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1438 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 3, JULY 2006
New Configuration of UPQC forMedium-Voltage Application
B. Han , Senior Member, IEEE , B. Bae , Student Member, IEEE , S. Baek, and G. Jang , Member, IEEE
Abstract—This paper proposes a novel configuration of a uni-fied power quality conditioner (UPQC) which can be connected tothe distribution system without series injection transformers. Theoperation of the proposed system was analyzed through simula-tions with PSCAD/EMTDC and experimental works with a scaledhardware model, assuming that the UPQC is connected with the22.9-kV distribution line. The proposed UPQC has the ultimate ca-pability of improving power quality at the point of installation onpower distribution systems. It has flexibility in expanding the op-eration voltage by increasing the number of H-bridge modules.
Index Terms—dynamic voltage restorer (DVR), H-bridge,
PSCAD/EMTDC, static synchronous series compensator (SSSC),Static var Compensator (STATCOM), unified power-flow con-troller (UPFC), unified power-quality conditioner (UPQC).
I. INTRODUCTION
AS MORE sensitive loads have come into wide use, power
qualityis a big issue of customers and utilities. The unified
power-quality conditioner (UPQC) has been widely studied by
many researchers as an ultimate device to improve power quality
[1]. The UPQC has two converters that share one dc-link capac-
itor.
The presently developed UPQC operates in much lower
dc-link voltage than the operation voltage of the distributionsystem. The restriction in dc-link voltage is due to the max-
imum sustained voltage of the switching element.
Series connection of the switching element was developed to
increase the dc-link voltage. However, the maximum allowable
number of switching elements is limited. Step-down trans-
formers are normally used to match the converter operation
voltage with the transmission voltage.
A multilevel converter was proposed to increase the converter
operation voltage, avoiding the series connection of switching
elements. However, the multilevel converter is complex to form
the output voltage and requires too many back-connection
diodes or flying capacitors [2].
A multibridge converter composed of several H-bridge mod-
ules for each phase, was proposed to increase the converter op-
eration voltage. The application of a multibridge converter for
STATCOM was first proposed in [3] and [4]. And the appli-
cation of a multibridge converter for static synchronous series
compensator (SSSC) and unified power-flow controller (UPFC)
Manuscript received January 26, 2005; revised April 15, 2005. This work was supported by the ERC program of MOST/KOSEF (Next-Generation Tech-nology Center). Paper no. TPWRD-00051-2005.
The authors are with the Department of Electrical Engineering, Myongji Uni-versity, Kyunggi-do 449-728, South Korea (e-mail: [email protected]).
Digital Object Identifier 10.1109/TPWRD.2005.860235
Fig. 1. Configuration of proposed UPQC.
were described in [5]–[7]. Recently, a dynamic voltage restorer
(DVR) with three H-bridge modules was proposed in [8].This paper proposes a new configuration of UPQC, in which
each phase consists of several pairs of H-bridge modules iso-
lated through a single-phase multiwinding transformer. The op-
eration of the proposed UPQC was verified through simulations
with PSCAD/EMTDC. The feasibility of hardware implemen-
tation was confirmed through experimental works with a scaled
model. The proposed UPQC can be directly connected to the
distribution system without a series injection transformer, which
struggles with core saturation and voltage drop.
II. PROPOSED SYSTEM
A. System Concept
Fig. 1 shows a configuration of the proposed UPQC based on
several pairs of H-bridge modules for each phase. Each pair has
two H-bridge modules connected in parallel through a common
dc-link capacitor. The H-bridge module in shunt part is con-
nected in series through a multiwinding transformer, while the
H-bridge in series part is directly connected in series and in-
serted in the distribution line.
B. Output Waveform
The proposed UPQC has a bypass function to remove the se-
ries converter from service during the distribution system fault.
0885-8977/$20.00 © 2006 IEEE
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HAN AND BAE: CONFIGURATION OF UPQC FOR MEDIUM-VOLTAGE APPLICATION 1439
Fig. 2. PWM pulse generation for H-bridge. (a) Carriers and reference signal.(b) Logic for gate pulse generation.
The bypass function is implemented by the operation of an insu-
lated-gate bipolar transistor (IGBT) bypass switch and mechan-
ical circuit breaker. The line overcurrent can be bypassed first
by the IGBT switch and then by the mechanical circuit breaker.
However, it is possible to attempt a bypass scheme using the se-
ries converter switches when the maximum fault current is lower
than the maximum current rating of the converter switches.
For the purpose of simulation, it is assumed that the shunt and
series converters have three H-bridge modules for each phase.
Fig. 2 shows the principle of pulsewidth-modulation (PWM)
gate-pulse generation for the H-bridge module. Fig. 2(a) shows
two carrier signals with a reference signal for converter module1. The frequency of carrier T1 and T2 is assumed to be 1 kHz.
Each of two carriers has 180 phase shift from each other. In
order to generate the gate pulse for other H-bridge module, the
other two carriers are required, which have 120 phase shift
from T1 and T2, respectively. Fig. 2(b) shows the logic diagram
to generate the gate pulse for the H-bridge module.
Fig. 3 shows the output voltage build-up of one phase and the
harmonic analysis results of the output voltage. Fig. 3(a) shows
the output voltage of each converter module , , , and
the output voltage of cascaded three converter modules .
is much closer to the sinusoidal waveform, compared with the
, , and . Fig. 3(b) shows the spectrum analysis result
for the output voltage of each converter module and the output
voltage of cascaded three converter modules.
A large number of high-level harmonics are involved in the
output of one module, while a significantly small number of
low-level harmonics are involved in the output of cascaded three
modules. If each carrier has a frequency of 1 kHz, the cascaded
output voltage of modules has an equivalent switching effect
of 2 1 kHz.
C. Controller Design
The UPQC controller was designed using the instantaneous
power method based on – – 0 transform. The instantaneous
power method makes it possible to generate the proper com-pensation signal by detecting a negative-sequence component,
Fig. 3. Waveform and harmonics analysis of output. (a) Output waveform of , , , and . (b) Spectrum analysis of , , , and .
Fig. 4. Positive-sequence voltage detector.
a zero-sequence component, and a harmonic component of the
source voltage. There are three major elements in the UPQC
controller, which are a positive-sequence detector, a shunt in-
verter control, and a series inverter control.
The positive-sequence detector shown in Fig. 4 extracts the
positive-sequence component from the disturbed three-phase
source voltage. The source voltage is measured to derive the
fundamental component of current with unity magnitudeand , passing it through the phase-
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HAN AND BAE: CONFIGURATION OF UPQC FOR MEDIUM-VOLTAGE APPLICATION 1441
Fig. 8. Simulation results with voltage sag.
Fig. 8 shows the compensated result when the voltage sagoccurs in the source side. It is assumed that phases A, B, and C
have 30%, 20%, and 10% of voltage sag, respectively, as shown
in the first graph. The second graph shows the load voltage
compensated by the UPQC. The third graph shows the output
voltage of the series inverter. The fouth, fifth, and sixth graphs
show the current waveform of the source, load, and shunt in-
verter. It is confirmed that the UPQC compensates the voltage
sag in the source and makes the load voltage constant.
Fig. 9 shows the compensated result when the voltage sag
occurs in the source side. It is assumed that phases A, B, and
C have 30%, 20%, and 10% of voltage swell, respectively,
as shown in the first graph. The seond graph shows the load
voltage compensated by the UPQC. The third graph shows theoutput voltage of the series inverter. The fourth, fifth, and sixth
Fig. 9. Simulation results with voltage swell.
graphs show the current waveform of the source, load, and
shunt inverter. It is also confirmed that the UPQC compensatesthe voltage swell in the source and makes the load voltage
constant.
IV. PROTOTYPE EXPERIMENT
A prototype was built and tested to confirm the feasibility
of actual hardware implementation. In order to simulate the
voltage sag and the voltage swell, a source simulator was used.
A dummy load was built to simulate the nonlinear load. UPQC
was also built in a panel with one solid-state switch and dig-
ital-signal-processing (DSP) processor. Fig. 10 shows the scaled
model of the proposed UPQC, which was used in the experiment
with the ac power source and the dummy load. Table II showsthe circuit parameters used in the experiment.
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1442 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 3, JULY 2006
Fig. 10. Scaled model of UPQC.
TABLE IICIRCUIT PARAMETERS OF THE EXPERIMENTAL MODEL
Fig. 11 shows the output voltage of one module and cascaded
three modules and their spectrum analysis results. The cascadedoutput has the same harmonic level with 3.6 kHz of switching
frequency when the carrier has a frequency of 600 Hz. There-
fore, the cascaded output has a much lower level of harmonics
through the carrier phase-shift scheme.
Fig. 12 shows the tracking performance of dc-link voltage
control when the reference dc-link voltage changes in step
mode. The two graphs confirm that the measured dc-link
voltage of each capacitor in one phase is properly regulated
through the controller as shown in Fig. 5.
Fig. 13 shows the current waveform when the shunt inverter
operates in active power filter mode. The load current is com-
pensated by the shunt converter current to make the source cur-
rent sinusoidal. The first two waveforms are the reference valueand measured value of shunt inverter current. The second two
Fig. 11. Harmonics analysis of an inverter’s output voltage.
Fig. 12. DC-link voltage control.
Fig. 13. Harmonic current compensation.
waveforms are the load current and the source current. There
are some transients in the source current, which are due to the
steepness of load current changes.
Figs. 14 and 15 show the experimental results when the
source voltage has unbalanced sag and swell. It is assumed that
the phase A, B, and C has 30%, 20%, and 10% of voltage sag orswell, respectively. Both results confirm that the load voltage is
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HAN AND BAE: CONFIGURATION OF UPQC FOR MEDIUM-VOLTAGE APPLICATION 1443
Fig. 14. Voltage sag compensation. (a) Source voltage. (b) Load voltage.
Fig. 15. Voltage swell compensation. (a) Source voltage. (b) Load voltage.
compensated properly as expected from the simulation resultsshown in Figs. 8 and 9.
Fig. 16. Conceptual diagram for system realization.
V. SYSTEM REALIZATION
The system realization aims at the development of a prac-
tical system that can be built with commercially available
components. A commercially available high-power dual IGBT,
FF200R33KF2 was considered for the building block of an
H-bridge. FF200R33KF2 has a peak offstate voltage of 3.3 kV
and a peak on-state current of 200 A. In order to guarantee safe
operation with enough margin, the operation voltage of 2.2 kV
and current of 140 A were considered for the system design.
The proposed UPQC is assumed to have a nominal operation
voltage of 22.9 kV and power rating of 3 MVA. The maximum
injection voltage in series part is assumed to be 50% of the op-
eration voltage, which is about 6.6 kV. Four pairs of H-bridge
modules for each phase are required with enough safety if IGBT
FF200R33KF2 is used for the system design. The turn-ratio of
primary winding to each secondary winding in the single-phase
multiwinding transformer is designed to be 8:1. The root-mean-
square (rms) voltage to be handled by each H-bridge is about
1.65 kV, which is much lower than 2.2 kV.
Fig. 16 shows the conceptual diagram of the proposed UPQC
including the distribution system. The proposed UPQC has four
pairs of H-bridge modules for each phase. There are a total of
12 pairs of H-bridge modules, in which each pair of H-bridge
modules has four dual IGBTs. Therefore, a total of 48 IGBTsare required in the design of the proposed UPQC.
VI. CONCLUSION
This paper proposes a new configuration of UPQC, in which
each phase consists of several pairs of H-bridge modules iso-
lated through a single-phase multiwinding transformer. The op-
eration of the proposed UPQC was verified through simulations
with PSCAD/EMTDC. The feasibility of hardware implemen-
tation was confirmed through experimental works with a scaled
model.
The proposed UPQC can be directly connected to the dis-
tribution system without a series injection transformer, whichstruggles with core saturation and voltage drop. It has flexibility
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1444 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 21, NO. 3, JULY 2006
in expanding the operation voltage by increasing the number of
H-bridge modules. The proposed UPQC might have the ultimate
capability of improving power quality at the point of installation
on the power distribution system.
REFERENCES
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[2] M. Aredes et al., “A combined series and shunt active power filter,” inProc. IEEE/KTH Stockholm Power Tech Conf., Stockholm, Sweden, Jun.1995, pp. 18–22.
[3] H. Fujita and H. Akagi, “The unified power quality conditioner: The in-tegrationof seriesand shunt active filters,” IEEE Trans. Power Electron.,vol. 13, no. 2, pp. 315–322, Mar. 1998.
[4] F. Peng, J. McKeever, and D. Adams, “A power line conditioner usingcascade multilevel inverters for distribution systems,” IEEE Trans. Ind.
Appl., vol. 34, no. 6, pp. 1293–1298, Nov./Dec. 1998.[5] F. Peng and J. Lai, “A multilevel voltage-source inverter with separate
DC source for static var generation,” in Proc. IEEE/IAS Annu. Meeting,
Orlando, FL, Oct. 8–12, 1995, pp. 2541–2548.[6] B. Han, S. Baek, H. Kim, and G. Karady, “Dynamic characteristic anal-
ysis of SSSC based on multibridge inverter,” IEEE Trans. Power Del.,
vol. 17, no. 2, pp. 623–629, Apr. 2002.[7] B. Han, H. Kim, and S. Baek, “Performance analysis of SSSC based on
three-level multi-bridge PWM inverter,” Elsevier Sci. Elect. Power Syst.
Rese., vol. 61, no. 3, pp. 195 –202, Jun. 2002.[8] B. Li, S. Choi, and D. Vilathgamuwa, “Transformerless dynamic voltage
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B. Han (S’91–M’92–SM’00) received the B.S. de-gree in electrical engineering from theSeoulNationalUniversity, Seoul, Korea, in 1976, and the M.S. andPh.D. degrees from Arizona State University, Tempe,in 1988 and 1992, respectively.
Currently, he is a Professor in the Department of Electrical Engineering, Myong ji University, Seoul,Korea. He was a Senior Research Engineer with theScience and Technology Center, Westinghouse Elec-
tric Corporation, East Pittsburgh, PA. His researchinterests include the high-power power electronics
and flexible ac transmission systems (FACTS).
B. Bae (S’05) received the B.S. and M.Sc. degrees inelectrical engineering in 2001 and 2003, respectively,from Myongji University, Seoul, Korea, where he iscurrently pursuing the Ph.D. degree.
His research interests include power-electronicsapplications for flexible ac transmission systems(FACTS) and custom power.
S. Baek received the B.S., M.Sc., and Ph.D. degreesin electrical engineering from Myongji University,Seoul, Korea, in 1997, 1999, and 2004, respectively.
Currently, he is an Associate Research Engineerin the Next-Generation Power Technology Center,
Myongji University. His research interests includepower-electronics application for flexible ac trans-
mission systems (FACTS) and custom power.
G. Jang (M’95) received the B.S. and M.S. degreesfrom Korea University, Seoul, Korea, and the Ph.D.degree from Iowa State University, Ames, in 1997.
Currently, he is an Associate Professor in theDepartment of Electrical Engineering, Korea Uni-versity. He was a Visiting Scientist in the Electricaland Computer Engineering Department, Iowa StateUniversity, for one year, and was a Researcher with
the Korea Electric Power Research Institute, Taejon,Korea, for two years. His research interests includepower quality and power system control.
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