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
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