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Improvement of the Power Quality with Series Active Filter according to the IEC 61000

S. P. Litrán, P. Salmerón, J. Prieto and R. S. Herrera Department of Electrical Engineering

Escuela Politécnica Superior, Universidad de Huelva Huelva (Spain)

salvador@uhu.es ; patricio@uhu.es; jpthomas@uhu.es; reyes.sanchez@die.uhu.es

Abstract— A combined system of passive and active filters for a four-wire three-phase system has been designed and simulated with Matlab/Simulink. A new control method based in the power vectorial theory and symmetrical components has been applied. The combined system mitigates the source current harmonics, compensates unbalance voltages, voltage harmonics, eliminates sags and reduces the problems of using only a shunt passive filter. The system is subjected to different voltage sources in accord to the IEC 61000. Simulation results are presented and discussed.

I. INTRODUCTION

Nowadays, problems with power quality can result in a high cost due to bad operation of sensitive load. The IEC 61000 [1] standardizes the limits for harmonic current emissions, power quality measurement and testing techniques. To improve the current waveform, shunt passive filters and shunt active filters are commonly applied to nonlinear loads to suppress harmonics and to improve the power factor in power systems.

Recently, combined systems with passive filter and active filter have been proposed [2-4] to improve the behavior of passive filters. In this work, a configuration with active filter connected in series to the source and a passive filter in parallel to the load (Figure 1) is analyzed. The shunt passive filter connected in parallel with a load suppresses the prevalent harmonics currents produced by the load, while the active filter connected in series with a source acts as a “harmonic isolator” between the source and the load.

Moreover, if the three-phase system is unbalanced, or has harmonics or presents sag, the active filter can eliminate these disturbances of the load voltages [8]. It is achieved generating the appropriate voltage waveforms with the inverter.

In this paper, a new control is presented, which improve the performance of shunt passive filter and restore the voltage on the load side to the desired amplitude and waveform even when the source voltage is unbalanced or distorted. The control is verified through extensive

computer simulation studies, according to the norm IEC 61000.

II. SYSTEM CONFIGURATION AND CONTROL

The active power filter designed consists of a three-phase PWM voltage source inverter, which is connected in series with the ac source impedance and load, through three single phase transformers (figure 1). A small rate passive filter to suppress switching ripples is connected between the transformers and the inverter.

The passive LC filter connected in parallel to the load is tuned to eliminate the fifth and seventh harmonics. For the fundamental harmonic, the passive filter also must supply the reactive power of the load.

In order to avoid the presence of harmonics at the source current, the active filter is controlled to present zero impedance at the fundamental frequency and a high impedance at the frequencies of the load harmonics. To achieve high impedance at the frequencies of the load harmonics, the output voltage of the series active filter must be proportional to the source current harmonics, that is,

SHCH IV k

When k>>ZS, ZF, then ISH 0

Figure 1. Scheme of active filter connected in series to the source

iS iL

iC

+ vC -

LS

LR L5 L7

C5 C7

Vdc Vdc

CR

IEEE MELECON 2006, May 16-19, Benalmádena (Málaga), Spain

1-4244-0088-0/06/$20.00 ©2006 IEEE 1086

The compensation characteristic becomes ideal when the active filter presents an infinite impedance, k= . In this way the passive filter problems, namely, the parallel resonance and the harmonic sink, are solved.

The harmonic component of the source current is calculated applying the vectorial theory of the electric power [9]. The reference current is calculated as follows

vii L 2refP

v

Where:

iL: current vector of the load and passive filter.

P: average power defined byT

dt'0

T

T

1P iu

u: voltage vector before active filter.

v: voltage vector of direct sequence.

v2: norm of v, defined by T

dtv'0

T2

T

1 vv .

T is the fundamental period and “·” denotes the dot product of vectors.

Moreover, if the three-phase system is unbalanced or distorted the active filter must eliminate these disturbances of the load voltages. It is achieved generating the appropriate voltage waveforms with the inverter. These reference signals are obtained applying the Fortescue transformation, defined by the following expression

c

b

a

a

a

a

vvv

aaaa

vvv

2

2

2

1

0

1

1

111

3

1

Where a=ej120` and zero, positive, and negative sequence components are denoted by subscripts 0, 1 and 2, respectively.

The fundamental harmonic of the positive component is calculated by the following relation

sen2

2cos

1

cos2

2sen

1

1

01

1

01

fa

Ta

fa

Ta

VdttvT

VdttvT

Where V f a1 denote the rms value of the fundamental

harmonic of va1, its initial phase and the fundamental frequency.

The instantaneous value is given by

ttVv fa

fa cossensencos211

With the inverse transformation is obtained a voltage system, balanced and free of harmonics

0

0

1

1

111

3

11

2

2 fa

c

b

a

vaaaa

vvv

It let obtain the reference voltage vref.

The inverter must generate the following waveform

refrefc viv vi kk

Figure 4 shows the control scheme.

The gating signals of the inverter are generated comparing the resultant reference signal with the output of the inverter through a bang-bang control.

III. SIMULATION RESULTS

The load to compensate is composed by three single phase non-controlled rectifiers, each of them connected between phase and neutral. Figure 2 shows the load current when the system is not compensated and the source voltages are balanced and not distorted.

Its frequency spectrum shows odd harmonics and the THD of the source current is 30.8%.

A passive filter is connected in parallel to the load. It is tuned at the 5th and 7th harmonics. The source current is shown in figure 3. These harmonics are eliminated but the THD rises from 30.8 to 65.7%.

Figure 3. Source current when passive filter si connected

0.2 0.22 0.24 0.26 0.28-20

-10

0

10

20

Time (s)

Figure 2. Load current

0.2 0.22 0.24 0.26 0.28

-10

0

10

Time (s)0.3

1087

The figure 5 shows the source current when the active filter is connected in series to the source. The active filter improves the harmonic contents of the source current. The circuit has been simulated with ki=20. The THD falls to 8.4%. It is possible to improve the current waveform if k is higher, but the capacity of the PWM converters required in the filter system is higher too.

A. Suply voltages unbalanced When the source voltages are unbalanced, the active

filter balances the voltages in the load side. A voltage systems with a 25% of unbalanced factor, according to IEC 61000 is applied to circuit. Figure 6 shows the voltages in the source side.

The active filter balances the load voltages under the limit of 2% that the IEC 61000 suggest. The active filter also adjusts its rms value to the level desired. Therefore it eliminates the zero and negative. Figure 7 shows the load voltage when the reference is 230 V.

B. Suply voltages distorted The IEC 61000 suggests different tests when the

source voltages are distorted. The test of the 5th harmonic is one of them. The voltage waveform in the source side is given by

ttVtv p 11 5sen12,0sen)(

Figure 7. Voltage in the load side when source voltages have a 25% of unbalanced factor

Time (s)0.2 0.22 0.24 0.26 0.28 0.3

-500

0

500

0.2 0.22 0.24 0.26 0.28-10

-5

0

5

10

Time (s)0.3

Figure 5. Source current when active filter is connected

Figure 6. Voltage in the source side with a 25% of unbalanced factor

Time (s)

-5000.2 0.22 0.24 0.26 0.28 0.3

0

500

Figure 4. Control scheme for the series active filter

ia iref

Fun

dam

enta

l ha

rmon

ic

For

tesc

ueT

rans

form

atio

n

TLow pass filter

u

i G

vc

va1

f 2

v

p(t)

u

P

va1f

rms

vf

Vn

vref

Low pass filter

Inve

rse

Tra

nsfo

rmat

ion

Ki

KV

Figure 8.Voltage in the source side with voltages distorted by 5th harmonic

Time (s)

-5000.2 0.22 0.24 0.26 0.28 0.3

0

500

1088

Where Vp denoted the voltage amplitude.

Figure 8 shows the source voltages applied to the circuit and figure 9 shows the load voltages, which are balanced, sinusoidal and regulated to 230 V rms.

C. Suply voltages with symetrical sag The tests for immunity in presence of sags are

established in IEC 61000. The sag can be of 0.70 or 0.40 p.u. and five cycles long. Figure 10 shows the waveform in the source side, which presents a sag of 0.40 p.u.

Figure 11 shows the load voltages. The sag is mitigated and can be considered eliminated.

IV. CONCLUSIONS

A combined system of shunt passive and series active filter for a four-wire three-phase system has been designed

and simulated with Matlab/Simulink. A new control method based in the power vectorial theory and instantaneous symmetrical components has been proposed.

The proposed filter system has the following features:

Decrease the harmonics component of the source currents, improving the THD.

Improve the behavior of the passive filter. The source impedance doesn’t influence the compensation characteristics.

It is able to compensate unbalanced voltage, regulates the voltage and eliminates harmonics and sag at the load terminals.

The control has been verified through extensive computer simulation studies according to IEC 61000.

ACKNOWLEDGMENT

This work is within the project “A New Technique for the Reduction the Harmonic Distortion in Electrical Installations using Active Compensation Equipment.” DPI2004-03501, supported by the ‘Comisión Interministerial de Ciencia y Tecnología CICYT’ of the Spanish Ministry of Science and Technology.

REFERENCES

[1] IEC 61000, ‘Electromagnetic compatibility (EMC) Part 3; Limits for harmonic current emissions (equipment up to and including 16 A per phase) and Part 4; Testing and measurement techniques (published by the International Electrotechnical Commission)

[2] F. Z. Peng and D. J. Adams, “Harmonics sources and filtering approaches,” in Proc. Industry Aplications Conference, October 1999, Vol, 1, pp. 448-455

[3] F. Z. Peng, H. Akagi, A. Nabae, ”A novel harmonic power filter,” in Proc. IEEE/PESC, April, 1988, pp. 1151-1159

[4] J. W. Dixon, G. Venegas and L.A. Morán, “A series active power filter based on a sinusoidal current-controlled voltage-source inverter,” IEEE Trans. Industrial Electronics. Vo, 44, no. 5, October 1997, pp. 612-620

[5] Z. Wang, Q. Wang, W. Yao and J. Liu, “A series active power filter adopting hybrid control approach,” IEEE Trans. Power Electronics, vol. 16, no. 3, May 2001, pp. 301-310

[6] F. Z. Peng, H. Akagi, A. Nabae, “A new approach to harmonic compensation in power systems-a combined system of shunt passive and series active filters,” IEEE Trans. Industry Applications. Vol, 26, no. 6, Nov/Dec 1990, pp. 983-990

[7] Y. S. Kim, J. S. Kim and S. H. Ko, “Three-phase three-wire series active power filter, which compensates for harmonics and reactive power,” IEE proc. Electr. Power Appl., Vol, 151, no. 3, May 2004, pp. 276-282

[8] A. Ghosh, A. K. Jindal, A. Joshi, “Design of a capacitor-supported dynamic voltage restorer (DVR) for unbalanced and distorted loads,” IEEE Trans. Power delivery. Vo, 19, no.1, January 2004, pp. 405-413

[9] P. Salmerón, J. C. Montaño, J. R. Vázquez, J. Prieto, A. Pérez, "Compensation in nonsinusoidal, unbalanced three-phase four-wire systems with active power line conditioner," IEEE Trans. Power Delivery, Vol. 19 (4), pp. 1968-1974, Oct. 2004

Figure 10. Source voltajes with a sag of 0.40 p.u.

Time (s)0 0.25 0.3 0.35 0.4 0.45 0.

-500

0

500

Figure 11. Load source when the source voltage present a sag of 0.40 p.u.

0 0.25 0.3 0.35 0.4 0.45 0.5-500

0

500

Time (s)

Time (s)

Figure 9.Voltage in the load side when source voltages are distorted

-5000.2 0.22 0.24 0.26 0.28 0.3

0

500

1089