Answer / ck misra

when the mechanical input to an alternator connected to
grid is changed it temporarily does increase the speed(RPM).
But an increase in speed means consequent increase in
frequency(as N=120f/P and P being the no. of poles is
constant)of the alternator.Now we know that the voltage and
frequency of grid busbar(what we call infinite busbar)are
constant(off course within some tolerance limit)and the
whole system has large electrical inertia which tends to
oppose any change in voltage level or frequency. So the
alternator is pulled back to synchronism by the grid.
BUT in case of stand alone alternator there is nothing to
pull the alternator back to synchronism and we can increase
the speed with a consequent rise in frequency....

Answer / madhu balaji

whenever there is change in mechanical input to the
alternator, there will be change in rpm. but when it is
synchronised with busbar, there will be change in real
power shared by that alternator(i.e. when there is increase
in input, then real power shared by that alternator also
increases and vice-versa)

[similarly when excitation level of particular alternator
(synchronous generator)is changed, then reactive power
shared by that machine will be changed.]

Answer / ashwin

it because the alternator it ynchronoed with the grid & it
output it to be kept constant as per grid ( Volt, frequency ),
frequency changes as there i an change in speed

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