Minimizing Cost of capacitor bank

8/12/2019 Minimizing Cost of capacitor bank

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Minimizing ost and Power loss by OptimalPlacement of apacitor using ET P

Pravin Chopade and Dr.Marwan BikdashComputational Science and Engineering Department ,Department of Electrical and Computer Engineering

North Carolina A T State UniversityGreensboro, USA

Email: [email protected]@gmail.com(LAuthor is doing Ph.D. at NCA T, USA and Assc. Professor at Bharati Vidyapeeth Deemed University College of Engineering Pune.INDIA)

AbstractLoads in a power distribution system network aremostly inductive and lead to poor power factor. In order to utilizetbe generated power optimaUy it is necessary to maintain close-to-unity power factor. Power factor correction is possible byintroducing the capacitive loads in the circuit, as to nullify theeffect of inductive loading. Due to simplicity of analysis of radialdistribution systems, most previous work I studied the effect ofnonlinear and capacitive loads on the optimal solution of theCapacitor Placement Problem (CPP) for radial distributionsystems only. In this paper, we study optimal capacitor placementon interconnected distribution systems in the presence ofnonlinear loads. The placement problem is solved using GeneticAlgorithms (GA) as implemented in the ETAP Power stationsoftware. Results (power losses, operating voltages and annualbenefits) are analyzed. Computational results show thatharmonic components affect optimal capacitor placement in allsystem configurations. If all loads were linear, interconnectedand loop system configurations offer lower power losses andbetter operating conditions than the radial system configuration.

Keywords Optimal placement of capacitors Reactive POII/CrETAP Software.

INTRODUCTIONThe leading current provided by a capacitor can effectively

cancel the lagging current demanded by reactive loadcomponents. Power factor is defined as the ratio of real power(kW) to total power (kVA). When the distribution systemsreactive load can be canceled by a capacitor placed at thereactive load center, the entire power delivery system will berelieved of KVAR, originally supplied from the powersupplier s generator. This makes the full capacity of thegenerator available to serve real power loads [1]. If a capacitoris connected to the distribution system either too far ahead of ortoo far beyond the system s inductive load center, the capacitorstill provides reactive loading relief, but the system will notgain the full advantages of voltage and loss improvementwhich would be afforded by proper capacitor placement [2].Electric power is supplied to final users by means of MediumVoltage (MY) or Low Voltage (LV) distribution systems, theirstructures and schemes can differ significantly according toloads location. Overhead lines with short interconnectioncapabilities are mostly employed in rural areas, whilst cableswith a great number of lateral connections for alternative

978-1-4244-9592-4/11/ 2500 2011 IEEE

supplies are widespread used in urban areas [3]. Most powerdistribution systems are designed to be radial, using only onepath between each customer and the substation. If powerflowing away from the substation to the consumer isinterrupted, complete loss of power to the consumer will follow[4]. The predominance of radial distribution is due to twooverwhelming advantages: it is much less costly than the othertwo alternatives (loop and interconnected systems) and it ismuch simpler in planning, design, and operation. An alternativeto purely radial feeder design is a loop system, which has twopaths between the power sources (substations, servicetransformers) and each customer [5]. Equipment is sized andeach loop is designed so that service can be maintained under asingle fault. In terms of complexity, a loop feeder system isonly slightly more complicated than a radial system [6]. Powerusually flows out from both sides toward the middle, and in allcases can take only one of two routes. Voltage drop, sizing, andprotection are only slightly more complicated than for radialsystems. Interconnected distribution systems are the mostcomplicated and costly but they are the most reliable methodof distributing electric power. An interconnected distributionsystem involves multiple paths between all points in thenetwork and provide continuity of service (reliability) farbeyond that of radial and loop designs. Interconnecteddistribution systems are more expensive than radial distributionsystems, but not greatly so in dense urban applications, wherethe load density is very high and the distribution must beunderground. Given that repairs and maintenance are difficultbecause of traffic and congestion, interconnected systems maycost little more than loop systems.

Interconnected systems require little more conductorcapacity than a loop system. The loop configuration required double capacity everywhere to provide increased reliability.Interconnected systems are generally no worse and often needconsiderably less capacity and cost, if that are well designed.The solution procedures of the Capacitor Placement Problem(CPP) start with performing a load flow analysis to analyze thesteady-state performance of the power system prior to capacitorplacement and after capacitor placement and to study theeffects of changes in capacitor sizes and locations [7].


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Load and power flow direction are easy to establish in aradial distribution system, and voltage profiles can bedetermined with a good degree of accuracy without resorting toexotic calculation methods; equipment capacity requirementscan be ascertained exactly; capacitors can be sized, located, andset using relatively simple procedures (simple compared tothose required for similar applications to non-radial (loop andinterconnected) system designs [8]. Due to the simplicity ofanalysis of radial distribution systems, all previous workstudied the effect of nonlinear loads on optimal solution ofCPPon only radial distribution systems [ ]

The study of the optimal placement and sizing of fixedcapacitor banks placed on distorted interconnected distributionsystems using Genetic Algorithms (GA) as used in ETAPSoftware [10] is presented in this paper. Results (power losses,operating conditions and .annual benefits) are compared withthat obtained from radial and loop distribution systems. Theradial, loop and interconnected distribution systems models areobtained by suitably simplification of a typical Power grid. TheCommercial package ETAP 7.1 program is also used for

. harmonic load flow analysis [10].Computational results obtained showed that harmonic

component distortion affects the optimal capacitor placement inall system configurations. When all loads were assumed to belinear, interconnected and loop system configurations offer thelowest power losses and best operating conditions rather thanthe radial system configuration. Radial system configurationoffers the best annual benefits due to capacitor placement. Indistorted networks, the interconnected system configuratioroffers lower power losses, best operating conditions, and bestannual benefits due to capacitor placement.

II C P CITOR SED POWER F CTOR CORRECTIONAs a rural power distribution system load grows, the systempower factor usually declines. Load growth and a decrease inpower factor lead to [ 3 5]

I Voltage regulation problems;2. Increased system losses;3. Power factor penalties in wholesale power contracts;

and4. Reduced system capacity.

In addition to improving the system Power Factor,capacitors also provide some voltage drop correction. Acapacitor's leading current cause a voltage rise on the system.But care must be exercised as not to cause too much voltagerise or provide too much leading current. Distributioncapacitors can also reduce system line losses, as long as thesystem power factor is not forced into a leading mode. Properlyplaced and sized capacitors can usually reduce system linelosses sufficiently to justify the cost of their installation [I, II].

BuLk power facilities have to use some of their capacity tocarry the inductive kVAR current to the distribution system.The resultant reactive current flow produces losses on the bulkfacilities as wel, introducing unnecessary costs. Generatorsprovide the reactive needs of distribution plant inductive loads

2reducing the generator's capacity to produce reef' power.designations.

III. PRO LEM FORMULTION

The current in branch (i,k) connecting buses i and k is givenby[I,2,4,12]L, = P it J Qik

V i 1where

lik Current through branch i k).?ik = Total real power flow in the branch i kQik = Total reactive power flow in the branch ( i, k).V i = Voltage at node i.

The Total Power Loss in the transmission lines is :n 2TPL L ; I Ik 1 R k

ik;1

wheren Current through branch (i

Rik Resistance of branch A branch curr nt has two component : e,rtive la and

reactive ( l).The total loss associated with the active andreactive components of a branch current can be written as

n,r(1I

ikand

n

TPLr I 12R ikik;1

The loss TPL associated with the active component ofbranch current cannot be minimized for a single - source radialnetwork because all active power must be supplied by thesource at the root bus. However, supplying part of the reactivepower demands locally, the loss TPL associated with thereactive components of branch currents can be minimized.

The capacitor draws a reactive current I, and for a radialnetwork it changes only the reactive component of current ofbranch set cThe current of other branches is unaffected by thecapacitor. Thus the new reactive current of the (i,k)h branch isgiven by

(2)where


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ik if branch R

otherwise.,Here I is the reactive current of branch in the original systemobtained from the load flow solution. The loss TPLrcomassociated with the reactive component of branch current in thecompensated system (when the capacitor is connected) can bewritten as

n

TPLomL U D ikc)2 R ikik = 1

The loss saving TLSis the difference between equation (2)and (3) and is given by .

TLs =TPLr- TPLrcomn n

L U ; ; 2 R ik - L U, D ik Ii )2 R ikik=1 ik = 1n= L (2D ikI,: D ik I/}R ik -- -ik =1

The capacitor current Ie that provides maximum loss savingcan be obtained from dS/d1e= 0

L (D iJ: +D ik I )R ik =0ik = 1

Thus the capacitor current form loss saving is given by

- L I;;' R ik ike ac ' k

ik

The corresponding capacitor size is =V Ic me

where c Capacitor size in KV ARV m Voltage magnitude of bus' m' in voltsIe Capacitor current in amps

The corresponding susceptance isS =-...Vm

The proposed technique can also be repeatedlyemployed tofurther optimizing saving of cost of energy by identifyingsequence of buses to be compensated for further loss reductionby optimal placement of capacitor.

3IV. CAPACITOR LOCATION

(3)

Maximum benefits are obtained by locating the capacitorsas near the inductive reactance kVAR loads as possible and bymatching the magnitude of the inductive reactance kV Rrequirement. Practical considerations of economics andavailability of a limited number of standard kVAR sizesnecessitate that capacitors be clustered near load centers.Computer modeling or rigorous evaluation ofconsiderable loadmetering data are absolutely necessary to make the propercapacitor placement decision and keep line losses as low aspossible. The loss reduction benefits possible with capacitoruse can be significant enough to economically justify feedermetering or a large share of SCADA system costs.

A textbook solution ] assume a uniform distribution ofconsumers, and suggests that as the distance from thesubstation increases, the number of consumers per main linemile of feeder increases.

To obtain maximum benefits in voltage improvement andreduction of loss on such a line, a permanently connected_:::: ':Cfixed)_capllcilor bank should be located at a.distance from the---substation which is 1/2 to 2/3 of the total length of theIine.

This location method is used strictly as a Rule of Thumbbecause few rural circuits contain such uniformly distributedloads.

Thus, the following method is better suited for locatingcapacitors: Use a computer model of electric system and allowthe computer program to place the capacitors on the system inblocks of the largest size that can be used to limit the voltagechanges to 3 volts per switched bank.

Computer models calculate proper capacitor placement bytrying the smallest size capacitor a system uses in each linesection of every feeder and calculating the total circuit losses.In this way, the computer selects the line secuon with thelowest net losses and then places subsequent additionalcapacitors in the same manner. The individual effect on feederlosses is tabulated for each capacitor placed, with eachsubsequent unit having less benefit. At some point at less thanunity power factor, an additional capacitor offers littleadditional benefit, and adding more actually increases losses.Capacitors should be located so as to reduce feeder losses asmuch as economically practical. The first capacitor placedprovides the most improvement per unit cost because it isusually a fixed capacitor and it increases power factor the most.Each subsequent unit is less economically practical [13).

OPTIMAL CAPACITOR PLACEMENT (OCp) USINGETA?: SYSTEM DESIGN WIT ETAP.

ETAP PowerStation [10) is a fully graphical power systemsanalysis program. ET AP PowerStation uses genetic algorithmtechnique for optimal capacitor placement.

V.


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'\m.t 4

hT t . FJ: 'ie. - jP. ' . U hf

D A i D. r o ;l,I;1lW 1 1/7 :I "', l ilflO v f; Yolf\ , &:0: 'f" ro L tl 1 4[) W \ W \ .\ flX(] Ij'j,l

Figure .1: Typical Single line diagram of Power Grid on ETAl>.

Most power systems that operate at a lagging power factor .'due to loads and delivery apparatus (lines and transformers) areinductive in nature. Therefore, power systems requireadditional VAR flow. This results in reduced system capacity,increased losses and decreased voltage [2, 10J.

To place shunt capacitors in power systems, the followingtasks are to be performed:I. Determine the bank size in KV AR2 Determine the connection location3. Determine a control method4 . D --'p. --, n . ? cor.'1ero':T\"' -pe ( r- / ' '.

Minimizing the cost while determining the capacitor sizeand location mathematically is an optimization problem.Therefore, we should employ an optimization approach. TheETAP Optima Capacitor Placement (OCP) module is apowerful simulation tool that is specifically designed for thisapplication. The OCP module helps to place capacitors forvoltage support and power factor correction while minimizingtotal cost. The advanced graphical interface gives the flexibilityto control the capacitor placement process and allows to viewthe results graphically. The precise calculation approachautomatically determines the best location and bank sizes. Inaddition, it reports the branch capacity release and the savingsduring the planning period due to VAR loss reduction.

OCP uses the present worth method to perform alternativecomparisons. It considers initial installation and operatingcosts, which include maintenance, depreciation, and lossreduction savings. It also provides interest rate and inflationconsideration.

The objeetive of optimal capacitor placement is tominimize the cost of the system. The cost includes four parts:1. fixed capacitor installation cost: $ 4369.75 /year2. capacitor purchase cost: $ 1860 US/year3. capacitor bank operating cost (maintenance anddepreciation) : $3588.24/year4. cost of real power losses: 7.56c1KWh

The main constraints for capacitor placement are

2. To ensure that, all voltage magnitudes of load(PQ) buses should be within the lower and upperbars;

3. To ensure that power factor (PF) should be greaterthan a threshold. It may be a maximum powerfactor bar.

The constraints are the power flow equations.VI. NUMLRICAL CALCULATIONS

The distribution network models are obtained by suitablysimplification of a typical Power grid [3]. The Single linediagram of the network simulated in ETAP is shown in Figure1 and the system data as follows:

Larger interconnected two 132 kV HV networks with thesame short circuit power MVAsc of 6000 MVA; Two HV /MVsubstations, comprising eaeh a 132 kV HV busbar, a 132/20 kV40 MV A transformer and a 20 kY MY busbar; A feeder,subdivided in three line sections (LOI , Ll2 and L23) of 3 kmeach with % positive sequence impedance (l00 MYA base)R=5. 17, X=4.23, Z=6.68.A series of further passive overhead,

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,

feeders; Link lines between various 4eeder (Lml and Table 2 : Values of Capacitor before and after OCP for radial, loopLm2);Configuration switches (S I, S2, S3, S4, S5 and S6). interconnected distribution systems for Test Case.Table 1 shows the system load data The GA optimization Variable After OCP for Test Case

method was applied to the test system- for three different Radial Loop Interconnecnetwork configurations: QCI(kVAR) 4050 3600 3900QCz(kVAR) 3900 4050 4050l. Radial configuration (SI, S2, S3, S4, S5, S6 all QC3(kVAR) 4050 3900 3600open); QC4(kVAR) 1500 1800 3600

2. Loop configuration (Slopen, S2 open, S3 closed, Qcs(kVAR) 3300 2400 3300S4 closed, S5 closed and S6 closed); QC6(kVAR) 1650 2700 2100Interconnected configuration (S 1closed, S2c1osed, QC7(kVAR) 3450 3300 34503. Qcg(kVAR) 750 3300 2400S3 open, S4 open, S5 open and S6 open) QC


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2

: ---- I lS OCOO 1y 0010 ;t

005

6capacito rs as nea r the inductive rea c tance kV AR loadsa s po ss ib l e.5. Lim ited number of st and ard kVAR sizes neces sitat eth at c ap ac it ors be c lus tered nea r loa d c en te rs.

Total Ct':-ti$/yeri s tter OCPjfo rTe.rtC ase

o To ;;1esr (Sh e';.r) befor ooI;,T< Coo., ACKNOWLEDGMENT

Th e study of the optim al capac itor placement oninte rcon ne cted dist rib uti on system s in the presence of nonlinea rloads usi ng ET AP is pres ente d in this paper. Resul ts (p owerlo sses , ope rat in g conditions and annual benefits) ar e compared_ . = with..:.fh at obtained. from . radial-and 19o p'" ne.W ork s-Ihe-ra dia j-, __= = [ D. g e R acvep . 2 .wer..m ana gem ..e.: -G H ill2000. _ _ _ .

. ---loo p ' and _ inte rconne cted dts tr ib ut ion- 1?y stems :: :"modefS ;iiLe_. l2f fan {andY ijay a, -"D istJ jbatio ni ffefrr -J.o J b bycap ac ltors", =:fobtained, by suitab le simplificat ion .of.a ty pic l powes gri d. -::. -Pr5 ' .of l 'iat i.unalC onference on cm i g Tr ends ir fEllg i_neeril g(2000 ), omputtion l results obt ined showed tht the harmonic Husur, -- .- ,-.-- [3] D .Rajicic &Y.Tamura (1988)IEEE tra ns. o n p owe rsystemsvolume-I.component affec ts the optim al capacitor placement in an 4s ys tem con fi gu rations. - - _[] Aok i k IchimoriT, K anezashi M. (1985), "Norma lstat e-optimal load _allocation in di str ibu tio n systems , IEE E Trans PowerDel iv, Volume 3 -(i ssue I), pp . 147-155.(5] S .I.W amo to & Y.T amura (19 81) ,IEEE trans. O n power apparatus &sy stems.[6 ] Y . Bag hzo uz,S . E rtem , "S hun tC apacitor S izin g forRadial D istributionF ee de rs w ith D istorted S ub st ation Volt ages", IEE ETrans. on PowerDeli ver y, Vol.5 , N o .2 , A pril 1990,pp650-65 .[7] ETAP Product O ve rview - Pow er S ystem Enterpris e S olu tion,

O perat ion T echno log y Inc .[8 M.Brenna, RFaranda and E'TriAri Non-convert iona DistributionNetwork Schemes Analysis \\'-:'.1 Vl.>trU.)ut(.;t,) G J.l:;.r-J.ltv.u., L -...::..Buchares t , 2004.[9] Larsson, (2000) C oordin at ed Voltage C ont rol in Elec tr ic Pow erS ys tem s. Doc tor al di sse rtat ion , Dep ar tm ent of Ind ustr ial Elec tric a lEngin ee rin g an d Aut om atio n, Lund Uni ve rsity .[l 0] P. M . Anderson ,A . A . Fou ad, Power S ys tem C ontrol and S tability,N ew York : IE EE Pre ss , 1992 .[II] R . H . Park , "Improve d relia bility of bulk power supply by fast loadcont rol," in Pr oceeding s of th e 196 8, American PowerCon fere nce, pp .44 5-45 7.

l Ratial SYStem.l.nop 1{'mll nSi .Jto rd1eC r l S.\ -"S TiWI

L. .._ ._ __ .__ _ _ ..___._ ._ _-_ _ - _-_jFigure4: Total C os t ($ fYear)bef ore and afterOCP

VII . C ONC LUSIO NS

When al l loads were assumed to be li nea r, in terconne ctedand loop system configur at ions offer low est pow er losses andbes t op erat ing conditions rather than th e radi al systemconfiguration while radi al sys tem configuration offer bes tannual benefi ts du e to capacitor pl acement. In dis tort ednetworks, in terconnected sy stem s configuration offer lowerpower los ses, best operatin g cond itions and bes t annua lbe nefi ts du e to capac itor placement .Capacitors can thus beused eff ectively for reac tive pow er compensa tion which helpsin impr oving th e power fac tor, redu cing system loss es,improving voltag e, inc re asing th e cap ac ity of feeders etc .

Th e study made above lead s to th e fo ll ow in g conc lusions: O ptimum valu e of th e capac itor required can be

determined.T he alg orithm finds out the proper location of thecapac itor.Th e res ul ts ar e encour aging w ith ref erence to th eimprovem ent in pow er fac tor and Volt age , th erebyincrea sin g th e fee der capacity .Maximum benef it s ar e ob tained by selecting th eoptimum size of th e cap acitor an d by lo catin g the

2.3.

4.

Th e authors gratef ully acknow ledge M r.D .M . Tagare,M an ag ing Director, Madhav C apac itors Ltd . Pu ne, India for hiscontribution for providing data on Reactive PowerM an ag ement f or e ff ec t of variation of sw itched c apac itor ban kon dail y power load. The Authors are grea tl y th ankful toDr. A jit D . Kelka r, D irec tor C omputa tio nal S c ience andEngin ee ring Dep artment, North. C aro lina A T S ta teUniversity, G ree nsboro, USA and the M anagem ent of BharatiVidyapeeth Pune, Bharat i V idyapee th Deemed Univ ersityPu ne, Dr. Anand R . Bhaler ao, Princ ip al, Bharati V id yapeethDeemed Unive rsity C ollege of Engineerin g, Pu ne,IND IA , f orth eir support.

REFERENCES

fl 2] Aok i K , Kuwahara H , S at oh t, Kan ezas hi M (1 988 ), "An ef fic ientalgo ri thm for load balanc ing of transformers and fee ders ".IEEE TransPow er Deliv,Vo lume 3 ( iss ue4), pp. 186 5-1872.[l3] IEE E RecommendedPrac tic esand Requ irement s fo r Harmonic C o n trolin Elec tric al Power S ys tem s, IEE E S td. 519 -199 2 , 199 3.

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Minimizing Cost of capacitor bank

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