Enhancement of Voltage Profile of Transmission Line by Using Static VAR Compensator-An Overview

Enhancement of Voltage Profile of Transmission Line by Using Static VAR Compensator-An Overview

J. N. Rai, Naimul Hasan*, Rishabh K. Gupta, Rahul Kapoor, Rajesh Garai Dept. of Electrical Engineering

Delhi Technological University Dept. of Electrical Engineering

Jamia Millia Islamia, New Delhi, India*

Abstract

One of the main prerequisite of the modern power system is the enhancement and control of voltage from varying from its desired value. For this we used many compensation techniques. In this paper a case study for a particular transmission line has been carried out to enhance and control the voltage profile using compensation. This paper presents the basic aspect of voltage profile enhancement and control without contingency by simple and efficient use of capacitor bank. The effectiveness of proposed experimental result demonstrated on model of 750 kV ,250 km,1000MW EHV lines.

Index terms–compensation, line compensation, series, shunt, static VAR compensation, contingency.

1. Introduction

   In a power system, given the insignificant electrical storage, the power generation and load must balance at all times. To some extent, the electrical system is self-regulating. If generation is less than the load, voltage and frequency drop, and thereby reducing the load. However, there is only a few percent margins for such regulation. If voltage is propped up with reactive power support, then the load increase with consequent drop in frequency may result in system collapse. Alternatively, if there is inadequate reactive power, the system may have voltage collapse.

   Electrical loads both generate and absorb reactive power. Since the transmitted load varies considerably from one hour to another, the reactive power balance in a grid varies as well. The result can be unacceptable voltage amplitude variations, a voltage depression, or even a voltage collapse. Therefore, transmission line require compensation for this varying reactive power.[4],[6]

2. Load Compensation

   Load compensation is the management of the reactive power to improve the power quality i.e. V profile and p.f. Here the reactive power flow is controlled by installing shunt compensation devices

   (capacitor/reactors) at the load end bringing about power balance between generated and consumed reactive power. This is most effective in improving the power transfer capability of the system and its voltage stability. It is desirable both economically and technically to operate the system near unity power factor. This is why some utilities impose a penalty on low pf loads. Yet another way of improving the system performance is to operate it under near balanced conditions so as to reduce the flow of negative sequence currents thereby increasing the systems load capability and reducing power loss.

   A transmission line has three critical loadings (1) natural loading (2) steady state stability limit and

   1. thermal limit loading. For a compensated line the natural loading is the lowest and before the thermal loading limit is reached, steady state stability is arrived.[4]

      The three main objectives of load compensation are:

      1. Better voltage profile

      2. p.f. correction

      3. load balancing.[5]

3. Line Compensation

   Line compensation can be defined as the use of electrical circuits to modify the electrical characteristics of the line so that the compensated electrical lines can achieved following objectives.

   1. Ferranti effect is minimized so that a flat voltage profile is observed on the line during all load conditions.

   2. Under excited as well as overexcited operation of line will be avoided and an economical means of reactive power management will be achieved.

   3. The power transfer capability of the system will be enhanced and system stability margins increase. In order to assess the effectiveness a compensated system a performance index in terms of length of line and the power to be transmitted is evaluated. It is very much necessary to fix this criteria as it is not possible to load the longer length line even to their natural loadings without compensation.[5]

      Figure1: Transmission on a no loss line

      1. SERIES COMPENSATION

         In series compensation, the FACTS is connected in series with the power system. It works as a controllable voltage source. Series inductance occurs in long transmission lines, and when a large current flow causes a large voltage drop. To compensate, series capacitors are connected. Series compensation increases transmission capacity, improve system stability, control voltage regulation and ensure proper load division among parallel feeders.[2],[7],[8]

         Figure 2: Series Compensation

      2. SHUNT COMPENSATION

         In shunt compensation, power system is connected in shunt (parallel) with the FACTS. It works as a controllable current source. Shunt compensation is used to influence the natural electrical characteristics of the transmission line to increase the steady state transmittable power and to control the voltage profile of the line. [2],[7],[8]

         Shunt compensation is of two types:

         1. Shunt capacitive compensation

            This method is used improve the power factor. Whenever an inductive load is connected to the transmission line, power factor lags because of lagging load current. To compensate, a shunt capacitor is connected which draws current leading the source voltage. The net result is improvement in power factor.

         2. Shunt inductive compensation

         This method is used either when charging the transmission line, or, when there is very low load at the receiving end. Due to very low, or no load – very low current flows through the transmission line. Shunt capacitance in the transmission line causes voltage amplification (Ferranti Effect). The receiving end voltage may become double the sending end voltage (generally in case of very long transmission lines). To compensate, shunt inductors are connected across the transmission line.[2],[7],[8]

         Fig.3: Shunt Compensation [10]

      3. STATIC VAR COMPENSATOR (SVC)

   A static VAR compensator is an electrical device for providing fast active, reactive power compensation on high voltage electricity transmission network. These comprise of three phase static capacitor bank fixed or switched (controlled) or fixed capacitor bank and switched reactor bank in parallel. The term static refers to the fact that SVC has no moving parts. [4],[6]

   Fig.4: Static Capacitor Bank

   A rapidly operating static VAR compensator (SVC) can continuously provide reactive power to control dynamic voltage swings under various system conditions and thereby improve the power system performance. Thus, these compensators draw reactive (leading or lagging) power from the line there by regulating voltage, improve stability (steady state and dynamic), control over voltage and reduce voltage flicker. These also reduce voltage and current imbalances. In HVDC application these compensators provide the

   required reactive power and damp out sub harmonic oscillations. Since static VAR compensator use switching for VAR control. These are also called static VAR switches or systems. It means that terminology wise

   SVC= SVS

   and we can use these interchangeably.[4]

   A reactance connected in shunt to line at voltage V draws reactive power V2/X. It is negative (leading) if reactance is capacitive and positive (lagging) if reactance is inductive.

   Fig.5: Typical one line diagram f a SVC PRINCIPAL OF OPERATION

   In the case of a no-loss line, voltage magnitude at receiving end is the same as voltage magnitude at sending end: Vs = Vr =V. Transmission results in a phase lag that depends on line reactance X.

   1. SERIES COMPENSATION

      FACTS for series compensation modify line impedance: X is decreased so as to increase the transmittable active power. However, more reactive power must be provided.[10]

   2. SHUNT COMPENSATION

      Reactive current is injected into the line to maintain voltage magnitude. Transmittable active power is increased but more reactive power is to be provided.[10]

   3. SVC TECHNOLOGY

   If |VR| is in line KV and XC is the per phase capacitive reactance of the capacitor bank on an equivalent star basis, the expression for the VARs fed into the line can be derived as under:-

   IC j

   VR KA 3XC

   jQC (3-phase) 3 V ( I

   jQC (3-phase) 3 V ( I

   R

   C*)

   3 VR VR

   3 3

   3

   MVA

   As it is a no-loss line, active power P is the same at any point of the line.

   QC (3-phase)

   VR 2

   XC

   MVAr

   Reactive power at sending end is the opposite of reactive power at receiving end:

   If inductors are employed instead, VARs fed into the line are:

   VR 2

   QC (3-phase)

   XL

   MVAr

   As is very small, active power mainly depends on whereas reactive power mainly depends on voltage magnitude.[10]

   Under heavy load conditions, when positive VARs are needed, capacitors are employed; while under light load conditions, negative VARs are needed, inductor banks are needed.[4]

   II.EXPERIMENTAL SET UP

   We can experimentally check the influence of capacitor bank on the voltage profile by making the given set up.

   1. Make connection as shown in figure (6).

   2. Switch on the power supply, keeping switch 2 and 3 in open position. Adjust sending end voltage VS and hence VR at about 100V by adjusting variac across supply. ILine is almost zero , the switch S2 is closed because of high impedance of voltmeter VR.

   3. Switch on load by closing S3. ILoad should not exceed rated wattmeter current which may otherwise get damaged. Note down VR,VS, IL.

   4. Close switch 3. Observe the VR rises. Adjust capacitance values (by making parallel or series compensation) so that VR = VS (full 100% compensation). Note VS,VR, ILine, ILoad , IC, WS.

   5. Repeat step 2 and 3 for different load voltages.

   The following observations have been made as according to the table(s):

   Table 1 Without Compensation

   Figure 6. Model of a Transmission line with compensation

   Table II: With Compensation

   S.No.	VS (Volt)	VR (Volt)	IS (Amp.)	IL (Amp.)	IC (Amp.)	WS (Watt)	WL (Watt)	KVARby Cap.	VARsby C*
   1.	140	138	1.58	0.54	1.54	120	70	1/3	137.16
   2.	140	140	1.22	0.74	1.44	100	56	1/3	134.99
   3.	160	164	1.74	0.52	1.78	135	79	1/3	185.22

   *written after calculations.

   Calculations

   Set 1:

   =138.46 VAR

   VAR supplied by Capacitor = 1/3*1000(140/220)2

   = 134.16 VAR

   Sending end VAR =

   =

   (VS * IS )2 (WS )2

   (140*1.58)2 (120)2

   Load VAR =

   (140*0.74)2 (56)2

   =185.82 VAR

   Set 3:

   = 87.16 VAR

   VAR supplied by Capacitor = 1/3*1000(VR/VC)2

   = 1/3*1000(138/220)2

   Sending end VAR =

   (160*1.74)2 (135)2

   =131.16 VAR

   Load VAR =

   =

   (VR * IL)2 (WL)2 (138*0.54)2 (70)2

   = 25.56

   = 243.40 VAR

   VAR supplied by Capacitor = 1/3*1000(164/220)2

   = 185.24 VAR

   VAR

   Set 2:

   Sending end VAR =

   (140*1.22)2 (100)2

   Load VAR =

   (164*0.52)2 (79)2

   = 32.20 VAR

4. CONCLUSION & DISCUSSION

   This paper described an efficient and practical method to enhance the voltage profile and control for the particular rating of transmission line. Thus we see within experimental limitations reactive power consumed by the load is equal to the reactive VAR supplied by the capacitor for 100% compensation. Since QC is proportional to the square of terminal voltage, for a given capacitor bank, their effectiveness tends to decrease as the voltage sags under full load conditions. Capacitor acts as short circuit when switched on. We should also take the precautions of a possibility of series resonance with the line inductance particularly at harmonic frequencies. Step less (smooth) VAR can be achieved using SCR (silicon controlled rectifier circuitry) as with them we can switch capacitor and inductor in steps.

5. REFERENCE

   1. Narain G. Hingorani, Laszlo Gyugyi Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems, Wiley-IEEE Press, December 1999. ISBN 978-0-7803-3455-7

   2. Xiao-Ping Zhang, Christian Rehtanz, Bikash Pal, Flexible AC Transmission Systems: Modelling and Control, Springer, March 2006. ISBN 978-3-540-30606-1. http://www.springer.com/3-540-30606-4

   3. A. Edris, R. Adapa, M.H. Baker, L. Bohmann, K. Clark, K. Habashi, L. Gyugyi, J. Lemay, A. Mehraban, A.K. Myers, J. Reeve, F. Sener, D.R. Torgerson, R.R. Wood, Proposed Terms and Definitions for Flexible AC Transmission System (FACTS), IEEE Transactions on Power Delivery, Vol. 12, No. 4, October 1997.

   4. Nagrath, I.J.,Kothari D.P. Modern Power System Analysis Third Edition TMH Publication.

   5. Wadhwa C.L. Electrical Power System Fourth Edition, New Age Publishers .

   6. K. R Padiyar , FACTS Controllers in Power Transmission & Distribution. New Age International (P) ltd., 2007.

   7. Giuseppe, Fusco / Mario, Russo, 2006, Adaptive Voltage Control in Power Systems: Modelling, Design and Applications (Advances in Industrial Control) Springer ISBN 184628564X November 13, 2006.

   8. P. Kundur. Power system stability and control. McGraw-Hill, New York, 1994.

   9. LEE S Y, Wu C Combined compensation of a static VAR compensator and an active filter for unbalanced three-phase distribution feeders with harmonic

      distortion. Electric Power System Research, 1998.

      WEB Reference

   10. www.ieeeexplore.ieee.org

   11. www.wikipedia.org