Time Delays on Speed Trip

[ReliabilityPF]

At our plant we have several generators running in parallel at 50HZ, there is a speed trip for the prime movers at 47Hz, is it recommended to put a time delay for this trip so the system can recover from an electrical upset. We have a load shedding system witch the last step is 47.5hz.

 

[ScottyUK]

What is the prime mover?

Some machines have a dual function underfrequency relay: Stage one is a time-delayed trip at (say) 47.5Hz to allow the load shedder to dump part of the load and the governor to open up. Stage 2 is an instantaneous trip set at (say) 47.0 Hz. If the governor is unable to regain control after the load shed then either the first stage trip times out or the second stage instantaneous trip operates as speed continues to fall.

Steam turbines often have a restriction on operation under load at abnormally low speeds which is measured in hours during the life of the machine. Gas turbines stall if load is sustained as frequency drops because the compressor efficiency falls away so quickly below nominal speed. At some point you must accept an instantaneous trip to protect the prime mover; the questions then are 'how low do you set it?' and 'what action can I take before speed drops that low?'. You should consult the OEM of the prime mover to determine where the limits are.


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