Alternator and Induction Generator

Without the field excitation, no electricity is generated. The alternator would turn very easily, but generate no electricity.

if your two generator are connected in parallel, and if the synchronization control failed to offload him automatically it could become a potential load to other unit as it will run freely without voltage output. he will be looks like one of your induction motor feeds by your prime power

Hi Soumik,

The generator will not start to generate power in the way you think it will because the rotor does not have shorted turns. If however the diode bridge on the rotor shorted out which would reduce the AVR driven field to zero and put a short across all rotor winding then it would seem possible that it would continue to generate power. I think it would depend on the loads connected. All such generators I have seen have capacitors connected across the phases giving a leading power factor. This would be an unlikely circumstance in normal networks.

Regards

Chas

Thanks Chas.

But the problem is that if starts running as an induction generator, then what will be the effect. Means I am asking physically and electrically. Because what I feel is that as there is not controlling power like AVR, the output voltage will go on increasing. Actually there is the problem. Because just last month we had tripping due to low excitation which happened be due to a faulty thyristor bridge in the AVR. I would really like to get a suggestion about that.

Hi Soumik

Under those circumstances I cannot see that the unit will produce anything.

I suspect you are referring to a much bigger machine than I have experience of, but I don't see that would make any difference.

How big is it and what are the circumstances (loads, parrallel units etc.)?

Chas

Pl understand prime mover will continue to turn the rotor.No power will be generated .In Induction generator there is requirement of feeding power to stator apart from requirements of shorted rotor etc.So when the rotor is turned above syn.speed the machine starts generating .this is not the case with alternator.

It is a very interesting situation. The condition you are referring to is

1 The main circuit breaker is closed i.e., the unit is connected to the grid system

2 The field breaker is open i.e., there will be loss of excitation and the excitation system becomes redundant

In such a situation, because of the damper bars provided on the pole faces of the synchronous generator to avoid hunting, the damper bars act as a squirril cage rotor and if the unit speed is over that of the grid frequency, the unit will act as an induction generator. If the unit speed drops below that of the grid frequency, it will tend to act as an induction motor. But the unit cannot run indefinately as an induction generator/ motor since the current in the damper windings will be high and it can damage the damper windings and the pole faces. Hence, the unit should be isolated from the grid as soon as possible. However, there is no immediate danger to the unit

Thanks Samuel

You have got the situation very appropriately. Well that was the situation when the unit tripped on low excitation protection. I am taking not very big but a 120 mw alternator. However we found the machine to be hotter than normal. But we were not able to attribute it to the possible reason. Whatever, now if the spedd is not higher than the grid frequency then what will happen because of this induction motor action?

Dear Mr Soumik Nath,

Glad to know that my comment was helpful to you.

In a hydro generator of 120MW capacity, the moment of inertia (GD2 or the flywheel effect) will be high since the rotor will be of the salient type and the no. of poles will be more since the unit speed will be less. The torque while running the synchronous generator as an induction generator will not be that high to maintain running the unit continuously as an induction generator and the unit will tend to slow down. As the unit slows down, the current in the damper windings increase enormously and will damage the damper windings and the pole faces. In my experience, such situations occurred in a GE (USA) make generator and BHEL (India) generator of 103.5MW capacity. Nothing happened to the GE unit but in the BHEL unit, few of the damper bars had deformed

In case of a thermal unit, the generator rotor will be non salient and will nrmally have only two poles (3000rpm). Hence the damage will be much more (since damper windings will be provided only on the two poles) and the damper windings and pole faces will get damaged much quicker. In my experience, only once this situation occurred in a Toshiba (Japanese) unit and the firm strongly advised to change the complete rotor. But we only changes the pole and with minor maintenance work, the unit was put back in the system and till date the unit is operating satisfactorily

Thanks Samuel

The things you discussed were very useful. Since induction motor'sdirection of rotation is the same as that of Induction generator but lower, is there is any chance of torsion in the shaft. Means I want to know, other than electrical problem any mechanical problem could happen in the shaft of the rotor or not. And by the way we are running thermal units.

Dear Soumik,

In case of hydro generator, normally nothing happens, but in a thermal generator, I am not sure. But from my experience, nothing happened to the shaft and the unit continues to run even today. But from the safety angle, it is best to isolate the generator from the grid as soon as possible when such an event occurs

SAMUEL CHELLIAH

Then you ask my ex-wife to step away from the control panel and rejoin the tour group.