VAR compensation involves the management of reactive power for the improvement ofelectric power system performance. Adequate reactive power control solves power qualityproblems like flat voltage profile maintenance at all power transmission levels, andimprovement of power factor, transmission efficiency and system stability. Series and ShuntVAR compensation techniques are used to modify the natural electrical characteristics ofelectric power system. Series compensation modifies the reactance parameter of thetransmission or distribution system, while shunt compensation changes the equivalent loadimpedance. In both cases, the line reactive power can be effectively controlled therebyimproving the performance of the overall electric power system. This paper presents anoverview of the reactive power and static VAR compensation technologies. In case study, anapplication example is illustrated. The results obtained by applying both series and shuntVAR compensation techniques individually to the case study illustrated are tabulated forcomparison.Key words: Reactive power, Transmission efficiency, Voltage regulation, Seriescompensation, Shunt compensation, Natural load.NotationsE or VS = Sendingendvoltage, V.Z = Circuit impedance, O.R = Circuit resistance, O.XL = Circuit inductive reactance, O.I = Line current, A.I = Load current, A.VR = Receivingendvoltage, V.F or F = Phase angle between VR and I.d or d = Phase angle between VS and VR.P+jQ = Power drawn by a practical inductive load, KVA.IR = Real power (or Inphase)component of current, A.INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGULVolume 2, No 1, 2011 Copyright 2010 All rights reserved Integrated Publishing AssociationRESEARCH ARTICLE ISSN 0976425929IX = Reactive (or Outofphase)component of current lagging the voltageby 90 0 , A.IC = Reactive (or Outofphase)component of current leading the voltageby 90 0 , A.1. IntroductionModern civilization depends mostly on electrical energy for agricultural, commercial,domestic, industrial and social purposes (B.R.Gupta, 1998). The electrical energy isexclusively generated, transmitted and distributed in the form of alternating current (a.c.). Ofthe three basic elements of electrical engineering, the resistor consumes ohmic energy whereas the inductor and capacitor store the electrical energy in magnetic field and electric fieldrespectively.The actual amount of power being used, or dissipated, in a circuit is called true or real power.Reactive loads such as inductors and capacitors dissipate zero power, yet they drop voltageand draw current giving the deceptive impression that they actually do dissipate power. Thisphantom power is called reactive power (Van Cutsem T., 1991). A reactor or a capacitorstores the reactive power generated by the a.c. power source during a quarter of a cycle andreturns the same power to the source in the next quarter cycle. In other words, the reactivepower oscillates between the source and the reactor or capacitor at a frequency equal to twicethe rated value. More precisely power dissipated by a load is referred to as true power whereas power merely absorbed and returned in load due to its reactive properties is referred to asreactive power. However in nature, most of the loads are inductive loads absorbing reactivepower and resulting in low lagging power factor.1.1 Effects of reactive power flow in linenetwork1.1.1 Poor transmission efficiencyLosses in all power system elements from the power station generator to the utilizationdevices increase due to reactive power drawn by the loads, thereby reducing transmissionefficiency.1.1.2 Poor voltage regulationDue to the reactive power flow in the lines, the voltage drop in the lines increases due towhich low voltage exists at the bus near the load and makes voltage regulation poor.1.1.3 Low power factorThe operating power factor reduces due to reactive power flow in transmission lines.1.1.4 Need of largesizedconductorThe low power factor due to reactive power flow in line conductors necessitates largesizedconductor to transmit same power when compared to the conductor operating at high powerfactor.INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGULVolume 2, No 1, 2011 Copyright 2010 All rights reserved Integrated Publishing AssociationRESEARCH ARTICLE ISSN 09764259301.1.5 Increase in KVA rating of the system equipmentThe reactive power in the lines directly affects KVA rating of the system equipment carryingthe reactive power and hence affects the size and cost of the equipment directly.1.1.6 Reduction in the handling capacity of all system elementsReactive component of the current prevents the full utilization of the installed capacity of allsystem elements and hence reduces their power transfer capability.A power system is expected to operate under both normal and abnormal conditions and underthese conditions it is desired that the voltage must be controlled for system reliability, thetransmission loss should be reduced and power factor should be improved (Rajesh Rajaramanet.al., 1998). In this paper the effect of line reactive power flow on transmission efficiency,voltage regulation and power factor with and without VAR compensation techniques areanalyzed and presented.1.2 Effect of reactive power flow on line voltage dropThe voltage variation is due to imbalance in the generation and consumption of reactivepower in the system. If the generated reactive power is more than the consumed one, then thevoltage levels go up and vice versa. However, if the two are equal, then the voltage profilebecomes flat and it happens only when the load is equal to natural load. Unfortunately thereactive power in a system keeps on varying and if the reactive power generation issimultaneously controlled, a more or less flat voltage profile could be maintained.The lamp characteristics in case of lighting loads are very sensitive to voltage changes(B.R.Gupta, 1998).. The voltage variations may cause erratic operations in case of powerloads. For example, if the supply voltage exceeds the rated value, the motor magnetic circuitmay be saturated and consequently draw large magnetizing current. On the other hand, if thevoltage is too below, the starting torque becomes low. Too wide variation of voltage causesexcessive heating of distribution transformers thereby reducing the transformer ratings. Sothe voltage profile must remain within +5 to 6% of the rated value for better and efficientoperation of various electrical equipment.As most of the loads operate at low lagging power factor, they require significant amount oflagging reactive power during peak load conditions (Kankar Bhattacharya and Jin Zhong,2001). However, if this significant amount of lagging reactive power is supplied by thegenerator from the sending end, all equipment starting from the sendingendmay be overloaded thereby causing low receiving end voltage. During off peak conditions, the linegenerates net VARs which must be absorbed to obtain voltage stability. Shunt compensationinvolves the use of shunt capacitors during peak load conditions to generate lagging VARs atthe receiving end and shunt reactors during off peak conditions to absorb line generatedVARs to avoid voltage instability (Van Cutsem T., 1991).Consider the Figure 1 showing the equivalent circuit of an a.c. generator supplying power toa practical inductive load and its phasor diagram.INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGULVolume 2, No 1, 2011 Copyright 2010 All rights reserved Integrated Publishing AssociationRESEARCH ARTICLE ISSN 0976425931Figure 1: Equivalent circuit of Alternator supplying electrical energy to a practical inductiveload and its phasor diagram.From the phasor diagram, it can be stated thatE2 = (V + DV)2 + (V ) 2= (V + IRcosj + IX sinj )2 + (IX cosj - IR sinj ) 2= (V + (RP /V ) + (XQ /V))2 + ((XP /V ) - (RQ /V )) 2Therefore DV = (RP + XQ) / V and V = (XP - RQ) / VFor V ppp (V + D V ), E -V = D V= (RP + XQ) / V= XQ / V (Since R ppp X )i.e., Voltage drop depends on Q. i.e., if reactive power flows over the transmission line, thereshall be a voltage drop.1.3 Effect of reactive power flow on transmission efficiency and power factorIt is desirable both economically and technically to operate the electric power systems at nearunity power factor (u.p.f). Usually power factor correction means to generate reactive poweras close as possible to the load which it requires rather than generating it at a distance andtransmit it to the load, as it results in not only need of a large sized conductor but alsoincreased losses thereby reducing transmission efficiency (M.W. Gustafson and J.S. Baylor,1988).2. Compensation techniquesArtificial injection of reactive power at the loads may relieve the transmission network fromreactive power flow and improves both transmission efficiency and operating power factorwhere as artificial injection of negative reactance in the lines may relieve the lines fromexcessive voltage drop and improves the voltage regulation (Kankar Bhattacharya and JinZhong, 2001). The methods available for the injection of both are static compensation andINTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGULVolume 2, No 1, 2011 Copyright 2010 All rights reserved Integrated Publishing AssociationRESEARCH ARTICLE ISSN 0976425932synchronous compensation. Static compensation involves capacitors and reactors where assynchronous compensation involves synchronous phase modifier.The principles of both shunt and series compensation techniques are presented in this paper.2.1 Shunt compensationAt buses where reactive power demand increases, bus voltage can be controlled byconnecting capacitor banks in parallel to a lagging load (Kankar Bhattacharya and Jin Zhong,2001). Capacitor banks supply part of or full reactive power of load, thus reducing magnitudeof the source current necessary to supply load. Consequently the voltage drop between thesending end and the load gets reduced, power factor will be improved and increased activepower output will be available from the source (M.W. Gustafson and J.S. Baylor, 1988).Depending upon load demand, capacitor banks may be permanently connected to the systemor can be varied by switching ON or OFF the parallelconnectedcapacitors either manuallyor automatically (M.L.Soni, P.V.Gupta and U.S.Bhatnagar, 1994).Figure 2 shows the singlelinediagram of a transmission line and its phasor diagram beforethe addition of the shunt capacitor and its phasor diagram.Figure 2: Singlelinediagram of an uncompensated transmission line and its phasor diagram.Voltage drop in the line with lagging power factor can be approximated asVD = IRR + IX X L VFigure 3: Singlelinediagram of a shunt compensated transmission line and its phasordiagram.INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGULVolume 2, No 1, 2011 Copyright 2010 All rights reserved Integrated Publishing AssociationRESEARCH ARTICLE ISSN 0976425933Figure 3 shows the singlelinediagram and its phasor diagram after the addition of the shuntcapacitor and its phasor diagram.Voltage drop can be approximated as VD = IRR + IX XL - IC X L VThe difference between the voltage drops is the voltage rise due to installation of thecapacitor and can be expressed as C L VR = I X VThe usage of shunt capacitor banks suffers from the following drawbacks:1. Shunt capacitors do not affect current or power factor beyond their point ofapplication.2. The reactive power supplied by the shunt capacitor banks is directly proportional tothe bus voltage.3. When the reactive power required is less on light loads, capacitor bank output willbe high. This disadvantage can be eliminated by connecting a number of capacitorsin parallel and then capacitance can be varied by switching ON or OFF dependingupon load requirement.2.2 Series compensationWhen the line has high value of reactance to resistance ratio, the inductive reactance of thetransmission line can be decreased by introducing series capacitors which results in lowvoltage drop (Rajesh Rajaraman et.al., 1998).When a load with lagging power factor is connected at the end, voltage drop in the line is( cos sin ) L VD = I R j + X j VIf a capacitance C with reactance Xc is connected in series with the line, then the reactancewill be reduced to (XLXc)and hence the voltage drop is reduced. Further the reactive powertaken by the line is also reduced.In Figure 4, the equivalent circuit of the line with series compensation and its phasor diagramare presented.Figure 4: Singlelinediagram of a series compensated transmission line and its phasordiagram.INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGULVolume 2, No 1, 2011 Copyright 2010 All rights reserved Integrated Publishing AssociationRESEARCH ARTICLE ISSN 0976425934It can be observed from the phasor diagram that line voltage drop isVD = I (Rcosj + (XL - X C )sinj ) VThus the use of series capacitors is to reduce the voltage drop in the lines with low powerfactor and improve the voltage at the receiving end particularly with low power factor loads.For variable load conditions, the voltage can be controlled by switching in suitable seriescapacitors in the line.Under short circuit condition, the produced high voltage may damage the capacitor and soseries capacitor has to be protected using a spark gap with a high speed contactor. The use ofseries compensation introduces few problems like Subsynchronousresonance, Ferroresonanceand high recovery voltage.3. Case StudyA typical case study has been carried out in view of improvement of voltage regulation,transmission efficiency and power factor with the objective of maintaining load voltageconstant.A threephaseinduction motor fed from a feeder is shown in Figure 5.Figure 5: Singlelinediagram of a threephasefeeder feeding a threephaseinduction motor.Table 1 shows the data