INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 Shunt versus Series compensation in the improvement of Power system performance Irinjila Kranti Kiran 1 , Jaya Laxmi.A 2 1– Associate Professor, Electrical and Electronics Engineering Department, MVGR College of Engineering, Vizianagaram (Dt.), India 2– Associate Professor, Electrical and Electronics Engineering Department, JNTUH College of Engineering, Hyderabad, India kranthiirinjila@yahoo.co.in ABSTRACT VAR compensation involves the management of reactive power for the improvement of electric power system performance. Adequate reactive power control solves power quality problems like flat voltage profile maintenance at all power transmission levels, and improvement of power factor, transmission efficiency and system stability. Series and Shunt VAR compensation techniques are used to modify the natural electrical characteristics of electric power system. Series compensation modifies the reactance parameter of the transmission or distribution system, while shunt compensation changes the equivalent load impedance. In both cases, the line reactive power can be effectively controlled thereby improving the performance of the overall electric power system. This paper presents an overview of the reactive power and static VAR compensation technologies. In case study, an application example is illustrated. The results obtained by applying both series and shunt VAR compensation techniques individually to the case study illustrated are tabulated for comparison. Key words: Reactive power, Transmission efficiency, Voltage regulation, Series compensation, Shunt compensation, Natural load. Notations E or VS = Sending­end voltage, V. Z = Circuit impedance, Ω. R = Circuit resistance, Ω. XL = Circuit inductive reactance, Ω. ¹ I = Line current, A. I = Load current, A. = Receiving­end voltage, V. = Phase angle between VR and I. δ or δ = 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.
VR ¹ Φ or Φ ¹ 28 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 IX by 90 0 , A. = Reactive (or Out­of­phase) component of current lagging the voltage IC = Reactive (or Out­of­phase) component of current leading the voltage by 90 0 , A. 1. Introduction Modern civilization depends mostly on electrical energy for agricultural, commercial, domestic, industrial and social purposes (B.R.Gupta, 1998). The electrical energy is exclusively generated, transmitted and distributed in the form of alternating current (a.c.). Of the three basic elements of electrical engineering, the resistor consumes ohmic energy where as the inductor and capacitor store the electrical energy in magnetic field and electric field respectively. 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 voltage and draw current giving the deceptive impression that they actually do dissipate power. This “phantom power” is called reactive power (Van Cutsem T., 1991). A reactor or a capacitor stores the reactive power generated by the a.c. power source during a quarter of a cycle and returns the same power to the source in the next quarter cycle. In other words, the reactive power oscillates between the source and the reactor or capacitor at a frequency equal to twice the rated value. More precisely power dissipated by a load is referred to as true power where as power merely absorbed and returned in load due to its reactive properties is referred to as reactive power. However in nature, most of the loads are inductive loads absorbing reactive power and resulting in low lagging power factor. 1.1 Effects of reactive power flow in line­network 1.1.1 Poor transmission efficiency Losses in all power system elements from the power station generator to the utilization devices increase due to reactive power drawn by the loads, thereby reducing transmission efficiency. 1.1.2 Poor voltage regulation Due to the reactive power flow in the lines, the voltage drop in the lines increases due to which low voltage exists at the bus near the load and makes voltage regulation poor. 1.1.3 Low power factor The operating power factor reduces due to reactive power flow in transmission lines. 1.1.4 Need of large­sized conductor The low power factor due to reactive power flow in line conductors necessitates large­sized conductor to transmit same power when compared to the conductor operating at high power factor.
29 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 1.1.5 Increase in KVA rating of the system equipment The reactive power in the lines directly affects KVA rating of the system equipment carrying the reactive power and hence affects the size and cost of the equipment directly. 1.1.6 Reduction in the handling capacity of all system elements Reactive component of the current prevents the full utilization of the installed capacity of all system elements and hence reduces their power transfer capability. A power system is expected to operate under both normal and abnormal conditions and under these conditions it is desired that the voltage must be controlled for system reliability, the transmission loss should be reduced and power factor should be improved (Rajesh Rajaraman et.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 are analyzed and presented. 1.2 Effect of reactive power flow on line voltage drop The voltage variation is due to imbalance in the generation and consumption of reactive power in the system. If the generated reactive power is more than the consumed one, then the voltage levels go up and vice versa. However, if the two are equal, then the voltage profile becomes flat and it happens only when the load is equal to natural load. Unfortunately the reactive power in a system keeps on varying and if the reactive power generation is simultaneously 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 power loads. For example, if the supply voltage exceeds the rated value, the motor magnetic circuit may be saturated and consequently draw large magnetizing current. On the other hand, if the voltage is too below, the starting torque becomes low. Too wide variation of voltage causes excessive heating of distribution transformers thereby reducing the transformer ratings. So the voltage profile must remain within +5 to 6% of the rated value for better and efficient operation of various electrical equipment. As most of the loads operate at low lagging power factor, they require significant amount of lagging reactive power during peak load conditions (Kankar Bhattacharya and Jin Zhong, 2001). However, if this significant amount of lagging reactive power is supplied by the generator from the sending end, all equipment starting from the sending­end may be over loaded thereby causing low receiving end voltage. During off peak conditions, the line generates net VARs which must be absorbed to obtain voltage stability. Shunt compensation involves the use of shunt capacitors during peak load conditions to generate lagging VARs at the receiving end and shunt reactors during off peak conditions to absorb line generated VARs to avoid voltage instability (Van Cutsem T., 1991). Consider the Figure 1 showing the equivalent circuit of an a.c. generator supplying power to a practical inductive load and its phasor diagram.
30 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 Figure 1: Equivalent circuit of Alternator supplying electrical energy to a practical inductive load and its phasor diagram. From the phasor diagram, it can be stated that E 2 = (V + DV )2 + (¶ V ) 2 = (V + IR cos j + IX sin j )2 + ( IX cos j - IR sin j ) 2 = (V + ( RP / V ) + ( XQ / V )) 2 + (( XP / V ) - ( RQ / V )) 2 Therefore DV = ( RP + XQ) / V and ¶V = ( XP - RQ ) / V
For ¶V ppp (V + DV ), 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, there shall be a voltage drop. 1.3 Effect of reactive power flow on transmission efficiency and power factor It is desirable both economically and technically to operate the electric power systems at near unity power factor (u.p.f). Usually power factor correction means to generate reactive power as close as possible to the load which it requires rather than generating it at a distance and transmit it to the load, as it results in not only need of a large sized conductor but also increased losses thereby reducing transmission efficiency (M.W. Gustafson and J.S. Baylor, 1988). 2. Compensation techniques Artificial injection of reactive power at the loads may relieve the transmission network from reactive power flow and improves both transmission efficiency and operating power factor where as artificial injection of negative reactance in the lines may relieve the lines from excessive voltage drop and improves the voltage regulation (Kankar Bhattacharya and Jin Zhong, 2001). The methods available for the injection of both are static compensation and
31 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 synchronous compensation. Static compensation involves capacitors and reactors where as synchronous compensation involves synchronous phase modifier. The principles of both shunt and series compensation techniques are presented in this paper. 2.1 Shunt compensation At buses where reactive power demand increases, bus voltage can be controlled by connecting 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 magnitude of the source current necessary to supply load. Consequently the voltage drop between the sending end and the load gets reduced, power factor will be improved and increased active power 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 system or can be varied by switching ON or OFF the parallel­connected capacitors either manually or automatically (M.L.Soni, P.V.Gupta and U.S.Bhatnagar, 1994). Figure 2 shows the single­line diagram of a transmission line and its phasor diagram before the addition of the shunt capacitor and its phasor diagram. Figure 2: Single­line diagram of an uncompensated transmission line and its phasor diagram. Voltage drop in the line with lagging power factor can be approximated as VD = I R R + I X X L V Figure 3: Single­line diagram of a shunt compensated transmission line and its phasor diagram.
32 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 Figure 3 shows the single­line diagram and its phasor diagram after the addition of the shunt capacitor and its phasor diagram. Voltage drop can be approximated as VD = I R R + I X X L - I C X L V The difference between the voltage drops is the voltage rise due to installation of the capacitor and can be expressed as VR = I C X L V The usage of shunt capacitor banks suffers from the following drawbacks: 1. Shunt capacitors do not affect current or power factor beyond their point of application. 2. The reactive power supplied by the shunt capacitor banks is directly proportional to the bus voltage. 3. When the reactive power required is less on light loads, capacitor bank output will be high. This disadvantage can be eliminated by connecting a number of capacitors in parallel and then capacitance can be varied by switching ON or OFF depending upon load requirement. 2.2 Series compensation When the line has high value of reactance to resistance ratio, the inductive reactance of the transmission line can be decreased by introducing series capacitors which results in low voltage drop (Rajesh Rajaraman et.al., 1998). When a load with lagging power factor is connected at the end, voltage drop in the line is VD = I ( R cos j + X L sin j ) V If a capacitance ‘C’ with reactance Xc is connected in series with the line, then the reactance will be reduced to (XL­Xc) and hence the voltage drop is reduced. Further the reactive power taken by the line is also reduced. In Figure 4, the equivalent circuit of the line with series compensation and its phasor diagram are presented. Figure 4: Single­line diagram of a series compensated transmission line and its phasor diagram.
33 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 It can be observed from the phasor diagram that line voltage drop is VD = I ( R cos j + ( X L - X C ) sin j ) V Thus the use of series capacitors is to reduce the voltage drop in the lines with low power factor 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 series capacitors in the line. Under short circuit condition, the produced high voltage may damage the capacitor and so series capacitor has to be protected using a spark gap with a high speed contactor. The use of series compensation introduces few problems like Sub­synchronous resonance, Ferro­ resonance and high recovery voltage. 3. Case Study A 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 voltage constant. A three­phase induction motor fed from a feeder is shown in Figure 5. Figure 5: Single­line diagram of a three­phase feeder feeding a three­phase induction motor. Table 1 shows the data of the system studied. Table 1: System Data Electrical component Induction motor Line conductor Parameter Type Frequency Connection H.P. Voltage Full­load efficiency Power factor Resistance Inductance Absolute magnitude Three­phase 60 Hz Star 500 4,160 88 % 0.75 lagging 0.1601 Ω per phase 1.029 mH per phase Table 2 shows the magnitudes of different parameters without any compensation technique.
34 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 Table 2: Base case (OR) Uncompensation Results Electrical parameter Load current, iLoad Line current, iLine Phase voltage drop, VD Ph Line voltage drop, VD L­L Sending­end line voltage, VS(L­L) Total active power loss, PL(Tot) Total reactive power loss, QL(Tot) Sending­end active power, PS Sending­end reactive power, QS System power factor, CosФSys Load power factor, CosФL Absolute magnitude 78.4352 A 78.4352 A 32.94 V 57.05 V 4,217.05 V 2.9548 KW 7.161 KVAR 426.8184 KW 380.973 KVAR 0.746 0.75 Series capacitors are usually selected such that the line inductive reactance is more than the capacitive reactance. Assuming each phase capacitive reactance equal to 0.25 Ω, Table 3 shows the magnitudes of different parameters with series compensation technique. Table 3: Series­Capacitive Compensation Results Electrical parameter Load current, iLoad Line current, iLine Phase voltage drop, VD Ph Line voltage drop, VD L­L Sending­end line voltage, VS(L­L) Total active power loss, PL(Tot) Total reactive power loss, QL(Tot) Sending­end active power, PS Sending­end reactive power, QS System power factor, CosФSys Load power factor, CosФL Absolute magnitude 78.4352 A 78.4352 A 16.55 V 28.66 V 4,188.66 V 2.9548 KW 2.547 KVAR 426.8184 KW 376.36 KVAR 0.75267 0.75 Assuming the power factor to be improved to 0.9, Table 4 shows the magnitudes of different parameters with shunt compensation technique. Table 4: Shunt­Capacitive Compensation Results Electrical parameter Capacitive current, iC Line current, iLine Phase voltage drop, VD Ph Line voltage drop, VD L­L Sending­end line voltage, VS(L­L) Total active power loss, PL(Tot) Total reactive power loss, QL(Tot) Sending­end active power, PS Sending­end reactive power, QS Absolute magnitude 23.389 A 65.363 A 27.452 V 47.55 V 4,207.55 V 2.052 KW 4.973 KVAR 425.9156 KW 210.26 KVAR
35 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 Capacitive reactive power, QC Line reactive power, QL System power factor, CosФSys Load power factor, CosФL 168.526 KVAR 205.28651KVAR 0.89668 0.75 The comparison among the effect of uncompensated line, series compensated line and shunt compensated line on line voltage, line flows and power factor are presented in Table 5. Table 5: Comparison of Results Parameter / Circuit arrangement VS(L­L) PL(Tot) QL(Tot) CosФSys CosФL Absolute magnitude Series Shunt Base case capacitive capacitive arrangement arrangement arrangement 4217.05V 4188.66V 4207.55V 2.9548 KW 2.9548 KW 2.052 KW 7.161 KVAR 2.547 KVAR 4.973 KVAR 0.746 0.75267 0.89668 0.75 0.75 0.75 4. Conclusions 1. To maintain constant load voltage, sending­end voltage to be maintained is low with Series– capacitive arrangement than with Shunt­capacitive arrangement. 2. Shunt­capacitive arrangement reduces the total active power loss while Series– capacitive arrangement does not affect it. 3. Series­capacitive arrangement reduces the total reactive power loss by a large margin as compared to Shunt­capacitive arrangement. 4. Shunt­capacitive arrangement improves the system power factor by a large margin as compared to Series­capacitive arrangement. 5. Load power factor remains always constant with and without any compensation technique. 5. References 1. Kankar Bhattacharya and Jin Zhong, (2001), “Reactive Power as an Ancillary Service”, IEEE Transactions on Power Systems, 16(2), pp 294 ­ 300. 2. M.W. Gustafson and J.S. Baylor, (1988), “Transmission loss Evaluation for Electric Systems”, IEEE Transactions on Power Systems, 3(3), pp 1026 ­ 1032. 3. Rajesh Rajaraman, Fernando Alvarado, Arthur Maniaci, Robert Camfield and Sasan Jalal, (1998), “Determination of Location and Amount of Series Compensation to Increase Power Transfer Capability”, IEEE Transactions on Power Systems, 13(2), pp 294 ­ 300.
36 INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011 © Copyright 2010 All rights reserved Integrated Publishing Association RESEARCH ARTICLE ISSN ­ 0976­4259 4. Van Cutsem T., 1991, “A method to compute reactive power margins with respect to voltage collapse”, IEEE Transactions on Power Systems, 6(1), pp 145 ­ 156. 5. B.M.Weedy, (1979), Electric power systems , John Wiley. 6. B.R.Gupta, (1998), Power System Analysis And Design, Third Edition , S.Chand and Company Ltd. 7. M.L.Soni, P.V.Gupta and U.S.Bhatnagar, (1994), A Course In Electrical Power,Dhanpat Rai and Sons Pvt Ltd. 8. S.Sivanagaraju and V.Shankar, (2006), Electrical Power Distribution And Automation, Dhanpat Rai and Co. Publications. 9. Dr.V.Kamaraju, (2008), Electrical Power Distribution Systems, Third Edition, Overseas Publishers Pvt. Ltd. 10. C.L.Wadhwa, (2005), Electrical Power Systems, Fourth Edition, New Age International Publishers. 11. T.J.E.Miller, (1982), Reactive power control in electric systems, John Wiley. 12. Turan Gonen, (1986), Electric Power Distribution System Engineering, McGraw­ Hill book company.
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