I I I I I I I I I I I I I I I I I I I I I I

ecause the incdence of harmonic-related problems in utility and industrial power systems is increasing, active power filters

have attracted great attention and have been ex- pected to be an effective remedy. Generally, an ac- tive filter has been considered to be a current source connected in parallel with the load (harmonic source). The approach is based on the principle of injecting harmonic current into the ac system, of the same amplitude ancl reverse phase to that of the load current harmonic:;. It has been wrongly be- lieved that the active filter is an ideal harmonic compensator whose compensation characteristics would not be influenced by the source impedance (as happens with a pas:,ive filter). In this article it will be shown, however, that such active filters (designated here as “parallel active filters”) are only effective for those nonlinear loads which can be considered as current -source type of harmonic source (“harmonic curr’mt source” herein), such as phase-controlled thyrktor rectifiers with large dc inductance for dc drives, etc.

Parallel active filters have been studied by many contributors since 1970s [l}-[9], and have been put into practical use [lo]-[ 121. Unfortunately, no paper has discussed the charact:eristics and application con- siderations of parallel aci:ive filters when they are ap- plied to nonlinear loads that are voltage-source type of harmonic source (“harmonic voltage source”), such as diode rectifiers with direct smoothing dc capaci- tors for ac drives, etc. This may be because traditional harmonic sources were mainly phase-controlled thyris- tor rectifiers and cycloconverters, which can be re- garded as current-source loads.

On the other hand, since more and more diode rectifiers with smoothing dc capacitors are used in electronic equipment, household appliances, and ac drives, harmonics generated by these loads have be- come a major issue. Naturally, attempts have been made to use parallel active filters for harmonic com- pensation of these diode rectifiers. However, it has been found in the field that the parallel active filters

not only cannot cancel the harmonics completely but also cause problems, such as enlarging the dc volt-age ripples and ac peak current of the rectifier. This is because a diode rectifier with smoothing dc capacitors behaves like a harmonic voltage source rather than as a harmonic current source. Another aspect is that there may be LC passive filters or power-factor correction capacitor banks connected on the load side (downstream) from the point where an active filter is connected. In this case, the equiva- lent circuit downstream seen from the connection point of active filter would not be a current source even ifthe main loads are a harmonic current source. When a conventional parallel active filter is applied to compensate a diode rectifier or a power system such that downstream contains passive filters and/or capacitor banks, the current injected by the active filter will flow into the diode rectifier or the load side that presents low impedance. As a result, har- monics of the source current cannot be completely canceled. Moreover, harmonic current flowing into the diode rectifier or the system downstream in- creases greatly, and overcurrent may occur due to the injected current.

A series active filter has been proposed to com- pensate for harmonics of diode rectifiers [ lb] , [17]. Although the series active filter is not found in com- mon practical use, [16] and E171 have shown that the series active filter is more suitable for harmonic compensation ofdiode rectifiers, i.e., harmonic volt- age sources. This paper puts more emphasis on the application issues of both parallel and series active filters in power systems. Their features and required operation conditions are clarified analytically and demonstrated through real field testing.

Two Types of Harmonic Sources

Current-Source ,Type of Harmonic Sources (Hurmonic Current Sources) As is well-known, thyristor converters are a com- mon source of harmonic currents. The distortion of

I I I I I I I I I I I I I I I I I I

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I This article was presented in its original f irm at the 7th Int‘l Conference on Harmonics and Quality of Power, Las

Vexas, Nevada. Pen(: is with Oak Ridge National Laboratory, P.0. Box 2009, Bldg. 91 02-1, Oak Ridge, T N ! 37831 -8038. He is a Stnior Member of the I E E E .

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4

/L

Current-Source Type AC Source Harmonic Source

(b) Fig. I . Typical current-source type of harmonic source. (d j Thyristor rectifier for dc drives, heater drive, etc. (b) Per-phase equivalent circuit of thyristor

Fig. 2. Typical voltage and current waveforms of thyristor rectifier. I I I I I I I I I I

the current waveform, i.e., the generation of har- monics, results from the switching operation. Fig.l(a) shows a typical thyristor rectifier, where a sufficient dc inductance produces a constant dc current. Fig.2 shows the source voltage and recti- fier current waveforms. Because the harmonic cur- rent contents and characteristics are less dependent upon the ac side, this type of harmonic source be- haves like a current source. Therefore, they are called current-source type of harmonic source (or harmonic current source) and represented as a cur- rent source shown in Fig. l(b).

Voltage-Source Type o f Harmonic Sources (Harmonic Voltage Sources) Nowadays, another common harmonic source is that of diode rectifiers with smoothing dc capaci- tors as shown in Fig. 3(a). Fig. 4 shows the current and voltage waveforms. Although the current is highly distorted, its harmonic amplitude is greatly affected by the impedance of the ac side, whereas the rectifier voltage (i.e., the voltage at the rectifier input terminal as shown in Fig. 4(b)) is characteris- tic and less dependent upon the ac impedance. Therefore, a diode rectifier behaves like a voltage source rather than a current source. Fig. 3(b) shows the equivalent circuit of the diode rectifier system, where the diode rectifier is represented as a volt- age-source type of harmonic source (or harmonic voltage source). Accordingly, the harmonic cur- rent originates from the rectifier voltage, and its contents are determined by, and dependent, upon the rectifier voltage and the ac impedance.

Characteristic Andysis of farale/ Active Filters A parallel active filter is a pulse-width modulation (PWM) inverter to be placed in parallel with a load (or a harmonic source) to inject a harmonic current with the same amplitude as that of the load into the ac system. Its control is implemented through ade- tection and extraction circuit of the load harmonic current. In order not to lose generality, the har- monic current source is represented as Norton’s equivalent circuit, and the harmonic voltage source as Thevenin’s equivalent circuit, respec- tively, as shown in Figs. 5 and 6. A pure cur- rent-source type of harmonic source is a special case of the Norton’s equivalent with ZL-+W. A pure voltage-source type of harmonic source is a special case of Thevenin’s equivalent with ZL--+O.

For Harmonic Current Sources Fig. 5 shows the basic p,rinciple of a parallel active filter compensating for a harmonic current source, where the harmonic source is represented as Nor- ton’s equivalent, 2 s is the source (line) impedance, ILO is the equivalent harmonic current source, ZL is the equivalent impedance on the load side which may include passive filters and power-factor cor- rection capacitors, and G is the equivalent transfer function of the active filter including the detection circuit of harmonics and the delay of the control circuit. In general, G has the function of notching the fundamental component, that is, IGlf=O at the fundamental, and IGlh= 1 for harmonics. In the fol- lowing analysis, all equations are represented in per unit (pz). From Fig. 5 , the following equations are obtained.

I , =GI , (1)

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z, (3) 1 vs I , = l G G .I,,, +-.

Z z, +- ZL z, +--I 1-G 1 -G

Focusing on harmonics, when the following equation

(4)

is satisfied, ( l) , (2) and (3) can be rewritten as

where, the subscripts, h and f, represent the har- monic components and the fundamental compo- nents respectively. ' ' I ' / ' ' represents the magnitude of a transfer function.

Equation (6) shows that the source current be- comes sinusoidal because of l l-Glh=O for har- monics when ( 4 ) is satisfied. Therefore, ( 4 ) is the required operating condition for the parallel ac- tive filter to cancel the load harmonic current. From ( 4 ) , i t is seen tha t only G can be pre-designed and determined by the active filter while Zs and ZL are determined by the system, i.e., the ac source imprdance and the load charac; teristics. Therefore, compensation characteristics of the active filter are determined not only by the active filter itself but also by the ac source and load impedance just like the case of conventional passive filters'. On the other hand, we have IZLI>>IZ~I for a pure current-source type of har- monic source such as a thyristor rectifier with a large dc inductance. So (2) and (4) can be reduced to the following equations, respectively.

1 LO

Equation (8) shows that compensation character- istics of the active filter are not influenced by the

VL

Voltage-Source Type AC Source Harmonic Source

(b) Fig. 3, Typical voltage-source type of harmonic source. (a) Diode rectifier for ac PWM drives, electronic equipment, etc. (b) Per-phase equivalent circuit of

Fig. 4 . Typical current and voltage waveforms of diode rectifier, (a) line current, (b) line-to-neutral voltage and line-to-line voltage at the rectifier inbut.

zs 's, k+

A - - - AC Source Parallel AF Harmonic Source

Fig. 7. Bask primipZe ofparazzeel active f i l ter for harmonic current source.

source impedance, Zs. So far, this property has been alleged as the advantage the active filter, making it superior to the passive filter. However, this superior property holds true only under the condition of

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Fig. 6. Basic principle of parallel active filter for harmonic voltage source. I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

lZ~l>>lZsl. Equation (9) can be easily satisfied by the active filter. G is determined by the active filter, and mainly dominated by the detection circuit of harmonics, delay time of the control circuit, and current response of the PWM inverter of the active filter. Ingeneral, ll-Glb=0.1-0.3, so the compensa- tion rate of harmonics, which is defined as (1-11s /I~0/).100%, ranges over 90% - 70%.

However, the load impedance, ZL, will become very low for harmonics when a parallel (or shunt) passive filter or power-factor improvement capaci- tor bank is connected on the side of the thyristor rectifier. Consequently, compensation characteris- tics of the active filter are influenced by the source impedance, Zs, because the condition, ~ZLI >>1Zsl, is no longer satisfied [13]. Moreover, it is clear from (7) that the current flowing into the passive filter 'connected on the load side is equal to ILh- ILOh=VSh/ZL. This current may be devastatingly large when the ac source is stiff and has appreciable voltage distortion, VJh. This means that ambient harmonics upstream will flow into the passive fil- ter. The load harmonic current, I L O ~ , will be en- tirely compensated by the active filter but not by the passive filter and the passive filter will absorb all ambient harmonics upstream instead. There- fore, special considerations are needed when a par- allel passive filter and a parallel active filter are placed in the same power system. If one tries to use a combined system of parallel active filter and par- allel passive filter to compensate load harmonics, the compensation responsibility of both the active filter and passive filter should be clear and well controlled. For example, using an active filter to compensate for the fifth- and seventh-order har- monics and a passive filter for higher order har- monics is a good responsibility-sharing of harmonic compensation, since an active filter is better for lower order harmonic compensation and a passive filter is better for higher order harmonic compensation. In this case, only the fifth- and sev- enth-order harmonics of the load current should be extracted as the reference of the active filter so that the active filter would not inject higher order har- monic current into the line.

For Harmonic Voltage Sources Fig. 6 shows the basic principle of parallel active filter compensating for a harmonic voltage source, where the load is represented as Thevenin's equiva- lent, i.e., a voltage source VL with an impedance ZL. From Fig. 6, we have the following equations.

I , =GI , (10)

L L

1-G z, +-

Therefore, when the following equation

is satisfied, the source current will become sinusoi- dal. That is,

I , = o

L

Equation (1 3) is the required operating condi- tion that should be satisfied when a parallel active filter compensates for a harmonic voltage source. However, it is difficult for a parallel active filter to satisfy (1 3 ) , because a harmonic voltage source usu- ally presents a very low internal impedance, ZL. For example, considering a diode rectifier with a large smoothing electrolytic dc capacitor, we have 12, = 01 as long as no series reactor is placed on the ac side of the rectifier. So (13) cannot be satisfied only with the source impedance, Zs, which is usu- ally under 10 percent (0.1 pu).

Providedthat lZjl=?%=O.O3pu, /l-G/h=O.l for the fifth-order harmonic, a series reactor of o.06pu (i.e., 6 percent) has to be placed on the ac side ofthe diode rectifier to let lZs+ZL/(l-G)I=3pu. More- over, it is evident from (12), (14) and (16) that (i) the parallel active filter makes the source imped- ance equivalent to zero as seen from the load side, thus lowering ac impedance to the load, (ii) har- monic current injected by the parallel active filter will flow into the load, and (iii) distortion of the source voltage, vsb, also causes a large harmonic

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current to flow into the load. These effects will largely increase the load harmonic current and the required volt-ampere (VA) rating ofthe parallel ac- tive filter, especially when ZL is small. These prob- lems will be shown later, in the discussion of field testing results.

Characferisfic Analysis of Series Active Filters A series active filter, as discussed in this paper, is to be placed in series between the ac source and the load (or harmonic source) to force the source cur- rent to become sinusoidal. The approach is based on a principle of harmonic isolation by controlling output voltage of the series active filter. In other words, the series active filter is to present high im- pedance to harmonic current, therefore blocking harmonic current flc’w from the load to the ac source and from the ac source to the load side. As in the preceding section, characteristics of series ac- tive filters are developed for harmonic current sources and harmonic voltage sources.

For Harmonic Curvent Sources Fig. 7 shows the basic principle of a series active fil- ter compensating for a harmonic current source, where Vc represents output voltage ofthe series ac- tive filter and the load (or harmonic source) is rep- resented as Norton’s equivalent. If the series active filter is controlled as

V, =KGI, , (17)

then we get the source current as follows,

where G is the equivalent transfer function of a de- tection circuit of harmonic current, including delay time of the control circuit. G is supposed to equal zero at the fundamental and approximately equal to 1 for harmonics, that is, IGk=O and /G/h=l . K is a gain with the dimension of ohms inpu. Distortion voltage of the ac source, VSh, usually is much smaller than harmonic current of the harmonic source. So when

is satisfied, we have

I s = o , (21)

that is, the source current becomes sinusoidal. Here, (19) is the required operating condition for the series active filter 1-0 compensate for a harmonic current source. Equation (19) requires that the gain,

Fig. 7. Basic principle of series active f i l t er fop a harmonic current source

AC Source Harmonic Source

Fig 8. Racic PrinciBle of ceriec a c h e fi l ter for a harmonic uoltage

K , should be large and the impedance of the load side, IZ&, be small for harmonics, in order to sup- press the source harmonic current. However, for a conventional phase-controlled thyristor rectifier, ZL is almost infinite, so (19) cannot be satisfied.

It is clear from (20) that the required output voltage of the series active filter, Vc, also becomes infinite. As a result, the series active filter cannot compensate for a current-source type of harmonic source theoretically. If a parallel passive filter is placed with the thyristor rectifier, however, ZL will become very small, (19) can be easily satisfied, and the required output voltage, Vc, becomes very small as well. This case is the combined system of series active filter and parallel passive filter, which has been discussed in 1141. In addition, it should be noted that the series active filter has a very impor- tant feature, that is, it provides harmonic isolation between the source and load. Equations (20) and (21) indicate that neither the source harmonics, VSh, will appear on the load side, nor the load har- monics, ILO, will flow into the ac source.

source. I I I I I I I I I I I I I I I

For Harmonic Voltage Sources I I l I I I I

Fig. 8 shows the basic principle of series active fil- ter compensating for a harmonic voltage source. If the series active filter is controlled as

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~

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

Basit Operating Printiple

Fig. 9. Hysteresis control method for series active filter.

Operates os a current source Operates us a voltage sourte

Fig. 10. Ramp-comparison control method for series active fi l ter.

Excellent and independent of the source impedance, Z,, for cur- rent-source loads, but depend on Z, when the load impedance, Z, , is IOW

the source current becomes

Excellent and independent of the source impedance, Z,, and the load impedance, Z,, for voltage-source loads, but depend on Z, when the loads are a current-source tvue

Therefore, when (24) is satisfied, we have ( 2 5 ) and (26) as:

I , = o ( 2 5 )

Equation (24) is the required operating condi- tion for the series active filter to compensate for a harmonic voltage source. To realize a large gain, K , the hysteresis-comparator control method shown

in Fig. 9, can be adapted. In this case, we have al- most K=-. Also, the ramp (or triangular wave) comparison control method shown in Fig. 10 can be used, where the reference of output voltage, Vi, is given by

V,* =G(KI , - V L ) . (27)

Hence, assuming that the active filter follows its reference bona fide, i.e.,V, =Vi , the source current becomes

V, - (1 - G)V, I , =

Z , + Z L +KG

When Vxi, is relatively small and

ll-Glb <<l (29)

is satisfied, the source current becomes sinusoidal even with K= lpu and IZ~+ZLIK, that is,

Equation (29) is the required operating condi- tion for the series active filter to compensate for a harmonic voltage-source load, which depends only on the series active filter itself. It is also clear from (30) that the compensation characteristics of the se- ries active filter are independent from the source im- pedance Zs and the load impedance ZL. Hence the series active filter can suppress harmonics of the source current effectively. These conclusions regard- ing the series active filter compensating for a harmonic voltage-source load are completely equivalent to those of the parallel active filter com-

Adaptive loads Inductive or current-source loads or harmonic SOUrceS, Capacitive Or voltage-source loads Or harmonic e.g., diode rectifiers with direct smoothing capacitors for ac drives

e.g., phase-controlled thyristor rectifiers of dc drives

Compensation Characteristics

Application Considerations A low impedance parallel branch (parallel passive filter or power-factor improvement capacitor bank) is needed when ap- died to an inductive or current-source load

Iniected current flows into the load side and may cause overcurrent when applied to a capacitive or voltage-source load

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pensating for a harmonic current-source load. Ex- perimental verification is shown two sections hence.

Comparison of P a d e l Active Filters and Series Active filters

In the previous sections, compensation characteris- tics of parallel active filters and series active filters were analyzed theorerically. The corresponding re- quired operation conditions of both parallel active filter and series active filter for harmonic current sources and harmonic voltage sources, respectively, were derived. In circuit configurations, duality re- lationships exist between the parallel active filter and the series active filter, that is, Fig. 5 is the dual ofFig. 8, and Fig. 6 is the dual ofFig.7. Therefore, the properties of the corresponding adaptive loads (harmonic sources) are each the dual of the other.

Table 1 summarim comparisons ofparallel active filters and series active filters, where their respective features and application considerations are listed.

Field Testing Results of Compensation for Harmorric Voltage Sources

For a harmonic current-source load as shown in Fig. 5 , compensation characteristics of the parallel active filters have been frequently discussed in the literature. This configuration has been considered as a typical case for studying active filters. The combined system ofa series active filter and aparal- le1 passive filter, which is the case in Fig. 7, has been presented in [14] and 1151. However, com- pensation character] stics for harmonic voltage sources have not been studied and reported so far. In addition, the author has found the difficulties and problems mentioned in previous sections when installing a parallel active filter to compensate for harmonics generated from ac drives. Therefore, for the typical harmonic voltage-source load-an ac drive's diode rectifier with smoothing dc capaci- tor-charactetistics Df both parallel active filter and series active filter are discussed in this section by simulation and experiment.

Parallel Active Filters Fig. 11 shows the practical system configuration discussed here by simulation and experiment, where the circuit constants are indicated in detail. L,, C, and Rr form a small passive filter to reduce the pulse width modulation (PWM) switching rip- ples generated from the inverter of the active filter. The load is a 60 k W ac drive and the active filter is a 50 kVA commercial product. The equivalent transfer function of the parallel active filter, G, is given by

where k is the equivalent gain (k=1+0.01-1k0.1, the error results from the precision of current sen-

Fig. 11. System configuration of parallel active filter.

40

(dB-pu)

0

-20

-49

Fig. 12. Compensation characteristics of parallel active filter for a harmonic voltage source w i t h different load impedances.

I

sors and current control), G, is the equivalent transfer function of the harmonic detection circuit (in the experimental system, first-order high-pass filter with cutoff frequency,fc=SOHz, is used on the synchronous reference frame, see [l5f and [8 ] for details), z is the delay time of the control circuit (a DSP digital control is used in the practical sys- tem, T = 30 psec), andaol(s+oo) is the transfer function of the isolation amplifier used in the con- trol circuit. From (1 l), the compensation charac- teristics can be obtained as

I " L l " , = O Iz, +&I Fig. 12 shows the calculated values plotted in

the solid lines. It is clear that, the smaller the load impedance, the worse are the compensation charac- teristics. Fig. 13 shows the simulation waveforms with Z ~ = 0 . 2 4 % (i.e., a small series ac reactor, as shown in Fig. 11, is placed). Harmonics remained in the source current after the parallel active filter was started. In addition, harmonic current of the load, especially the peak value, increases largely

I I I I I I I I I I I I

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Fig. 13. Simulated waveforms of parallel active filter for a harmonic voltage source (diode rectifier) wi th ZL=O.24%.

(4 (b)

Fig. 14. Field testing results ofparallel active filter for a harmonic voltage source (diode rectifier) wi th ZL=0.24%, IS , IC, and IL: 2OOAldi6 time: Smsldiv.

Fig. 15. Field testing results of parallel active filter for a harmonic voltage source (diode rectifier) wi th ZL=7.30%, IS , IC, and IL: 2OOAldiv, time: Smsldiv.

Fig. 16. System configuration of series active filter for harmonic compensation of a diode rectifier.

I

due to the injected harmonic current from the ac- ’

tive filter, which may cause overcurrent. When the series inductance ZL is reduced to zero, the parallel active filter will form a positive feedback, because the injected current will completely flow into the load side and then will be picked up by the active filter itself as its current reference.

With the same conditions, experiments were performed in the field. The experimental wave- forms are shown in Fig. 14, which exactly agree with the simulation waveforms shown in Fig. 13. The FFT results of the experimental waveforms are plotted as “X” in Fig. 12, which agree with the cal- culated results very well. In Fig. 14, the dc voltage of the parallel active filter was 8OOV, the rms VA rating and the peak VA rating of the active filter were 76% and 123% of that of the load, respec- tively. (The peak VA rating is defined as the prod- uct of peak voltage value and peak current value, divided by 2, that is, the peak VA rating =

(V, / f i . ( I , / f i . Therefore, it is not economi- cal and practical for a parallel active filter to com- pensate for a harmonic voltage-source load, especially when the load-side impedance is low, because the required VA rating of the parallel ac- tive filter may be even larger than that ofrhe load.

In the case of applying a parallel active filter to a harmonic voltage source, a large series reactor must be placed on the load side to enhance the load im- pedance. Fig. 15 shows experimental waveforms with larger series inductance, Z ~ = 7 . 3 % . In this case, the source current, Is, becomes sinusoidal, and the rms and peak VA ratings of the parallel ac- tive filter were 33% and 49% of that of the diode rectifier, respectively. To compensate for a har- monic voltage source, therefore, a minimum 6% of series inductance should be placed on the load side to meet the required operation conditions as men- tioned previously in the discussion of parallel ac- tive filters for harmonic voltage sources.

Series Aciwe Filters From the above-mentioned analytical and experi- mental results, it is evident that the injected har- monic current from a parallel active filter flows into the load side rather than into the source side for a harmonic voltage-source load, and is thus un- able to cancel the harmonic current of the source and enlarging harmonic current of the load in- stead. To solve the above problems, a large series reactor should be placed on the load side. However, a large series reactor is bulky, increases costs, and causes a fundamental voltage drop, hence it is un- desirable. Since it has been shown in the previous analysis that series active filters are better suited for harmonic compensation of a harmonic voltage source, a series active filter is applied to harmonic compensation of the diode rectifier in this section. The validity is corroborated by experiment.

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Fig. 16 shows the system configuration of a se- ries active filter used to compensate for the diode rectifier. The series active filter is placed between the ac source and the load through a three-phase transformer, the main circuit of which is the same circuit used in the parallel active filter shown in Fig. 11, composed of three-phase bridge PWM in- verter. Lr, Cy, and R, form a switching ripple trap just like the switching ripple filter used in the par- allel active filter. The hysteresis control method shown in Fig. 9 is usvd, the average switching fre- quency is 4 kHz, and the dc voltage of the series ac- tive filter is 340V, which is much lower than that of the parallel active filter of Fig. 11.

Figs. 17, 18, and 19 show experimental and simulated waveforms under the same conditions. Note that no series reiwtor is placed on the rectifier side, that is, ZL=O. After the series active filter was started, the source current became sinusoidal. The output voltage of the series active filter, Vc, was 65V. Without the aforementioned problems of the parallel active filter, ]:he series active filter has ex- cellent compensation characteristics. The rms VA rating of the series active filter was 25% of that of the load. Also, it is clear that since the current (in this case, the source current is equal to the load cur- rent) becomes sinusoidal, the peak value of the load current, and the tipples ofthe dc voltage of the rec- tifier, are minimized. In addition, the series active filter and the diode rectifier can share the same dc capacitor (source) by selecting an appropriate turns ratio for the isolation rransformer [l8]. In this way, the dc voltage control will become very easy. Futher, the switching ripple filtering inductor, L,, can be incorporated into the transformer to reduce component count.

Cc tnclusions In this article, comm'm nonlinear loads have been characterized into two types of harmonic sources, current-source type of harmonic source and volt- age-source type of harmonic source. Compensation characteristics of both parallel active filters and se- ries active filters have been discussed analytically and experimentally for these two types ofharmonic sources. The corresponding required operation conditions, features, application issues, and adap- tive harmonic sources of both filters have been pre- sented. The fact that the traditional active filter, the parallel active filter, is not panacea to harmonic compensation, and that and one cannot use it blindly, has been clearly addressed. The parallel ac- tive filter will increase harmonic current and may cause overcurrent of the load when the load is a har- monic voltage source. Instead, it has been verified that the series active filter is better suited for com- pensation of a harmonic voltage source such as a di- ode rectifier with smoothing dc capacitor. The conclusions of this article also imply that when a parallel active filter is installed in a power system

(a) (b)

Fig. 17. Field testing results of series active filter, VTa: 635Vldiv, ISa: ZOOAldiu, VCa: 254Vldiv, time: Smsldiv. (a) before being started, (b) aft6 being started.

(a) Before Started (b) After Started

Fig. 18. Field testing results of series active filter, upper trace=VLab: 63SVldiq middle trace=ISa: 2OOAldiv, lower trace=VCa: 254Vldiv, time: Smsldiv.

I

Fig. 19. Simulated results of series active filter.

network such as at a point of common coupling, the network impedance and main harmonic sources downstream from the installation point should be investigated in order to get good petfor- mance and to minimize influence to the loads downstream. In some cases, a combined system of parallel active filter and series active filter may be necessary by utilizing the harmonic isolation func- tion of the series active filters. No doubt active fil- ters are superior to passive filters if used in their niche applications.

References 111 H. Sasaki and T. Machida, "A new method ro eliminare AC

harmonic currenrs by magnetic compensation-consider- arions on basic design," l E E E Tram PAS , Vo1.90, no. 5 , p. 2009, 1971.

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@ Iff€ lndustry Applirotions Mogozine September/Ortober 1998

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[21] A. Mansoor and R. J. Ferraro, “Characterizing ASD power quality application issues,” presented at PQA97, Colum- bus, OH.

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