REACTIVE POWER -VOLTAGE CONTROL
1. What are the sources of reactive power? How it is controlled?
Ø The sources of reactive power are generators, capacitors, and reactors.
Ø These are controlled by field excitation.
Ø Give some excitation system amplifier. The excitation system amplifiers are,
a) Magnetic amplifier
b) Rotating amplifier
c) Modern electronic amplifier.
2. When is feedback stability compensation used?
Ø High loop gain is needed for static accuracy but this causes undesirable dynamic response, possibly instability.
Ø This conflicting situation is resolved by adding feedback stabling compensation to the AVR loop.
3. Give the characteristics of line compensators?
Ø The characteristics of line compensators are,
a. Ferranti effect is minimized.
b. Under excited operation of synchronous generator is not required.
4. What is known as bank of capacitors? How it is adjusted?
Ø When a number of capacitors are connected in parallel to get the desired capacitance, it is known as bank of capacitors.
Ø These can be adjusted in steps by switching (mechanical).
5. What is the disadvantage of switched capacitors are employed for compensation?
Ø When switched capacitors are employed for compensation, these should be disconnected immediately under light load conditions to avoid excessive voltage rise and Ferro resonance in presence of transformers.
6. What are the effects of capacitor in series compensation circuit?
Ø The effects of capacitor in series compensation circuit are, Voltage drop in the line reduces.
Ø Prevents voltage collapse.
Ø Steady state power transfer increases.
Ø Transient stability limit increases.
7. Give two kinds of capacitors used in shunt compensator?
Ø The two kinds of capacitors used in shunt compensator are, a. Static Var Compensator (SVC) :
Ø These are banks of capacitors ( sometimes inductors also for use under light load conditions).
8. What is synchronous condenser?
Ø It is a synchronous motor running at no-load and having excitation adjustable over a wide range.
Ø It feeds positive VARs into the line under overexcited conditions and negative VARs when under excited.
9. Write about Static VAR Compensator (SVC).
Ø These comprise capacitor bank fixed or switched or fixed capacitor bank and switched reactor bank in parallel.
Ø These compensators draw reactive power from the line thereby regulating voltage, improve stability (steady state and dynamic), control overvoltage and reduce voltage and current unbalances.
Ø In HVDC application these compensators provide the required reactive power and damp out sub harmonic oscillations.
10. What is Static VAR Switches or Systems?
Ø Static VAR compensators use switching for var control.
Ø These are also called static VAR switches or systems.
Ø It means that terminology wise SVC=SVS.
Ø And we will use these interchangeably.
11. Give some of the Static compensators schemes.
a. Saturated reactor
b. Thyristor- Controlled Reactor (TCR)
c. Thyristor Switched capacitor (TSC)
d. Combined TCR and TSC compensator.
12. What is tap changing transformers?
Ø All power transformers and many distribution transformers have taps in one or more windings for changing the turn's ratio.
Ø It is called tap changing transformers.
13. Write the types of tape changing transformers.
a. Off- load tap changing transformers.
b. Tap changing under load transformers.
14. What is the use of off-load tap changer and TCUL ?
Ø The off- load tap changers are used when it is expected that the ratio will need to be changed only infrequently, because of load growth or some seasonal change.
Ø TCUL is used when changes in ratio may be frequent or when it is undesirably to de-energize the transformer to change the tap.