VOLTAGE SAG AND SWELL COMPENSATION BY USING DVR …

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Int. J. Elec&Electr.Eng&Telecoms. 2016 K Anil Kumar and K V Dhanalakshmi, 2016

VOLTAGE SAG AND SWELL COMPENSATION BYUSING DVR WITH A BESS

K Anil Kumar1* and K V Dhanalakshmi2

*Corresponding Author: K Anil Kumar,[email protected]

Power quality problems become a major anxiety of industries due to enormous loss in terms oftime and money. In this paper, different voltage injection schemes for Dynamic Voltage Restorer(DVR’s) are analyze with a particular focus on a different technologies for DVR to compensatethe voltage sag, swell and harmonics. One of the most powerful FACTS devices is the DVR toalleviate the voltage sag and swell. The SRF theory has been used for estimating the referenceDVR voltages by converting voltage from rotating vectors to stator vectors form. A comparisonof the DVR with different schemes has been performed with a PWM controlled VSC and SVPWMcontrolled VSC of DVR. In the simulink model the In-Phase compensation technique for voltagequality improvement is used.

Keywords: Dynamic Voltage Restorer (DVR), Power quality, Unit vector, SVPWM, Voltageharmonics, Voltage sag, Voltage swells

INTRODUCTIONPower quality problems in the present-daydistribution systems are addressed in theliterature due to the increased use of sensitiveand critical equipment pieces such ascommunication network, process industries,and precise manufacturing processes. Powerquality problems such as transients, sags,swells, and other distortions to the sinusoidalwaveform of the supply voltage affect theperformance of these equipment pieces.Technologies such as custom power devicesare emerged to provide protection against

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Int. J. Elec&Electr.Eng&Telecoms. 2016

1 PG Scholar, Teegala Krishna Reddy Engineering College, Hyderabad, India.2 Assoc.Professor, Teegala Krishna Reddy Engineering College, Hyderabad, India.

power quality problems. Custom powerdevices are mainly of three categories suchas series-connected compensators known asDynamic Voltage Restorers (DVRs), shunt-connected compensators such as distributionstatic compensators, and a combination ofseries and shunt-connected compensatorsknown as unified power quality conditioner. TheDVR can regulate the load voltage from theproblems such as sag, swell, and harmonicsin the supply voltages. Hence, it can protectthe critical consumer loads from tripping andconsequent losses. The custom power devices

Research Paper

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are developed and installed at consumer pointto meet the power quality standards such asIEEE-519.

Voltage sags in an electrical grid are notalways possible to avoid because of the finiteclearing time of the faults that cause the voltagesags and the propagation of sags from thetransmission and distribution systems to thelow-voltage loads. Voltage sags are thecommon reasons for interruption in productionplants and for end-user equipmentmalfunctions in general. In particular, trippingof equipment in a production line can causeproduction interruption and significant costsdue to loss of production. One solution to thisproblem is to make the equipment itself moretolerant to sags, either by intelligent control orby storing “ride-through” energy in theequipment. An alternative solution, instead ofmodifying each component in a plant to betolerant against voltage sags, is to install a plantwide uninterruptible power supply system forlonger power interruptions or a DVR on theincoming supply to mitigate voltage sags forshorter periods. DVRs can eliminate most ofthe sags and minimize the risk of load trippingfor very deep sags, but their main drawbacksare their Standby losses, the equipment cost,and also the protection scheme required fordownstream short circuits. Many solutions andtheir problems using DVRs are reported, suchas the voltages in a three-phase system arebalanced and an energy-optimized control ofDVR is discussed. Industrial examples ofDVRs are given in, and different controlmethods are analyzed for different types ofvoltage sags in. A comparison of differenttopologies and control methods is presentedfor a DVR in. The design of a capacitor-

supported DVR that protects sag, swell,distortion, or unbalance in the supply voltagesis discussed in. The performance of a DVRwith the high-frequency-link transformer isdiscussed in. In this paper, the control andperformance of a DVR are demonstrated witha reduced-rating Voltage Source Converter(VSC). The Synchronous Reference Frame(SRF) theory is used for the control of the DVR.

OPERATION OF DVRThe schematic of a DVR-connected systemis shown in Figure 1a. Voltage injection orcompensation methods by means of a DVRdepend upon the limiting factors such as; DVRpower ratings, various conditions of load, anddifferent types of voltage sags. Some loadsare sensitive towards phase angel jump andsome are sensitive towards change inmagnitude and others are tolerant to these.Therefore the control strategies depend uponthe type of load characteristics. The voltageVinj1 is inserted such that the load voltage Vload

is constant in magnitude and is undistorted,although the supply voltage Vs is not constantin magnitude or is distorted. Figure 1b showsthe phasor diagram of different voltageinjection schemes of the DVR. VL(pre-sag) is avoltage across the critical load prior to thevoltage sag condition. During the voltage sag,the voltage is reduced to Vs with a phase lagangle of . Now, the DVR injects a voltage suchthat the load voltage magnitude is maintainedat the pre-sag condition.

According to the phase angle of the loadvoltage, the injection of voltages can berealized in four ways. Vinj1 represents thevoltage injected in-phase with the supplyvoltage. With the injection of Vinj2, the load

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voltage magnitude remains same but it leadsVs by a small angle. In Vinj3, the load voltageretains the same phase as that of the pre-sagcondition, which may be an optimum angleconsidering the energy source. Vinj4 is thecondition, which may be an optimum angleconsidering the energy source. Vinj4 is thecondition where the injected voltage is inquadrature with the current, and this case issuitable for a capacitor-supported DVR as thisinjection involves no active power. However, aminimum possible rating of the converter isachieved by Vinj1. The DVR is operated in thisscheme with a Battery Energy Storage System(BESS).

Figure 2 shows a schematic of a three-phase DVR connected to restore the voltageof a three-phase critical load. A three-phase

supply is connected to a critical and sensitiveload through a three-phase series injectiontransformer. The equivalent voltage of thesupply of phase AVMa is connected to the Pointof Common Coupling (PCC) VMa throughshort-circuit impedance ZSa. The voltageinjected by the DVR in phase A VCa is suchthat the load voltage VLa is of rated magnitudeand undistorted. A three-phase DVR isconnected to the line to inject a voltage inseries using three single-phase transformersTr, Lr, and Cr represent the filter componentsused to filter the ripples in the injected voltage.A three leg VSC with Insulated-Gate BipolarTransistors (IGBTs) is used as a DVR, and aBESS is connected to its dc bus.

CONTROL OF DVRThe compensation for voltage sags using aDVR can be performed by injecting orabsorbing the reactive power or the real power.

Figure 1: (a) Basic Circuit of DVR,(b) Phasor Diagram of the DVR Voltage

Injection Schemes

Figure 2: Schematic of theDVR-Connected System

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When the injected voltage is in quadrature withthe current at the fundamental frequency, thecompensation is made by injecting reactivepower and the DVR is with a self-supporteddc bus. However, if the injected voltage isinphase with the current, DVR injects realpower, and hence, a battery is required at thedc bus of the VSC. The control techniqueadopted should consider the limitations suchas the voltage injection capability (converterand transformer rating) and optimization of thesize of energy storage.

Control of DVR With PWMTechnique and BESS for VoltageSag, Swell and HarmonicsCompensationFigure 3 shows a control block of the DVRin which the SRF theory is used for reference

signal estimation. The voltages at the PCCVs and at the load terminal VL are sensed forderiving the IGBT’s gate signals. Thereference load voltage VL* is extracted usingthe derived unit vector Load voltages (VLa,VLb, VLc) are converted to the rotatingreference frame using abc–dqo conversionusing Park’s transformation with unit vectors(sin, , cos, ) derived using a phase-lockedloop as

...(1)

Similarly, reference load voltages (VLa*,VLb*, VLc*) and voltages at the PCC Vs are alsoconverted to the rotating reference frame.

Figure 3: Control Block of the DVR that Uses the SRF Method of Control

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Then, the DVR voltages are obtained in therotating reference frame as

...(2)

...(3)

The reference DVR voltages are obtainedin the rotating reference frame as

...(4)

...(5)

The error between the reference and actualDVR voltages in the rotating reference frameis regulated using two Proportional-Integral(PI) controllers.

Reference DVR voltages in the abc frameare obtained from a reverse Park’stransformation taking VDd* from (4), VDq* from(5), VDo* as zero as

...(6)

Reference DVR voltages (Vdvra*, Vdvrb*,Vdvrc*) and actual DVR voltages (Vdvra*, Vdvrb*,Vdvrc*) are used in a pulse width modulated(PWM) controller to generate gating pulses toa VSC of the DVR. The PWM controller isoperated with a switching frequency of 10 kHz.

Control of DVR with SVPWM andBESS for Voltage Sag, Swell, andHarmonics CompensationComparison of Sinusoidal PWM andSpace Vector PWM• In the Figure 4 below, U is the phase-to-

center voltage containing the triple orderharmonics that are generated by spacevector PWM, and U1 is the sinusoidalreference voltage. But the triplen order

harmonics are not appeared in the phase-to-phase.

• SPWM only reaches to 78% of square-wave operation, but the amplitude ofmaximum possible voltage is 90% ofsquare-wave in the case of space vectorPWM. The maximum phase-to-centervoltage by sinusoidal and space vectorPWM are respectively. voltage as well. Thisleads to the higher modulation indexcompared to the SPWM.

Vmax = Vdc/2: for Sinusoidal PWM; And

Vmax = Vdc/3, where, Vdc is DC-Linkvoltage: for Space Vector PWM

SVPWM control strategy is to adopt aspace vector of the inverter voltage to get betterperformance of the exchange is gained in lowswitching frequency conditions. Carrier basedSVPWM allow fast and efficientimplementation of SVPWM without sectordetermination. The technique is based on theduty ratio profiles that SVPWM exhibits (asshown in Figure 6). By comparing the duty ratioprofile with a higher frequency triangularcarrier the pulses can be generated, basedon the same arguments as the sinusoidal pulse

Figure 4: Phase-to-Center Voltageby Space Vector PWM

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width modulation. Figure 5 shows a controlmodel of the DVR in which the SRF theory isused for the control of DVR with SVPWMtechnique and fuzzy controller. In this paper weintroduce a fuzzy logic controller in place of PIcontroller of Vdc error generator.

The fuzzy controller used in this voltagereference generator has ‘7’ input membershipfunctions and seven ouput membershipfunction. The membership functions can beseen in Figure 7. By using the fuzzy logictracking error and transient overshoots ofSVPWM can be considerably reduced.Voltages at the PCC VS are converted to therotating reference frame using abc–dqo

Figure 6: SVPWM Gate Pulse Signalsto the VSC

Figure 5: SRF Control for VSC by Using SVPWM and FUZZY Techniques

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(LPFs). The components of voltages in the d-and q-axes are

...(7)

...(8)

In this method the VSC is controlled bySpace Vector PWM. Space Vector generatedpulses to the VSC by the following equation.

...(9)

where is the offset voltage

Space Vector PWM can produce about15% higher than Sinusoidal PWM in outputvoltage.

The compensating strategy forcompensation of voltage quality problemsconsider that load terminal voltage should beof rated magnitude and undistorted.

Figure 7: Fuzzy Input and OutputMembership Functions

Figure 8: MATLAB-Based Model of the BESS-Supported DVR-Connected System

conversion using Park’s transformation. Theharmonics and the oscillatory components ofthe voltage are eliminated using lowpass filters

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MODELING AND SIMULATIONThe DVR-connected system consisting of athree-phase supply, three-phase critical loads,and the series injection transformers shown inFigure 2 is modeled in MATLAB/Simulinkenvironment along with a sim power systemtoolbox and is shown in Figure 8. An equivalentload considered is a 10-kVA 0.8-pf lag linearload. The parameters of the consideredsystem for the simulation study are given inthe Appendix. The performance of the PWMcontrolled and SVPWM controlled DVR isdemonstrated for different supply voltagedisturbances such as voltage sag and swell inFigures 10 and 11 respectively. Figures 10and 11 shows the transient performance of thesystem under voltage sag and voltage swellconditions. At 0.2 s, a sag in supply voltage iscreated for five cycles, and at 0.4 s, a swell in

Figure 9: Fuzzy Rule Identification

Figure 10: Dynamic Performance of PWM Controlled DVR with In-Phase Injection DuringVoltage Sag and Swell Applied to Critical Load

the supply voltages is created for five cycles. Itis observed that the load voltage is regulatedto constant amplitude under both sag and

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Figure 11: Dynamic Performance of SVPWM Controlled DVR with In-Phase InjectionDuring Voltage Sag and Swell Applied to Critical Load

Figure 12: Voltage Magnitude at the PCCand Load During Sag and Swell

swell conditions PCC voltages VS loadvoltages VL DVR voltages VC, amplitude of loadvoltage VL and PCC voltage VS sourcecurrents iS reference load voltages VLref , anddc bus voltage Vdc are also depicted in Figure11. The load and PCC voltages of phase Aare shown in Figure 12, which shows the in-phase injection of voltage by the DVR.

The load voltage is maintained sinusoidalby injecting proper compensation voltage bythe DVR. The Total Harmonics Distortions(THDs) of the voltage at the PCC, supplycurrent, and load voltage are shown in Figures14 and 15, respectively.

It is observed that the load voltage THD isthe level of 0.60% for PWM controlled DVRand the load voltage THD is the level of 0.64%

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for the SVPWM controlled DVR. Thecompensation of voltage by using SVPWMcontrolled DVR is much better than PWMcontrolled DVR but in point of THD theSVPWM controlled DVR is little lesser in therange of 0.04%. The magnitudes of the voltageinjected by the DVR for mitigating the samekinds of sag in the supply with different anglesof injection are observed. The injected voltage,series current, and kilovoltampere ratings ofthe DVR for the four injection schemes are

Figure 13: Comparison of OutputCompensation Voltages Between

Sinusoidal PWM and SVPWM TechniquesDuring Sag and Swell

Figure 14: THD for PWM Controlled DVR

Figure 15: THD for SVPWM Controlled DVR

Table 1: Comparison of DVR Rating for SagMitigation

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given in Table 1. In Scheme-1 in Table 1, thein-phase injected voltage is Vinj1 in the phasordiagram in Figure 1. In Scheme-2, a DVRvoltage is injection at a small angle of 30%,and in Scheme-3, the DVR voltage is injectedat an angle of 45°.

The injection of voltage in quadrature withthe line current is in Scheme-4.

The required rating of compensation of thesame using Scheme-1 is much less than thatof Scheme-4. The injected voltage is highercompared with an inphase injected voltage(Scheme-1).

When a deep sag occurs in the line thePWM controlled DVR only compensateminimum level of voltage in the line where asSVPWM controlled DAR can compensate thevoltage if the level of sag is more is shown inthe Figure 13.

The fuzzy rule identification as follows inFigure 9.

SUMMARYIn this chapter we have seen simulink modeldesigned and the corresponding results areobtained for the proposed system.

CONCLUSIONIn this paper, study and performance of DVRfor mitigating the voltage sags/swell indistribution power systems is presented. Thevarious techniques show that the DVR issuitable for compensation of voltage sag andswell with the help of these different controllingtechniques which are discuses in this paper.The controlling of DVR is simpler as comparedto other customs power devices. A comparisonof the performance of the DVR with different

schemes has been performed with a PWMcontrolled VSC and SVPWM controlled VSCof DVR are illustrated. The reference loadvoltage has been estimated using the methodof unit vectors, and the control of DVR hasbeen achieved, which minimizes the error ofvoltage injection. The SRF theory has beenused for estimating the reference DVRvoltages. The controlling of DVR is simpler ascompared to other customs power devices.

REFERENCES1. Bhavraju V B and Enjeti P N (1996), “An

Active Line Conditioner to BalanceVoltages in a Three Phase System”,IEEE Trans. Ind. Appl., Vol. 32, No. 2,pp. 287-292.

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5. Ghosh A and Ledwich G (2002a), “PowerQuality Enhancement Using CustomPower Devices”, Kluwer, London, UK.

6. Ghosh A and Ledwich G (2002b),“Compensation of Distribution SystemVoltage Using DVR”, IEEE Trans. PowerDel., Vol. 17, No. 4, pp. 1030-1103.

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Trans. Power Del., Vol. 13, No. 1,pp. 278-282.

8. Moreno-Munoz A (2007), “Power Quality:Mitigation Technologies in a DistributedEnvironment”, Springer-Verlag, London,UK.

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AC line voltage: 415 V, 50 Hz

Line impedance: Ls = 3.0 mH, Rs = 0.01

Linear loads: 10-kVA 0.80-pf lag

Ripple filter: Cf = 10 F, Rf = 4.8

DVR with BESS

DC voltage of DVR: 300 V

AC inductor: 2.0 mH

Gains of the d-axis PI controller: Kp1 = 0.5, Ki1 = 0.35

Gains of the q-axis PI controller: Kp2 = 0.5, Ki2 = 0.35

PWM switching frequency: 10 kHz

DVR with dc bus capacitor supported

DC voltage of DVR: 300 V

AC inductor: 2.0 mH

PWM switching frequency: 10 kHz

Series transformer: three-phase transformer of rating 10 kVA, 200 V/300 V

APPENDIX

VOLTAGE SAG AND SWELL COMPENSATION BY USING DVR …

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