Compensating Characteristics of a Series-ShuntActive Power Filter Considering Unbalanced

Source Voltage and Unbalanced Load

Kiyotake Nohara*, Akiteru Ueda*, Akihiro Torii*, Kae Doki** Aichi Institute of Technology, 1247 Yachigusa, Yakusacho, Toyota, Aichi 470-0356, Japan

Abstract- Recently, the suppression of the harmoniccomponent in the electric power system has become animportant problem. In this paper, we analyze compensatingcharacteristics of a series-shunt active power filter bysimulation and experiment when source voltage or loadcurrent becomes unbalanced. The harmonic andunbalanced voltages are detected and compensated in aseries active filter using dqO transformation. The harmonicand unbalanced currents are detected and compensated in ashunt active filter using pq-theory. The results of analysisshow that the THD of the compensated voltage is 1.0% bysimulation and 8.3% by experiment. The THD of thecompensated current is 3.0% by simulation and 7.2% byexperiment. It is shown that the series-shunt active filter cancompensate the voltage and the current simultaneously andit has excellent compensating characteristics even whenunbalanced components occur in the electric power system.

Index Terms-Active power filter, Harmonic componentdetection, Unbalanced component detection

I. INTRODUCTIONRecently, the suppression of harmonic component in

the electric power system has become an importantproblem. As for its countermeasure, active power filtersplaced near the harmonic source are effective. Manyresearch works on series active filters and shunt activefilters have been published [1]-[3]. The series active filteracts as a voltage source. It is suitable to compensatecapacitive or voltage source load such as a capacitorinput-type diode rectifier. The shunt active filter acts as acurrent source. It is suitable to compensate inductive orcurrent source load such as a thyristor rectifier.

In recent years, various loads are connected in thepower systems. Harmonic voltages and currents are hardto compensate by solely using the series active filter orthe shunt active filter. To deal with these problems, aseries-shunt active filter is suited to compensate bothvoltages and currents simultaneously. We have proposedthe series-shunt active filter using a moving average high-pass-filter (HPF) [4].

In this paper, we discuss the application of theproposed active filter. When an earth fault or a short-circuit occurs, the three-phase source voltage becomesunbalanced. We analyze compensating characteristics ofthe series-shunt active filter by simulation andexperiment under conditions in which source voltage orload current becomes unbalanced.

II. SYSTEM CONFIGURATION

The system configuration is shown in Figure 1. Theseries-shunt active filter consists of a series active filterand a shunt active filter. The series active filter and theshunt active filter are connected with the common DCsmoothing capacitor Cd,. The voltage detection method ofthe series active filter is source voltage detection, and thecurrent detection method of the shunt active filter is loadcurrent detection. The series active filter compensates theharmonic and unbalanced voltage by using the dqOtransformation. The shunt active filter compensates theharmonic and unbalanced current by using the pq-theory[5]. The pq-theory uses phase voltage and the loadcurrent. The series active filter is connected at the powersupply side, therefore the compensated voltage can beused for the source voltage when pq calculation isperformed. The filter with L, C and R is used to removethe switching ripple.

III. HARMONIC AND UNBALANCE DETECTION

A. Harmonic and unbalanced voltage detectionFigure 2 shows the harmonic and unbalanced voltage

detection using dqO transformation. When source voltagewith distortion and unbalance is transformed to dqOcoordinates, the fundamental voltage becomes dccomponents and the unbalanced voltage becomes thesecond order ac components with dc components. Usingthe moving average HPF, the dc components are removedand the ac components are detected. The harmonic andunbalanced voltages are detected using reverse dqOtransformation. With the detection of the above-mentioned, the compensated voltage becomes the averageof the three-phase source voltage that is lower than therated value. When the voltages should be compensated tothe rated value, the value which makes the reference isadded to the ac components of Vq.

B. Harmonic and unbalanced current detection andcapacitor voltage control

Figure 3 shows the harmonic and unbalanced currentdetection and capacitor voltage control. Harmonic andunbalanced current are detected by the pq-theory. The u-Porthogonal coordinates transform the source voltage andthe load current from three-phase to two-phase. Theinstantaneous real power p and the instantaneousimaginary power q are calculated by the pq-theory. The

1-4244-0844-X/07/$20.00 ©2007 IEEE. 1692

is

Fig. 1. System configuration of a series-shunt active filter.

dc components are removed from p by the movingaverage HPF. The harmonic and unbalanced currents are

detected using reverse pq-calculation. The control valuePdc is subtracted from real power p so that the capacitorvoltage Vdc remains constant.

C. Moving average HPFThe moving average is a kind of digital filter. The

moving average is calculated by shifting the average

period in calculation at each sampling time [6] [7]. In dqOtransformation and pq-theory, the fundamentalcomponents are transformed into dc components and theunbalanced components are transformed into ac

components. The harmonic components are transformedinto 6mxf Hz components of vd, vq, p and q where m isan integer and f is a source frequency. The unbalancedcomponents are transformed into 2f Hz components ofvd, vq, p and q. So, it is necessary to remove the dccomponents to detect the unbalanced voltage and current.The average period is given by

T = 1/(2f).

The average period is 8.33 ms when the source

frequency is 60 Hz.Moving average HPF can detect unbalanced voltage

and current with high accuracy, because the gain is 1.0and the phase difference is 00 at 2fHz that corresponds tothe unbalanced voltage and current.

IV. SIMULATION

A. Simulation conditionThe circuit in Figure 1 is simulated by using the circuit

parameters in Table I. The three-phase source voltage is100 V and the source frequency is 60 Hz. The amplitudeof the distortion of the 5th, 7th and 11th ordercomponents with 10% is added to the fundamentalcomponents. Assuming one line earth fault, phase U ofthe source voltages is decreased 30°0. In the case of a

balanced load, the three-phase diode rectifier and thethree-phase thyristor rectifier are connected in parallel as

shown in Figure 4(a). In the case of an unbalanced load,the single phase thyristor rectifier is used instead of thethree-phase thyristor rectifier as shown in Figure 4(b).The phase control angle of the thyristor rectifier is 300.The compensating characteristic is examined in threecases; (a) source voltage and load current are balanced,(b) source voltage is unbalanced, and (c) load current isunbalanced.

In each case, total harmonic distortion (THD) andharmonic compensation factor (HCF) are given by

40 /

THD[%o} = Zv2 1Vax100,n=2

40 />40

HCF[%] = I -Y Van2 /YE Vbn2 )X100,n=2 n=2

1693

-II

IL

Vsu

Vsv

Source voltage(with distortion)

Load current

Harmonic andunbalanced voltage

Moving Average HiPF

Fig. 2. Harmonic and unbalanced voltage detection.

-armonic anuunbalanced current

Source voltage

Vdc

Fig. 3. Harmonic and unbalanced current detection and capacitor voltage control.

where Van is a voltage of the n-th order components aftercompensation and Vbn is a voltage of the n-th ordercomponents before compensation. In the case of currentcalculations, THD and HCF are derived by replacing Vnwith In. HCF shows the rate of decrease of the distortion,and it is important to evaluate compensation performanceof the active filter.

B. Simulation Results1) In the case that source voltages and load currents

are balancedThe simulation result waveforms are shown in Figure

5 and the compensation characteristic is summarized inTable II. The source voltage and the load current withdistortion are compensated to the sinusoidal waveforms.The THD of the voltage, which was 17.4% beforecompensation, is approximately 1.0% after compensation,and the HCF of the voltage is approximately 94.000. TheTHD of the current, which was approximately 51.O0%before compensation, is approximately 3.000 aftercompensation, and the HCF of the current isapproximately 94.00/.

2) In the case that source voltages are unbalancedThe simulation result waveforms are shown in Figure

6 and the compensation characteristic is summarized inTable III. The unbalanced source voltage with distortionis compensated to the sinusoidal waveforms andbalanced. The load current with distortion is compensatedto the sinusoidal waveforms. The THD of the voltage,which was 17.3-24.6% before compensation, isapproximately 1.5% after compensation, and the HCF ofthe voltage is approximately 94.00/. The THD of thecurrent, which was approximately 51.00% before

compensation, is approximately 3.00/ after compensation,and the HCF of the current is approximately 94.5°/O. Theharmonic and unbalanced components are compensatedexcellently in the case of the unbalanced source voltage.

3) In the case that load currents are unbalancedThe simulation result waveforms are shown in Figure

7 and the compensation characteristic is summarized inTable IV. The source voltage with distortion iscompensated to the sinusoidal waveforms. Theunbalanced load current with distortion is compensated tothe sinusoidal waveforms and balanced. The THD of thevoltage, which was approximately 17.5% beforecompensation, is approximately 1.0% after compensation,and the HCF of the voltage is approximately 95.00/. TheTHD of the current, which was 60.0-84.9% beforecompensation, is approximately 4.00/ after compensation,and the HCF of the current is approximately 94.00/. Theharmonic and unbalanced components are compensatedexcellently in the case of the unbalanced load current.

V. EXPERIMENT

A. Experiment conditionThe circuit shown in Figure 1 is examined by using

circuit parameters in Table V. The three-phase sourcevoltage is 20 V and the source frequency is 60 Hz. Theamplitude of the distortion of the 5th, 7th and 11th ordercomponents with 10% is added to the fundamentalcomponents. The three phase source voltages withdistortion are synthesized by using function generatorsand power amplifiers. Assuming one line earth fault,phase W of the source voltages is decreased 5000. The

1694

"Rt

Cd

(a) Balanced load (b) Unbalanced loadFig. 4 Load circuit for simulation.

10

100

0

70

25[ms] Phase u Phase v Phase w

Fig. 5. Simulation result in the case of balanced source voltageand balanced load current.

100

- 0

t 100

-00

>

U 3 W

M03

0

25ns] _Phase u Phase v Phase w

Fig. 6. Simulation result in the case of unbalanced source voltage.

100

>>

0

V 3 - A f W (X b ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~t......ol'''S ArJ ''' i'''' :.' '.'S''',~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~........3

o: t

0 X25[-s] Phase u Phase v Phase w

TABLE ICIRCUIT PARAMETERS IN SIMULATION

Ls | 0.3[mH] Rt 160[Q]Cdc 500[uF] Lt 50[mH]R lO[Q] Rd 200[Q IL 5[mH] Cd 1500[uF]o 20[uF] LL 2[mH]Lc 5[mH] fsw 15[kHzl

TABLE IICOMPENSATION CHARACTERISTIC IN THE CASE OF

BALANCED SOURCE VOLTAGE AND BALANCED LOAD CURRENT

THD [Ol HF[obefore after

phase u 17.4 1.0 94.1voltage phase v 17.4 1.0 94.0

phase w 17.4 1.1 93.8phase u 51.0 2.8 94.4

current phase v 50.6 2.9 94.3phase w 50.9 2.7 94.6

TABLE IIICOMPENSATION CHARACTERISTIC

IN THE CASE OF UNBALANCED SOURCE VOLTAGE

THD [Ool HOF [obefore after

phase u 24.6 1.4 94.4voltage phase v 17.3 1.1 93.8

phase w 17.3 1.1 93.4phase u 50.5 2.8 94.4

current phase v 51.0 2.6 94.8phase w 50.7 2.8 94.4

TABLE IVCOMPENSATION CHARACTERISTIC

IN THE CASE OF UNBALANCED LOAD CURRENT

THD [Ool HOF [obefore after

phase u 17.4 0.8 95.3voltage phase v 17.4 0.7 95.8

phase w 17.5 1.2 93.2phase u 60.0 3.7 93.9

current phase v 69.3 4.0 94.2phase w 84.9 3.5 95.8

Fig. 7. Simulation result in the case of unbalanced load current.

1695

Lt Lt

Rd

Fig. 8. Load circuit for experiment.

a 400 :;

IO2 C >

-9 40

U 0.5

0*

...................................... ................ ................... ................................................................................................................................................................................................................

Fig. 9. Experiment result in the case of balanced source voltageand balanced load current.

40

o

t 40

0

U 0.5

-go O o

0

Fig. 10. Experiment result in the case of unbalanced sourcevoltage.

three-phase diode rectifier as shown in Figure 8 is used asthe load circuit. Source voltage, compensated voltage andload current are detected and taken into a digital signalprocessor (DSP). The harmonic components arecalculated in DSP by using dqO transformation, pq-calculation and a moving average high pass filter. ThenPWM control signals are generated by feed-back controlof the compensating voltage and the compensatingcurrent.

B. Experiment Results1) In the case that source voltages and load currents

are balancedThe experiment result waveforms are shown in Figure

9 and the compensation characteristic is summarized inTable VI. The source voltage and the load current withdistortion are compensated to the sinusoidal waveforms.The THD of the voltage, which was 17.10% beforecompensation, is approximately 4.5% after compensation,and the HCF of the voltage is approximately 74.0%. The

TABLE VCIRCUIT PARAMETERS IN EXPERIMENTU I. I I- ..wX-V L, I _L1-.1 2.-Z- 1 11tvill . 1

Ls 0.3[mH] Lc 20[mH]Cdc 1410[uF] Rd 120[Q IR 20[QI Ld 200[mH]L I O[mH] fsw I O[kHzlC 20[uF]

TABLE VICOMPENSATION CHARACTERISTIC OF EXPERIMENT IN THE CASE OFBALANCED SOURCE VOLTAGE AND BALANCED LOAD CURRENT

THD [Ol HF[obefore after

phase u 17.1 4.1 75.9voltage phase v 17.1 4.5 73.9

phase w 17.1 4.5 74.0phase u 25.6 8.6 66.3

current phase v 25.7 9.1 64.7phase w 26.0 8.6 66.8

TABLE VIICOMPENSATION CHARACTERISTIC OF EXPERIMENTIN THE CASE OF UNBALANCED SOURCE VOLTAGE

THD [Ool HOF [obefore after

phase u 17.0 4.2 75.3voltage phase v 17.1 4.3 74.9

phase w 17.0 4.1 76.1phase u 24.6 7.7 68.5

current phase v 24.8 9.8 60.5phase w 26.7 9.3 65.1

THD of the current, which was approximately 26.0%before compensation, is approximately 9.000 aftercompensation, and the HCF of the current isapproximately 65.0%. Compensation ofharmonic voltageand current is not sufficient compared with the simulationresults. Furthermore, there is more ripple component inthe compensated voltage and the source current.

2) In the case that source voltages are unbalancedThe experiment result waveforms are shown in Figure

10 and the compensation characteristic is summarized inTable VII. T

he unbalanced source voltage with distortion iscompensated to the sinusoidal waveforms and balanced.The load current with distortion is compensated to thesinusoidal waveforms. The THD of the voltage, whichwas approximately 17.0% before compensation, isapproximately 4.0% after compensation, and the HCF ofthe voltage is approximately 75.0%. The THD of thecurrent, which was 24.6-26.7% before compensation, is7.7-9.8% after compensation, and the HCF of the current

1696

is 60.5-68.5%. Compensation of harmonic voltage andcurrent is not sufficient compared with the simulationresults. Furthermore, there is more ripple component inthe compensated voltage and the source current. A tuningerror of the experimental control circuit seems to causethe insufficient characteristics.

VI. CONCLUSIONThe compensating characteristics of a series-shunt

active filter in three different cases are analyzed by thesimulation and experiment. From the results of thesimulation, the voltage and the current are compensatedto the sinusoidal waveforms and the harmonic andunbalanced components are compensated excellently inall cases. It is shown that the series-shunt active filter cancompensate the voltage and the current simultaneously.Therefore, effectiveness of the system is confirmed. Fromthe result of the experiment, the source voltage and theload current with distortion are compensated to thesinusoidal waveforms and unbalanced components arecompensated. However, compensation of harmonicvoltage and current is not sufficient compared with thesimulation result.

REFERENCE

[1] H. Fujita, H.Akagi: "The Unified Power Quality Conditioner:The Integration of Series- and Shunt-Active Filters", IEEETransactions On Power Electronics, Vol. 13, No. 2, 1998

[2] Gu Jianjun, Xu Dianguo, Liu Hankui, Gong Maozhong"Unified Power Quality Conditioner (UPQC): the Principle,Control and Application" , Proceedings of power conversionconference-Osaka 2002, pp.80-85

[3] Mehrdad Tarafdar Haque, T. Ise: "Implementation of Single-Phase pq Theory", Proceedings of power conversion conference-Osaka 2002, pp.761-765

[4] H. Hayashi, A. Ueda: "Compensating Characteristics of a Series-Shunt Active Power Filter", Proc. on IPEC Niigata, 2005, S 14-1

[5] H. Akagi, Y. Kanazawa, A. Nabae "Instantaneous ReactivePower Compensators Comprising Switching Devices withoutEnergy Storage Components", IEEE Transactions on IndustryApplications, Vol. IA-20, pp.625-630, 1984.

[6] A. Nakata, A. Ueda, A. Torii: "A Method of Current Detector foran Active Power Filter Applying Moving Average to pq-Theory",IEEE Power Electronics Specialists Conference 1998, pp.242-247.

[7] Y. Oono, A. Ueda, A. Torii, A. Ooyagi "CompensatingCharacteristic of Both Harmonic Currents and Reactive Power byan Active Power Filter Using a Moving Average HPF",International Power Electronics Conference - Tokyo 2000,pp. 1478-1483.

1697