Understanding of Dynamic Voltage

ThammasatInt. J. Sc. Tech.,Vol. 11, No. 3, July-September2006

Understandingof DynamicVoltage RestorersThrough MATLAB Simulation Paisan Boonchiam and Nadarajah Mithulananthan ElectricPower SystemManagement,EnergyField of Study,Asian Instituteof Technology, P.O. Box 4, Klong Luang,PathumThant 12120,Thailand; E-mail: mithulan(@,ait. ac.th Abstract This paper presentsthe applicationof dynamic voltage restorers(DVR) on power distribution systemsfor mitigation of voltagesags/swellsat critical loads. DVR is one of the compensatingtypes of custom power devices. An adequatemodeling and simulation of DVR, including controls in MATLAB, show the flexibility and easinessof the MATLAB environment in studying and understandingsuch compensatingdevices.The DVR, which is basedon forced-commutated voltage sourceconverter(VSC) has been proved suitablefor the task of compensatingvoltage sags/swells. Simulationresultsare presentedto illustrateand understandthe performancesof DVR in supporting load voltagesundervoltagesags/swellsconditions. Keywords: custompower,power quality, voltagesags,voltageswells,DVR. 1. Introduction Modem power systems are complex networks,where hundredsof generatingstations and thousandsofload centersare interconnected through long power transmission and distribution networks [1]. The main concernof consumersis the quality and reliability of power suppliesat various load centerswhere they are locatedat. Even though the power generationin most well-developedcountriesis fairly reliable, the quality of the supply is not so reliable. Power distribution systems, ideally, should provide their customerswith an unintemrpted flow of energy at smooth sinusoidalvoltage at the contractedmagnitude level and frequency l2l However, in practice, power systems, especially the distribution systems, have numerous nonlinear loads, which significantly affect the quality of power supplies.As a result of the nonlinear loads, the purity of the waveform of supplies is lost. This ends up producing many power quality problems.Aparl from nonlinearloads,some systemevents,both usual (e.g. capacitorswitching, motor starling) and unusual(e.g.faults) could also inflict power quality problems[3]. The consequence of power quality problems could range from a simple nuisanceflicker in the electricallampsto loss of thousandsofdollars due to productionshutdown. A power quality problem is definedas any manifestedproblemin voltage/currentor leading

to frequencydeviationsthat result in failure or misoperation of customer equipment 13-41 Power quality problems are associatedwith an extensive number of electromagnetic phenomenain power systemswith broad ranges of time framessuch as long durationvariations, short durationvariationsand other disturbances. Short duration variationsare mainly causedby either fault conditions or energizationof large loads that require high starting currents. Dependingon the electricaldistancerelated to impedance,type ofgrounding and connectionof transformersbetween the faulted/loadlocation and the node, there can be a temporaryloss of voltage or temporaryvoltage reduction(sag) or voltage rise (swell) at different nodes of the system[5]. Voltage sag is defined as a sudden reductionof supply voltage down 90% to 10Vo of nominal, followed by a recoveryafter a short period of time. A typical duration of sag is, accordingto the standard,l0 ms to I minute. Voltage sag can cause loss of production in automatedprocessessincevoltagesag can trip a motor or cause its controller to malfunction. Voltage swell, on the other hand,is definedas a suddenincreasingof supply voltageup I l0% to 180%in rms voltageat the network fundamental frequencywith durationfrom 10 ms to 1 minute. Switching off a large inductive load or energizing a large capacitor bank is a typical

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system event that causes swells t I ] To compensatethe voltage sag/swell in a power distributionsystem,appropriatedevicesneed to be installedat suitablelocations.Thesedevices are typically placed at the point of common c o u p l i n gt P C C I w h i c h i s d e f i n e da s t h e p o i n t where the ownership of the network changes. The DVR is one of the custom power devices which can improve power quality, especially, voltage sags and voltage swells. As there are more and more concerns for the quality of supply as a result of more sensitiveloads in the systemconditions,a betterunderstandingof the devicesfor mitigating power quality problemsis important.This would allow us to make use of the functions of such devices in a better way with efficient control techniques.Hence,in this paper an attempt is made to understandthe functionsof DVR with the help of MATLAB. 2. Custom Power Technology The concept of custom Power was introducedby N.G. Hingorani in 1995. Like Flexible AC Transmission Systems (FACTS) for transmissionsystems,the term custompower pertains to the use of power electronics controllersin a distribution system,especially, to deal with various power quality problems. Just as FACTS improves the power transfer capabilitiesand stabilitymargins, custompower makes sure customersget pre-specifiedquality and reliability of supply. This pre-specified quality may contain a combination of specificationsof the following !l: low phase unbalance,no power intemrptions,low flicker at the load voltage,low harmonicdistorlionin load duration of voltage, magnitude and overvoltages/undervoltageswithin specified limits, acceptanceof fluctuations,nonlinearand poor factor loads without significant effect on the terminalvoltage. These can be done on the basis of an large customer, industrial/ individual, commercial parks or a supply for a high tech community on a wide areabasis.Custompower technology is a general term for equipment capableof mitigating numerouspower quality problems. Basic functions are fast switching, and current or voltage injection for correcting anomaliesin supply voltageor load current, by injectingor absorbingreactiveand activepower, respectivelyt6l, t7l

The power electronic controllers that are used in the custom power solution can be a nelwork reconfiguringtype or a compensating type. The network reconfiguratingdevices are usually called switchgearswhich include current limiting, current breaking and current transferring devices. The solid state or static versions of the devices are called: solid state currentlimiter (SSCL), solid statebreaker(SSB), and solid state transfer switch (SSTS). The compensatingdeviceseither compensatea load, i.e. its power factor, unbalanceconditions or improve the power quality of suppliedvoltage, etc. Thesedevicesare either connectedin shunt or in seriesor a combinationof both. This class o f d e ri c e s i n c l u d e s t h e d i s t r i b u t i o ns t a t i c compensator(D-STATCOM), dynamic voltage restorer (DVR), and unified power quality conditioner (UPQC) [2]. Among compensatrng devices,a DVR can deal with voltage sagsanc swells which are consideredto have a severe impact on manufacturing places such as semiconductors and plastic products, food placesandpapermills. processing 3. Dynamic Voltage Restorers A DVR is a device that injects a in seriesto dynamicallycontrolledvoltage V,,,(41 the bus voltage by means of a booster transformeras depicted in Figure L There are three single phase booster transformers connectedto a threephaseconverterwith energy storage system and control circuit [8]. The amplitudesof the three injected phasevoltages are controlled such as to eliminate any detrimental effects of a bus fault to the load voltage V1@. This means that any differential voltage causedby transientdisturbancesin the ac feederwill be compensatedby an equivalent voltage generatedby the converterand injected on the medium voltagelevel throughthe booster transformer. The DVR works independentlyof the type of fault or any eventthat happensin the system, provided that the whole system remains connectedto the supplygrid, i.e. the line breaker does not trip. For most practical cases,a more economical design can be achieved by only compensating the positive- and negative sequencecomponentsof the voltagedisturbance seen at the input of the DVR. This option is reasonablebecausefor a typical distributionbus conhguration, the zero sequence part of a

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disturbancewill not passthrough the step down transformersbecauseof infinite impedancefor this component. For most of the time the DVR has,virtually, "nothing to do," except monitoring the bus voltage. This means it does not inject any voltage (V,,ift) : 0) rndependentof the load current.Therefore,it is suggestedto particularly focus on the losses of a DVR during normal operation.Two specific featuresaddressingthis loss issuehave been implementedin its design, which are a transformer design with a low impedance,and the semiconductordevicesused for switching. An equivalentcircuit diagram of the DVR and the principle of seriesinjectionfor sagcompensationis depictedin Figure2. Mathematically expressed, the injection satisfies: SuDply Poir't

n^^-,-. T.---r^---.

( on'umer

(1)

vL(.t)=v"(t)+vi,iQ)

wherevr.ft)is the load voltage,v,(t/ is the sagged supply voltage and vi,1ft)is the voltage injected by the mitigation device as shown in Ftg. 2. Under nominal voltage conditions, the load power on eachphaseis given by (2): S, - rrti = e,. jQr

Q)

where I is the load current, and Pr, and Qt are the active and reactivepower taken by the load, respectively, during a sag/swell. When the mitigation device is active and restores the voltagesback to normal,the following appliesto eachphase: S t = P t j Q r = @ - . j Q , ) + ( P , , i -j Q . i )

(3)

where the sag subscript refers to the sagged supply quantities.The inject subscriptrefers to quantitiesinjectedby the mitigationdevice.

Fig. I Schematicdiagramof DVR System.

circuitof DVR. Fig. 2 Equivalent

:

Fig. 3 Compensationstrategyof DVR for voltagesags.

4. Modeling of DVR in MATLAB The compensationof voltagesag/swellcan be limited by a number of factors, including finite DVR power rating, loading conditions, power quality problemsand types of sag/swell. lf a DVR is a successfuldevice,the control is able to handle most sags/swells and the performancemust be maximized according to the equipment inserted. Otherwise, the DVR may not be ableto avoid tripping and evencause to the loads. additionaldisturbances The control strategy should be able to compensatefor any of voltage sag/swell and considerthe limitation the DVR. Figure 3 shows the supply voltage vector during the pre-sag stage which is representedsas V5',,"-,orft)on the dp,"-.up 0Xis.in which the rotatingphaseangle d is derived from Phase Lock Loop (PLL) [9], !0l.Initially, the load voltagevector V1,ft)islhe and is assumedto be 1.0 p.u. sameas Vt,o,"-,or(t) if the voltagedropsacrossthe seriestransformer are neglected.When the voltage sagsoccur, the actual source voltage vector V5ft) is moved to Vs.-r(t).To restorethe load voltagevector V/t), is provided by an injected voltage vector V1,,1(t) t h e D V R . A s i m i l a rc o m p e n s a t i osnt r a t e g yc a n be drawn in the form of a phasor diagram for voltaseswellas well.

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Figure 4 shows the basic control scheme and parameters that are measured for control purposes.When the grid voltageis at its normal level the DVR is controlledto reducethe losses in the DVR to a minimum. When voltage sags/swellsare detected,the DVR should react as fast as possibleand inject an ac voltageto the grid. It can be implementedusing a feedback control techniquebasedon the voltagereference and instantaneousvalues of supply and load voltage.The control algorithmproducesa threephasereferencevoltage to the seriesconverter that tries to maintain the load voltage at its referencevalue [0]-[2]. The voltage sag is detectedby measuringthe error betweenthe dqvoltage of the supply and the referencevalues. The d-referencecomponent is set to a rated voltage and the q-referencecomponentis set to zero. The MATLAB/Simulink environmentis a useful tool to implement this study becauseit has many tool boxes that can be used in this work and is easyto understand. In Figure4, the supplyvoltageis connected to a transformation block that convefts stationary frame to crp-frame. Output of this block is connectedto a phase lock loop (PLL) and anothertransformationblock that converts a,B-frameto rotating frame (dq), which detects the phase and changesthe axis of the supply voltage.The detectionblock detectsthe voltage sag/swell.If voltagesag/swelloccurs,this block generates the reference load voltage. The injection voltageis also generatedby difference betweenthe referenceload voltage and supply voltageand is appliedto the VSC to producethe preferredvoltage, with the help of pulse width modulation(PWM). 5. Simulation results In order to understandthe performanceof the DVR along with control, a simple distribution network as shown in Figure 5, is implemented. Voltage sags/swellsare simulatedby temporary connectionof differentimpedancesat the supply side bus. A DVR is connectedto the system througha seriestransformerwith a capabilityto insert a maximum voltageof 50ohof the phaseto-ground system voltage. Apart from this a series filter is also used to remove any high frequency components of power. The load consideredin the study is a 10 MVA capacity with 0.9 p.f., lagging.

.\

"N

VSC

qYu/ !,

dot PWM

Vaq T

e

J

+

d tr/,"t, m;,"'l'}*f

0

Fig. 4 Control structureof DVR.

IOMV

Fig. 5 Simple distributionnetworkwith DVR. 5.1 VoltageSags First, a caseof symmetricalsag is simulated by connecting a three-phasereactanceto the busbar. The results are shown in Figure 6. A 30% voltage sag is initiated at 400 ms and it is kept until 550 ms, with total voltage sag durationof 150 ms. Figure 6 (a), (b) and (c) show the seriesof voltage componentsinjected by the DVR and compensatedload voltage, respectively. As a result of DVR, the load voltage is kept at 1.00 p.u. throughout the simulation, including the voltage sag period. Observethat during normal operation,the DVR is doing nothing. It quickly injects necessary voltage componentsto smooththe load voltage upon detectinga voltagesag. In order to understandthe performanceof the DVR under unbalancedconditions,a singleline-ground(SLG) fault at supplybus bar at 400 ms is simulated.As a result of SLG fault. an unbalancedvoltage sag is createdimmediately after the fault as shown in Figure 7 (a), the supply voltage with two of the phasevoltages dropped down to 80%. The DVR injected voltageand the load voltageare shownin Figure 7 (b) and (c), respectively.As can be seenfrom the results, the DVR is able to produce the required voltage components for different phases rapidly and help to maintain a balanced andconstantloadvoltageat 1.00p.u.

ThammasatInt. J. Sc. Tech.,Vo1. 11,No. 3, July-September2006

!

". \) i i

l

b)g a ^\

Fig. 6 Simulationresultof DVR responseto a balancedvoltagesag.

i

Fig. 8 Simulationresultsof DVR responseto a balancedvoltageswell. Ph.a.rt.{es

a)

b' )ti ,,

0{

Fig. 7 Simulationresultof DVR responseto an unbalancedvoltagesag.

5.2 Voltage Swells Next, the performanceof DVR for a voltage swell condition is investigated.Here, voltage swell is generatedby energizing of a large capacitor bank and the correspondingsupply voltage is shown in Figure 8 (a). The voltage amplitude is increasedabout l25o/oof nominal voltage.The injectedvoltagethat is producedby DVR in order to coffect the load voltageand the load voltage,are shown in Figure 8 (b) and (c), respectively. As can be seenfrom the results, the load voltage is kept at the nominal value with the help of the DVR. Similar to the caseof voltagesag,the DVR reactsquickly to inject the appropriatevoltage component(anti phasewith the supply voltage or negative voltage magnitude)to correctthe supplyvoltage. The performance of the DVR with an unbalancedvoltage swell is shown in Figure 9. In this case, the unbalancedvoltage swell is createdby partly rejectingthe load. This results in an unbalancedvoltageswell wheretwo phase voltagesare equaland the otherphasevoltageis slightly higher that the first two phasesvoltages. The anti phase unbalancedvoltage component injectedby the DVR to correctthe load voltage is shownin Figure9(b) and the load voltageis

Fig. 9 Simulationresultof DVR responseto an unbalancedvoltageswell. given in Figure 9(c). Notice the constant and balanced voltage at the load throughout the simulation, including during the unbalanced voltageswell event. 6. Conclusion In this paper, performanceof a DVR in mitigating voltage sags/swellsis demonstrated with the help of MATLAB. A forcedcommutated voltage sources converter is considered in the DVR along with energy storageto maintain the capacitorvoltage. The DVR handles both balanced and unbalanced situationswithout any difficulties and injectsthe appropriatevoltage componentto correct any anomaly in the supply voltage to keep the load voltage balanced and constant at the nominal value. In the caseof a voltage sag, which is a condition of a temporary reduction in supply voltage, the DVR injects an equal positive voltagecomponentin all threephases,which are in phasewith the supplyvoltageto correctit. On the otherhand,for a voltageswell case,which is a condition of a temporary increasein supply voltage, the DVR injects an equal negative voltagein all threephases,which are anti-phase with the supply voltage. For unbalanced conditions, the DVR injects an appropriate unbalanced three-phase voltage components

Thammasatlnt. J. Sc. Tech.,Vol. 11, No. 3, July-September2006

positive or negativedependingon whether the condition is an unbalanced voltage sag or unbalancedvoltageswell. 7. References [1] A. Ghoshand G. Ledwich, PowerQuality Enhancement Using Custom Power Devices, Kluwer Academic Publishers, 2002. [2] N.G. Hingorani,IntroducingCustomPower, in IEEE Spectrum,32,pp. 4l-48, 1995. t3l C. Sankaran,Power Quality, CRC Press, 2001. l4l S. Santoso, Electrical Power System Quality, Mcgraw-Hill, 2002. t5l Y.H. Chung, G.H. Kwon, T.B. Park, and K.Y. Lim, Voltage Sag and Swell Generatorfor Evaluationof CustomPower Devices, in IEEE. Power Eng. Society, 4, pp.2507,2003. Application Nilsson, S, Special t6l Considerationsfor CustomPower Systems, rn IEEE Power Eng. Society,2, pp. ll27I l3l. 1999. l7l M.M. Osborne, R.H. Kitchin, and H.M. Ryan, Custom Power Technology in Distribution Systems:an Overview, in IEE

North Eastern Centre Power Section p ,p . 1 0 / 1 - 1 0 / 1 11,9 9 5 . Symposium 'Protectionfrom Voltage Dips R. Buxton, t8] with Dynamic Voltage Restorer', in IEE H alf day coIloquium,(1998). t9l Ming Hu and Heng Chen, Modeling and Controlling of Unihed Power Quality Component,in APSCOM-00, 2, pp. 431435,2000. [10] J.W. Liu, S.S.Choi, and S. Chen,Designof StepDynamic Voltage Regulatorfor Power IEEE Enhancement, in Quality Transactionson Power Delivery, 18, pp. 1 4 0 3 - 1 4 0 92.0 0 3 . 1] A. Ghosh, A.K. Jindal, and A. Joshi, [l InverterControl Using Output Feedbackfor Power Compensating Devices, in TENCON-2}}J,I, pp. 15-l'7.2003. [2] G. Sybille and P. Giroux, Simulation of FACTS Controllers Using the MATLAB Power System Blockset and Hypersim Real-Time Simulator, in IEEE Power Eng. Society,I, pp.488-491 ,2002. [3] A. Sanninoand J. Svensson,Static Series Compensatorfor Voltage Sag Mitigation SupplyingNonlinearLoads,in IEEE Pov'er Ereg.Society,2,2002.