A New Control Strategy of UPQC in Three-phase Four-wire System

Tan Zhili1,2,Li Xun1,Chen Jian1,Kang Yong1 Zhao Yang1

(1. School of Electrical & Electronics Engineering, Huazhong University of SCI&TECH, Wuhan, 430074, China; 2. School of Applied Geophysics & Space Information, China University of Geosciences,

Wuhan, 430074,China)Abstract This paper introduces the p-q-r instantaneous power theory firstly, and then an improved p-q-r theory is proposed. Based on this theory, it presents a composite control strategy of unified power quality conditioner (UPQC), which is the combination of the ordinary direct and indirect control strategy. An algorithm of calculating the compensation current and the compensation voltage are introduced. A principle analysis of the proposed control strategy is described in particular. Meanwhile the control formulas on the p-q-r coordinate are deduced in detail. The control schematic diagram based on these formulas is presented. Simulation results show that, when the UPQC applying such control strategy is used for the compensation of the nonlinear and unbalance three-phase four-wire system, the harmonic current, reactive power of loads as well as neutral current are compensated well, load voltage get balanced and rated, power factor of power source is about unity, which verified the effectiveness of applying such control strategy in UPQC. Key word improved p-q-r theory three-phase four-wire systemUPQC the composite control strategy

INTRODUCTION

The use of nonlinear and impact loads bring about harmonics and reactive power loading variance in power system, which has a strong impact on the other loads in the same system. Employment of UPQC (unified power quality conditioner) could decrease impact on transmission and distribution harmonics and neutral-line current caused by unbalance and nonlinear load, enhance custom power quality meanwhile supply balance and sinusoidal voltage to load and enhance power distribution reliability [1]-[3]

Fig.1 shows the circuit configuration of the proposed UPQC, which is a three-phase four-wire UPQC, being formed of series compensator and shunt compensator. Usually there are two control scheme of UPQC,one is most used ,known as indirect control strategy, in which series compensator work by way of voltage source compensating mainly voltage distortion and fundamental wave deviation supplying rated balance sinusoidal

voltage for load and shunt compensator as current sourcecompensating the harmonics, reactive current in load. The other is direct control strategy in which series compensator work as sinusoidal current source shunt compensator as sinusoidal voltage source. The power factor of power line can be unity because of series compensation current having the same phase with system voltage and the load can get balance, rated sinusoidal voltage. Employing this strategy, series compensator isolate the voltage disturbance between power line and load as well as shunt compensator prevent the reactive power, harmonic and neutral current on the load side into power line .Additionally, another benefit from the direct control strategy is that it is not necessary to change the work mode when power line dumping or restoring, for shunt compensator all along is controlled as sinusoidal voltage source .[4]- [8]This paper presents a method of detecting compensation signals

and a control scheme based on it. Because the p-q-r transformation is sophisticated, this paper presents a improved p-q-r algorithm, which simplify the calculations. Based on the improved p-q-r theory, the calculating method of compensating current and voltage are proposed. With introducing its principle and control schematic diagram in detail, a composite control strategy combining of the ordinary direct and indirect control strategy is presented, too. Simulation results using MATLAB/ SIMULINK show that the harmonic current and reactive power of load as well as neutral current are compensated well .So the proposed strategy is feasible and effective.

THE IMPROVED P-Q-R THEORY

Voltage at three-phase a-b-c coordinates can be transformed to - -0 as

0

0

1 1 1 2 2

2 3 3 0 3 2 2

1 1 1 2 2 2

a a

b b

c c

e e e

e e e

e ee

1

If rotating axis in - -0 with 1( )t to the direction of the e ,which is the components of voltage space vector e inthe - coordinate, we get the axis ,at the same time

axis rotate with 1( )t to ( q ),where 1( )t arccos ee

,

Fig.1 Circuit configuration of the proposed UPQC

10601-4244-0655-2/07/$20.00©2007 IEEE

e 2 2e e .Then p axis can be formed by rotating

with 2 ( )t to the direction of voltage space vector ,as well as r axis formed by rotating 0 axis with 2 ( )t in 0coordinate, q axis being seen the axle center , where 2 ( )t

0

0

arcsine

eand 0e 2 2 2

0e e e , the transformations are

shown in Fig.2(a) and(b). For the rotating voltage or current space vector x , when

choosing and ae be and ce as the coordinate reference wave,

the x components in p-q-r coordinates defined by (2)

0

0 0 0

000

0 0 0

0

p

q

r

ee ee e e

x xe e

x xe e

x xe e ee e

e e e e e

2

If x has the same direction with reference vectore , then 0

00

p

q

r

x x

x

x

3

where 2 2 20 0x x x x

If the system voltage space vector v is chosen as the coordinate reference and the system current is i ,theinstantaneous active power p , instantaneous reactive power qq on the q axis and instantaneous reactive power qr on the r axis can be described as

p p

q p r

r p q

v ip

q v i

q v i

4

Its apparent power 2 2 2q rs p q q

In this case, the concept of instantaneous active power and instantaneous reactive power becomes clearer. qi and ri and are

orthogonal with the p axis and reference voltage v , having no effect on active power. The q axis and r axis relate to reactive

power. The q axis lies on - coordinates, having something to do with phase-shifted angle with the reference voltage and harmonic wave deviated from it, other than the r axis relates to zero sequence components, such as zero sequence voltage and or the neutral current. Generally speaking, to a measured vector x ,

such as the system voltage and current vector, px and qx include

the dc and ac components, while rx include only ac components. The dc components come from the positive sequence component of x , as well as the ac components from negative sequence and harmonic components of it. The value of dc components of qx is decided by its phase-shifted angle with the positive sequence component of reference voltage.[9]- [11]

From above analyses it can be seen that the calculation of p-q-r transformation is sophisticated. Especially, if p-q-r coordinate reference arefe , brefe and crefe are sinusoidal and balanced, as shown in Fig.2(c) ,the components in , and 0 axis after transformation are satisfied with (5)

0

3 sin t

3 cos t

0

ref

ref

ref

e E

e E

e

(5)

where, E is the root-square value of phase voltage. Hence equation 2 can be rewritten as (6)

0 0

sin t cos 0 cos sin t 0 0 0 1

p

q pqr

r

x x xtx t x x

x xx

(6)

This is the improved p-q-r transformation formula. The sine and cosine items could be obtained through the Phase Lock Loop.Obviously, this improved p-q-r transform is simpler than the direct p-q-r transform.

THE CONTROL SYSTEM

The compensating purpose of UPQC includes two aspects. One is assuring the balance and rated loads voltage even if sag, wave, distortion or unbalance occurred in source voltages, the other is that to supply the power line of balanced and sinusoidal current with same phase as the source positive voltage even if loads current are reactive, distortion or unbalance. Thus under the ideal situation, both source currents and load voltages are balance and sinusoidal with the same phase as source positive voltage. If chose the positive sequence components of source voltage sav

sbv and scv as coordinates reference voltage, under ideal condition, from equation (3) we can know that both source currents and load voltages are include only the dc components on p axis after p-q-r transform. This method has definite physical significance and can simplify the calculation thus the compensating purpose of UPQC can be direct described as that to compensate the AC component at p axis , q axis components and r axis component of source currents and load voltages so that both of them have only dc components This paper chose

e

( )q

e

e1(t)

1(t)0e

p

r

0e

e2 (t)

q

2 (t)

refe

v i

qv qi

pvpi

rv ri

1(t)

(a) (b) c

Fig.2 Physical meaning of the - -p q r transformation

sai

sbi

sci

sav

sbv

scv

cav

cbv

ccv

1ai

1bi

1ci

1av

1bv

1cv

12 dcV

12 dcV

lav

lbv

lcv

Fig.3 Series compensator equivalent circuit

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sav , sbv and scv as the coordinates reference voltage arefe ,

brefe and crefe . It can be seen from (5) that refe has the same phase as the a phase voltage, thus t can be got by the PLL of the a phase voltage, then sin t and cos t can be calculated out.

In the following analysis, supposing that T[ ]a b c= x x xX 7

T0 0[ ]X ax x x 8

T[ ]X pqr p q rx x x 9Where x may be voltage or current in the system, X is their vectors, such as T

1 1 1 1[ ]a b c= v v vV

A. Series Compensator Control

Fig.3 shows the series compensator equivalent circuit. For the series compensator, according the control purpose, its primary side voltage should be the difference between source voltage and rated load voltage, and its current be in phase with positive sequence source voltage. Because of choosing the positive sequence component of source voltage as the coordinate reference voltage, the dc component of loads active current comes from its positive sequence component, which should be in-phase with supply current. So it can be seen that supply current should offer load the positive sequence current component , namely should contain the lpi ,Because of choosing the positive sequence component of source voltage as the coordinates reference voltage, the dc component of active current of loads comes from its positive sequence component ,which should be in-phase with supply current. So it can be seen that supply current should offered load the positive sequence current component ,namely spi should contain the lpi .In addition supply current should contain the active current which is consumed by the compensator as well as maintain voltage constant on dc side .

Because the value of C1 is very small, current flowing through it as well as the series transformer magnetization current can be neglected. Considering the transformer ration 2 1:n n N ,thecurrent supplied by series compensator can be calculated by (10).

* *1

* *1

* *1

/ /

/ 0

/ 0

p sp lp comp

q sq

r sr

i i N i N I

i i N

i i N

10

Where compI is used for compensating the active powers that the converter and condenser consume, which can get by measuring the voltage of condenser. All mentioned dc components can obtain from the LPF.

It can be seen from Fig.3 that the output voltage 1V can be

described as (11) S1 1 1

1 C 1 1 1 C Sdd

dt dtL

L

L RL R

N NII

V V I V I 11

0

0 0 0 0

0 0 0

pqrpqr pqr

d

dt dt

X XX C 12

Transforming the equation 11 to - -0 and using (12),then

1 1

1 1p1 1

1 1

1 111

0

0

0 0

p L

p cp

q Lq q cq

r r crLr

di Rdt L i v v

di RL L i v v

dt Li v vRdi

Ldt

13

11 1 1 1 1

11 1 1 1 1

11 1 1 1

pp cp q L p

qq cq p L q

rr cr L r

div v Li L R i

dtdi

v v Li L R idt

div v L R i

dt

14

For the compensating voltage cV ,its ideal value is * *.( )c l sNV V V .When adopting the proportional- integral(PI)

controller ,its given value is * *

* *

* *

[ ( ) ]

[ ( ) ]

[ ( ) ]

cp PI lp sp cp

cq PI lq sq cq

cr PI lr sr cr

v k N v v v

v k N v v v

v k N v v v

15

The series compensator output voltage order can be described as (16)

1 1 1 1

1 1 1 1

1 1 1

( )

( )

( )

p cp q PID p p

q cq p PID q q

r cr PID r r

v v Li k i i

v v Li k i i

v v k i i

* * *

* * *

* * *

16

Series compensator control block diagram can be gotten from (15) and (16),as shown in Fig.4.

B. Shunt Compensator Controller

Fig.5 shows the shunt compensator equivalent circuit. According the control purpose, ideal load voltage should be

*1 0ri

1pi

compIdcV

*dcV

1qi

lri

*1 0qicpv

cqv

crv

1L

1L

*1pi

spv

*lpv

sqv

* 0lqv

srv

* 0lrv

*1pv

*1qv

*1rv

lpilpi

Fig.4 Series compensator control block diagram

lavlbvlcv

3ai

3bi

3ci

ai2

bi2

ci2

av2

bv2

cv2

12 dcV

12 dcV

Fig.5 Shunt compensator equivalent circuit

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sinusoidal , balance and in-phase with the positive sequence component of source voltage ,which is supplied by shunt compensator .On the other hand ,because the source supply only the dc current component lpi for the load ,the harmonic and reactive current of load have to be supplied by shunt compensator ,thus the output current of shunt compensator can be expressed by (17)

*3

*3

*3

p lp

q lp

r lr

i i

i i

i i

17

It can be seen from Fig.5 that the output voltage and current can be described as (18) and (19)

22 2 2 2

ddtI

V V Il LL R 18

2 2 3ddtV

I IlC 19

Transforming the equation 11 and (19) to - -0 and using

(12),then 20 and 21 can be derived.

22 2 2 2 2

22 2 2 2 2

22 2 2 2

pp lp q L p

qq lq p L q

rr lr L r

div v Li L R i

dtdi

v v Li L R idt

div v L R i

dt

20

2 3

2 2 2 3

2 3

0 0 0 0

0 0 0

lp

p l p p

lqq lq q

r lr rlr

dv

dti v idv

i C C v idt

i v idvdt

21

From 20 and 21 ,the control algorithm can be derived , which is described as 22 and 23

2 3 2

2 3 2

2 3

( )

( )

( )

p p lq PI lp lp

q q lp PI lq lq

r r PI lr lr

i i C v k v v

i i C v k v v

i i k v v

* * *

* * *

* * *

22

2 2 2 2

2 2 2 2

2 2 2

( )

( )

( )

p lp q PI p p

q lq p PI q q

r lr PI r r

v v Li k i i

v v Li k i i

v v k i i

* *

* *

* *

23

We can get the shunt compensator control block diagram from (22) and (23), as shown in Fig.6. The output voltage order *

2pv , *2qv

and *2rv can be transformed to 0 coordinate and modulate with

SVPWM, or transformed to abc coordinate and modulate with SPWM.

. SIMULATION RESULT

The simulations with MATLAB/SUMLINK were performed for the purpose of analyzing the operation of the proposed system. The power circuit is modeled as a three-phase four-wire system. The circuit parameters that were used are shown in Tab.1.

The simulations take into account two working conditions. One is shown in Fig.7.Under this condition, source is under

voltage and load is balanced R-L, whose parameter are shown in Tab.1.

Fig.7 a shows the source voltage and load voltage . The source voltage is under voltage ,but the voltage maintains rated value by the support of shunt compensator .From Fig.7(b)and(c) we can know that the output current of shunt compensator supplying the load for reactive current ,at the same time ,the component of source current on p axis spi compensates the dc

component of load active current lpi ,as shown in Fig.7(d).From Fig.7(f),we can see that UPQC has realized its functions. The

other simulation results are shown in Fig.8.UPQC works under the condition of unbalance source voltage and nonlinear with unbalance load, whose parameters are shown in Tab.1

Tab. 1 Circuit parameter of the UPQC

Under voltage Peak value,264V,50Hz Source voltage Unbalance Peak.value,358V,311V and 264V, 50Hz

capacitor 6600 FCDC-LinkReference voltage 768V Filter L,C 2 2mHL , 2 100 FCShunt

compensator Switching Freq. 10kHz

Filter L,C 1 5.8mHL , 1 33 FCSwitching Freq. 10kHz

Series compensa

tor transformer 2 1: 3.464n n ,7kVA Balance R-L load 8R , L 25mH

Load Unbalance and nonlinear load

a phase, 10R , 30mHLb phase, 8R , 25mHLc phase, 6R , 20mHLnonlinear load,25kVA

2L

* 380lpvlpv

2C

* 0lqv

lqv

2C

lpi

lqi

2 pi

2qi

*2 pi

*2qi

* 0lrv *2ri

lrv

2ri

2L

lri

*2pv

*2qv

*2rv

Fig.6 Shunt compensator control block diagram

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Fig.8(a) shows the source and load voltage under this condition. It is obviously that load voltage is balance and rated after compensation. Because load current is unbalance and contain harmonic component ,the output current of shunt compensator

3 pi , 3qi and 3ri include the current component generated by the

unbalance and nonlinearity of load ,which known as lpi , lqi and lri ,as shown in Fig.8(c), (d) and(e).For the series compensator ,its output voltage cpv , cqv and crv are the difference between source voltage and rated load voltage ,which described as the given value of compensator voltage *

cpv , *cqv and *

crv . Fig.8(k), (l) and

a b c

d e (f)

Fig.7 The simulation waveforms when source voltage is under voltage with balanced L-R loads.(a) source voltage and load voltage.(b)the q axis component of load current and output current of shunt compensator.(c)the r axis component of load current and output current of shunt compensator.(d) the p axis component of load current and source current.(e) the given value and actual value of output voltage of series compensator.(f) voltage and current of a phase.

a b c

d e f

g h i

Mag

(% o

f Fun

dam

enta

l)

j k l m

Fig.8 Simulation waveforms when source voltage is unbalanced with nonsinusoidal and unbalanced loads.(a) source voltage and load voltage.(b) load current.(c) the p axis ac component of load current and p axis component output current of shunt compensator.(d) the q axis component of load current and output current of shunt compensator.(e) the r axis component of load current and output current of shunt compensator.(f) the p axis component of load current and source current.(g) the given value and actual value of output voltage of series compensator on p axis.(h) the given value and actual value of output voltage of series compensator on q axis.(i) the given value and actual value of output voltage of series compensator on r axis.(j) voltage and current of a phase.(k) frequency spectrum of load current.((l) frequency spectrum of source current.(m) frequency spectrum of load voltage.

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(m) shows the frequency spectrum of load current, source current and load voltage respectively. The THD of source current is 0.38%, which is better than that of load current. The load voltage is rated with low THD value. As shown in Fig.7 and Fig.8,it can be seen that UPQC performance well when proposed control strategy is supplied in it.

. CONCLUSION

This paper describes a new control strategy used in a proposed UPQC, which mainly compensate voltage sag and swell, reactive power, and harmonics. To simply the calculation, this paper improves the ordinary p-q-r theory. Based on the improved p-q-rtheory, the control strategy, combining the ordinary direct and indirect control strategy, is proposed. The block diagram of control system is proposed in particular. The simulation results shows that, when unbalance and nonlinear occur in load current or unbalance and sag in source voltage ,the above control algorithms eliminate the impact of distortion and unbalance of load current on the power line, making the power fact of it unity. Meanwhile, the series compensator isolate the loads voltages and source voltage, shunt compensator provide three-phase balanced and rated voltages of sine for loads. All above mentioned have realized the function of UPQC.

REFERENCES

[1] H.Fujit and H. Akagi “The unified power quality conditioner: The integration of series and shunt-active filters,”IEEE Trans. On Power Electronics, vol. 13, no.2, pp.315–322,Mar.1998

[2] B.Han, B.Bae ,H. Kim, and S.baek, “Combined Operation of Unified Power-Quality Conditioner with Distributed Generation,” IEEE Trans. On Power Delivery, vol.21,no.1, pp.330-338,January 2006

[3] Y. Chen, X. Zha, and J. Wang, “Unified power quality conditioner (UPQC): The theory, modeling and application,” in Proc. Power System Technology Power Con Int. Conf., vol. 3, 2000, pp. 1329–1333.

[4] ZHU Pengcheng, LI Xun,KANG Yong,and CHEN Jian,“Study of control strategy for a unified power quality conditioner,” Proceedings of the CSEE,vol.24,pp.67-73,Aug.2004.

[5] Xun Li, Pengcheng Zhu, Yinfu Yang, and Jian Chen,“A New Controlled Scheme for Series-Parallel Compensated UPS System” .IEMDC'03.Madison WI, USA. 2003. vol.2, pp. 1133~1136

[6] da Silva,S.A.O., Donoso-Garcia, P.F.,Cortizo, P.C., and Seixas P.F.,“Acomparative analysis of control algorithms for three phase line-interactive UPS systems with series-parallel active power-line conditioning using SRF method,” PESC00. 18-23 June 2000.vol.2 ,pp.1023~1028

[7] Monteiro, L.F.C.; Aredes, M.,and Moor Neto, J.A.,“A control strategy for unified power quality conditioner” IEEE International Symposium on Industrial Electronics(ISIE '03) ,9-11 June 2003 , vol.1,pp.391~ 396

[8] Chen Jian,Dai Ke,LI Xun,ZHU Pengcheng,and Liu Peiguo, “Series- Parallel Compensated UPS with Double Converters,” Journal of Power Supply vol.1,pp.262~271, Jan. 2003.

[9] Hyosung Kim,F.Blaabjerg, B.Bak-Jensen, and Jaeho Choi, “Instantaneous power compensation in three-phase systems by using p-q-r theory,” IEEETrans. On Power Electronics, vol.17, pp.701-710,Sept. 2002

[10] Hyosung Kim,F.Blaabjerg ,and B.Bak-Jensen,“Spectral analysis of instantaneous powers in single-phase and three-phase systems with use of p-q-r theory,” IEEE Trans. on Power Electronics, vol.17,pp.711-720,Sept. 2002

[11] M.C.Benhabib, S.Saadate, “New control approach for four- wire active power filter based on the use of synchronous reference frame, ”Electric Power System Research,vol.73,pp. 353-362,2005

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