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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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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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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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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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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

[1] M. Aredes, K. Heumann, and E. H. Watanabe, “An universal activepower line conditioner,” IEEE Trans. Power Del., vol. 13, no. 2, pp.545–551, Apr. 1998.

[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

restorer,” Proc. Inst. Elect. Eng., Gen., Transm. Distrib., vol. 140, no. 3,pp. 263–273, May 2002.

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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