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C H A P T E R 6

P A R TIC L E S W A R M O P T I M I Z A T IO N B A S E D D Y N A M I CVO L T AG E RE S TO RE R

6.1. INTRODUCTION

The recent grouzh in the use of power electronics has cauxd a greaterdHareness on power quality. Voltage sags. swells and harmonics can cause ancqulpment to fail or shut down. and also create huge current imhalanccs ujhich couldblow fuses or trip the breakers. These etyects can he v e y expensive fbr the customer.ranging from minor quality variations to production downtime and equipmentdamage. Utilities are interested in keeping their customers satisfied and also keepingthem on-line and drawing kilowatts. creating more revenue for the utility. All of thisInterest ha? resulted in a variety of' devices designed for mitigating power disturbances\uch as voltage sags. One class ol.the device is the Dynamic Voltage Restorer (INK).

I . Jauch et al. 11351 have demonstrated the in-phase voltage ~njectiontechnique where the load voltage is assumed to be in-phase with the presag voltaget i ~ rhe DVR control. N .A Samara et al. 11361 have incorporated the I N K nto adlstrihution network and analyzed the perfkrmance of D VK for highly sensitivetndustrial loads hased on reactive power compensation. Alexander Kara el al. 11 371have presented the technical aspects of designing a dynamic voltage restorer to meettile stringent requirements of voltage dips mitigation with respect to the magnitude of\oltage dip, fault duration. permissible line voltage deviations and response time.S W.Middle Kauff et al. [I381 have proposed that almost d l voltage disturbances are%\socialed with some degree of phase shifi for .wries custom power devices. Poh('hlang Loh et al. 11391 have presented the implementation and control of a highrollage dynamic voltage restorer for compensation of utility voltages using amultilevel inverter topology. John Godsk Nielsen el al. (1401 have proposed differentDVR control methods to reduce voltage disturbances caused by voltage sap withphase jump technique for very sensitive loads. Hyosung Kim et al. 11411 haveexploited various operation modes and boundaries such as inductive operation.

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capacitive operation and minimum power operation as an effective and economicsolution to overcome voltage sags. Chris f i t m a al. [I421 have proposed a novelstate-space matrix method for computation of the phase shift and voltage reduction ofthe supply voltage much quicker than the fourier transform or a phase locked loop(PLL). F. Jurado et al. [I431 have proposed a neural network control strategy forprotection of sensitive loads from th e effe cts of voltage sags.

In this chapter, PSO based approach identifies the required value of phaseadvancement angle corresponding to minimum energy injection from the energystorage element o f the DVR such as a capacitor or a ban ery. T he proposed PSO basedenergy optimization method is tested using a case study for a balanced 3-phasesystem. The energy stored in the DVR after implem enting PSO technique is lesserthan that of conventional in-phase voltag e injection and phase advance compensationmethods.

6.2. OVERVIEW OF A DYNAMIC VOLTAGE RESTORER

l'he dynamic voltage restorer is a custom power device for series connectionInto a distribution line. When connected in xries between a source and a load. theDVK can control the voltage applied to the load by injecting a voltage of arbitraryamplitude. phase and harmonic content into the line. This enables the voltage seen bythe load to be compensated to a desired magnitude in the face of upstreamd~sturbances.

The DVR is capable of supplying and ab sorbing both real and reactive power.In many cases, small disturbances can be restored through the exchange of reactivepower only . For large r disturbances, it is necessary for the DVR to supply real powerto the load. The reactive power exchanged is generated by the inverter without anyenergy storage d evices. Th e real pow er exchange requires energy storage. Therefore,the DVR should be provided with a storage device in the form of a battery or acapacitor bank. When the line returns to normal following a disturbance, the storedenergy i s replenished from the distribution system by the DVR.

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Fig. 6.1. Scbemntie block dingrnm of power distribution systemcompensated by a DVRA schematic diagram of the DVR incorporated into a distribution network is

shown in the Fig. 6.1. V, is the supply voltage, VI is the incoming supply voltagehefore compensation. VZ is the load voltage after compensation, Vdvris the series~njectedvoltage of the DVR and 1 is the line current. Dynamic voltage restorerconsists of an injection transformer in which the secondary winding of the transformerIS connected in series with the distribution line. Also a voltage-source pulse widthmodulation inverter bridge is connected to the primary of the injection transformerand the energy storage device is connected at the dc-link of the inverter bridge. Theinverter bridge output is filtered in order to mitigate the switching frequencyharmonics generated in the inverter. The series injected voltage of the DVR, Vdvr.ssynthesized by modulating pulse width of the inverter-hridge switches. The injectionof-anappropriate Vdvr.n the face of upstream voltage disturbances demands a certainamount of real and reactive p w e r requirement from the DVR.

6.3. EXISTING DVR STRATEGIES

As was recognized by many researchers. the compensation correctioncapability of the restorer concentrates to improve the voltage quality by adjusting theboltage magnitude. wave shape, and phase shift during the occurrenceof voltage sag.

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~xtensivelyused in present DVR control is the w called in-phase voltage inject~ontechnique and phase advance compensation t ec hn ~q uc .6.3.1. In-Phase Voltage Injection technique

In this technique the load voltage is assumed to be in-phase with the presagboltage by injecting AC voltage in series with the incoming three phase network [I 351and [144]. This strategy is applied to both halanced and unbalanced voltage sags.Ilowever, this method does not take into account the phase shifl of the voltaged~sturbances.Therefore the power needed to inject from the DVR energy storage unltInto the distribution system was maximum. Hence this technique does not take intoaccount the minimization of the energy required to achieve a required voltagerestoration. For sags of long duration, this could result In poor load ride-throughcapability.

Th e steady state active pow er injection from the DVR when using the in-phaseboltage injection technique is given [77] as follows.

where V2 S the balanced output voltage,I is the balanced load current,0 s the load power factor angle,V, s the source sid e voltage,6 is the supp ly voltage phase angle,the subscript j represents j" phase and j = I , 2 . 3 .

Similarly, the steady state reactive power injection from the DVR when usingthe in-phase voltag e injec tion is given [77] as follows,

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63.2. Phase Advance Compensation (PAC) technique

The function of the D V R hown in Fig. 6.1 is to ensure that any load voltagedisturbances can be compensated effectively and the disturbance is transparent to theload. The corresponding phasor diagram describing the electrical conditions duringthe voltage sag compensated by PAC scheme is depicted in Fig. 6.2 wherr only theaffected phase is shown for clarity. Let I. @. 6 and a represent the load current, loadpower factor angle, supply voltage phase angle, and load voltage advance angle.respectively. Unlike the in-phase voltage injection technique considered in 11351 and11441, the phase advance compensation (PAC) technique 1801 is realized hy theadjustment in load voltage advance angle a. One major advantage of the proposedscheme is that less real power needs to be injected from DVR energy storqe unit intothe distribution system. Compared to the conventional in-phase injection method, thephase advance compensation scheme permits the DVR o help the load ride throughmore severe voltage .sags. However. the advancement of load voltage advance angle uat the beginning of compensation as well as the restoration of phase angle at the endof sag must he carried out gradually in order not to disturb or interrupt the operationof sensitive loads.

Fig. 6.2. Phrser diagram of power distribution system during sag

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6.3.2.1. DVR Power Flow

The power flow calculation of the D V R under the phase advancecompensation technique [SO] is considered as follows.

I. D V R Power Flow

If P,, and P,,, are the input power from the source and the load power.respectively [SO]. then

Assume a balanced Load(IJ 1) and a balanced output voltage (VZ, Vl)Po, = 3v, I Cos (Q )Let Pd, be the real power supplied by the DVR, hen from (6.3) nd (6.5)

Pdvr = Pout - P,npdvr = 3v21c= ( e ) v,, I , cm ( 0 - m + s l ) (6.6)

V ISimilarly ifQ, and Q be the input reactive power from the source and loadrespectively [80], thenQ,, x ~ , , l , ~ i n ( @ - a + G , ) (6.7)

VJ

Reactive power supplied by D V R will beQ , = Q , - Q .

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From (6.6) and (6.10) it is obvious that the control of real power and reactivep)wer exchange between DVR and distribution system IS possible only with theadlustmen1of th e phase angle n or a given value of 6.a, ,. V2.

11. Minimum Power Operation

The real power and reactive powers suppl~edby the DVR depends on thenature of voltage disturbance expenenced as well as the d lrmt~on f the DVR injected\.oltage with reference to the presag voltage. Pdvr depends on the advance angle a for agven 6 and V I, as shown in Fig. 6.2. Based on the values of n used, the minimum~a lue f P,jw can be negative. This impl~eshat the real power 1s bang absorbed byL)VR However, there is no technical and economical advantage hy operating this wayduring the sag period, the DVR should be exporting energy to support the load insteadcf drawing more power from the source A negatlve Pd,, may cvcn aggravate the sagsltuatlon. A larger energy storage facility will be requ~rcd o cater for the absorbedpower for no obvlous technical advantage

The possibil~ty f operating at Pdw= 0 during sag 1s an Interesting proposition.The following analysis is therefore carried out to explore thls possibility bydetermining the corresponding value of load voltage advance angle a for such anoperation [80].

Case A. Operation at Pdn= 0, m quation (6.6)3V,ICos(O)- V,, 1, Cos (O - a +8 , ) = 0 (6.1 1)

v,Let X=CV,,COS(S,), Y=CV,,SI~(S,),hen, following some simple

manipulations, the phase advance angle cr that corresponds to Pdvr= 0 is given by

~v,Icos@)-C V,, , Cos(Q, - a + 6 , ) = 0v,

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( X ' +Y : l 0 ' = I V , , CO S (& , ) + x ~ , , ~ i n ( f i , )[i. 1; ! , I;]"

Hence.

where p = tan-'(y/x), it can be seen from the expressions already shown thatfor balanced sags. p = 8 and G,,s the optimum value of phaw advancement angle forminimum power operation.

The necessary condition for the existence of a , is given by(X 2 +Y2)O' 2 3v2cos(@) (6.22)Thus, voltage correction with zero power injection is possible only if the

condition imposed by the above equation is satisfied.

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If the voltage :eg is so severe that equation (6.22) cannot he sat~sfied,hen theoptimum value of load voltage advance angle a can he calculated bysettlngdPd, /d a = 0 . At this operating point, the DVR suppl~cs ~ n ~ m u meal power tothe external system to keep VI = 1 P.U.

Case B. Optimal operation when Pd, > 0 [80]:Asllf 0 , use equatlon (6.6)and sct dPd,lda = 0. This means that

Z V , , S I ~ ( O - ~ + ~ , )0I

The corresponding DVR ~njection eal power requirement under a,,,controlstrategy is given as

p z :v,Icos(@)-~ V , , I C O S ( Q ,a,, + 6,)6.4. PROBLEM FORMUI.ATION

A powerful PSO based phase advancement compensatlon strategy 1sdeveloped for optimizing the energy storage capacity of the DVR in order to enhancethe voltage restoration property of the device

6.4.1. Objective Function

The proposed work aims at minimizing the objective function designed toopt~mizehe energy injection from the energy storage element of the DVR such as acapacitor or a battery. The mathematical model is changed to the followinggeneralized objective function which is [80] given as,

M~nrrnrzePdvr= Pout - P,, (6 .25)where Pdvris the real power supplied by the DVR,

P, and P are the input power from the source and the load power.respectively.

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Subject toL,oad voltage advance angle constraint, in which

h a d voltage advance angle (a)uring each compensation strategy should bewlthin the permissible range

6.5. A1,GORITHM O F THE PROPOSED METHOD

1. Input the parameters of the system such as three phase voltage magnitudcand angles, supply side voltage angle (6). tlme durat~on f voltage sag,load side power factor angle (0)nd number of iterations.

2. Specify the lower and upper boundaries of load voltage advance angle (a).3. Init~alize teration loop, particle position and the particle velocity.4. Calculate the input power flow of each phase (P, . , . Pln2,P,,,,).he total

power flow (Tp,,) and the power from DVR (P,,,,) for each particle.5. Compare each particles evaluation value. Pdvr, with its pbest. The best

evaluation value among the pbest is denoted as gbest.6. Update the inertia weight W as given by (2.9).7. Modify the velocity V of each particle according to (2.7).

If V>Vm" then V = V"""If VcVm'" then V= Vm'"

8. Modify the position of each particle according to (2.8). If a particleviolates its position limits in any dimension, set ~ t sosition at the properlimit.

9. Each particle is evaluated according to its updated position. If theevaluation value of each particle is better than the previous pbest, thecurrent value is set to be pbest. If the best pbest is bmer than gbest, thevalue is set to be gbest.

10. If the stopping criteria is satisfied then go to Step 12.1I. Othmrise,go o Step 4.12. l k article that generates the latest gbcst is the optimal value.

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6.6 SIMULATIONRESULTS

The result of the analysis for the proposed PSO based phase advancementcompensation (PSO-PAC) strategy is illustrated with the following case. Under theproposed PSO-PAC method, DVR uses the power h m he source-side healthyphases to minimize active power supply h m he stored energy source. As anillustration, consider a single-phase sag occuning in a balanced three phase systemwhere the post-sag voltages are (l L O 0, lL - 12O0.0.4L - 240" ). respectively. Assumea 2-MVA, 0.85 (lag) power factor load at 22 KV. The load power factor angle can bereadily evaluated and is found to be 31.78". Under presag conditions. each phasesupplies 566.7 KW. During in-phase injection technique. power supplied from thephases is 566.7 KW. 566.7 KW, and 226.6 KW, respectively, while the LWR willsupply the balance power of 340 KW. The source-side input power for the threephases with the phase adv anceme nt comp ensation is 666.6 KW . 666.6 KW. and 266.6KW . respectively. and hence the remaining 100 KW is supplied from the DVRstorage device. The reactive power obtained from the DVR is 210.671 KVAr al theload voltage advance angle a value of 151.7833'. Therefore. from the above results itis clear that with the PAC method. the healthy phases of the source provide moreenergy thus reducing the energy storage burden on the DVR storage device. However.there is a significant increase in reactive power supplied by the IIV R. A reactivepower of 1054 KVAr i s supplied by the DVR with the PAC scheme while 210 KVAris supplied with in-phase injection technique.

The results of the analysis shown above are considered for the typicalarrangement a s show n in Fig. 6.1 whe re the sensitive load is assum ed to have a powerfactor of 0.8 5 lag and the presag load volIage and current a re at 1 p.u. The volIage sagis considered as 60% sag on any one pha se of the three phase system . Th e simulationp a m e t e r s considered for the above test case are shown in Table 6.1

Using the proposed PSO based PAC scheme. it can be seen that the DVR canrestore the voltage sag by reduced real power. Here the real power supplied from

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DVR is 99.906 KW for a balanced sag level of 60%. It is observed that the amount ofreal power obtained in three phases are 666.71 KW. 666.71 KW. 266.68 KW.respectively, and the total power supplied to the load is 1600.1 KW. Thecorresponding value of optimized energy is 832.5511 Wan-hours. The optimum phaseadvance angle obtained using the proposed method is 151.7883'. The reactive powerfrom DVR is 1053.6 KVAr and the line current is 52.48638 A. Hence the proposedPSO based PAC scheme finds the optimum phase advancement angle which is almostclose to that of the angle found by the conventional PAC scheme. This indicates thecorrectness of the proposed method.

Table 6.1. Parameters used in PSO method -Single phase sap

Parameters Values

-. -Population Size-.

Vm'" -180

1 Acceleration coefficients c, and C) 1 2.0, 2.0

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Table 6.2. Comparison of resultswith PAC and In-Phase injection scheme

Injection Phase-Advance Proposed PSO based PhaseScheme Compensation Advance CompensationScheme (PAC) Scheme(PS0-PAC)1 P ~ I 566.699KW 666.705 KW

1 666.71 KW

226.679 KW 266.682 KW 266.68 KW1600.1 KW

1700.099 W atts

1 Energy I 28 11 49 1 Watt-h 833.375 Wan-h I 832.551 I Wan-h--From the above comparison it is clear that PSO based PA C scheme requires

minimum power for a 60 % voltage sag in any on e phase lasting for 30 ms for a inputboltape of 22 K V and save s around 94 W . Based on the aho ve analysis. it can he seenthat the am oun t of storage ene rgy can he reduced. thus resulting in a m ore econom icalrestorer in term s of a m ore com pact design . It is evident that the energy saving fromthe proposed m ethod is significantly better than other conven tional metho ds.

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GenerationsFig. 6.3.

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i. More reasonable performance measure compared to conventional in-phasevoltage injection and PAC scheme.

ii. Energy supplied from the DVR to correct a given voltage sag is reducedwhen compared to conventional techniques.