Frequency Insensitive System Impact on Generator

Many thanks for your detailed reply.

I just want to further clarify the point you mentioned , If we operate our generator (connected to National Grid) in Isochronous (fixed frequency) mode & grid frequency starts drifting down then does this drifting down in frequency will enforce our prime mover speed to slow down in Isochronous mode?

If it slows down our prime mover speed then would it allow to the governors to inject extra energy in Isochronous mode to compensate this drifting down in speed which could lead to potential threat of Tripping?

Looking forward for your expert opinion.

Thanks

Isochronous mode is normally used when the generator is the only source in the network, also known as black-start or isolated mode. In isochronous mode the governor will only compare the speed of the turbine-generator against an internal reference frequency and adjust the steam valves to maintain that speed. As the load increases incrementally the governor sees the speed drop and admits more steam to maintain the speed, it knows nothing about load and will keep opening the valves until the stops are reached, so it is up to the operator to carefully manage the additional loading.

So if you insist on improperly operating in isochronous mode while attached to the grid your governor will try its best to counteract the speed drop without any regard for the load that is being imposed on it. The threat (guarantee) of tripping is the fact that unless your generators represent a significant portion of the grid loading (80% or more) then there will quickly be a speed difference between your generators and the rest of the grid, and it is this speed difference which will cause the generators to lose synchronism with the grid.

It is no different than when you are driving your manual transmission car 50km/h on a level surface and you encounter a hill. You (the governor) want to maintain the 50 km/h speed up the hill, so you increase the fuel supply but the engine speed never changes. If the hill is too steep or the load in the car is too great then the car slows down once the pedal reaches the floor, you have exceeded the rating of the motor. If your motor can provide the power but the clutch slips then it is equivalent to slipping poles; i.e., there is not enough magnetic field to couple the turbine power from the rotor to the generator stator. Be careful as you reach the top of the hill because if you keep the pedal buried you will surely overspeed the engine as you reach the crest and gravity aids you instead of impeding you.

Hope this helps, and thanks for the GA.