minimizing cost and power loss by optimal

7/30/2019 Minimizing Cost and Power Loss by Optimal

1/6

1

Minimizing Cost and Power loss by OptimalPlacement of Capacitor using ETAP

Pravin Chopade1 and Dr.Marwan Bikdash

Computational Science and Engineering Department ,Department of Electrical and Computer EngineeringNorth Carolina A & T State University

Greensboro, USAEmail: [email protected], [email protected]

(1.Author is doing Ph.D. at NCAT, USA and Assc. Professor at Bharati Vidyapeeth Deemed University College of Engineering Pune.INDIA)

AbstractLoads in a power distribution system network are

mostly inductive and lead to poor power factor. In order to utilize

the generated power optimally it is necessary to maintain close-

to-unity power factor. Power factor correction is possible by

introducing the capacitive loads in the circuit, as to nullify the

effect of inductive loading. Due to simplicity of analysis of radial

distribution systems, most previous work [1] studied the effect of

nonlinear and capacitive loads on the optimal solution of theCapacitor Placement Problem (CPP) for radial distribution

systems only. In this paper, we study optimal capacitor placement

on interconnected distribution systems in the presence of

nonlinear loads. The placement problem is solved using Genetic

Algorithms (GA) as implemented in the ETAP Power station

software. Results (power losses, operating voltages and annual

benefits) are analyzed. Computational results show that

harmonic components affect optimal capacitor placement in all

system configurations. If all loads were linear, interconnected

and loop system configurations offer lower power losses and

better operating conditions than the radial system configuration.

Keywords- Optimal placement of capacitors, Reactive Power,

ETAP Software.

I. INTRODUCTION

The leading current provided by a capacitor can effectivelycancel 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 system'sreactive 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 capacitor

still 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 (MV) 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

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 two

overwhelming 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].

978-1-4244-9593-1/11/$26.00 2011 IEEE 24


2/6

2

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 of CPPon only radial distribution systems [9].

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 configurationoffers lower power losses, best operating conditions, and bestannual benefits due to capacitor placement.

II. CAPACITOR BASED POWER FACTOR CORRECTION

As a rural power distribution system load grows, the systempower factor usually declines. Load growth and a decrease inpower factor lead to [ 3, 5]

1. 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. Distribution

capacitors 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 [1, 11].

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 well, introducing unnecessary costs. Generatorsprovide the reactive needs of distribution plant inductive loads

reducing the generator's capacity to produce real power.designations.

III. PROBLEM FORMULATION

The current in branch (i,k) connecting buses i and kis givenby [1, 2, 4, 12]

(1)ik ik

ik

i

P jQI

V

where

Iik= Current through branch (i, k).

Pik= Total real power flow in the branch (i, k).

Qik= Total reactive power flow in the branch ( i, k).

Vi = Voltage at node i.

The Total Power Loss in the transmission lines is :

ik

n

ik

ik RI

1

2||TPL

where

Current through branch (, )

Resistance of branch (, )ik

n i k

R i k

A branch current has two components: active (aI ) and

reactive (r

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

ik

n

ik

aika RI

1

2||TPL

and

ik

n

ik

rikr RI

1

2||TPL

The loss TPLa 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 TPLr associated with thereactive components of branch currents can be minimized.

The capacitor draws a reactive current Ic and for a radialnetwork it changes only the reactive component of current of

branch set . The current of other branches is unaffected by thecapacitor. Thus the new reactive current of the (i,k)thbranch isgiven by

(2)cikikrik

newrik IDII

where

25


3/6

3

1, if branch (, )

0, otherwise.

ikD i k

Here rikI is the reactive current of branch in the original system

obtained from the load flow solution. The loss TPLrcomassociated with the reactive component of branch current in thecompensated system (when the capacitor is connected) can be

written as

(3))(TPL 2

1

comr ikcik

rik

n

ik

RIDI

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

rcomrLS TPLTPLT

ikcikik

rik

n

ik

ik

n

ik

rik RIDIRI

2

11

2 )()(

ikcikrikik

n

ik

RIDID )2( 2

1

The capacitor current Ic that provides maximum loss savingcan be obtained from dS/dIc= 0

0)(

1

ikcikrikik

n

ik

RIDID

Thus the capacitor current form loss saving is given by

ikik

ik

ikrik

c

R

RI

I

The corresponding capacitor size is

cI

mV

cQ

where

ampsincurrentCapacitor=

in voltsm''busofmagnitudeVoltage=

KVARinsizeCapacitor=

c

m

c

I

V

Q

The corresponding susceptance is

mV

cIS

The proposed technique can also be repeatedly employed to

further optimizing saving of cost of energy by identifying

sequence of buses to be compensated for further loss reduction

by optimal placement of capacitor.

IV. CAPACITOR LOCATION

Maximum benefits are obtained by locating the capacitorsas near the inductive reactance kVAR loads as possible and bymatching the magnitude of the inductive reactance kVARrequirement. 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 of considerable 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 [1] 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(fixed) capacitor bank should be located at a distance from thesubstation which is 1/2 to 2/3 of the total length of the line.This location method is used strictly as a "Rule of Thumb"because 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 section 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].

V. OPTIMAL CAPACITOR PLACEMENT (OCP) USINGETAP : SYSTEM DESIGN WITH ETAP.

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

26


4/6

4

Most power systems that operate at a lagging power factordue 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, 10].

To place shunt capacitors in power systems, the followingtasks are to be performed :

1. Determine the bank size in KVAR2. Determine the connection location3. Determine a control method4. Determine a connection type ( Y or )

Minimizing the cost while determining the capacitor sizeand location mathematically is an optimization problem.Therefore, we should employ an optimization approach. TheETAP Optimal 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 savings

during 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 objective of optimal capacitor placement is tominimize the cost of the system. The cost includes four parts:

1. fixed capacitor installation cost: $ 4369.75 /year

2. capacitor purchase cost : $ 18601.35/year

3. capacitor bank operating cost (maintenance anddepreciation) : $3588.24/year

4. cost of real power losses : 7.56c/KWh

The main constraints for capacitor placement are

1. Used to meet the load flow constraints;

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. NUMIRICAL CALCULATIONS

The distribution network models are obtained by suitablysimplification of a typical Power grid [3]. The Single line

diagram 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 each a 132 kV HV busbar, a 132/20 kV40 MVA transformer and a 20 kV MV busbar; A feeder,subdivided in three line sections (L01, L12 and L23) of 3 kmeach with % positive sequence impedance (100 MVA base)R=5.17, X=4.23, Z=6.68.A series of further passive overhead

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

27


5/6

5

feeders; Link lines between various feeder (Lm1 andLm2);Configuration switches (S1, S2, S3, S4, S5 and S6).

Table 1 shows the system load data The GA optimizationmethod was applied to the test system- for three differentnetwork configurations:

1. Radial configuration (S1, S2, S3, S4, S5, S6 allopen);

2. Loop configuration (S1open, S2 open, S3 closed,S4 closed, S5 closed and S6 closed);

3. Interconnected configuration (S1closed, S2closed,S3 open, S4 open, S5 open and S6 open)

Table 1: Test system load data

BusNo.

Load(80% Motor, 20 % static

MW MVAR

1 5.25 5.356

2 5.25 5.356

3 5.25 5.3564 4.5 2.18

5 4.5 2.18

6 4.5 2.18

7 4.5 2.18

8 4.5 2.18

9 4.5 2.18

10 4.5 2.18

11 4.5 2.18

12 4.5 2.18

Commercially-available capacitor sizes with realcosts/kVAR were used in the analysis. It was decided that thelargest capacitor size Qcmax should not exceed the total reactiveload, i.e., 35688 kVAR. The yearly costs of capacitor sizes asdescribed in [4].

The Optimum shunt capacitor sizes have been evaluated forthe test case where all loads are assumed to be linear and Kpwas selected to be $168/kW. The voltage limits on the rmsvoltages were selected as Vmin= 0.95pu, and Vmax=1.05, for testcase and Vmin= 0.93 pu, and Vmax=1.05 pu.

Table 2 shows the values of capacitors for radial, loop andinterconnected distribution systems and Table 3 showscomparison of power loss and voltages before and after OCPfor radial, loop and interconnected distribution systems for Test

Case. This table shows significant use of OCP with respect tosaving $/year.

Figure 3 shows Power losses before and after Capacitorplacement and Figure 4 shows total cost $/year before and aftercapacitor placement.

Plots of Comparison for Power losses and Total cost($/year) before and after OCP.

Table 2 : Values of Capacitor before and after OCP for radial, loop andinterconnected distribution systems for Test Case.

Variable After OCP for Test Case

Radial Loop Interconnected

Qc1(kVAR) 4050 3600 3900

Qc2(kVAR) 3900 4050 4050

Qc3(kVAR) 4050 3900 3600

Qc4(kVAR) 1500 1800 3600Qc5(kVAR) 3300 2400 3300

Qc6(kVAR) 1650 2700 2100

Qc7(kVAR) 3450 3300 3450

Qc8(kVAR) 750 3300 2400

Qc9(kVAR) 3450 3450 2400

Qc10(kVAR) 1800 2400 1200

Qc11(kVAR) 2250 2550 2100

Qc12(kVAR) 2700 1650 2250

Total capacitor(kVAR) 32850 35100 34350

Table 3 : Comparison of results before and after OCP for radial, loop andinterconnected distribution systems for Test case

Minvoltage(pu)

Min voltage (pu) before OCP Min voltage (pu) after OCP

0.91 0.91484 0.92671 0.96535 0.97364 0.97521

Maxvoltage(pu)

Max voltage (pu) before OCP Max voltage (pu) after OCP

0.95516 0.95516 0.94462 0.98778 0.98860 0.98554

Power

losses

(kW)

Power losses (kW)before OCP

Power losses (kW)after OCP

1244.1 1211.1 1206.5 812.8 795.7 796.8

Cap.Cost($/year)

------ ----- ----- 6142.8 6301.65 6156.9

Total

Cost($/year)

Total Cost

($/year)before OCP

Total Cost

($/year) after OCP209008 203464 202692 142685 139904 139952

Benefits

($/year)

Benefits($/year)before OCP

Benefits($/year) after OCP

----- ---- ----- 66323 63560 62740

Figure 3: Power losses before and after OCP

28


6/6

6

Figure 4: Total Cost ($/Year) before and after OCP

VII. CONCLUSIONS

The study of the optimal capacitor placement oninterconnected distribution systems in the presence of nonlinearloads using ETAP is presented in this paper. Results (powerlosses, operating conditions and annual benefits) are comparedwith that obtained from radial and loop networks. The radial,

loop and interconnected distribution systems models areobtained by suitable simplification of a typical power grid.Computational results obtained showed that the harmoniccomponent affects the optimal capacitor placement in allsystem configurations.

When all loads were assumed to be linear, interconnectedand loop system configurations offer lowest power losses andbest operating conditions rather than the radial systemconfiguration while radial system configuration offer bestannual benefits due to capacitor placement. In distortednetworks, interconnected systems configuration offer lowerpower losses, best operating conditions and best annualbenefits due to capacitor placement .Capacitors can thus beused effectively for reactive power compensation which helpsin improving the power factor, reducing system losses,improving voltage, increasing the capacity of feeders etc.

The study made above leads to the following conclusions :

1. Optimum value of the capacitor required can bedetermined.

2. The algorithm finds out the proper location of thecapacitor.

3. The results are encouraging with reference to theimprovement in power factor and Voltage, therebyincreasing the feeder capacity.

4. Maximum benefits are obtained by selecting the

optimum size of the capacitor and by locating the

capacitors as near the inductive reactance kVAR loadsas possible.

5. Limited number of standard kVAR sizes necessitatethat capacitors be clustered near load centers.

ACKNOWLEDGMENT

The authors gratefully acknowledge Mr.D.M. Tagare,Managing Director, Madhav Capacitors Ltd. Pune, India for hiscontribution for providing data on Reactive PowerManagement for effect of variation of switched capacitor bankon daily power load. The Authors are greatly thankful toDr. Ajit D. Kelkar, Director Computational Science andEngineering Department, North Carolina A & T StateUniversity, Greensboro, USA and the Management of BharatiVidyapeeth Pune, Bharati Vidyapeeth Deemed UniversityPune, Dr. Anand R. Bhalerao, Principal, Bharati VidyapeethDeemed University College of Engineering, Pune,INDIA , fortheir support.

REFERENCES

[1] D.M.Tagare Reactive power management Mc-Graw Hill 2000.

[2] Rani and Vijaya, Distribution system loss reduction by capacitors,Proc.of National Conference on Emerging Trends in Engineering(2000),Husur.

[3] D.Rajicic &Y.Tamura(1988) IEEE trans. on power systems volume-I.

[4] Aoki k, Ichimori T, Kanezashi M. (1985), Normal state optimal loadallocation in distribution systems. IEEE Trans Power Deliv, Volume 3(issue 1), pp. 147-155.

[5] S.I.Wamoto & Y.Tamura (1981),IEEE trans. On power apparatus &systems .

[6] Y. Baghzouz, S. Ertem, Shunt Capacitor Sizing for Radial DistributionFeeders with Distorted Substation Voltages, IEEE Trans. on PowerDelivery, Vol. 5, No. 2, April 1990, pp. 650-65.

[7] ETAP Product Overview Power System Enterprise Solution,Operation Technology Inc.

[8] M.Brenna, R.Faranda and E.Trioni, Non-conventional Distribution

Network Schemes Analysis with Distributed Generation, TEQREP,Bucharest, 2004.

[9] Larsson, M. (2000) Coordinated Voltage Control in Electric PowerSystems. Doctoral dissertation, Department of Industrial ElectricalEngineering and Automation, Lund University.

[10] P. M. Anderson, A. A. Fouad, Power System Control and Stability,New York: IEEE Press, 1992.

[11] R. H. Park, "Improved reliability of bulk power supply by fast loadcontrol," in Proceedings of the 1968, American Power Conference, pp.445-457.

[12] Aoki K, Kuwabara H, Satoh t, Kanezashi M (1988), An efficientalgorithm for load balancing of transformers and feeders. IEEE TransPower Deliv, Volume 3 (issue 4), pp.1865-1872.

[13] IEEE Recommended Practices and Requirements for Harmonic Controlin Electrical Power Systems, IEEE Std. 519-1992, 1993.

29

minimizing cost and power loss by optimal

Documents