IEEE ISIE 2006, July 9-12, 2006, Montreal, Quebec, Canada

Hybrid Filter Applications for Power QualityImprovement of Power Distribution Networks

Utilizing Renewable Energies

Ghasem AhrabianFaculty of Electrical and Computer

Engineering, University of Tabriz, Tabriz, Iranahrabian@tabrizu.ac.ir

Farhad ShahniaEastern Azarbayjan Electric PowerDistribution Company, Tabriz, Iran

shahnia@tabrizu.ac.ir

Mehrdad Tarafdar HaqueCenter of Excellence for Mechatronics,

University of Tabriz, Tabriz, Irantarafdar@tabrizu.ac.ir

Abstract-A hybrid filter constructed of a shunt active filterand distributed passive filters used for power qualityimprovement in dispersed generation system is presented. Thedistribution system consists of several kinds of renewableenergies as DG units where the power quality problems due todispersed generation LV systems are introduced and thenecessity of using hybrid filters instead of active or passive filtersalone are discussed. The simulations are done withPSCAD/EMTDC for a distribution system with dynamicnonlinear loads and wind turbines, photovoltaic cells and dieselgenerators and several dynamic nonlinear load without anyfilters, with usage of just shunt active filter or passive filters andthe hybrid filter structure, introduced in the paper. Studying andcomparing the waveforms, harmonic contents and THD of thesystem current with different filter structures mentioned, provespower quality improvement by the applied hybrid filter.

I. INTRODUCTION

In recent years, there has been a considerable interest ininstallation of dispersed generation systems such as windfarms or photovoltaic cells in medium and low voltage powerdistribution systems. Because of installation of DG units lowpower quality problems such as harmonics, voltage flicker andfluctuation, reactive power consumption and low power factorare imposed to the distribution system. The circumstancebecomes more serious because of harmonics and high THD ofthe current waveform of nonlinear and dynamic loads in thedistribution system. Therefore, it is necessary to use effectivepower quality improvement facilities for improving the powerquality of the distribution system including the reduction ofharmonics in the system current, compensate reactive powerand improve power factor.Usage of passive or active filters can be a successful

procedure in limitation and removal of harmonics, reactivepower consumption and improvement of power factor andTHD of the system current. But because of dynamic loads,using passive filters alone cannot compensate the harmonicsgreatly; also it might cause resonances in the system, so theycannot be effective regardless of their low costs. On the otherhand, using active filters alone with the ability for removingsystem current harmonics of dynamic loads can increase thecosts of the project for high rating of active filters. Therefore,usage of hybrid filters would be the best solution for powerquality improvement because the problems of using passive

and active filters alone will not be any problem in such astructure, also the advantages of both of these filters can begained together. The schematic diagram of the powerdistribution system with several nonlinear loads and differentkinds of DG units such as wind turbines, PVs and dieselgenerators is shown in Fig. 1.

In this paper, power quality problems of dispersedgeneration LV or MV systems are introduced and thenecessity of using hybrid filters instead of active or passivefilters alone are discussed. A hybrid filter constructed of a PVconnected shunt active filter and distributed passive filtersused for power quality improvement in dispersed generationsystems is presented and the parameters of the passive filtersand the control system of the active filter are discussed.

Fig. 1. Schematic of the simulated study case with PSCAD/EMTDC.

II. POWER QUALITY PROBLEMS OF DG UNITS

Technical studies of dispersed generation systems prove lowpower quality issues of these systems such as voltagefluctuation and flicker or high harmonic contents of the systemcurrent [1-2].

In the case of installation of DG units to distributionsystems, voltage fluctuation is remarkable especially at

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connection points resulting of DG units switching or suddenfluctuations in their output power. So, load flow analysis ofthese systems should consider the voltage fluctuation not toexceed the standards for preventing power instruments andcostumers from damage. According to the distribution systemsstandards, voltage fluctuation of MV systems should be lessthan 2%-3% for limiting LV fluctuations less than 10%.

Another problem of dispersed distribution systems is anincrease in the fault level of the distribution system. This iscaused when a ground fault is occurred in the system becauseDG units still feed the fault although the power system feedersare disconnected.The most important power quality problem of dispersed

generation systems is the existence of high contents ofharmonics and interharmonics in the waveforms of the systemcurrent. For limiting the harmonics level of the system lessthan the standards, it is required to limit maximum outputpower ofDG units in the system or use effective power qualityimprovement or harmonic removal facilities in the powersystem to be able to gain more power form DG unites. In thispaper, an effective solution for this problem is proposed andpresented.

III. POWER QUALITY IMPROEVEMNT FILTER STRUCTURE

A. Passive FiltersPassive filters as classic methods for power quality

improvement of distribution systems consist of series LCtuned for removing a specific harmonic or blocking abandwich of severe harmonics of nonlinear loads current.These filters have low impedances for the tuned frequenciessuch as 5th and 7th and for these frequencies, the lowerimpedance of the filter in comparison with system impedance,the better filtering characteristics of the passive filter. Lowcost is a great benefit of these filters but because of their LCconstant parameters, they cannot be efficient power qualityimprovement facilities for dynamic nonlinear loads. Anotherproblem of installation of passive filters is probableresonances between the impedance of passive filter and thesystem resulting in increasing harmonics of system current.

B. Active FiltersActive filters as modem applications of power electronic

inverters are used to fulfill the necessity of compensating theharmonics of power systems for dynamic conditions such asswitching on and off loads and DG units. Shunt active filtersare used at main bus of distribution systems for reducing theamount of current harmonics of the system being inserted toHV network. These filters are controlled as current sources forproducing non sinusoidal currents according to non sinusoidalcurrent of loads or system for removing the harmonics andmaking system current sinusoidal. These filters suffer fromhigh ratings that increase the cost of project.

C. Hybrid FiltersHybrid filters constructed of active and passive filters with

different structures are used for removing the disadvantages ofpassive filters such as probability of resonances and nondynamic responses and also high costs of active filters, while

Different structures of hybrid filters utilized in powersystems are shown in Fig. 2.

A)

Source Non Linear

I LP . PassivAcivFe ltFeitersFiltFer | lte

couplingTransformer

B) i_|Z

IV. HYRI FITE STRUCTURE

ConsumingrthePV syon lineartemperatureofu350rcentigrad degrees te o tvLoadtiappoxiFilt0

I Active l Il

totrnfr h Csource voltgehoonLitV earAassive p o tsfadFllters p i a

fAtictive10 Fillter |

Fig. 2. Different structures of hybrid filters utilized in power distributionnetworks.

IV. HYBRID FILTER STRUCTUREThe DC side of the shunt active power filter is connected to

the output DC voltage of the PVs, therefore the required DCvoltage of the active filter is applied through the PV.Consuming the PV system with the parameters shown inAppendix and for a reference radiation ofless and ambienttemperature of 350 centigrade degrees, the output voltage ofthe PV system is approximately 0.5 kV DC. A DC-ACinverter and an appropriately selected transformer are neededto transform the DC output voltage of the PV system to 0.4 kVAC of the distribution system while producing the sufficientinsulation for the PV system. But the most important fact isthat the PV cells can only generate power in a specific periodof the 24 hours a day as shown in Fig. 3, therefore, the activepower filter for power quality improvement of the distributionsystem can be utilized in this period only. This facts does notaffect the basis of the proposed method because we canassume, at night times, lower power is needed which can beobtained from the distribution system while less power qualityproblems occur at night times because of less industrial powerconsumption [5].

PVt Gene0fitng Power

Day time3. .c rai. io .

Fig. 3. PV cell operation time in 24 hour period.

24 Hours a day

using the advantage of both of the filters with lower costs[3-4].

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In this paper, a Modified PQ reference current calculationmethod is used for controlling the active filter where thecompensating reference currents are as follows:

fa 1 FvaF )q- A LVI

- VA PL(OS)a ILQL

(1)

(2)A=v2 +v2

Utilizing a PI controller for keeping the DC voltage bus to aconstant value, the output of controller is multiplied bycapacitor current to calculate Pc losses. This power is added tothe compensated active power, and then the compensatingreference currents are calculated as:

F1fa 1 FVa -v1FiPL(os) + PcL'f# A Lvq VA ]QL

(3)

The hysteresis current control is applied to the modified PQcompensation method for PWM controlling of the switchingsof the inverter.

V. SIMULATION RESULTS

Fig. 1 shows a 0.4 kV, 60 Hz power distribution systemwith a PV connected to the main bus through an inverterbehaving as a shunt active filter and four dynamic nonlinearloads with distributed passive filters connected to their mainbuses, simulated with PSCAD/EMTDC [6]. The powerdistribution system involves several DG units such as windturbines with induction generators, PVs and diesel generator.The nonlinear load structure in the distribution system isconstructed of thyristor controlled RL loads in series with lowRL loads where the parameters of the nonlinear loadsincluding the magnitude of resistances and reactances,thyristors firing degree, THD% and 5th and 7th harmonic levelsof nonlinear load bus are shown in the Appendix. All otherparameters of the simulation study case including theparameters ofDG units are also shown in the Appendix [7-8].The simulation is done for the distribution system without anyfilters, with usage of just shunt active filter or passive filtersand the hybrid filter structure introduced in the paper [9-10].The current waveforms of the distribution system without

any filters are shown in Fig. 4 which proves high THD andharmonic contents of the system. The low power factor of thesystem can also be studied through the difference between thevoltage and current waveforms of the system as shown in Fig.5. The active and high reactive power consumption of thissystem is also shown in Fig. 6.

Fig. 5. Voltage-Current waveform of the distribution system without anyfilters.

.a P

Fig. 6. Active and reactive power waveforms of the distribution systemwithout any filters.

Using 5th and 7th shunt passive filters can greatly reduce theharmonic contents and compensate the reactive power butwith lower performance at the load changes. The currentwaveforms, power factor and active and reactive powerconsumption of the system are shown in Figures 7-9 where thepassive filters are switched on at 0.5 sec..

Fig. 7. Current waveforms of the distribution system using passive filters.

Fig. 8. Voltage-Current waveform of the distribution system using passivefilters.

Fig. 4. Current waveforms of the distribution system without any filters.

Fig. 9. Active and reactive power waveforms of the distribution system usingpassive filters.

Using a PV connected shunt active filter also reduces theharmonic contents and compensates the reactive power but forhigher costs. It should be noted that the power system can onlyutilize shunt active filter at day times according to Fig. 3. Thecurrent waveforms, power factor and active and reactive

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power consumption of the system are shown in Figures 10-12where the active filter is switched on at 0.55 sec. It is obviousthat the active filter can start compensation at dynamicsituations much faster than passive filters. Fig. 13 also showsthe active power consumption of the bare shunt active filterstructure.

Fig. 10. Current waveforms of the distribution system using PV connectedshunt active filter.

- a

Fig. 14. Current waveforms of the distribution system using proposed hybridfilter.

.

Fig. 15. Voltage-Current waveform of the distribution system using proposedhybrid filter.

Fig. 11. Voltage-Current waveform of the distribution system using PVconnected shunt active filter.

Fig. 12. Active and reactive power waveforms of the distribution system usingPV connected shunt active filter.

'- (;aefie fiiter m Padiive fRiier

Fig. 13. Active and reactive power consumption of the active filter whileusing only a PV connected shunt active filter.

Fig. 16. Active and reactive power waveforms of the distribution system usingproposed hybrid filter.

- a atiTe filter e P acive filte

p.iFig. 17. Active and reactive power consumption of the active filter whileusing hybrid filter.

Comparing the results of current THD and harmoniccontents through the simulation for the system without anyfilters, with usage of just 5th and 7th shunt passive filters, withusage ofjust a shunt active filter and the hybrid filter structureproposed in this paper, prove the efficiency of the proposedhybrid filter structure in current harmonic reduction as shownin TABLE I.

Using the proposed hybrid filter structure consisting of a PVconnected shunt active power filter and 5th and 7th shuntpassive filters has the best performance in harmonic reduction,power factor improvement and reactive power compensationwith lower active power consumption, lower costs and highercapabilities at dynamic situations as shown in Figures 14-17.

TABLE ITHD AND HARMONIC CONTENTS OF THE CURRENT OF THE POWER SYSTEM

WITH DIFFERENT FILTER STRUCTURES

System and Filter ConfigurationsWithout any filtersWith only 5th & 7th Shunt Passive FilterWith only PV Connected Shunt Active FilterWith Hybrid Filter (AF+PFs)

THD%

15.868.070.070.04

h5 h70.1389 0.07250.0337 0.06860.0005 0.00040.0003 0.0001

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).is 1.1

VI. CONCLUSIONIn this paper, a hybrid filter was used for power quality

improvement of the low voltage power distribution systemwith several installed DG units. The hybrid filter isconstructed of a PV connected shunt active filter at the mainbus and distributed passive filters at the buses of nonlinearloads. High harmonic contents of the distribution systembecause of nonlinear loads and DG units were removed andrestricted by the use of the active and passive filters. Studyingand comparing the waveforms, harmonic contents and THD ofthe system current without any filters, with PV connectedshunt active filter, with passive filters and with the hybridfilter structure introduced in the paper, proves theeffectiveness of the proposed hybrid filter structure.

VII. APPENDIXTABLE II

PARAMETERS OF PHOTOVOLTAIC CELLS OF DG UNITS

Number of modules connected in seriesNumber of module string in parallelNumber of cells connected in seriesNumber of cell strings in parallelReference radiationReference cell temperatureEffective cell areaSeries resistanceShunt resistanceDiode ideality factorBand gap energySaturation current at reference temperatureSpectral average responsibilityTemperature coefficient of photo current

20203621000250.01 m20.01 ohm1000 ohm1.51.103 eVIE-9 A0.005 A/W0.0017

TABLE IIIPROPERTIES OF WIND TURBINES OF DG UNITS

Turbine Rated MVAGenerator Rated MVAMachine Rated Angular SpeedRotor RadiusRotor AreaAir DensityGear Box Efficiency

2 MVA2 MVA376.99 rad/s.38 m4500 m21.229 kg/m20.97pu

TABLE IVPROPERTIES OF NONLINEAR LOADS OF THE DISTRIBUTION SYSTEM

LoadNo.

234

RLoadQ31

0.85

LLoadmH150300700400

Thyr.deg85756555

RseriesQ

0.2650.5250.35

LseriesmH0.6351.32

0.3 10

TABLE VPROPERTIES OF INDUCTION GENERATOR OF WIND TURBINES

Rated RMS Phase VoltageRated RMS Phase CurrentBase Angular Frequency (rad/s)Stator ResistanceFirst Cage ResistanceSecond Cage ResistanceStator Unsaturated Leakage CurrentRotor Unsaturated Mutual ReactanceUnsaturated Magnetizing ReactanceSecond Cage Unsaturated ReactancePolar Moment of Inertia (J=2H)Mechanical Damping

2.4 kV2.77 kA376.990.066 pu0.298 pu0.018 pu0.046 pu3.86 pu0.122 pu0.105 pu1200.00.008 pu

TABLE VITHD AND HARMONIC CONTENTS OF SYSTEM CURRENT

AT THE MAIN BUS OF NONLINEAR LOADS

Load No. THD% h5 h7

1 24.45 0.192 0.1262 20.58 0.169 0.1053 24.76 0.187 0.1274 10.43 0.094 0.038

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