generator motoring
generator motoring
(OP)
A stream turbine/generator (70mw) trips and the main steam valve closes, the generator breaker is still closed and is tied to the grid. 15 mins later the operator notices the turbine/gen is still turning at 3600rpm.( with no power output). The 32 relay was checked under trip conditions 3 months previously and worked flawlessly as it has in the past.The relay was not picked up during this incident. Why was the turbine/gen still running at speed after 15mins? When the gen. breaker was opened manually the speed dropped off to zero.Was this equipment motoring?






RE: generator motoring
How did you test the relay? If bench-tested, that still leaves open the possibility that the CTs or PTs are mis-wired.
RE: generator motoring
The reverse power relay acts as a wattmeter measuring positive power under generating conditions. If the unit senses a negative power signal i.e. machine motoring then a trip signal is initiated. However, the relay uses the instrument current and voltage transformers common to all other protection relays.
The actual power required to motor the generator and turbine is a small fraction of the rated output power of the generator (approximately 3%). In order to detect this small level of power the voltage and current transformers must be very accurate.
Under normal conditions when the generator is de-loaded to be removed from service the current reduces but the voltage level is maintained. The relay is optimised to operate over a small range of current and power. Therefore as the machine is de-loaded the current and power reduce together and the errors introduced by the instrumentation are small. Thus the relay operates reliably.
However, certain conditions with high reactive loads can produce phase errors in the instrumentation. Under these conditions the relay appears to be measuring positive power even though the unit is in a motoring condition. Therefore the relay will not operate.
Do you know what the reactive load on your generator was at the time of trip?
RE: generator motoring
RE: generator motoring
When the unit is tripped or removed from service the governor valves are closed thus removing input energy to the turbine. However, entrained steam still remains within the pipework and turbine casing. Therefore, the main generator breaker is left closed to allow this stored energy to be dissipated into the electrical network under a controlled condition. If this were not the case then there would be danger of over-speeding the machine.
RE: generator motoring
RE: generator motoring
RE: generator motoring
The simplest / easiest way we found was to automatically drive the AVR pot to zero on 'turbine trip+ESV closed' condition instaed of changing the Rev Pow relay. This was because during one such incident an operator reduced the excitation and the relay did pick-up and it was later on conclusively prooved that the heavy VAR loading ('heavy/large'compared to watt component at that time)does affect the CT/Relay combined performance.
Good luck and best regards