Primer Frequency Control

More information is required:

How big is your generator?

How big is your system?

How long is your transmission line to the rest of the system?

Who regulates your system frequency?

There are more detailed questions, any additional information you have will help our understanding of your situation.

175mva unit 1

175mva unit 2

our system frequency is interested in rpm? ı dont understand your question.

Are these two units attached to the same bus?

Is there any other generation in your system?

I made no statement about speed (rpm), just wanted to know who sets the frequency, but please remember that speed and frequency are directly related to each other.

My other previous questions are trying to determine how or what is your master frequency set point controller since it is possible to have slightly different frequencies when there is a "long" distance between generators (this does not apply to isolated systems where there is no interconnection to another utility).

1. First reason for not getting any more power if frequency drops to 49.5 Hz is that unit is already giving all the power it can. A reserve of 4 MW is not much for a 150 MW unit. 4 MW could easily be lost by an increase of ambient or water temperature above the rated values of the set. Or maybe the commercial guys have "forgotten" they ought to have a reserve and operate at maximum anyhow (even though this may have serious implications for maintenance or plant life).
2. The second is that you have a controller loop working on plant MW [active power] which has no input from frequency. Initially, governor droop may increase power when frequency falls to 49.5 Hz - but then MW control loop pulses the governor setting down until unit is back at the normal power.
3. You should find out if your power controller has a continuous frequency or speed input or a switched input - e.g. from under-frequency relay.