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Electric Power Systems Research 77 (2007) 219–226

Theory and application of distribution electronic power transformer

Dan Wang a,∗, Chengxiong Mao a, Jiming Lu a, Shu Fan b, Fangzheng Peng c

a Department of Electrical Engineering, Huazhong University of Science & Technology, Wuhan 430074, Chinab Department of Electrical Engineering and Electronics, Osaka Sangyo University, Osaka 574-8530, Japan

c Department of Electrical & Computer Engineering, Michigan State University, United States

Received 13 June 2005; received in revised form 5 October 2005; accepted 28 February 2006Available online 17 April 2006

bstract

Three-phase and 4-wire Distribution Electronic Power Transformer’s (DEPT’s) operation principle is analyzed in this paper. Based on thenalysis, the control scheme is established. In this control scheme, the input stage is controlled as a three-phase balanced current source and makeshe primary current sinusoidal and power factor easily adjusted. While the output stage is controlled as a three-phase balanced voltage source andeep the load voltage sinusoidal and nominal. In order to meet the requirement of the single phase or unbalanced loads, each phase is an independent

oltage source. With the proposed control strategy, the characteristics of DEPT are studied by simulations. And further detailed simulations arearried out to validate the power quality control function of DEPT. The results show that DEPT has very good static and dynamic performances,nd it can not only realize the functions of conventional power transformer, but also can prevent from voltage sags, swells, flickers and harmonicsnfecting the loads while avoid loads impacting the primary system.

2006 Elsevier B.V. All rights reserved.

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eywords: Distribution electronic power transformer; Distribution system; Inp

. Introduction

Transformers are widely used in electric power system toerform the primary functions, such as voltage transformationnd isolation. Because of the bulky iron cores and heavy copperindings in the composition, transformers are one of the heavi-

st and most expensive parts in an electrical distribution system.he power throughput density of the transformer is inverselyroportional to frequency, so increasing the frequency allowsigher utilization of the steel magnetic core and reduction inransformer size [1]. In the recent decades, the power qualityroblem is becoming worse with increasing nonlinear loads inhe distribution system. So many active power filters are intro-

uced and the structure of the distribution system is becomingore complicated.Recently, a new type power transformer, which is based on

ower electronics and high frequency link, has been studiedxtensively [1–9].

∗ Corresponding author at: 1037#, Luoyu Road, Hongshan district, Wuhan30074, Hubei, China. Tel.: +86 27 87542669; fax: +86 27 87542669.

E-mail addresses: wangdenver@hotmail.com (D. Wang),xmao@mail.hust.edu.cn (C. Mao), lujiming8215@sohu.com (J. Lu),anshu@hotmail.com (S. Fan), fzpeng@egr.msu.edu (F. Peng).

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378-7796/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.epsr.2006.02.012

put characteristic; Power quality

Since Ref. [2] first introduced the concept of a high fre-uency link, many different configuration power electronicsased transformers have been discussed. However, these dis-usses are emphasized on the configuration designs.

In this paper, the power electronics based distribution trans-ormer with the high frequency link, termed as Distributionlectronic Power Transformer (DEPT), is further explored. The

ocus of the paper is to study the load characteristics and appli-ation of the DEPT.

This paper is organized as follows. A brief survey of theonfiguration and principle of DEPT is presented in Section. Section 3 introduces the control strategy of the DEPT. Theharacteristics of DEPT are analyzed in the following section.nd the application as power quality controller is presented inection 5. Finally, the relative simulations are carried out.

. Principle and configuration of DEPT

.1. Principle of DEPT

Fig. 1 shows a basic diagram of the DEPT with a primarynd secondary static converter and high frequency transformer.s can been seen, the power-frequency (50 or 60 Hz) input sin

220 D. Wang et al. / Electric Power Systems Research 77 (2007) 219–226

le dia

wttimittp

wpcsMm

2

tw3ado

c

asfthhveti

eitep

3

Dvid

Fig. 1. Princip

ave voltage is first converted into a high frequency signal byhe primary side converter, and then, magnetically coupled tohe secondary. In the secondary side, the high frequency signals unfolded into a power-frequency waveform. Here, the pri-

ary function of the high frequency transformer is to achievesolation between the primary and secondary system. Becausehe transformer size is inversely proportional to the frequency,he high frequency transformer will be much smaller than theower-frequency transformer.

There are two approaches to realize the DEPT. The one isithout dc link [2–7], and the other is with dc link [8,9]. Com-ared with the first scheme, the second has many attractiveharacteristics. For instance, the voltage or current in eitheride of DEPT can be flexibly controlled through Pulse Width

odulation (PWM) technology. So, it is likely to become theainstream of the future DEPT.

.2. Configuration of DEPT

In a distribution system, in order to meet the requirement ofhe single-phase load or line-to-line load, the 3-phase and 4-ire supply source is needed. So, DEPT must be designed as a-phase and 4-wire transformer. Fig. 2 shows a typical 3-phasend 4-wire DEPT topology. As can be seen, this is a three-part

esign that includes an input stage, an isolation stage and anutput stage.

The input stage is a 3-phase PWM rectifier, which is used toonvert the primary power-frequency voltage into the dc volt-

i

ma

Fig. 2. The typical circuit of 3-

gram of EPT.

ge. The isolation stage consists of a front-end H-bridge voltageource converter (VSC), a multi-windings high frequency trans-ormer and three back-end H-bridge VSCs. The dc voltage fromhe input stage is fed to the front-end VSC, modulated into aigh frequency square wave, coupled to the secondary of theigh frequency transformer and rectified to form three dc linkoltages. The output stage consists of three single-phase invert-rs with their output terminals YN-connection. They convert thehree dc voltages into three phase balance ac sinusoidal voltages,ncluding a neutral line.

This type DEPT has the following features: (1) In terms oflectrical performance, it and the conventional transformer aredentical. Such as, they both can achieve the functions of voltageransformation and isolation. (2) It provides 3-phase and 4-wirelectrical source for users. (3) It has good voltage regulationerformance.

. Proposed control strategy for DEPT

As the basic conversion device in the distribution system, theEPT should achieve the following demands. It can performoltage transformation and isolation. For its secondary system,t can provide high quality electric power even much harmonicistortion exists in the primary system. For its primary system,

t can’t inject the harmonics into the primary system.

Because the control method directly influences the perfor-ance of the DEPT, we will design the control system to fulfill

bove demands.

phase and 4-wire DEPT.

D. Wang et al. / Electric Power Systems Research 77 (2007) 219–226 221

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L

L

wl

[

[

a

T

tdvtcs

ad

i

tpttf

b

L

L

w

3

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Fig. 3. Mathematic model of the input stage.

.1. Control scheme for the input stage

The input stage of the DEPT is directly connected with therimary system. To prevent harmonics from being injected tohe grid, the input current should be sinusoidal and in phaseith the input voltage to achieve unity input power factor. Toatch the demand, the input stage is controlled as a controllable

urrent source. The mathematic model of the input stage can beresented in stationary a–b–c reference frame or synchronousotating d–q reference frame. But the control developed in rotat-ng d–q reference frame, would get better performance [10].

The mathematic model of the input stage in synchronousotating d–q reference frame is presented as (1) and shown inig. 3.

1di1d

dt= ωL1i1q − v1d + ed,

1di1q

dt= −ωL1i1d − v1q + eq (1)

here L1 is the input inductance; and ω is the synchronous angu-ar velocity of the grid voltage and

i1d

i1q

]= T (ωt)

⎡⎢⎣

i1a

i1b

i1c

⎤⎥⎦ ,

[v1d

v1q

]= T (ωt)

⎡⎢⎣

v1a

v1b

v1c

⎤⎥⎦ ,

ed

eq

]= T (ωt)

⎡⎢⎣

ea

eb

ec

⎤⎥⎦

nd

(ωt) = 2

3

[sin ωt sin(ωt − 120◦) sin(ωt + 120◦)

cos ωt cos(ωt − 120◦) cos(ωt + 120◦)

]

Eq. (1) and Fig. 3 show that there is a cross coupling betweenhe d axis and the q axis and that will influence the systemynamic performance. In order to solve this problem, the d–q

oltage decouplers are designed and suitable feed-forward con-rol components of the input source voltage are added in theontrol. To realize constant dc voltage and keep input currentinusoidal, the double control loops, a dc voltage outer loop

V

w

Fig. 4. Input stage control diagram.

nd an ac current inner loop, are adopted. The complete controliagram is shown in Fig. 4.

As can be seen from Fig. 4, the reference for the active current∗1d is derived from the dc voltage outer loop. The reference forhe reactive current i∗1q is set independently or derived from theower factor loop. Here, i∗1q is set to zero to get unity power fac-or. The current error signals are input the current regulators andhen form the modulation signals. If the d axis of the referencerame is aligned to the grid voltage, we obtain eq = 0.

According to Fig. 4, the input stage is decoupled. Eq. (1) cane simplified as (2).

1di1d

dt=

(KiP + KiI

s

)(i∗1d − i1d),

1di1q

dt=

(KiP + KiI

s

)i1q (2)

here, KiP and KiI are control coefficients.

.2. Control scheme for the isolation stage

In the isolation stage, the dc voltage coming from input stageould be modulated to a high frequency square wave, coupled to

he secondary and then reconverted into the low dc voltage. Here,he functions of the high frequency transformer are isolation andoltage levels transformation. The power electronic converteran change the voltage level directly, but, which would makeemiconductor devices bear too high stress [11].

To simplify the control system design, an open loop PWMontrol is applied for the front-end H-bridge VSC. In the back-nd H-bridge VSCs, the diode rectifiers are adopted if onlyonsidering single-directional power flow. This control methodrovides an absorbing additional feature that the synchroniza-ion problem becomes easy to solve. So, the isolation stage cane seen as a proportional amplifier. The simplified model of thesolation stage is presented as (3):

dc2 = 1

kVdc1 (3)

here k is the transformation ratio.

222 D. Wang et al. / Electric Power Systems Research 77 (2007) 219–226

tage control strategy.

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C

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Table 1Principal parameters

Parameter Value

Capability 1600 kVAGrid frequency 50 HzInput inductance 10.2 mHDC-link capacitance 5 mFOutput filter inductance 0.8 mHOutput filter capacitance 470 �FO

4

ctsltspf

4

ishown in Fig. 8. It is evident that the output voltages have verylittle fluctuation and maintains very short time.

Fig. 5. Output s

.3. Control scheme for the output stage

The output stage of the DEPT is directly provided to the endsers. Ideally, it should provide a clean and constant output volt-ge. As described above, there are three single-phase invertersn the output stage. Each inverter is an output phase indepen-ently, so the control of each inverter can be independent. Toeet the requirement of the loads, each inverter is controlled

s a single-phase sinusoidal voltage source. As we know, thenverter can be seen as a proportional amplifier when the switchrequency is much higher than the frequency of reference signal.o, the mathematic model of a phase of the output stage can beescribed as (4).

fdvo

dt= iL − io, Lf

diL

dt= −vo + vi (4)

n the distribution system, the loads are regarded as a passive sys-em. So the constant ac voltage control based on instantaneousalue feedback can be implemented [12]. The control schemef one phase of the output stage is shown in Fig. 5.

According to Fig. 5, there are two loops in the control scheme.wo signals are introduced as feedback: output voltage RMSalue (Vo) and output voltage instantaneous value vo. The outputoltage RMS value is introduced for the regulation of the outeroop to achieve constant output voltage RMS value. The outputoltage instantaneous value is introduced for the inner loop tobtain output voltage sinusoidal. The output of the outer loop isaken as the reference of the inner loop after being multiplied byhe ideal sinusoidal signal.

Theoretically, the output voltage RMS value would keep con-tant even when the loads change. At the same time, the outputoltage also can track the sinusoidal waveform. Here, to avoidhe phase angle error enlarging, a P controller, but not a PI con-roller, is adopted for the inner loop.

The control schemes are the same for the three phases. Thenly differences are phase angles of the ideal sinusoidal signals.or instance, the phase angle of phase a is set as 0◦, so phase b

s +120◦ and phase c is −120◦.

. Characteristics of DEPT

The input/output characteristics are important for the DEPT.ext we will investigate the input/output characteristics by the

imulations, which are based on Matlab/Simulink. In the sim-lation, the primary voltage of the DEPT is 10 kV and theutput voltage is 400 V. The principle parameters are shown inable 1.

perating frequency of high frequency transformer 1000 Hz

.1. Steady-state characteristics

The steady-state characteristics of the DEPT include no loadharacteristic and full load characteristic. Figs. 6 and 7 illus-rate several important waveforms recorded from the steady-tate simulations at nominal voltage and under no load or ratedoads with a lagging load power factor 0.8, respectively. Fromhe figures, it is clear that the output voltages are constant andinusoidal. Furthermore, the input voltage and current are inhase and the current is sinusoidal, regardless of output poweractor.

.2. Dynamic state characteristics

Performance of the DEPT was also simulated with switch-ng on full loads at 0.85 s. The output voltages and currents are

Fig. 6. Output phase voltage waveforms under no load.

D. Wang et al. / Electric Power Systems Research 77 (2007) 219–226 223

Fig. 7. Simulation results under full load: (a) input voltage and current of A phase, where, the current is enlarged eight times, (b) output phase voltages, (c) outputphase currents.

Fig. 8. Simulation results under switching full load: (a) output phase voltages, (b) output phase currents.

Fig. 9. Simulation results under voltage sag and swell: (a) input phase voltages, (b) output phase voltages.

224 D. Wang et al. / Electric Power Systems Research 77 (2007) 219–226

Fig. 10. Simulation results under voltage flicker: (a) input phase voltages, (b) output phase voltages.

ics: (a

5

DutcfS

ssan

Fig. 11. Simulation results under voltage harmon

. Power quality control

In comparison to the conventional power transformer, theEPT offers an attractive additional feature, which is to besed as power quality controller. Fast voltage regulation, reac-

ive power compensation, harmonic suppression and waveformontrol etc., are incorporated into the DEPT. These additionaleatures are guaranteed when the control system is designed inection 3.

v2l

Fig. 12. Simulation results under single-phase rectifier loa

) input phase voltages, (b) output phase voltages.

In order to verify the effectiveness of the proposed controlcheme to ensure the DEPT realizing power quality control,ome simulations are carried out under the conditions of volt-ge sag, voltage swell, voltage flicker, voltage harmonics andonlinear loads.

Fig. 9 shows the simulation results under the conditions ofoltage sags and swells arising in the primary grid. At 0.82 s, a0% voltage sag appears in the 10 kV distribution system andasts 0.02 s. And then, at the moment of 0.86 s, a 20% voltage

d: (a) output phase voltage, (b) output phase current.

ystem

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D. Wang et al. / Electric Power S

well arises and also lasts 0.02 s in the primary system. Theesults show that the DEPT can prevent from voltage sag andwell of primary grid infecting the output voltages. Furthermore,he voltage sag and swell are compensated without any timeelay.

The simulation results for a 10 Hz flicker in the primary sys-em are shown in Fig. 10. It can be seen that the output voltagesf the DEPT are not infected.

Fig. 11 shows the simulation results when the 20% 5th and theth order harmonics appear in the primary grid voltages. Underhis condition, the output voltages can also keep sinusoidal andonstant.

The simulation results that the DEPT operates under single-hase rectifier load are shown in Fig. 12. Where, the load ofhe single-phase rectifier is 5 � resister. According to Fig. 12,lthough the output current is severely distorted, the output volt-ge is still sinusoidal. The THD of the output voltage is less than.49%.

. Conclusion

DEPT can perform the functions of conventional transformernd power quality controller. Based on the principle analysis ofhe 3-phase and 4-wire DEPT, a simple and available controlcheme is proposed in the paper. In this control scheme, thenput stage is controlled as a 3-phase controllable current sourcend the output stage is controlled as three independent single-hase voltage sources. By simulations, the characteristics of theEPT are analyzed. The results show the distribution EPT hasood dynamic and steady state performances under proposedontrol strategy.

Based on the proposed control, further detailed simulationsonfirm that the DEPT can realize power quality control: whileealizing voltage levels change, magnetic isolation and energyransmission, the DEPT can prevent from dynamic power qualityroblems infecting output voltage and avoid load shock impact-ng the primary system.

cknowledgements

The authors gratefully acknowledge the support by Programor New Century Excellent Talents in University and by thexcellent Young Teachers Program of M0E, PR China.

ppendix A

ist of symbols, b, c denotes three quantitiesf output filter capacitance, q denotes rotating reference frame quantitiesd, eq grid voltage1a, i1b, i1c instantaneous ac supply currents

1d, i1q instantaneous ac supply currentsL output inductance currento load current

transformation ratio of high frequency transformer

gs

Jf

s Research 77 (2007) 219–226 225

iI integral constant of PI controlleriP proportional constant of PI controller1 input inductance

time1a, v1b, v1c three phase rectifier instantaneous input voltages1d, v1q three phase rectifier ac input voltagesi instantaneous output voltage of invertero instantaneous output voltagedc1 dc bus voltage of input stagedc2 dc bus voltage of output stage

angular frequency of grid

eferences

[1] M. Kang, P.N. Enjeti, I.J. Pitel, Analysis and design of electronic trans-formers for electric power distribution system, IEEE Trans. Power Electr.14 (1999) 1133–1141.

[2] W. McMurray, Power converter circuits having a high frequency link, USPatent 3,517,300, June 23, 1970.

[3] G. Venkataramanan, B.K. Johnson, A. Sundaram, An ac–ac power con-verter for custom power applications, IEEE Trans. Power Delivery 11(1996) 1666–1671.

[4] J.L. Brooks, Solid State Transformer Concept Development, Naval Mate-rial Command, Civil Engineering Laboratory, Naval Construction BattalionCtr., Port Hueneme, CA, 1980.

[5] EPRI Report, Proof of the principle of the solid-state transformer: theAC/AC switch mode regulator, EPRI TR-105067, Research Project 8001-13, Final Report, August 1995.

[6] K. Harada, F. Anan, K. Yamasaki, M. Jinno, Y. Kawata, T.Nakashima, K. Murata, H. Sakamoto, Intelligent transformer, in: Pro-ceedings of the 1996 IEEE PESC Conference, 1996, pp. 1337–1341.

[7] M.D. Manjrekar, R. Kieferndorf, G. Venkataramanan, Power electronictransformers for utility applications, in: Proceedings of the 2000 IEEE-IAS Annual Meeting, 2000, pp. 2496–2502.

[8] E.R. Ronan, S.D. Sudhoff, S.F. Glover, D.L. Galloway, A power electronic-based distribution transformer, IEEE Trans. Power Delivery 17 (2002)537–543.

[9] M. Marchesoni, R. Novaro, S. Savio, AC locomotive conversion systemswithout heavy transformers: is it a practicable solution, in: Proceedings ofthe 2002 IEEE International Symposium on Industrial Electronics, 2002,pp. 1172–1177.

10] M.E. Fraser, C.D. Manning, B.M. Wells, Transformerless four-wire PWMrectifier and its application in ac–dc–ac converters, IEEE Proc. Electr.Power Appl. 142 (6) (1995) 410–416.

11] J. Kassakian, M. Schlecht, G. Verghese, Principles of Power Electronics,Addison-Wesley, 1991.

12] G.B. Zhang, Z. Xu, G.Z. Wang, Steady-state model and its nonlinear controlfor VSC-HVDC system, in: Proceedings of the CSEE, vol. 22, no. 1, 2002,pp. 17–22.

an Wang was born in Jiangxi, China, in 1977. He received his B.S. and M.S.egrees in Department of Electrical Engineer, from Huazhong University ofcience & Technology (HUST), Hubei, China, in 1999 and 2002 respectively,nd is pursuing the Ph.D. degree in HUST. His interest is the excitation controlf synchronous generator and applications of high power electronic technologyo power system.

hengxiong Mao was born in Hubei, China, in 1964. He received his B.S., M.S.nd Ph.D. degrees in electrical engineering, from HUST, in 1984, 1987 and991, respectively. Presently, he is a Professor of HUST. His fields of interestre power system operation and control, the excitation control of synchronous

enerator and applications of high power electronic technology to powerystem.

iming Lu was born in Jiangsu, China, in 1956. He received his B.S. degreerom Shanghai Jiaotong University, Shanghai, China, and received his M.S.

2 ystem

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26 D. Wang et al. / Electric Power S

egree from HUST. His research is focused on the excitation control based onicrocomputer.

hu Fan received his B.S., M.S. and PhD. degrees in Department of Electricalngineer, from HUST, in 1995, 2000 and 2004, respectively. Presently he works

n Osaka Sangyo University, Japan. His research interests are power systemperation and intelligent control system.

angzheng Peng received the B.S. degree from Wuhan University, Wuhan,hina, in 1983, and the M.S. and Ph.D. degrees from Nagaoka University ofechnology, Nagaoka, Japan, in 1987 and 1990, respectively, all in electricalngineering. From 1990 to 1992, he was a Research Scientist with Toyo Electricanufacturing Company, Ltd., engaged in research and development of active

E2ciH

s Research 77 (2007) 219–226

ower filters, flexible ac transmission systems (FACTS) applications, and motorrives. From 1992 to 1994, he was a Research Assistant Professor with Tokyonstitute of Technology, where he initiated a multilevel inverter program forACTS applications and a speed-sensorless vector control project. From 1994o 2000, he was with Oak Ridge National Laboratory (ORNL) and, from 1994o 1997, he was a Research Assistant Professor at the University of Tennessee,noxville. He was a Staff Member and Lead (Principal) Scientist of the Power

lectronics and Electric Machinery Research Center at ORNL from 1997 to000. In 2000, he joined Michigan State University, East Lansing, as an Asso-iate Professor in the Department of Electrical and Computer Engineering. Hes also currently a specially invited Adjunct Professor at Zhejiang University,angzhou, China.