(a)
Give a brief outline of the care and maintenance that
should be given to the stator and rotor of an AC generator. (6m)
(b) State what is likely to occur if the
driving power of an AC generator suddenly fails when two generators are
operating in parallel. (4m)
Answer :-
(a)
Before any generator work can be carried out:
1) prime
mover must be shut down and secured.
2) Generator
circuit breaker is locked off.
3) Generator
electric heaters isolated.
Maintenance that should be given to
the stator and rotor of an AC generator:
1) check
all wiring for damaged or fractured. Check for sign of water and oil
contamination of cable.
2) Check
cooling air intake and exhaust are free from dirt and dust.
3) Inspect
and clean generator rotor and stator windings.
4) Check
insulation and tightness of terminal insulation correction.
5) For
generator with slip rings, it must be checked for even wear and carbon brushes
for free movement. Correct brush pressure can be checked by pull type spring
balance.
6) Replace
brushes if faulty are bed in the slip ring curvature with glass paper. Clean
dusty area with vacuum cleaner after bedding.
7) For
brushless generator, check excitation transformer, AVR components and rotating
diodes for oil and dirt.
8) Measure
insulation of the stator and rotor windings to earth and between stator phases.
9) After
maintenance carry out, run on no load. If all OK, check on load foe excessive
temperature rise and load sharing stability when running in parallel.
(b)
When prime mover of a generator fails while it is
running parallel with another, the generator of the failed prim mover acts as
synchronous motor placing additional load on the other generator.
At the preset value, reverse power
protection detects this faults and trips failed generator circuit breakers.
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