Generator Islanding

It depends upon whether it is on manual or AVR control. The AVR will try and maintain the setpoint voltage by reducing the excitation current, in manual the voltage will rise slightly until saturation becomes an issue.

What makes you think the speed will increase uncontrollably? You do have some form of overspeed limit on your governor don't you?

Dear sir, Thanks for yr reply. Actually as per my understanding suddenly the load of generator decrease from its capacity at which it has been running in such case input mechanical power will not change immediately which result high rotor speed & high real power. But Based on yr explation I am still not understand why the terminal voltage will change because generator terminal voltage is function of field excitation.Please explain this ppont little more.

Have you observed this or are you asking a theoretical question? Your statement "...because generator terminal voltage is function of field excitation..." needs these words "...and internal voltage drop as a function of saturation..." You described a generator at full load, which means that it is somewhere on the the knee of the saturation curve.

When you suddenly unloaded it the stator current started to decay, which means the internal voltage drop decays as well. Since in manual the excitation current doesn't change neither does the internal voltage behind the stator impedance, so the terminal voltage rises a bit. Exactly how much and how fast depends upon where you are on the saturation curve and the machine's time constants.

You have also exhibited a common misconception about the prime mover's response to the sudden change in load; i.e., that the prime mover's speed suddenly increases uncontrollably. Ask yourself "where does the accelerating torque come from?"

It can only come from what little fuel is left in the fuel rails after the governor reduces the flow rate. Granted it happens at mechanical speeds instead of electrical speeds, so while the voltage rise takes 5-8 cycles to reach steady state, the frequency should stabilize in 50-200 cycles, not a long time in the life of a generator. If it hunts it's usually because the governor isn't tuned properly.

I want to learn the behavior of generator with changing loads when connected to grid or other generators as well as in the condition of isolated generator . Can u give me any paper/web address/ppt etc so that I can understand such things more precisely.My email address is electrical009@rediffmail.com. Thanks ®ards

If AVR is there, it will take care, to control the voltage.

If not the voltage will buildup, and if HIGH VOLTAGE PROTECTION is available, it will trip the EXCITATION, so that the Voltage will be theoritically ZERO, but in practise there will some voltage due to RESIDUAL MAGNETISM.

DHAYANANDHAN.S