davidbeach
Electrical
- Mar 13, 2003
- 9,509
I was looking at an event report triggered when a generator was tripped off-line this afternoon and would like some conformation - or alternate explanations - for what I think happened.
Generator is question is a 2MW diesel engine driven gen, 480V, new generator installed on the engine following a messy failure of the previous generator. Generator was running in parallel with the utility at the time. The relay that tripped the generator was between the generator and any other connections, load or utility. The voltage at the bus was rock steady and the angular separation of the three phases was 120[°][±]0.5[°]. Therefore the currents should have been very close. But, it wasn't even close - 440A, 450A, and 505A (in round numbers, there was some fluctuation).
As I understand the situation, when the generator is running in PF/VAr control mode, power into the shaft, from the prime mover, determines power out and the rotor settles at the necessary angle relative to the utility to meet the requirements of the power transfer equation. The equation can be evaluated on a three-phase basis or it can be evaluated phase-by-phase. With a nearly perfectly balanced system voltage and a common internal voltage, assuming identical impedances, each phase should solve to the same value, should it not? The excitation system will control the amount of reactive power, but again, assuming identical impedances, there is nothing that would act differently on different phases, is there?
My contention is that there is a serious problem somewhere between the CTs of the relay and the neutral of the generator, something that is presenting quite different impedances to the different phases. Is there anything beyond the relay (remember nearly perfect voltages) that could cause a current unbalance when running in parallel with the utility in PF or VAr control mode? Any control system (AVR/Excitation/etc.) failures that could do this?
Thanks.
Generator is question is a 2MW diesel engine driven gen, 480V, new generator installed on the engine following a messy failure of the previous generator. Generator was running in parallel with the utility at the time. The relay that tripped the generator was between the generator and any other connections, load or utility. The voltage at the bus was rock steady and the angular separation of the three phases was 120[°][±]0.5[°]. Therefore the currents should have been very close. But, it wasn't even close - 440A, 450A, and 505A (in round numbers, there was some fluctuation).
As I understand the situation, when the generator is running in PF/VAr control mode, power into the shaft, from the prime mover, determines power out and the rotor settles at the necessary angle relative to the utility to meet the requirements of the power transfer equation. The equation can be evaluated on a three-phase basis or it can be evaluated phase-by-phase. With a nearly perfectly balanced system voltage and a common internal voltage, assuming identical impedances, each phase should solve to the same value, should it not? The excitation system will control the amount of reactive power, but again, assuming identical impedances, there is nothing that would act differently on different phases, is there?
My contention is that there is a serious problem somewhere between the CTs of the relay and the neutral of the generator, something that is presenting quite different impedances to the different phases. Is there anything beyond the relay (remember nearly perfect voltages) that could cause a current unbalance when running in parallel with the utility in PF or VAr control mode? Any control system (AVR/Excitation/etc.) failures that could do this?
Thanks.