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Abstract

- The main drawback of the current-source activepower filter is the heavy and bulky dc side filter. The large dc fil-ter is needed to store the energy of the compensated harmoniccomponents. In this paper a new smaller dc filter structure isproposed for the current-source active power filter. In the pre-sented dc filter structure the energy of the most important har-monics are stored in resonant circuit which makes it possible todecrease the overall size of the filter. The function of the pro-posed dc filter structure is examined with both simulations andexperimental tests.

I

. I

NTRODUCTION

In recent years, active power filters have been widely investi-

gated for the compensation of harmonic currents in electrical

power systems. These active power filters are divided into two

types: voltage-source active filter (VSAF) and current-source

active filter (CSAF). CSAF has advantages of excellent current

control capability, easy protection and high reliability over

VSAF [1]. The main drawbacks of the CSAF has been so far the

lag of proper switching devices and large dc side filter. The new

IGBTs with reverse blocking capability are being launched on

the markets which are suitable for CSAF [2]. However, thebulky and heavy dc side filter is still a problem.

Fig. 1(a) shows the most common main circuit structure of

the current-source active power filter (e.g. [3]-[5]). The line

current characteristics are improved by injecting the current

components opposite to the harmonics of the load current.

The energy of the injected harmonic components is stored in

and restored from the dc circuit which makes ripple in the dc

current. In order to keep this ripple in an acceptable level rel-

atively large dc filter inductor is needed.

In this paper a new smaller dc filter structure is presented

for the current-source active power filter. The proposed dc fil-

ter structure is shown in Fig. 1(b). In the presented dc filter the

energy of the most important harmonics are stored in reso-

nant circuit which makes it possible to decrease the overall

size of the filter.

II

. P

ROPOSED DC FILTER STRUCTURE

The most important harmonics in typical nonlinear loads are

5th and 7th harmonic components which produce 6th harmonic

component in the dc circuit of the active power filter. Next im-

portant harmonic component in the dc circuit of the CSAF is the

12th harmonic which is caused by the compensation of 11th and

13th load current harmonics. Other components in the dc circuit

have order of 18, 24, 30 etc. However, the 6th harmonic compo-

nents makes the biggest ripple in the dc current because of its

large magnitude and low frequency.

The proposed dc filter structure shown in Fig. 1(b) is designed

to damp the 6th harmonic component effectively. The proposed

dc filter structure uses a parallel resonant circuit which is tuned

for 6th harmonic component. Other harmonics of the dc circuit

are filtered with the inductor connected in series with the res-

onant circuit. With the proposed modified dc filter structure the

amount of the total filter inductance can be significantly reduced.

Impedance of the conventional dc filter structure is

(1)

where is the resistance of the dc filter. For modified dc filter

structure can be written as

(2)

Ldc

Z s( ) Rdc sLdc+=

Rdc

Z s( ) Rdc sLdcLrs Rr+

LrCrs2

RrCrs 1+ +----------------------------------------+ +=

A Current-Source Active Power Filter with a New DC Filter Structure

Mika Salo

Department of Electrical Engineering, Institute of Power Electronics

Tampere University of Technology

P.O.Box 692, FIN-33101 Tampere, Finland

(a)

(b)Fig. 1. The current source active power filter (a) with conventional dc filterstructure and (b) with modified dc filter structure.

Ldc

Powersupply

Load

Ls

Cs

Rectifier

us

T1

T6

T5

T4

T3

T2

Supplyfilter

bridge dc-filter

i ldAidc

sAi rAitAi

Lf

Ldc

Powersupply

Load

Ls

Cs

us

T1

T6

T5

T4

T3

T2

Supplyfilter

bridge dc filter

ildA

idc

sAi rAitAi

Lf

Lr Cr

icr

Rectifier

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where , and are the inductance, capacitance and resis-

tance of the parallel resonant circuit respectively.

Impedances of the conventional (--) and modified (-) dc filter

structures are plotted in Fig. 2 as a function of the angular fre-

quency. With conventional dc filter =170 mH and =8

.

The modified dc filter have parameter values: =30 mH,

=3

,

=15 mH, = 2

and =18.7

F. The resistanc-

es are approximate values at 300 Hz. Fig. 2 shows also the im-

pedance curve with 45 mH dc filter inductor (.-) which is the

total inductance of the modified dc filter structure. These param-

eter values are used with the active power filter of which nominal

power is 5 kW.

Fig. 2 shows that impedance of the modified dc filter is in-

creased rapidly at 1900 rad/s (300 Hz). This is caused by the par-

allel resonant circuit which is tuned for this frequency (6 th

harmonic component). At this frequency the impedance of themodified dc filter is higher than the impedance of the conven-

tional filter which indicates that the proposed filter structure can

store effectively the energy of the filtered 5th and 7th load cur-

rent harmonics without large ripple in dc current. Fig. 2 shows

also that the impedance of the modified filter is very low at 2700

rad/s (420 Hz). This is caused by the series resonance of the pro-

posed dc filter structure. The frequency of the series resonance is

determined by and the parallel connection of and .

When selecting the parameters for the modified dc filter struc-

ture it should be made sure that the frequency of the series reso-

nance is not near to 12th harmonic component which is the next

important harmonic component in the dc circuit of the active fil-

ter. Otherwise, the series resonance should neither be near 6th

harmonic component. This can be avoided when the ratio of

and is between 1/2-2.

III

. C

ONTROL OF CSAF

The proposed dc filter topology can be used with any con-

trol system of CSAF. Fig. 3 shows a control system [6]

which is practical for current-source topology. It is realized

in the synchronously rotating reference frame where the ac-

tive power of the active filter can be simply controlled withreal axis component and the reactive power with imagi-

nary axis component of the filter current. The superscript

s in space vector variables and x/y in space vector components

refers to a synchronously rotating coordinate system. The har-

monic compensation is based on the feedforward control of

the load currents. The active filter currents are controlled

in an open-loop manner.

The reference values for active filter current vector are cal-

culated as follows:

(3)

Lr Cr Rr

Ldc Rdc

Ldc

Rdc Lr Rr Cr

Cr Ldc Lr

Lr

Ldc

isx

isy

isxy

isx*

iff x*

idc x*+=

and

(4)

where (both components combined in one expression),

and are outputs of the feedforward, dc current and

the reactive power controls respectively. These two compo-

nents form the rectifier current reference vector which is

transformed to the stationary reference frame and fed to the

modulator. Due to the open-loop control of the active filter

currents the currents references are not realized accurately

because the supply filter takes capacitive currents. Also, oscil-

lations may occur in supply currents due to the LC-filter res-

onance if the active filter current references are rapidly

changed. Detailed describtion of the control methods to solve

both of these problems can be found in [6].

A. Dc current control

The task of the active filter is to compensate the harmonics of

the non-linear load. The magnitude of the dc current is changed

as the energy of the harmonic components is stored in and re-

stored from the dc circuit. This ripple in the dc current is the ba-

sic feature in the active power filter and for that reason the dc

current control should not try to remove it. However, the dc cur-

rent control should work effectively when the reference value of

the dc current is changed. For that reason, a non-linear PID con-

troller, where the input of the controller is the square of the errorsignal, is proposed for the control system shown in Fig. 3. With

small error values the controller acts slowly and when the error

value is increased faster control dynamics is achieved. Fig. 4

shows the block diagram of the non-linear PI controller where

the modified proportional gain depends on the error val-

ue. In practice, it is reasonable to limit between 0 and

, which is done with Saturation

block.

To understand how is constructed in the dc-current

control we can first consider that

(5)

isy*

iff y* iqy

*+=

iffxy*

idc x* iqy

*

i rs*

Pdck 1+

Pdck 1+

Pdcmax

idc x

3

2

---usx idcx udcbridc=

102

103

104

0

20

40

60

80

Impedance[dB]

Fig. 2. Impedances of the convential dc filter when =170 mH (--) and=45 mH (.-) and impedance of the modified dc filter (-) as function of the

angular frequency.

LdcLdc

Angular frequency [rad/s]

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i.e. that the ac and dc active powers of the converter are

equal in steady state if the converter losses are ignored.

is the dc-voltage of the rectifier bridge. By solving (5)

for and by using the reference values of and

we have

(6)

which is used in Fig. 3 to transform the dc-voltage reference

of the rectifier bridge to vector variable.

IV

. S

IMULATION RESULTS

The proposed control methods are tested with the simula-tion model. The simulation model is built in discrete form to

have close analogy with the microcontroller implementation.

Simulation is based on the control system shown in Fig. 3.

Sampling time of the feedforward and dc current controllers

is 50

s and the modulation frequency 10 kHz. The supply fil-

ter is realized with parameters: =0.6 mH and =8

F.

Figs. 5 and 6 show the simulation results of CSAF with mod-

ified and conventional dc filter structure respectively. As a non-

linear load a three-phase diode rectifier with RL-load is used.

The parameters of the modified dc filter are =15 mH,

=10 mH and =28.1

F and conventional =100 mH.

udcbr

idc x idcx udcbr

idcx* 2

3usx-----------udcbr

*idc cudcbr

*idc= =

Ls Cs

Ldc

Lr Cr Ldc

In both simulations the the ripple of the dc current is about

1 A. The total harmonic distortion (THD) of the load current

in both simulations is 26.8%. THDs of the supply current with

modified dc filter is 4.2% and with conventional filter 4.0%.

Fig. 6(d) shows the current of the resonant circuit capaci-

tor. Its amplitude is about 3A and frequency 300 Hz. This ac

current is caused by the 300 Hz ac voltage component across

the resonant circuit. This 300 Hz ac voltage component is

caused by the compensation of the 5th and 7th load current

harmonics as was explained earlier.

Figs. 7 and 8 show the simulation results of CSAF with

icr

Ldc

Powersupply isA

idcCSAFildA

itA

3->2

ild

ejs

ilds

PID(e2)

idc

idc*

*

+

Modulator

HPF

iffx*

ildx

ildy

idcx*

++

++

*iffy

ir

us

isx*isy

*

irA

Ls

Cs

iqy*

+

+

HPF

CDC

c

udcbr*

FG

Load

-1 -1

CDC

Feedforwardcontrol

Reactivepower control

Dc current

ir

s*= i

s

s*

ildx^

ildy^ 1

Lf

udcbr

ejs

control

*

s

s

L Cr r

Fig. 3. Control system of CSAF with modified dc filter topology.

Ts

Tdc

Delay

+

+

Saturation

+ Ts

Ddc

+

+

+

Saturation

Delay

idck*,

idck+1 +1

*

abs

Saturation

Pk+1

Pdc

dc

udcbrk*, +1

Fig. 4. Non-linear PID controller for dc current control.

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modified and conventional dc filter structure respectively

when as a non-linear load a three-phase diode rectifier with

RC-load is used. In this case the parameters of the modified

dc filter are =30 mH, =15 mH and =18.7F and con-

ventional =170 mH. The size of is increased in both

modified and conventional dc filter solutions because RC-type

diode load contains more harmonics than RL-type load. Also,

Ldc Lr Cr

Ldc Ldc

(a) (b)

(c) (d)

t[s] t[s]

t[s] t[s]

Fig. 5. Simulation results of CSAF with modified dc filter structure when the current harmonics are produced using a three-phase diode rectifier withRL-load. (a) Load current , (b) supply current , (c)dc current and (d) current of the resonant circuit .ild A itA idc icr

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

icr[A]

(a) (b)

t[s] t[s]

t[s]

(c)

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

Fig. 6. Simulation results of CSAF with conventional dc filter structure when the current harmonics are produced using a three-phase diode recti fierwith RL-load. (a) Load current , (b)supply current and (c)dc current .ild A itA idc

the size of is increased in order to keep the series resonance

of the modified filter far enough from the parallel resonance.

Furthermore, smaller decreases the current of the resonant

circuit capacitor. Anyway, Figs. 5(d) and 7(d) shows that

is much larger with RC-type load than RL-type load due to

the larger amount of harmonics included in RC-load which

increases also the harmonics of the dc circuit. THD of is

Lr

Cr

icr

ildA

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87.9% and THDs of with modified and conventional dc

filter 7.3% and 7.6% respectively.

V. EXPERIMENTAL INVESTIGATION

The prototype of CSAF is built using 1200 V, 50 A IGBTs.

The control system realization is based on the Motorola

itA MPC555 32-bit single-chip microcontroller. The supply filter

parameters and the sampling times of the control system are

the same as used in simulation model.

Figs. 9 and 10 show the experimental results of CSAF with

modified and conventional dc filter structure respectively. As

a non-linear load a three-phase diode rectifier with RL-load is

used. The dc filter parameters are same as used in simulations.

(a) (b)

(c) (d)

t[s] t[s]

t[s] t[s]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

icr[A]

Fig. 7. Simulation results of CSAF with modified dc filter structure when the current harmonics are produced using a three-phase diode rectifier withRC-load. (a) Load current , (b)supply current , (c)dc current and (d) current of the resonant circuit .i ld A itA idc icr

(a) (b)

t[s] t[s]

t[s]

(c)

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

Fig. 8. Simulation results of CSAF with conventionald dc filter structure when the current harmonics are produced using a three-phase diode rectifierwith RC-load. (a) Load current , (b)supply current and (c)dc current .i ld A itA idc

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By comparing Figs. 5, 6, 9 and 10 it can be seen that the

simulation and experimental results are in good agreement.

THD of in Figs. 9(a) and 10(a) is 27.1% and THDs of

with modified and conventional dc filter 3.3% and 3.2% respec-

tively.

Figs. 11 and 12 show the experimental results of CSAF

with modified dc filter in two cases when RC-type diode rec-

ildA itA

tifier load is used. In the first case shown in Fig. 11 the load

current contains only 1.3% of 3rd harmonic current. In the

case of Fig. 12 the amount of 3rd harmonic is 7.1%. The 3rd

harmonic component in load currents causes 2nd harmonic

voltage component in the dc circuit. However, the impedance

of the modified dc filter for 2nd harmonic component is very

low as can be seen in Fig. 2. As a result, the ripple in dc cur-

(a) (b)

(c) (d)

t[s] t[s]

t[s] t[s]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

icr[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

Fig. 9. Experimental results of CSAF with modified dc filter structure when the current harmonics are produced using a three-phase diode rectifierwith RL-load. (a) Load current , (b)supply current , (c)dc current and (d) current of the resonant circuit .ild A itA idc icr

(a) (b)

t[s]t[s]

t[s]

(c)

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it

[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

Fig. 10. Experimental results of CSAF with conventional dc filter structure when the current harmonics are produced using a three-phase diode rectifierwith RL-load. (a) Load current , (b)supply current and (c)dc current .ild A itA idc

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rent is much larger in Fig. 11(c) than in 12(c) due to the sig-

nificant 100 Hz component (2nd harmonic).

In principle, symmetric three-phase load should not contain

3rd harmonic component which was also confirmed with sim-

ulations. The 3rd harmonic components seen in measured

load current is caused propably by the distorted supply volt-

ages.

The experimental results of the conventional dc filter are

shown in Fig. 13. In Figs. 11-13 the THD of is around

82%. The THDs of shown in Figs. 11(b), 12(b) and 13(b) are

5.7%, 7.5% and 6.0%.

According to simulations and experimental investigation it

can be concluded that the proposed dc filter structure works

well if the load currents are symmetrical and do not contain

ildA

itA

(a) (b)

(c) (d)

t[s] t[s]

t[s] t[s]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

icr[A]

Fig. 11. Experimental results of CSAF with modified dc filter structure when the current harmonics are produced using a three-phase diode rectifierwith RC-load. Load current contains only small amount of 3rd harmonic component. (a) Load current , (b)supply current , (c)dc currentand (d) current of the resonant circuit .

ild A itA idcicr

(a)(b)

(c)(d)

t[s] t[s]

t[s] t[s]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

icr[A]

Fig. 12. Experimental results of CSAF with modified dc filter structure when the current harmonics are produced using a three-phase diode recti fierwith RC-load. Load current contains large amount of 3rd harmonic component. (a) Load current , (b)supply current , (c)dc current and(d) current of the resonant circuit .

i ld A itA idcicr

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3rd harmonic component. It seems that in practice three-

phase diode rectifier with RC-type load generates 3rd har-

monic in load currents and is not practical for proposed filter

structure. However, the modified filter can be used if the 3rd

harmonic component is not compensated. In the case of RL-

type diode rectifier load the amount of 3rd harmonic is mini-

mal and the proposed filter structure can be used. In this case

the amount of total inductance needed in the modified dc filter

is decreased to 1/4 compared to the conventional dc filter.

VI. CONCLUSIONS

In this paper a new smaller dc filter structure is pro-

posed for the current-source active power filter. In the

presented dc filter structure the energy of the most impor-

tant harmonics are stored in resonant circui t which makes

it possible to decrease the overall size of the filter. After

simulations and experimental tests it was found that theproposed filter structure works well with symmetrical

loads if the load current doesnt contain 3rd harmonic

component.

REFERENCES

[1] Y. Hayashi, N. Sato and K. Takahashi, A novel control of a current-

source active filter for ac power system harmonic compensation,IEEE Trans. Ind. App., Vol. 27, No. 2, pp. 380-385, March/April1997.

[2] A. Lindemann, Characteristics and applications of a reverseblocking IGBT, PCIM Europe, pp.12-16, January-February 2001.

[3] S. Fukuda and T. Endoh, Control method and characteristics of ac-tive power filters, 5th European Conference on Power Electronicsand Applications, Vol 8, pp. 139-144, 1993.

[4] S. Fukuda and T. Endoh,Control method for a combined active filtersystem employing a current source converter and a high pass filter,IEEE Trans. Ind. App., Vol. 31, No. 3, pp. 590-597, 1995.

[5] M.-X W ang and H. Pouliquen, Performance of an active filter usingPWM current source inverter, 5th European Conference on PowerElectronics and Applications, Vol. 8, pp. 218-223, 1993.

[6] M. Salo and H. Tuusa, H., A novel open-loop control method for acurrent-source active power filter,IEEE Trans. Ind. Electr., Vol. 50,No. 2, pp. 313-321, 2003.

(a) (b)

t[s] t[s]

t[s]

(c)

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

ild[A]

0 0.02 0.04 0.06-15

-10

-5

0

5

10

15

it[A]

0 0.02 0.04 0.060

5

10

15

idc[A]

Fig. 13. Experimental results of CSAF with conventional dc filter structure when the current harmonics are produced using a three-phase diode rectifierwith RC-load. (a) Load current , (b)supply current and (c)dc current .ild A itA idc