Reactive power compensation

Description of Role of reactive power flow in power system and its application .

1. REACTIVE POWER COMPENSATION Submitted By : Indubhushan Kumar PhD*, M.Tech , B.Tech

2. CONTENTS Introduction Need for reactive power compensation Methods of Reactive Power Compensation Shunt Compensation Series compensation Advantage & disadvantage of Compensators Conclusion References

3. Introduction Reactive power (VAR) compensation is defined as the management of reactive power to improve the performance of ac systems. There are two aspects :- (a ) Load Compensation : The main objectives are to increase the power factor of the system to balance the real power drawn from the system compensate voltage regulation to eliminate current harmonics . (b) Line compensation – The main purpose is to decrease the voltage fluctuation at a given terminal of transmission line. Therefore the VAR compensation improves the stability of ac system by increasing the maximum active power that can be transmitted.

4. Need for reactive Power Compensation Voltage variation at a node is indication of the unbalance between reactive power generated & consumed by load. P = ( V 1 V 2 )sin δ / X , Q = V 1 ( V 1 – V 2 cos δ ) / X If reactive power generated is greater than consumed by the node , the voltage goes up & vice versa. L ack of reactive power can cause voltage collapse in a system. It is therefore important to supply/absorb excess reactive power to/from the network. Ferranti effect is minimized so that flat voltage profile will exist on line for all loading condition. Power Transfer capability of system will be enhanced ,hence stability margins increases.

5. Methods of Reactive Power Compensation Shunt compensation Series compensation Synchronous condensers Static VAR compensators Static compensator

6. Shunt Compensation A device that is connected in parallel with a transmission line is called a shunt compensator A shunt compensator is always connected at the end point and / usally in the middle of the transmission line. It can be provided by either by shunt reactor or a shunt capacitor. Shunt-connected reactors are used to reduce the line over-voltages by consuming the reactive power, while shunt-connected capacitors are used to maintain the voltage levels by compensating the reactive power to transmission line. In an SMIB power system it improves voltage profile power-angle characteristics improved power factor load stabilisation

7. Transmission Line With Shunt Compensation

8. Series Compensation When a device is connected in series with the transmission line it is called a series compensator . A series compensator can be connected anywhere in the line. There are two modes of operation – capacitive mode of operation and inductive mode of operation. Main idea of series compensation is to cancel to cancel part of series inductive reactance of the line by use of series capacitors which results in : increase in maximum transferrable power Increase in virtual impedance loading Reduction in transmission angle for certain amount of power transfer From practical point of view , it is not desirable to exceed series compensation beyond 80 % .

9. Transmission Line With Series Compensation

10. Sub Synchronous Resonance If the line is 100 % compensated , the line will behave as a purely resistive element & would result into series resonance even at fundamental frequency, Known as subsynchronus frequency (SSR). Under this resonating condition , even for small disturbance in rotor angles of the terminal synchronous machine would results into flow of large currents. It is very difficult to control voltage & currents during disturbances . The location of series capacitor is decided partly by economical factors and partly by severity of faults currents. It is a combined electrical –mechanical resonance phenomenon.

11. Contd … Sub synchronous Frequency is given by : Where , Natural frequency Degree of series compensation = Degree of compensation varies between 25 %-65% , thus this resonance take place at sub harmonic frequency i.e .

12. Thyristor Controlled Series Compensator Like the SVC, a thyristor controlled series compensator ( TCSC ) is a thyristor based series compensator that connects a thyristor controlled reactor ( TCR ) in parallel with a fixed capacitor. By varying the firing angle of the anti-parallel thyristors that are connected in series with a reactor in the TCR, the fundamental frequency inductive reactance of the TCR can be changed. This effects a change in the reactance of the TCSC . it can be controlled to produce either inductive or capacitive reactance.

13. Diagram of TCSC

14. Advantage Of Series Compensation Series capacitors are self regulating & a control system is not required. For same performance , series capacitor are often less costly than SVCs & losses are very low. For voltage stability , series capacitors lower the critical or voltage. Series capacitors posses adequate time overload capability. Series capacitor & switched series capacitors can be used to control loading of lines to minimize active & reactive power losses.

15. Disadvantage Of Series Compensation Series capacitors are line connected & compensation is removed for outage . During heavy loading, the voltage on one side of the series capacitor may be out of range. Shunt reactors may be needed for lighted load compensation. Subsynchronous resonance may be call for expensive counter measure s.

16. Static VAR Compensators The Static Var Compensator (SVC) is a shunt device using power electronics to control power flow and improve transient stability on power grids . The SVC regulates voltage at its terminals by controlling the amount of reactive power injected into or absorbed from the power system. When system voltage is low, the SVC generates reactive power (SVC capacitive). When system voltage is high, it absorbs reactive power (SVC inductive). The variation of reactive power is performed by switching three-phase capacitor banks and inductor banks connected on the secondary side of a coupling transformer. Each capacitor bank is switched on and off by three thyristor switches ( Thyristor Switched Capacitor or TSC).

17. Contd.... Reactors are either switched on-off ( Thyristor Switched Reactor or TSR) or phase-controlled ( Thyristor Controlled Reactor or TCR). It is the first generation shunt compensator. An SVC is constructed using the thyristor technology and therefore does not have gate turn off capability. The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor. Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage.

18. Fig : Configuration of various SVC

19. Thyristors Switched Reactor

20. Advantage of SVC Static VAR compensation (SVCs) control the voltage directly. It control Temporary overvoltage's rapidly. Higher reliability Faster in operation Smoother control and more flexibility

21. Static Synchronous Compensator The Static Synchronous Compensator (STATCOM) is a shunt device using power electronics to control power flow and improve transient stability on power grids. The STATCOM regulates voltage at its terminal by controlling the amount of reactive power injected into or absorbed from the power system. When system voltage is low, the STATCOM generates reactive power (STATCOM capacitive). When system voltage is high, it absorbs reactive power (STATCOM inductive). The variation of reactive power is performed by means of a Voltage-Sourced Converter (VSC) connected on the secondary side of a coupling transformer .

22. Contd.. The VSC uses forced-commutated power electronic devices (GTOs, IGBTs or IGCTs) to synthesize a voltage from a DC voltage source. It is operated as a shunt –connected static Var Compensator whose capacitive or inductive current can be controlled independent of ac system voltage. A STATCOM usually contains an SVS that is driven from a dc storage capacitor and the SVS is connected to the ac system bus through an interface transformer. A capacitor connected on the DC side of the VSC acts as a DC voltage source. The transformer steps the ac system voltage down such that the voltage rating of the SVS switches are within specified limit .

23. Structure Of STATCOM Basically, the STATCOM system is comprised of Power converters, Set of coupling reactors or a step up transformer, Controller.

24. Comparison Of V- I Characteristics

25. Advantages of STATCOM R eactive components used in the STATCOM are much smaller than those in the SVC. The characteristics of STATCOM are superior. The output current of STATCOM can be controlled up to the rated maximum capacitive or inductive range. Reduction of the capacity of semiconductor power converter and capacitor bank to one half of those for the conventional SVC. Better transient response of the order of quarter cycle. Reduction of harmonic filter capacity. Reduction of size of high value air-cored reactor. Reduction of equipment volume and foot-print.

26. CONCLUSION Increased demands on transmission lines , absence of long term planning , need to provide open access to generating companies & customers have resulted in less security,& quality of supply. Thus power compensation is must for improving the performance of the ac system. By reactive power compensation we can control the power factor and reduce the consumption of electricity.

27. References [1] Juan Dixon , JOSÉ RODRÍGUEZ “Reactive Power Compensation Technologies: State-of-the-art Review” VOL. 93, No. 12, December 2005. [2] BF Wollenberg “Transmission Systems Reactive Power Compensation” Ieee 2002. [3] C L Wadhwha “Electrical Power Systems”, New Age International (P) Limited Publishers 5 th Edition [4] D P Kothari , I.J Nagrath “Modern Power Systems”, TMH Publication 4 th Edition [5] http://nptel.ac.in/courses/Webcourse-contents/IIT-KANPUR/power-system/chapter_10/10_1.html

28. THANK YOU……