EE 1401 POWER SYSTEM OPERATION AND CONTROL(2 marks)

UNIT- III RECATIVE POWER AND VOLTAGE CONTROL

1. What is the relationship between voltage and reactive power?



Q1, Q2 –Sending end and receiving reactive power respectively
V1, V2- Sending end and receiving Voltages respectively
X –Total transfer reactance between sending end and receiving end side.
2. What are the Methods of Voltage Control?
i. Excitation control with AVR in generating stations
ii. Tap changing transformers using in sending and receiving end of the transmission line.
iii. Shunt reactors – Low loads
iv. Shunt capacitors – high loads or low p.f load
v. Series capacitors –long EHV transmission line
vi. Synchronous condenser or compensator
vii. Static VAR compensator (SVC)
3. What is exciter?
The fields winding of synchronous machines (alternator) are always supplied with d.c from d.c generators called exciter.

4. What are the various functions of excitation system?
Exciter is the main component in the AVR loop .It delivers the d.c power to the generator field. It must have adequate power capacity and sufficient speed response.
5. What are the types of excitation system?
1. D.C Excitation system
2. A.C Excitation system
3. Static Excitation system.
6. What are effects of AVR loop?
1. AVR must regulate the terminal voltage V within required static accuracy limit
2. It must have sufficient speed response
3. It must be stable.
7. What is the necessity of stability compensation in AVR loop?
Stability compensation improves the dynamic response characteristics without affecting the static loop gain.
8. What are the power system components are generate and absorb the reactive power?
Synchronous machine under over excited condition, Shunt capacitor and cable are generating reactive power.
Shunt reactor, overhead lines, transformer and Load are absorbing the reactive power.
9. Compare shunt and series capacitors.
S.No Shunt capacitors Series capacitors
1 Shunt capacitors supply capacitive reactive power to the system at the point where they are connected. Series capacitors are connected in series with the line and are used to reduce inductive reactance between supply point and load
2 It is mainly used for power factor correction at the load terminal of low voltage It is mainly used to compensate the effect of series reactance.
3 If the loadVAR requirement is small, shunt capacitors are of high use. If the load VAR requirement is small, series capacitors are of small use
4 If the total line reactance is high, shunt capacitors are not effective. If the total reactance is high series capacitors are very effective and stability is improved
10. What is a synchronous compensator?
A Synchronous condenser is a synchronous machine running without a prime mover or a mechanical load under over excited condition. It has wide variation excitation control. It can be made to either generate or absorb reactive power.
11. Where synchronous condensers installed?
The synchronous compensator is connected to the tertiary winding of the main transformer for voltage and reactive power control at both transmission and sub transmission level. A neutral point is provided by the earthling transformer.
12. What are the advantages and disadvantages of synchronous compensator?
Advantages:
1. Flexibility of operation for all conditions
2. As the losses are considerable compared with static capacitors and the power factor is not zero.
Disadvantages:
1. The cost of installation is high
2. Losses of synchronous condensers are much high compared to those of capacitors.
13. How is voltage control obtained by using tap changing transformers?
Voltage is controlled with the help of line drop compensator, voltage sensitive regulating relay, time delay relay, etc
By changing the turns ratio of transformer the voltage ratio and the secondary voltage is changed and the voltage control is obtained. So, voltage control of transmission and distribution system is obtained by tap changing.
14. What is Static VAR Compensator (SVC)? Where it is used?
SVCs are Shunt connected static generators and/or absorber whose outputs are varied so as to control specific parameters of the electric power system.
The term static is used to indicate the SVC, have no moving, or rotating main components. Thus an SVC consists of static VAR generator or absorber devices and suitable devices.
SVC s are located in receiving substations and distribution systems for smooth and steeples variation of compensation of reactive power injected into the line, by shunt capacitors and shunt reactors.
15. What are the different types of SVC?
The following are the basic types of reactive power control elements which make up all or part of any static VAR system.
i. Saturated reactor(SR)
ii. Thyristor controlled reactor(TCR)
iii. Thyristor controlled Transformer(TCT)
iv. Thyristor Switched Reactor(TSR)
v. Thyristor Switched capacitor(TSC)
vi. Self or line commutated converter(SCC/LCC)
16. Write the some applications of SVC.
i. Control of temporary (power frequency) over voltages
ii. Prevention of voltage collapse
iii. Enhancement of transient stability
iv. Enhancement of damping of system oscillations