Voltage Control Interview Questions and Answers:
1. Why is it necessary to keep the receiving-end voltage constant within specified limits ?
Ans. For satisfactory operation of equipment/machinery supplied by the electrical power network it is necessary to keep the receiving-end voltage constant within specified limits. Too wide variation of voltage may cause erratic operation or even malfunctioning of consumers appliances. This is statuary requirement also.
2. What are the limits within which consumer’s terminal voltage should be maintained ?
Ans. According to IE rules the declared consumer’s voltage is 415/240 V (i.e., 415 V between phases and 240 V between phase and neutral). The higher and lower limits are 440/254 during light load hours and 400/230 V during peak load hours.
3. How is voltage control achieved ?
Ans. Voltage control is achieved by installing voltage control equipment at suitable places such as at (i) generating stations (ii) transformer stations supplying to feeders and (iii) at load end.
4. Name the different methods of controlling voltages in ac supply systems.
Ans. The different methods used to control voltages in ac supply system are (i) excitation control at generating stations and by using (ii) tap-changing transformers (iii) induction regulators (iii) booster transformers (iv) series compensation (v) shunt compensation and (vi) synchronous phase modifiers.
5. Why the excitation control is not possible beyond certain limits ?
Ans. Excitation below a certain level may cause unstability of the system and excitation above a certain limit may cause overheating of the rotor. So excitation control is not possible beyond certain limits.
6. Why is excitation system required in a power station ?
Ans. The excitation system is required to provide the necessary field current to the rotor winding of a synchronous machine. The amount of excitation depends on the power factor, speed of the machine and load current. For large currents, lower speeds and lagging power factors, the excitation requirement is more.
7. What are the main requirements of excitation system ?
Ans. The main requirements of an excitation system are reliability under all conditions of service, simplicity of control, ease of maintenance, stability and high transient response.
The initial cost of excitation equipment should be a secondary consideration in the selection of excitation system. Loss of excitation of a unit on the bus results in a more serious disturbance than that resulting from outage of the generator unit from the bus as the remaining units must not only pick up the load dropped but also supply the large reactive current drawn by the unexcited generator. In view of this, an excitation system with better reliability is preferred, even if the initial cost is more. Simplicity of control and ease of maintenance are always preferred for any equipment and excitation system is no exception.
8. Distinguish between centralized and individual excitation systems.
Ans. An excitation system may be individual one, in which each alternator is provided with its own exciter in the form of small generator on an extension of the main shaft, or centralised excitation system having two or more exciters feeding a bus-bar to which field systems of all the alternators in the power plant are connected. Though centralised excitation system is a cheaper arrangement but a fault in this system adversely affects all the alternators in the power plant. As such individual excitation system is widely used.
9. Why have dc excitation systems been superseded by ac excitation systems ?
Ans. The main drawbacks of dc excitation system are large time constant (about 3 seconds) and commutation difficulties. As such the dc excitation systems have been superseded by ac excitation systems.
10. Why excitation circuit should not be opened suddenly ?
Ans. A sudden opening of the excitation circuit may cause a inducement of a high voltage in the inductive field winding because of sudden release of energy stored in it. As such the excitation circuit should not be opened suddenly.
11. In a rotating thyristor excitation system, the main alternator field rectifier usually consists of four parallel thyristors each having a fuse in its circuit. Why so ?
Ans. The main alternator field rectifier usually consists of four parallel thyristors each having a fuse in its circuit so that in case of failure of any thyristor, the fuse opens thereby allowing the system to continue operation. The thyristor assembly is so rated that with the loss of one thyristor unit, the alternator field needs are met with sufficient margin.
12. Why is brushless excitation system becoming more and more popular ?
Ans. With the increase in rating of alternators, the problem of brush maintenance gets more and more complicated, therefore, the brushless excitation system is becoming more and more popular.
13. What are the advantages of static excitation system ?
Ans. The advantages of the static excitation system are eliminaÂtion of exciter windage loss and commutator wearing and winding maintenance resulting in reduced operating costs and electronic speed response. The fact that in static exciÂtation the voltage is proportional to the speed, affords a major advantage in load rejection.
14. Why the excitation control is not possible beyond certain limits ?
Ans. Excitation below a certain level may cause unstability of the system and excitation above a certain limit may cause overheating of the rotor. So excitation control is not possible beyond certain limits.
15. Why is automatic voltage regulating equipment essential in operation of synchronous machines ?
Ans. Synchronous generators have inherently large internal reactance and, therefore, high voltage regulation. So, in order to maintain terminal voltage of the synchronous generators under various operating conditions, provision of automatic voltage regulation equipment is essential.
16. What is the operating principle of AVR ?
Ans. An automatic voltage regulator operates on the principle of detection of error. The output voltage of an ac generator obtained through a PT is rectified, filtered and compared with a reference. The difference of the actual voltage and the reference voltage, known as error voltage, is amplified through an amplifier (rotary, magnetic or static) and supplied to the field circuit of the main exciter or pilot exciter. Thus, the amplified error signals controls the excitation of the main or pilot exciter through a buck or boost action. Exciter output control leads to the control of the main alternator terminal voltage.
17. What are the main functions of AVRs ?
Ans. The main functions of an AVR are as follows:
- Control of system voltage within prescribed limits and have the operation of the machine nearer to the steady state stability limit.
- Proper division of reactive load between the alternators operating in parallel.
- Prevention of dangerous overvoltages on the occurÂrence of sudden loss of load on the system.
- Increase of excitation under system fault conditions so that maximum synchronising power exists at the time of clearance of fault, to prevent loss of synchronism.
18. What are the drawbacks of electromechanical AVRs ?
Ans. Electromechanical AVRs have the following drawbacks:
- The variations of resistances in the exciter circuit results in a variable time constant. Thus, the response time of the exciter will vary with different excitation conditions.
- The response time of the regulator and main exciter field circuit is too slow. Dead band does not suit to generators when operating near steady-state limit.
- With the increase in the rating of the machines, the excitation need is increased and it is difficult for regulator contacts to handle this power.
Electromechanical AVRs suffer from the wear and friction and need fairly regular attention. Friction reduces the sensitivity and causes a dead zone of between 0.5 and 1% within which no control action is available. The contacts of vibrating type AVR need regular attention because of erosion caused by the sparking. The rate of response is limited by the mass of the moving parts. Even when providing full corrective action they cannot force the main exciter to go negative in order to limit alternator voltage on occurrence of sudden loss of the load on the system.
19. What are the benefits of electronic voltage regulators ?
Ans. Electronic voltage regulators provide the benefits of accuracy of control, high speed response, high reliability and minimum maintenance.
20. What is a booster transformer ?
Ans. Booster transformer is one which is often used toward the end of a power line in order to raise the voltage to the desired value.
21. What special precautions must be taken when using a booster transformer ?
Ans. There must be no fuse on the hv side, or primary; since the booster transformer is similar to a current transformer, if the fuse should blow, an extremely high voltage could be built up on the secondary side.
22. State whether the tap changer in a transformer is provided on hv side or lv side ?
Ans. On hv side.
23. Why are both of the primary and secondary windings of a single-phase induction regulator usually designed to provide 2 poles or 4 poles ?
Ans. Both of the primary and secondary windings of a single-phase induction regulator are usually designed to provide 2 poles or 4 poles because the smaller the number of poles, the larger the mechanical angle corresponding to one pole pitch and the easier the voltage regulation.
24. Why is the single-phase induction regulator is provided with the short-circuited compensating winding ?
Ans. If the single-phase induction regulator were not provided with the short-circuited compensating winding, its operation would not be satisfactory (voltage regulation would be very poor with adjustments for intermediate voltages).
25. No compensating or tertiary winding is required in case of a 3-phase induction regulator. Why ?
Ans. No compensating or tertiary winding is required in case of a 3-phase induction regulator because, in this case, for every position of setting of the regulator each secondary winding is magnetically coupled to one or more of the primary windings.
26. What are the applications of induction regulators ?
Ans. The induction voltage regulator finds its greatest application in distribution systems for raising or lowering the line voltage to maintain the load voltage at a constant value under all load conditions because it varies the voltage uniformly, not in steps.
27. Name few commonly used static VAR schemes.
Ans. Many SVS schemes are in operation. Some of commonly used schemes are as follows :
- Thyristor controlled reactor (TCR).
- Thyristor switched capacitor (TSC).
- Self saturated reactor (SR).
- Thyristor controlled reactor-Fixed capacitor (TCR-FC).
- Thyristor switched capacitor-Thyristor controlled reactor (TSC-TCR).
28. In case of TCR-FC scheme of VAR compensation, harmonics are generated because of switching operations. How these harmonics can be eliminated ?
Ans. Small reactors are usually connected in the fixed capacitor branches, so as to tune these branches as filters for the fifth and seventh harmonics. The TCR and coupling transformer secondary are connected in delta so as to cancel third harmonics.
29. What are the advantages of TSC-TCR scheme over TCR-FC scheme.
Ans. The TSC-TCR scheme has the following advantages over TCR-FC scheme.
- Improved performance during large system disturbances.
- Lower power losses.
30. Give the advantages of SVS.
Ans. The static VAR system (SVS) offer the advantages of (i) Increased power transfer capability of lines, (ii) Improved transient stability of the system, (iii) Improved dynamic system stability owing to increased damping provided, (iv) Possibility of damping of subsynchronous frequency oscillations, (v) Very fast dynamic response, (vi) Controlled steady state and temporary overvoltages, (vii) Improved load power factor, reduced line losses and improved system efficiency.
31. What are the advantages of synchronous compensation ?
Ans. Synchronous compensation has the following advantages: (i) Same voltage can be maintained at both ends of a transmission line. (ii) Power factor correction is affected at times of heavy loads. (iii) A better protection for the line is possible due to possibility of using high terminal reactances.
32. What are the shortcomings of synchronous compensation ?
Ans. Synchronous compensation has the following shortcomings : (i) Short-circuit current of the system is increased. (ii) Risk of interruption of supply becomes more due to falling of synchronous condensers out of synchronism.