of LC filter in harmonics reduction, Power and Energy Conference, PECon Proceedings. National Nov Page(s): IX. BIOGRAPHIES Qipeng Song was born in Ruzhou, china, in 98. From 2000 to 2004, he studied in hengzhou University, Henan province, and received the B.S. Then he worked in High-voltage Apparatus Research Institute in Pingdingshan as an assistant engineer for two years. Since 2006, he has been in North China Electric Power University, where he is a post-graduate student in School of Electrical Engineering, mainly engaging in research of power electronics, power quality and their control systems. hongdong Yin was born in Wuhan, China, in 968. He received the B.S., M.S.,and Ph.D. degrees in Wuhan University of Hydraulic and Electric Engineering, Wuhan, China, in 99, 993 and 997, respectively. From 997 to 999, he was with Department of Electrical Engineering and Applied Electronic Technology, Tsinghua University, China, as a postdoctoral research associate. He was with Industry Electronics and System Laboratory, Mitsubishi Electric Corporation, Japan, as a senior researcher from 999 to 2002 and a senior researcher in Advanced Technology R&D Center, Mitsubishi Electric Corporation, Japan, from 2002

7 7 to Since 2004, he has been with North China Electric Power University, where he is currently a vice professor in School of Electrical Engineering. His research interests include power electronics, FACTS, distributed generation, power quality and their control systems. Jinhui Xue was born in hangjiakou, china, in 982. From 2002 to 2006, he studied in Yanshan University, Hebei province, and received the B.S. Since 2006, he has been in North China Electric Power University, where he is a post-graduate student in School of Electrical Engineering, mainly engaging in research of power electronics, power quality and their control systems. His research intertest is connection-grid of renewable energy. Lixia hou was born in Xingtai, China, in 982. She received the M.S. degree in North China Electric Power University, Beijing, China in Since 2006, she has been with North China Electric Power University, where she is currently a Ph.D candidate in School of Electrical and Electronic Engineering. Her research interests include reactive power theory, power quality and their control systems.