REDUCTION OF THD IN POWER SYSTEM USING GENERALIZED …

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Int. J. Elec&Electr.Eng&Telecoms. 2013 P Ramya and C N Arpitha, 2013

REDUCTION OF THD IN POWER SYSTEM USINGGENERALIZED UPQC

P Ramya1* and C N Arpitha1

*Corresponding Author: P Ramya, [email protected]

This paper deals with Unified Power Quality Conditioners (UPQC’s), which aim at the integrationof series-active and shunt-active filters. The main purpose of a UPQC is to compensate forvoltage flicker/imbalance, reactive power, negative-sequence current, and harmonics. In otherwords, the UPQC has the capability of improving power quality at the point of installation onpower distribution systems or industrial power systems. This paper presents a modifiedSynchronous-Reference Frame (SRF)-based control method to Shunt active filter andInstantaneous PQ (IPQ) theory based control technique for series active filter to compensatePower-Quality (PQ) problems through a three-phase four-wire Unified PQ Conditioner (UPQC)under unbalanced and distorted load conditions. The proposed UPQC system can improve thepower quality at the point of common coupling on power distribution systems under unbalancedand distorted load conditions. The simulation results based on Matlab/Simulink are discussed indetail in this paper.

Keywords: Unified Power Quality Conditioner (UPQC), Phased Locked Loop (PLL), ActivePower Filter (ACP), Synchronous Reference Frame (SRF)

INTRODUCTIONPower quality has become an important factorin power systems, for consumer andhousehold appliances with proliferation ofvarious electric and electronic equipment andcomputer systems. The main causes of a poorpower quality are harmonic currents, poorpower factor, supply-voltage variations, etc. Atechnique of achieving both active current

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Int. J. Elec&Electr.Eng&Telecoms. 2013

1 EEE Department, KSRMCE, Kadapa 516003, Andhra Pradesh, India.

distortion compensation, power factorcorrection and also mitigating the supply-voltage variation at the load side iscompensated by unique device of UPQCpresented in this paper.

This paper deals with Unified Power QualityConditioners (UPQC’s), which aim at theintegration of series active and shunt-activefilters. The main purpose of a UPQC is to

Research Paper

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compensate for supply voltage flicker/imbalance, reactive power, negative-sequence current, and harmonics. In otherwords, the UPQC has the capability ofimproving power quality at the point ofinstallation on power distribution systems orindustrial power systems. The UPQC,therefore, is expected to be one of the mostpowerful solutions to large capacity loadssensitive to supply voltage flicker/imbalance.

In this paper, much attention is paid to thegeneralized UPQC consisting of a seriesactive and shunt-active filter. The series-activefilter eliminates supply voltage flicker/imbalance from the load terminal voltage, andforces an existing shunt-passive filter to absorball the current harmonics produced by anonlinear load. Elimination of supply voltageflicker, however, is accompanied by lowfrequency fluctuation of active power flowinginto or out of the series-active filter. The shunt-active filter performs dc-link voltage regulation,thus leading to a significant reduction ofcapacity of the dc capacitor.

DEFINITION OF POWERQUALITYPower quality is a term that means differentthings to different people. Institute of Electricaland Electronic Engineers (IEEE) StandardIEEE1100 defines power quality as “theconcept of powering and grounding sensitiveelectronic equipment in a manner suitable forthe equipment.” As appropriate as thisdescription might seem, the limitation of powerquality to “sensitive electronic equipment”might be subject to disagreement. Electricalequipment susceptible to power quality ormore appropriately to lack of power quality

would fall within a seemingly boundlessdomain.

All electrical devices are prone to failure ormalfunction when exposed to one or morepower quality problems. The electrical devicemight be an electric motor, a transformer, agenerator, a computer, a printer,communication equipment, or a householdappliance. All of these devices and othersreact adversely to power quality issues,depending on the severity of problems.

A simpler and perhaps more concisedefinition might state: “Power quality is a setof electrical boundaries that allows a piece ofequipment to function in its intended mannerwithout significant loss of performance or lifeexpectancy.” This definition embraces twothings that we demand from an electricaldevice: performance and life expectancy. Anypower-related problem that compromiseseither attribute is a power quality concern. Inlight of this definition of power quality, thischapter provides an introduction to the morecommon power quality terms. Along withdefinitions of the terms, explanations areincluded in parentheses where necessary. Thischapter also attempts to explain how powerquality factors interact in an electrical system.

UNIFIED POWER QUALITYCONDITIONER (UPQC)The provision of both DSTATCOM and DVRcan control the power quality of the sourcecurrent and the load bus voltage. In addition, ifthe DVR and STATCOM are connected on theDC side, the DC bus voltage can be regulatedby the shunt connected DSTATCOM while theDVR supplies the required energy to the loadin case of the transient disturbances in source

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voltage. The configuration of such a device(termed as Unified Power Quality Conditioner(UPQC)) is shown in Figure 1. This is aversatile device similar to a UPFC. However,the control objectives of a UPQC are quitedifferent from that of a UPFC.

UPQC that combines the operations of aDistribution Static Compensator(DSTATCOM) and Dynamic VoltageRegulator (DVR) together.

One of the serious problems in electricalsystems is the increasing number of electroniccomponents of devices that are used byindustry as well as residences. These devices,which need high-quality energy to workproperly, at the same time, are the mostresponsible ones for injections of harmonicsin the distribution system. Therefore, devicesthat soften this drawback have beendeveloped.

One of them is the Unified Power QualityConditioner (UPQC). It consists of a shuntactive filter together with a series-active filter.This combination allows a simultaneouscompensation of the load currents and thesupply voltages, so that compensated currentdrawn from the network and the compensatedsupply voltage delivered to the load aresinusoidal, balanced and minimized. Theseries and shunt-active filters are connectedin a back-to-back configuration, in which theshunt converter is responsible for regulatingthe common DC-link voltage.

SRF MethodsAmong the several methods presented in theliterature, the Synchronous Reference Framemethod (SRF) is one of the most common andprobably it is widely used method. This sectionis organized as to describe succinctly the SRFmethods. The three methods presented in thissection with some results obtained with theabove mentioned methods. The nonlinear loadconsidered is a three-phase diode bridgerectifier. A Synchronous Reference Theory (SRF)

Figure 1: A Unified Power QualityConditioner

Figure 2: Equivalent Circuit of Ideal UPQC

A Unified Power Quality Conditioner(UPQC) is a device that is similar inconstruction to a Unified Power FlowConditioner (UPFC). The UPQC, just as in aUPFC, employs two voltage source inverters(VSIs) that connected to a dc. energy storagecapacitor. One of these two VSIs is connectedin series with ac. line while the other isconnected in shunt with the ac system. An

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In the SRF (Akagi et al., 2007), the load currentsignals are transformed into the conventionalrotating frame d-q. If theta is the transformationangle, the transformation is defined by:

c

b

a

q

d

XXX

XXX

21

21

21

34sin

32sinsin

34cos

32coscos

32

0

...(1)

Figure 3 shows the basic configuration ofsynchronous reference frame. In the SRF is atime varying angle that represents the angularposition of the reference frame which isrotating at constant speed in synchronism withthe three phase ac voltages. In the SRF is atime varying angle that represents the angularposition of the reference frame which isrotating at constant speed in synchronism withthe three phase ac voltages. To implement theSRF method some kind of synchronizingsystem should be used. In Akagi et al. (2007)Phase-Locked Loop (PLL) is used for the

implementation of this method. In this case thespeed of the reference frame is practicallyconstant, that is, the method behaves as if thereference frame’s moment of inertia is infinite.The fundamental currents of the d-qcomponents are now dc values. The harmonicsappear like ripple. Harmonic isolation of thed-q transformed signal is achieved byremoving the dc offset. This is accomplishedusing High Pass Filters (HPF). In spite of ahigh pass filter, a low pass filter is used to obtainthe reference source current in d-q coordinates.

Figure 3: Control Scheme Using UPQC

Figure 4: Basic Principle of SRF

Instantaneous Current Component(id–iq) TheoryThe Modified Synchronous Frame method ispresented in Graovac et al. (2007). It is calledthe instantaneous current component (id-iq)method. This is similar to the SRF framemethod. The transformation angle is nowobtained with the voltages of the ac network.The major difference is that, due to voltageharmonics and imbalance, the speed of the

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reference frame is no longer constant. It variesinstantaneously depending of the waveform ofthe three phase voltage system. In this methodthe compensating currents are obtained fromthe instantaneous active and reactive currentcomponents and of the nonlinear load. In thesame way, the mains voltages V(a, b, c) andthe polluted currents I(a, b, c) in -components must be calculated as given by(2), where C is Clarke Transformation Matrix.However, the load current components arederived from a synchronous reference framebased on the Park transformation, whererepresents the instantaneous voltage vectorangle (3).

IlcIlbIla

cii

11

...(2)

11

cossinsincos

11

ii

qidi

vv1tan ...(3)

Figure 5 shows the block diagram SRFmethod. Under balanced and sinusoidal mainsvoltage conditions angle is a uniformly

increasing function of time. This transformationangle is sensitive to voltage harmonics andunbalance; therefore d/dt may not be constantover a mains period. With transformation (2)and (3) the direct voltage component is:

ilil

vvvv

vvilqild

2

1

icq

icdvvvv

vvicic

2

1

icic

caIcompaIcompaIcomp

T

,,,

...(4)

Figure 5: Principal of the SynchronousReference Frame Method

Figure 6: Simulink Diagram of Proposed UPQC System

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Figure 7: Subsystem of UPQC

Figure 8: Simulink of Series Active Filter

Figure 9: Simulink of Shunt Active Filter

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SIMULATION RESULTSIn this study a new control algorithm for UPQCis evaluated by using simulation results givenin Matlab/simulink software. In simulationstudies, the results are specified before andafter UPQC system are operated. In this

system a breaker is connected such that if thebreaker is opened UPQC is disconnected andif breaker is closed UPQC is connected.

The proposed UPQC control algorithm hasthe ability to compensate both harmonics andreactive power of the load is eliminated.

Figure 10: Source Current Without Compensation

Figure 11: Source Voltage with Compensation

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Figure 12: Source Current with Compensation

Figure 13: THD for Source Current Before Compensation

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Figure 14: THD for Source Current Before Compensation

The tabular column shown below indicatesthe difference in the system having UPQC andwithout UPQC.

CONCLUSIONThis paper has dealt with UPQC’s, the aim ofwhich is not only to compensate for currentharmonics produced by nonlinear loads, butalso to eliminate voltage flicker/imbalanceappearing at the receiving terminal from theload terminal. Theoretical comparison among

three types of control methods for the series-active filter has clarified that the combinationof current and voltage-detecting methods issuitable for voltage flicker/imbalanceelimination and harmonic compensation. Theflow of instantaneous active and reactivepowers has shown that installation of the shunt-active filter is effective in performing dc-voltageregulation.

The proposed control strategy use onlyminimum measurement like loads and mainsvoltage measurements for series APF basedon the modified PLL with synchronousreference frame theory. The instantaneousreactive power theory is used for shunt APFcontrol algorithm by measuring mains voltage,currents and capacitor voltage. But theconventional methods require measurementsof the load, source and filter voltages andcurrents. The simulation results show that,

I source 0.045 A 23.66 2.45 A 0.55

V source 360 V 0.79 360 V 0.05

I load 3.5*10^-4 A 0.8 2.45 A 0.61

V load 360 V 0.91 360 V 0.31

Tabular Column 1: Comparison of THDBetween with and Without UPQC

Without UPQC With UPQC

VALUES THD% VALUES THD%

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when unbalanced and Nonlinear load currentor unbalanced and distorted mains voltageconditions, the above control algorithmseliminate the impact of distortion andunbalance of load current on the power line,making the power factor unity. Meanwhile, theSeries APF isolates the loads voltages andsource voltage, the shunt APF provides three-phase balanced and rated currents for theloads.

REFERENCES1. Akagi H and Fujita H (1995), “A New

Power Line Conditional for HarmonicCompensation in Power Systems”, IEEETrans. Power Del., Vol. 10, No. 3,pp. 1570-1575.

2. Akagi H, Watanabe E H and Aredes M(2007), Instantaneous Power Theory andApplications to Power Conditioning,April, Wiley IEEE Press.

3. Aredes M (1995), “A Combined Seriesand Shunt Active Power Filter”, in Proc.

IEEE/KTH Stockholm Power Tech Conf.,June, pp. 18-22, Stockholm, Sweden.

4. Sankaran C (2002), Power Quality, p. 202,CRC Press, Boca Raton.

5. Fujita H and Akagi H (1998), “The UnifiedPower Quality Conditioner: TheIntegration of Series and Shunt-ActiveFilters”, IEEE Trans. Power Electron.,Vol. 13, No. 2, pp. 315-322.

6. Graovac D, Katie A and Rufer A (2007),“Power Quality Problems Compensationwith Universal Power Quality ConditioningSystem”, IEEE Transaction on PowerDelivery, Vol. 22, No. 2.

7. Han B, Bae B, Kim H and Baek S (2006),“Combined Operation of Unified Power-Quality Conditioner with DistributedGeneration”, IEEE Transaction on PowerDelivery, Vol. 21, No. I, pp. 330-338.

8. Walling R A, Saint R, Dugan R C, Burke Jand Kojovic L A (2008), “Summary ofDistributed Resources Impact on PowerDelivery Systems”, IEEE Trans. PowerDelivery, Vol. 23, No. 3, pp. 1636-1644.

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