Answer / raghavendran

Consider a Generator operating in three different conditions

when operating at rated load,the AVR applies rated DC
excitation to the field.But consider a condition when there
is a sudden load throw off, the terminal voltage at the
stator will be increased.To compensate for the sudden
increase in voltage level, the DC Exc is reduced
drastically.This is called as 'Negative Ceiling Voltage'.

But when there is a sudden increase in load the terminal
stator voltage drops due to external reaction effect.Hence
the drop in terminal voltage is compensated by sudden
increase in DC Exc.This is called as 'Positive Ceiling
Voltage'.For this the response time of the AVR has to be quick.
The minimum and maximum ceiling voltages may range upto 1.5
times the DC Exc voltage at No-Load and Full Load respectively.

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