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6-9 April 2008 Nanjing China
1
Zero-Sequence Harmonics Current
Minimization Using Zero-Blocking Reactor and
Zig-Zag Transformer
Qipeng Song, Zhongdong Yin, Jinhui Xue and Lixia Zhou
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 (ZSBR) and Zig-Zag transformer. Zig-Zag
transformer is a special connection of three-phase transformer’s
windings. The ZSBR is also a special connected transformer,
whose three windings are wounded in the same core. The ZSBR
has zero reactance for positive and negative-sequence
components but giving three times of self-reactance for zerosequence reactance. The ZSBR placed in series with the source
provides high zero-sequence impedance while the Zig-Zag
transformer placed parallel with the load provides low zerosequence impedance. Thus, the zero-sequence harmonics
currents tend to flow through the Zig-Zag 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 ZigZag transformer and ZSBR under ideal and non-ideal power
conditions. The simulation and laboratory test results indicate
that the combination of ZSBR and Zig-Zag 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; ZigZag transformer
w
common power system problems include [5][6][8][9]:
1) 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. Zero-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 [11][13]. 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.1.
IS
Ih
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]102 )
Qipeng Song, Zhongdong Yin, Jinhui Xue and Lixia Zhou are with Key
Laboratory of Power System Protection and Dynamic Security Monitoring
and Control under Ministry of Education (North China Electric Power
University), Beijing, 102206 (e-mail: laoguai2002@sohu.com)
978-7-900714-13-8/08/ ©2008 DRPT
I NS
IF
I NL
Fig.1. 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;
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6-9 April 2008 Nanjing China
-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[1][2][12]. 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.
2
Fig.2. Zig-Zag transformer. (a) Circuit connection and (b) phase diagram
Utility Voltage
iLa
i Lb
III. BASIC THEORY
Load
iLc
Zig-Zag 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 ( ia 0 , ib 0 and ic 0 ) have
the same amplitude and the same phase, and they can be
represented as
(1)
ia 0 (t ) = ib 0 (t ) = ic 0 (t )
ZS
iza
Zz
i zb
i zc
Zig − Zag
Transformer
Z sn
isn
i zn
iLn
Z Zn
Fig.3.The system configuration of three-phase four wire distribution power
system with the Zig-Zag transformer
The neutral current in (t ) is the sum of three-phase zerosequence currents, and it is represented as
(2)
in (t ) = 3ia 0 (t )
Because the turn ratio of the transformer’s windings is 1:1
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
(3)
iza (t ) = izb (t )
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 ZigZag 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.4. The zero-sequence equivalent circuit
IV. ANALYSIS OF ZIG-ZAG TRANSFORMER IN THETHREEPHASE FOUR-WIRE SYSTEM
Fig.3.shows the system configuration of the Zig-Zag
transformer applied in the three-phase four-wire distribution
power systems. In Fig. 3, Z Ln is the impedance of neutral
conductor between the load and the Zig-Zag transformer, Z Sn
is the impedance of neutral conductor between the utility and
the Zig-Zag transformer and ZS is the impedance between the
utility and the Zig-Zag transformer. The current flowing
through the Zig-Zag transformer is only the zero-sequence
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6-9 April 2008 Nanjing China
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 vs0(t) is
a zero sequence voltage source caused by the unbalanced
utility voltages. Assuming the thee-phase voltages ( van (t ) ,
vbn (t ) , vcn (t ) )are unbalanced, the zero-sequence voltage
can be expressed as[7]:
1
vs 0 (t ) = (van (t ) + vbn (t ) + vcn (t ))
3
(6)
iL0 (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
1
iL 0 (t ) = (vLa (t ) + vLb (t ) + vLc (t ))
3
(7)
In Fig.4, Z zn is zero-sequence impedance of the Zig-Zag
transformer. The effects of vs0 (t) and iL0 (t) to the neutral
current of the utility side after using the Zig-Zag transformer
can be analyzed by using the superposition theory. For
considering the effect of iL0 (t) , vs0 (t) should be assumed to be
a short circuit in Fig. 4. Then, the utility side neutral current
i 'sn (t ) caused by iL0 (t) can be expressed as
i 'sn (t ) =
Z zn
iL 0 (t )
(Z Sn + Z S ) + Z zn
(8)
Equation (8) indicates that the magnitude of the utility side
neutral current caused by iL 0 (t ) will be reduced after applying
the Zig-Zag transformer. If is Z zn educed or is Z S increased,
i 'sn (t ) in the utility side can be further attenuated.
For considering the effect of vs 0 (t ) and iL 0 (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 ) =
1
vs 0 (t )
( Z Sn + Z S ) + Z zn
(9)
Equation (9) shows that the Zig-Zag transformer supplies a
path for the zero-sequence current flowing between the utility
and the Zig-Zag transformer. However, the impedance of the
utility system, the Zig-Zag 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 Zig-Zag
transformer even if under a very small unbalanced utility
voltages. This significant neutral current may result in the
burn-down of the Zig-Zag transformer, the neutral conductor
and the distribution power transformer. It violates original
intention of the filter.
Fig.5. The winding arrangement of ZSBR
So this paper presents a novel method to avoid this problem,
it is to insert a zero-sequence blocking rector (ZSBR) between
the utility and the Zig-Zag transformer. The impedance of the
ZSBR is Z z , shown in Fig. 3. The winding arrangement of
ZSBR is shown in Fig.5.The ZSBR is also a special connected
transformer, whose three windings are wounded in the same
core [10]. 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
( Z Sn + Z S + Z zn ) + Z z and Z z >> ( Z Sn + Z S + Z zn ) , greatly
reducing the zero-sequence current i ''sn (t ) caused by
unbalanced utility voltages vs 0 (t ) . Thus, the zero-sequence
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 Zig-Zag transformer and ZSBR 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.
Table1
MAJOR PARAMETERS USED IN THE SIMULATION
Utility voltage
220V 50HZ
Zsn
0.05Ω 0.1mH
ZLn
0.01Ω 0.1mH
Zs
0.01Ω 0.1mH
Zzn
0.002Ω 0.01mH
Zz
R
C
0.002Ω 10mH
1000Ω
8000μf
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A. Zig-zag Transformer’s Performance under balanced
Utility Voltages
The simulation results of the Zig-Zag 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 0.617A to 0.01A after applying the Zig-Zag
transformer. The neutral current on the utility side is very
small and only 1.18% of that on the load side. This result
shows that the Zig-Zag 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 187.14% because the 3rd harmonic current is
attenuated by the Zig-Zag transformer.
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B. Zig-zag Transformer’s Performance without ZSBR under
Unbalanced Utility Voltages
Fig.8. Simulate results of phase A under the unbalanced nonlinear loads
Fig.9. Current harmonic spectrums
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, 10.).
(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, 11.).
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 Zig-Zag transformer. This coincides
with the above analysis that the use of Zig-Zag 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 Zig-Zag
transformer. At the same time, the current flowing through the
Zig-Zag transformer is also as high as 42 A. These results are
very consistent to (9).
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C. Zig-zag Transformer’s Performance with ZSBR 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 Zig-Zag transformer to become
larger than that before applying the Zig-Zag transformer, is
depended on the impedances of Z S , Z zn and Z Sn . In some
5
the load side; the THD of the utility current is reduced
from276.7% to 187.9%. That is to say, the zero-sequence
current is mostly flowing between the zig-zag transformer and
the load.
VI. LABORATORY TESTS
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 ZSBR between the utility and the zig-zag
transformer, it has only zero-sequence impedance Z z (shown
in Fig.4 and Table1). It takes off the impact of unbalance
voltage and intensify the effect of the Zig-Zag transformer for
attenuating the neutral current.
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.10. Simulate results of phase A under the unbalanced nonlinear loads
Fig.12.Experiment equipments installation and wiring diagram
Fig.13. Phase current waveforms of the utility side before and after the zig-zag
transformer and ZSBR were installed
Fig.11.Current harmonic spectrums
From Fig.10.11, We can see clearly that the performance of
combination of zig-zag transformer and ZSBR 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 1.2% of that on
Fig.14. Phase current harmonic spectrums of the utility side before and after
the zig-zag transformer and ZSBR were installed.
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6-9 April 2008 Nanjing China
[3]
[4]
6
filter for the neutral conductor,” in Proc. IEEE IAS, vol. 1, 2002, pp. 64–
69.
P. P. Khera, “Application of Zig-Zag transformers for reducing
harmonics in the neutral conductor of low voltage distribution system,”
in Proc. IEEE IAS, vol. 2, 1990, pp. 1092–1096.
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 1, April 2005 Page(s):1168 1173.
Books:
Fig.15. Neutral current waveforms of the utility side before and after the zigzag transformer and ZSBR were installed.
[5]
[6]
[7]
Wang Zhao’ an, Yang Jun, Liu Jinjun. Harmonic suppression and var
compensation [M]. Beijing: China Machine PRESS.1998
Xiao Xiangneng. Analysis and Control of Power Quality. China power
press.2004.2
Li Guangqi. Transient stability analysis of power system. XI’AN
JIAOTONG UNIVERSITY. Water and electricity power Press.1984
Papers from Conference Proceedings (Published):
[8]
Fig.16. Neutral current harmonic spectrums of the utility side before and after
the zig-zag transformer and ZSBR were installed.
[9]
Fig.15, 16 shows the zero-sequence neutral current is
remarkably decreased after the zig-zag transformer and ZSBR
are installed. The results of the experiments are very
consistent to the theoretical analysis and simulation.
[10]
VII. CONCLUSIONS
[11]
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 Zig-Zag transformer and ZSBR 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:
(1) The Zig-Zag 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 Zig-Zag
transformer;
(3) The insertion of ZSBR 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 Zig-Zag transformer under the unbalanced
utility voltages and the distorted utility voltages with zerosequence harmonic components;
(4) The insertion of ZSBR in the utility side can increase
the attenuated rate of the utility side neutral current.
VIII. REFERENCES
Periodicals:
[1]
[2]
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. 1565–1572, 1994.
K. Wada and T. Shimizu, “Mitigation method of 3rd-harmonic voltage
for a three-phase four-wire distribution system based on a series active
[12]
[13]
Halasz, S.; Csonka, G.; Hassan, A.A.M., Sinusoidal PWM techniques
with additional zero-sequence harmonics, Industrial Electronics, Control
and Instrumentation, 1994. IECON '94., 20th International Conference
on, Volume 1, 5-9 Sept. 1994 Page(s):85 - 90 vol.1
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. 1997, pp. 683–686.
Syafrudin, M.; Hadzer, C.M.; Sutanto, J., Zero-sequence harmonics
current minimization using zero-blocking transformer and shunt LC
passive filters, Power System Technology, 2002. Proceedings.
PowerCon 2002. International Conference on, Volume 1, 13-17 Oct.
2002 Page(s):116 - 120 vol.1.
I.Volkov Prof,Dr, “Universal Harmonic Filter LINEATOR new
approach to harmonic mitigation” 10th International Conference on
Harmonics and Quality of Power. Proceedings (Cat. No.02EX630),
2002, pt. 2, p 743-7 vol.2.
C. A. Quinn, N. Mohan, and H. Mehta, “A four-wire, currentControlled converter provides harmonic neutralization in three-phase,
four-wire systems,” in Proc. IEEE APEC, 1993, pp. 841–846.
Mahamad, N.; Hadzer, C.M.; Masri, S., Application of LC filter in
harmonics reduction, Power and Energy Conference, 2004. PECon
2004. Proceedings. National 29-30 Nov. 2004 Page(s):268 - 271
IX. BIOGRAPHIES
Qipeng Song was born in Ruzhou, china, in 1981.
From 2000 to 2004, he studied in Zhengzhou
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.
Zhongdong 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 1991, 1993 and 1997,
respectively.
From 1997 to 1999, 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 1999 to 2002 and a senior researcher in Advanced
Technology R&D Center, Mitsubishi Electric Corporation, Japan, from 2002
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6-9 April 2008 Nanjing China
to 2003. 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 Zhangjiakou, china, in 1982.
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 Zhou was born in Xingtai, China, in 1982. She
received the M.S. degree in North China Electric
Power University, Beijing, China in 2006. 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.
7