1. What is the major control loops used in large generators? The major control loops used in large generators are
· Automatic voltage regulator (AVR)
· Automatic load frequency control (ALFC).
2. What is the use of secondary loop?
Ø A slower secondary loop maintains the fine adjustment of the frequency, and also by reset action maintains proper MW interchange with other pool members.
Ø This loop is insensitive to rapid load and frequency changes but focuses instead on drift like changes which take place over periods of minutes.
3. What is the adv of AVR loop over ALFC?
Ø AVR loop is much faster than the ALFC loop and therefore there is a tendency, for the VR dynamics to settle down before they can make themselves felt in the slower load frequency control channel.
4. What is the diff. between large and small signal analysis?
Ø Large signal analysis is used where voltage and power may undergo sudden changes of magnitude that may approach 100 percent of operating values.
Ø Usually this type of analysis leads to differential equations of non-linear type.
Ø Small signal anaysis is used when variable excursions are relatively small, typically at most a few percent of normal operating values.
5. What is the exciter?
Ø The exciter is the main component in AVR loop.
Ø It delivers the DC power to the generator field.
Ø It must have adequate power capacity and sufficient speed of response (rise time less than 0.1 sec).
6. What is the function of AVR?
Ø The basic role of the AVR is to provide constancy of the generator terminal voltage during normal, small and slow changes in the load.
7. Explain about static AVR loop?
Ø In a static AVR loop, the execution power is obtained directly from the generator terminals or from the station service bus.
Ø The AC power is rectified by thyristor bridges and fed into the main generator field via slip rings. Static exciters are very fast and contribute to proved transient stability.
8. Write the static performance of AVR loop?
Ø The AVR loop must regulate the terminal |V| to within required static accuracy limit.
Ø Have sufficient speed of response. Be stable.
9. What is the dis.adv of high loop gain? How is to be eliminated?
Ø High loop gain is needed for static accuracy but this causes undesirable dynamic response, possibly instability.
Ø By adding series AND/OR feedback stability compensation to the AVR loop, this conflicting situation can be resolved.
10. What are the effects of generator loading in AVR loop?
Ø Added load does not change the basic features of the AVR loop, it will however affect the values of both gain factor Kf and the field constant.
Ø High loading will make the generator work at higher magnetic saturation levels.
Ø This means smaller changes in |E| for incremental increases in if, translating into the reduction of KF.
Ø The field time constant will likewise decreases as generator loading closing the armature current paths.
Ø This circumstance permits the formation of transient stator currents the existence of which yields a lower effective field induction.
11. What are the functions of ALFC?
The basic role of ALFC‟s is to maintain desired MW output of a generator unit and assist in controlling the frequency of large interconnection.
Ø The ALFC also helps to keep the net interchange of power between pool members at predetermined values.
Ø Control should be applied in such a fashion that highly differing response characteristics of units of various types are recognized.
Ø Also unnecessary power output changes should be kept at a minimum in order to reduce wear of control valves.
12. Specify the dis.adv of ALFC loop?
Ø The ALFC loop will main control only during normal changes in load and frequency.
Ø It is typically unable to provide adequate control during emergency situations, when large MW imbalances occur.
13. How is the real power in a power system controlled?
Ø The real power in a power system is being controlled by controlling the driving torque of the individual turbines of the system.
14. What is the need for large mechanical forces in speed-governing system?
Ø Very large mechanical forces are needed to position the main valve against the high stream pressure and these forces are obtained via several stages of hydraulic amplifiers