3.4. Methodologyof UPQC Operating Conditions 24 Base on the above analysis the different modes of operation arediscussed conditions.

Case I: Reactive powerflow of diagramsDuring the normal operating condition when UPQC is not connectedto the network as shown in the Figure (a). In this condition the reactive powerrequired to the load is supplied by the source only. When the UPQC is connectedin the network and the shunt APF is put into the operation, the reactive powerdemanded by the load is provided by the shunt APF alone; such that no reactivepower support is put on the mains.

So as long as the shunt APF is ON, it ishandling all the reactive power even during voltage sag, voltage swell andcurrent harmonic compensation condition from the load side. The series APF isnot taking any active part in supplying the load reactive power. The reactivepower flow during the entire operation of UPQC is shown in the Figure3.3 (b).In this case no active power transfer takes place via UPQC, named as Zero Active PowerConsumption Mode.a)     No UPQC                                                 b) With Shunt APFFigure3.3: Reactivepower flow of diagramsCASE II: Active Power Flow during Voltage SagConditionIf k< 0, i.e.

Vt < VL, then PSr will bepositive, means series APF appends the active power to the load. This conditionis possible during the utility voltage sag condition, and is will begreater than the normal rated current. Hence we can say that the requiredactive power is taken from the utility itself by taking more current so as tomaintain the power balance in the network and to keep the dc link voltage atdesired level. This active power flows from the source to shunt APF.  The load would get the desired power evenduring voltage sag condition. Therefore in such cases the active power absorbedby shunt APF from the source is equal to the active power supplied by theseries APF to the load. Since series APF supplies active power, termed as ActivePower Delivering Mode.

The overall active power flow is shown in theFigure3.4.Figure 3.4:  Active Power Flowduring Voltage Sag ConditionKey: Ps’-powersupplied by the source to the load during voltage sag conditionPsr’-powerinjected by series APF in such a way that sum Ps”+Psr” will be therequired load power during normal working conditionPsh’=powerabsorbed by shunt APF during voltage sag condition Psr’= Psh’CASE:III Active power flow during voltage swellcondition If k > 0, i.e. vt > vL, then PSr will be negative,this means series APF is absorbing the extra real power from the source. Thiscondition is possible during the voltage swell problem.

Again is willbe less than the normal rated current. Since VS is increased, the dc link voltage canincrease. To maintain the dc link voltage at constant level the shunt APFcontroller reduces the current drawn from the supply. In other words we can saythat the UPQC feeds back the extra power to the supply system.

Sinceseries APF absorbs active power, termed as Active Power Absorption Mode.The overall active power flow is shown in the Figure3.5.Figure 3.5:Active power flow during voltage swell conditions Key: Ps”-powersupplied by the source to the load during voltage swell conditionPsr”-powerinjected by series APF in such a way that sum Ps”- Psr” will be the required load power during normalworking condition        Psh”=powerdelivered by shunt APF during voltage sag condition Psr”= PshCASEIV: Active Power Flow during Normal Working ConditionIf k = 0, i.

e. vt = vL, then there willnot be any real power exchange though UPQC. This is the normal operatingcondition and active power flow is shown in the Figure 3.6.Figure 3.6:  Active Power Flow during Normal WorkingConditionKey: Ps-powersupplied by the source to the load during voltage swell conditionPsr-powerinjected by series APF in such a way that sum Ps”=Psr” will be the required load power duringnormal working condition      Psh=powerdelivered by shunt APF during voltage sag condition Psr= PshCASE V: Voltage Harmonic Compensation ModeIf the supply voltage is distorted which containing severalharmonics, in such cases the series APF injects harmonic voltages equal to thesum of all harmonics voltage at PCC but in opposite direction. Thus the sum ofvoltage injected by series APF and distorted voltage at PCC will get cancelledout.

During this voltage harmonic compensation mode of operation the series APFdoes not consume any real power from sources since it injects only harmonicsvoltage. Here UPQC works in zero active power consumption modes.CASE VI: Current Harmonic Compensation ModeIf the load is a nonlinear one producing harmonics, in such casesthe shunt APF injects current equals to the sum of harmonics current but inopposite direction, thus cancelling out any current harmonics generated bynonlinear load. During this current harmonics compensation mode of operationthe shunt APF does not consume any real power from the source since it injectedonly harmonics currents. Here UPQC works in zero active powerconsumption modes. 3.5.

Dynamic dqo transformation The d-q-model convenience for control system design stationarysymmetrical AC variables to DC reference frame.Figure3.7…..

…..

……

…..

. Park’s (abc_to_dq0)transformation for the reference current sourceThe inverse dq0 transformationThe Park’s and its inverse is the same for the reference voltagesource. 3.

6.   CONTROL STRATEGY OF UPQCThe control strategy proposed hereaims to generate reference signals for both shunt and series APFs of the UPQC.The proposed control technique is capable ofextracting most of the load current and source voltage distortions. The seriesAPF is controlled to eliminatethe supply voltage harmonics; whereas the shunt APF is controlled to the supplycurrent harmonics and negative sequence current. Inthis paper, d-q frame theory is used to control both series and shuntcontroller.

3.6.1. Descriptionof implementation of series controllerThe control strategy of seriescontroller is shown in figure{?} in which voltage from the load and source isconverted to its equivalent dqo components, by using the angles from thediscrete three phase PLL. The angles for the calculation are generated by usingload voltage. The resultant voltage is then transferred back to the 3 phasecomponent using reverse transformation.

The resultant abc component is the fedto the discrite pulse width modulation generator (PWM) to produce gate pulses.The dqo transformation is done by parks transformation. The same formula can beused for current transformation. Inverse parks transformation for generation ofreference signal.Figure 3.8: Control Strategy for Series Controller3.

6.2.Description of implementation of shunt controllerThe control strategy of shunt controlleris shown in Figure 3.8.

it is same as that for series controller the differencelies in the fact that input in place of control voltage wave having magnitudeof  1 p.u controlled by the angle drawnfrom the pll(phase lock loop). Load control is given as input to pll. The anglefor the calculation is generated by using load current by parks transformationequation.

Resultant reference signal is fed to PWM generator which produce gatesignal.Figure3.9: Control Strategy for Shunt Controller 3.7.

 General Simulink Representations In this model, the load is supplyfrom the utility of 200V and 50 Hz as a source. A step up transformer is usedto step up the utility voltage of 200V to440V. Non-linear loads are chosen for the purpose of investigation of bothsingle line to ground fault and double-line to ground fault.

The series and shunt compensator ofUPQC is connected through an inductor so that to remove the harmonics from theinjected voltage and to remove thedistortions in the injected current. Here, two compensators of UPQC works asDynamic Voltage Restorer (DVR) andDistribution Static Compensator (DSTATCOM), the series compensator works as DVRand the shunt compensator works asDSTATCOM. Figure3.10: Matlab/Simulink model of UPQC 1

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