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APPLICATION OF A SHUNT ACTIVE POWER FILTER

TO COMPENSATE MULTIPLE NON-LINEAR LOADS

Hanny H. Tumbelaka and Chem V. Nayar

School of Electrical and Computer EngineeringCurtin University of Technology

Lawrence J. Borle

Electrical and Electronics EngineeringUniversity of Western Australia

Abstract

In this paper, the implementation of a shunt active power filter with a small series reactor for athree-phase system is presented. The system consists of multiple non-linear loads, which are a

combination of harmonic current sources and harmonic voltage sources, with significant

unbalanced components. The filter consists of a three-phase current-controlled voltage sourceinverter (CC-VSI) with a filter inductance at the ac output and a dc-bus capacitor. The CC-VSI

is operated to directly control the ac grid current to be sinusoidal and in phase with the gridvoltage. The switching is controlled using ramptime current control, which is based on theconcept of zero average current error. The simulation results indicate that the filter along with

the series reactor is able to handle predominantly the harmonic voltage sources, as well as theunbalance, so that the grid currents are sinusoidal, in phase with the grid voltages andsymmetrical.

1. INTRODUCTION

Non-linear loads, especially power electronic

loads, create harmonic currents and voltages inthe power systems. For many years, variousactive power filters (APF) have been developedto suppress the harmonics, as well as compensate

for reactive power, so that the utility grid willsupply sinusoidal voltage and current with unitypower factor [1]-[3].

Conventionally, the shunt type APF acts toeliminate the reactive power and harmonic

currents produced by non-linear loads from thegrid current by injecting compensating currentsintended to result in sinusoidal grid current with

unity power factor. This filter has been proven tobe effective in compensating harmonic currentsources, but it cannot properly compensate for

harmonic voltage sources. Many electronicappliances, such as switch mode power supplies

and electronic ballasts, are harmonic voltagesources. A voltage sourcing series active powerfilter is suitable for controlling harmonic voltagesources, but it cannot properly compensate for

harmonic current sources [4].

In many cases, non-linear loads consist of

combinations of harmonic voltage sources andharmonic current sources, and may containsignificant load unbalance (ex. single phase loads

on a three phase system). To compensate for

these mixed non-linear loads, a combined systemof a shunt APF and a series APF can be effective

[4].

In this paper, a combination of a grid currentforcing shunt APF with a series reactor installed

at the Point of Common Coupling (PCC) isinvestigated to handle the harmonic andunbalance problems from mixed loads (seeFigure 1).

Figure 1. Active Power Filter configuration

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2. SHUNT ACTIVE POWER FILTER

OPERATION

The three-phase shunt active power filter is a

three-phase current controlled voltage-sourceinverter (CC-VSI) with a mid-point earthed,split capacitor in the dc bus and inductors in the

ac output (It is essentially three independentsingle phase inverters with a common dc bus).

Conventionally, a shunt APF is controlled insuch a way as to inject harmonic and reactivecompensation currents based on calculated

reference currents. The injected currents aremeant to cancel the harmonic and reactive

currents drawn by the non-linear loads.However, the reference or desired current to beinjected must be determined by extensivecalculations with inherent delays, errors and slow

transient response.

2.1 Series Inductance

A key component of this system is the addedseries inductance XL (see Figure 2), which is

comparable in size to the effective gridimpedance, ZS. Without this inductance (or aseries active filter), load harmonic voltage

sources would produce harmonic currentsthrough the grid impedance, which could not becompensated by a shunt APF. Currents from the

APF do not significantly change the harmonicvoltage at the loads. Therefore, there are stillharmonic voltages across the grid impedance,

which continue to produce harmonic currents.The inductance XL takes the place of a seriesactive power filter at significantly less cost,

providing the required voltage decouplingbetween load harmonic voltage sources and thegrid.

2.2 Direct Control of the Grid Current

In this scheme (see Figure 1), the CC-VSI isoperated to directly control the ac grid currentrather than its own current. The grid current is

sensed and directly controlled to followsymmetrical sinusoidal reference signals in phasewith the grid voltage. Hence, by putting the

current sensors on the grid side, the grid currentis forced to behave as a sinusoidal current sourceand the grid appears as a high-impedance circuitfor harmonics. By forcing the grid current to besinusoidal, the APF automatically provides theharmonic, reactive, negative and zero sequence

currents for the load, following the basic current

summation rule:

igrid= iAPF+ i loads (1)

The sinusoidal grid current reference signal isgiven by:

iref= k vgrid-1 (2)

where vgrid-1 is the fundamental component of thegrid voltage, and k is obtained from an outercontrol loop regulating the CC-VSI dc-bus

voltage. This can be accomplished by a simple PIcontrol loop. This is an effective way of

determining the required magnitude of activecurrent required, since any mismatch betweenthe required load active current and that beingforced by the CC-VSI would result in the

necessary corrections to regulate the dc-busvoltage. In the VSI topology used in the APF,the dc-capacitor voltage must be greater than the

peak of the ac grid voltage.

Figure 2. Circuit equivalent for harmonics

2.3 Ramptime Current Control

The performance and the effectiveness of the

filter are enhanced by the use of the ramptimecurrent control technique to control the CC-VSI.

The principle operation of ramptime currentcontrol is based on the concept of zero averagecurrent error (ZACE) [7], [8], [9]. In this

application, the current error signal is thedifference between the actual grid current andthe desired/reference grid current waveform. The

ramptime control produces switching instantswhich result in the current error signal crossingzero at intervals of half the desired switching

period. Hence the current error signal spendshalf the time on alternate sides of zero, resulting

in an average value of zero, a close following ofthe reference signal, and a switching period (and

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hence switching frequency) very close to the

desired value.

As mentioned, in this application, the CC-VSI is

switched so as to regulate the grid current ratherthan its own current. Controllability is ensuredby the proper relative sizing of the inverter filter

inductance Linv and the choice of the dc busvoltage so that the two output pwm states (perphase) will always result a corresponding

opposite polarity current error signal slopes. Thecontrollability of the ac grid current isguaranteed since the CC-VSI can be constructed

so that its minimum di/dtexceeds the maximumdi/dt permitted by the inductance XL. This

provides the CC-VSI complete control over theac grid current.

3. A SHUNT ACTIVE POWER

FILTER WITH HARMONIC

VOLTAGE SOURCING LOADS

3.1 Compensation for Harmonic Voltage

Sources

To show a compensation for harmonic voltagesources, a simulation was conducted using circuit

constants from the literature [4] based on a three-phase ac system with a grid voltage of 400V-50Hz, a 60kW diode rectifier load with dc filter

capacitor, a filter inductance (Linv) of 0.45mH(5.3%), ZS of 1.8%, and XL of 1.8%, without ahigh frequency filter. The circuit equivalent from

the harmonic point of view is shown in Figure 2.

From computer simulation results shown in

Figure 3, the three-phase shunt APF successfullyforces sinusoidal current from the grid, as shownin Figure 3(a) and 3(b). In doing this, the APF

compensates the harmonic voltages because theload harmonic voltage in Figure 3(c) (in spectralperformance up to 1kHz) appears across XL in

Figure 3(d). These same harmonic voltagesappear (in relative proportion to the inductances)in the inverter voltage in Figure 3(e) and across

the inverter inductance in Figure 3(f). Thus, theload harmonic voltages do not appear across ZSand load harmonic currents are not created

through this grid impedance. Also, assuming thegrid voltage harmonics are negligible, the ac gridvoltage at the PCC will be sinusoidal.

It is apparent that the CC-VSI generatesharmonic voltages with the same characteristics

as the load harmonic voltages. Moreover, the

harmonic voltage of CC-VSI yields equal-but-

opposite voltage on the filter inductance (Linv) to

keep harmonic voltage at the PCC close to zero.This result leads the output harmonic current of

the active filter to match the harmoniccomponents ofiloads.

Figure 4 shows that whenXL is reduced to 0.5%,the filter cannot suppress the harmonics properly,so that the grid currents are still distorted and

contain significant amount of harmonics. Theload harmonic voltage cannot be removedcompletely by the harmonic voltage on XL,

because the inverter cannot produce sufficientharmonic voltage to compensate load harmonic

voltage. Then, harmonic voltages still occuracross grid impedance. As a result, the inverterloses its controllability; and the compensation bythe active filter cannot be accomplished.

3.2 Series InductanceXL

There are several ways to determine the size ofXL [4], [6]. In [4], it is suggested that theminimum value ofXL is 6%. In [6], theXL is used

for a different purpose and not related toharmonic voltage type loads.

The practical choice ofXL is that it should be assmall as possible to minimize cost. Furthermore,if the APF can directly force the grid current to

be sinusoidal, the voltage at the PCC will havesimilar characteristics to the grid (except verysmall fundamental voltage drop and very small

phase shift). In order to make the loads operatein the similar operating point to which they wereconnected directly to the grid, then the size ofXL

should be chosen close to ZS XS in per-unitvalue (usually the resistance of the grid

impedance is very small compared to itsinductance).

From the above simulation, it is proven that withthe XL = 1.8%, the compensation is successful.

The value of XL could be lower than 1.8%provided that minimum di/dtofLinv exceeds themaximum di/dtpermitted by the inductance XL.Otherwise, the value ofLinv has to be reduced.

However, decreasing the Linv will increase thehigh switching frequency ripple in the ac gridcurrents.

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Figure 3. Simulation results for XL = 1.8%; (a)Igrid, (b) Igrid spectrum, (c) spectrum of V load

harmonics, (d) V on XL, (e) V output CC-VSI,(f) V on filter inductance, (g) V at PCC

4. A THREE-PHASE SHUNT ACTIVE

POWER FILTER WITH

MULTIPLE NON-LINEAR LOADS

By directly controlling the grid current, a three-phase shunt APF can be provided for all non-linear loads at the PCC instead of compensating

each load individually. The system is simpler

and more efficient because only one currentsensor for each phase is located in the grid side.

Figure 4. Simulation results for XL = 0.5%; (a)Igrid, (b) Igrid spectrum, (c) spectrum of V loadharmonics, (d) V on XL, (e) V output CC-VSI,

(f) V on filter inductance, (g) V at PCC

From the preceding explanation, the shunt APFwith a series reactor can compensate the

harmonic voltage sources in the loads. This filtercombination can also succeed for harmoniccurrent sources. In this case, the reactor will

function to limit the slope of the falling andrising edges of the load current [6]. For mixed

loads, it is practical to provide a series reactor fortotal loads. The reactor is installed at the PCCand integrated with the APF. The size can be

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chosen for the possible maximum power of

harmonic voltage sources.

A three-phase shunt APF has been proven for

balanced loads [5]. However, the system maycontain significant amounts of load unbalance asin commercial buildings with non-linear single-

phase computer type loads. Such loads producelarge negative sequence and harmonic currents.Hence, the filter has to inject the inverse of the

negative sequence current to balance theunbalanced loads. The shunt APF discussedpreviously has the ability to balance the

asymmetrical current. This is because the CC-VSI is operated to directly control the ac grid

current to follow a three-phase balancedsinusoidal reference signal without measuringand determining the negative sequencecomponent. Once the grid currents are able to

follow the reference signal, the inverter createsthe inverse of the negative sequence currentsautomatically. At the PCC, all three currents

according to (1) are potentially accessible to bedirectly controlled by the CC-VSI.

The system in Figure 1 is tested using computersimulation to verify the concepts discussed inprevious sections. The circuit constants are the

same as of the above section. The mixed loadsare 45kW three-phase diode rectifier with LCfilter on dc side and a 15kW single-phase diode

rectifier with capacitive filter on the dc sideconnected phase-to-phase across the ac grid. Thethree-phase current waveforms of the mixed

loads are shown in Figure 5. It shows clearly thatthe currents are not sinusoidal and areunbalanced.

4.1 Mixed-Type Harmonic Sources And

Unbalanced loads

Figures 6 and 7 show results with several non-linear loads to demonstrate the validity of the

filter. In Figure 6, the shunt active power filtercombined with the series reactor is able tosuccessfully compensate the total mixed loads

that produce harmonic and unbalanced currents.The grid currents become sinusoidal and in phasewith the grid voltage (in this graph, only phase A

of the grid voltage is shown). The magnitude isdetermined by the active power required by thesystem.

Furthermore, the grid currents are symmetrical inmagnitude and phase. These currents are

balanced because the CC-VSI is able to generate

three different currents for each phase. For each

phase, the current controller is able to force the

average current error, which is the differencebetween the reference signal and the actual

current to be zero. Then, the individual phasecurrent can follow its reference signal closely.From Figure 7, it is obvious that phase B of the

inverter current is not the same as other twophases, since the single-phase load is connectedbetween phase A and C. Hence, the inverter not

only generates harmonics to eliminate the loadharmonics but also provide balancing to createthe symmetrical grid currents.

Figure 5. Three-phase load currents

Figure 6. Three-phase grid currents aftercompensation

Figure 7. Three-phase output currents of the

CC-VSI

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3.2 DC BusFigure 8 shows the simulation results of thedynamic condition of the dc-bus voltage. It can

be seen that the dc-capacitor voltage is decreasedwhen the load is increased. This is because theactive power demanded by the load is higher

than that supplied from the grid. The dc-bus hasto provide the active power to fulfill the powerbalance.

Figure 8. Dynamic state of dc-bus when theload is changing; upper graph: load and grid

currents - phase A; lower graph: dc-bus voltage

Once the transient interval is finished, the dc-busvoltage is recovered and remains at the reference

voltage 800V (by using a PI controller), andthe magnitude of the grid active currents is fixedat a designated value. At this time, the total

active power demanded by the load is suppliedfrom the grid, because the active power filteronly supplies the reactive power.

This same process will occur when the load isdecreased. In this case, the dc-capacitor voltage

will increase in a transient state. Hence, the dcbus capacitor must be sized not only to minimizethe ripple but also to provide maximum expected

power unbalance until the PI loop again achievessteady state. The above result shows that theamplitude of the grid currents is regulated

directly by controlling the dc bus voltage, andthe calculation process of the grid currentamplitude can be eliminated. Figure 8 also shows

that the dc-bus contains a ripple voltage at thesecond harmonic frequency since the system hasa single-phase diode rectifier load.

4. CONCLUSION

This paper proposes the implementation of athree-phase active power filter together with a

decoupling reactor in series with the loadoperated to directly control the ac grid current tobe sinusoidal and in phase with the grid voltage.

From the simulation results, this system providesunity power factor operation of non-linear loadswith harmonic current sources, harmonic voltage

sources, reactive, and unbalanced components.

5. REFERENCES

[1] M. El-Habrouk, M.K. Darwish and P.Mehta, Active Power Filter: AReview, IEE Proc. Electr.Power.Appl,pp. 403-413, Sept 2000

[2] B. Singh, K. Al-Haddad and A.Chandra, A Review of Active Filter forPower Quality Improvements, IEEE

Trans. on Industrial Electronics, pp.960-971, Feb 1999

[3] Fang Zheng Peng, Harmonic Sources

and Filtering Approaches, IEEEIndustry Applications Magazine, pp.18-25, July 2001

[4] Fang Zheng Peng, Application issuesof Active Power Filters, IEEE IndustryApplications Magazine, pp. 21-30, Sept

1998[5] H.L. Jou, Performance Comparison of

the Three-phase Active-power-filter

Algorithms, IEE Proc. Gener. Trans.Distrib., pp. 646-652, Nov 1995

[6] Adil M. Al-Zamil and D.A Torrey, A

Passive Series, Active Shunt Filter forHigh Power Applications, IEEE Trans.on Power Electronics, pp. 101-109,

January 2001[7] L. Borle, Method and control circuit

for a switching regulator, U.S. Patent

US5801517, granted 1-September-1998[8] L. Borle, Zero average current error

control methods for bidirectional AC-

DC converters, PhD thesis, CurtinUniversity of Technology and theAustralian Digital Theses Program:

http://adt.caul.edu.au/[9] L. Borle and C. V. Nayar, Ramptime

Current Control, IEEE Applied PowerElectronics Conference (APEC96),March, 1996, pp 828-834.