Determine the correct setting for Under Frequency Relay (UFR) for Generator

[powering2002]

Dear all,
We operate four generators in our steam power plant. 2 X 100 MW and 2 X 200 MW. They are Toshiba Generator. Built in 1980 and 1987. Our Grid Frequency is 50 Hz. According to Our National Grid Code, our generator must continuous supply the grid in the frequency range of 47,5 - 52 Hz. But, based on our equipment operation manual (published by Toshiba), it was stated that the permissible continuous operating range is 48,5 - 50, 5 Hz. This is done to reduce the possibility of damage due to High Cycle Fatigue. My question is,
1. Could you recommend us the correct setting for Our Under Frequency Relay (UFR) based on the condition mentioned above?
2. Could you give us explanation about the natural frequency of the steam turbine?
..Thanks a lot.

 

[davidbeach]

Does your Grid Code allow exceptions for machine limitations?

 

[dpc]

I suspect the Toshiba requirements are for continuous operation. A momentary excursion below or above this should not be an issue. But I'd set the UF to pickup below the 48.5 Hz with a inverse time function if possible. Or two levels of UF with different time delays. Basically, I'd set the relays to protect the machine and let someone higher up argue with the Grid Authority - unless they plan to buy you a new turbine-generator when this one fails.

Natural turbine frequencies are resonant frequencies of the mechanical system where vibration issues can get amplified. Basically, you should not operate at a critical frequency - you need to accelerate through the critical frequency and don't linger.

dpc