Please give me calculation process of FSNL of Mark-4 GT.

I'm working in a GT power Plant whose capacity is 34.5 MW. I several time found that when we disconnect from Grid, GT frequency instantaneously increase until change of DROP control Mode, even the ma

 

You use the term "disconnect from the grid." If you mean that the 52G generator breaker is still closed and the disconnect is from a line breaker, the Mark 4 may not treat this as a FSNL condition. When 52G trips, the normal setting for speed control is 100.3%. If 52G remains closed, unless there has been an islanding mode design, the speed/load control will likely cause the speed to increase to near the load setpoint. (If you were at base load, this would typically be 104%.) Instantaneouly, the speed may go even higher (up to 107%) due to the sudden loss of load. Also, I am assuming the unit had been in droop control mode, not isochronous.

The cure for the problem is to add logic for islanding operation. It is not a difficult change, but you need to check with GE on this, since type of combustion system and other factors influence the design. Since you have Mark 4 control, I doubt that you have DLN combustion system, which usually makes islanding mode simpler. (DLN is unstable in isochronous operation.)

 

Depending on the vintage of the Mark IV used on the turbine, it's not very scientific.

How long has this problem been occurring? What changed just before the problem started occurring?

What have you done to try to troubleshoot the problem?

It's not clear what you were trying to say about the condition which occurs after some breaker opens.

You <b>seem</b> to be implying that when the unit is separated from the grid that the turbine-generator is still operating to supply some "local" load. This condition is sometimes called "islanding", operating to supply a "local" load independent of a larger grid, sometimes with other generators, sometimes independently.

So, there seems to be a utility "tie" breaker which connects the turbine-generator and the "local" load together. When the turbine-generator and the local load are connected to the grid through the tie breaker, the unit should be being operated in Droop Speed Control mode.

When the tie breaker opens but the generator breaker is still closed the generator is supplying the local load with the generator, and in that case the unit should be operating in Isochronous Speed Control mode. Generally, there is a signal which comes from the utility tie breaker to indicate that it has opened (and probably that the generator breaker is still closed) to automatically switch the Speedtronic to Isochronous Speed Control Mode from Droop Speed Control Mode.

If the unit does not automatically switch from Droop Speed Control mode to Isochronous Speed Control mode when the utility tie breaker opens then it is incumbent on the operator to manually switch from Droop to Isoch. If the operator doesn't do this very quickly, the frequency can increase significantly, which seems to be what you were trying to tell us.

In the above situation, the calculation of FSNL is not applicable. If the unit is still supplying a load when separated from the grid, then FSNL (Full Speed-No Load) is not applicable. FSNL is only applicable when there is no load, when the generator breaker is open.

If you are saying that when the generator breaker opens, and the unit is not supplying any load, that the speed of the turbine (and therefore the frequency of the generator terminal voltage) increases, then there is likely something else wrong. Something is causing the fuel flow-rate to be excessive for FSNL operation. Or, an LVDT has not been properly calibrated. Or, a gas fuel valve is not closing fully. Or the Liq. Fuel Bypass Control Valve is not opening fully. Or a Control Constant has been changed improperly. Or some combination of the above.

But, you haven't told us when this problem started. How long has this problem been occurring? Did it start after some maintenance outage? After some Control Constants were changed? After some LVDT calibrations?

What fuel is the unit being operated on? What is the fuel supply pressure when the frequency increases? If the unit is running on liquid fuel, is the liquid fuel supply pressure (to the inlet of the High Pressure Liquid Fuel Pump) greater than approximately 8 barg (approximately 120 psig)? If the unit is running on gas fuel, is the gas fuel supply pressure greater than approximately 25 barg (approx. 360 psig)?

We just don't have enough information, and your post was truncated. If you want to continue this thread, you will need to tell us more about the problem and the configuration of the site when the problem occurs (island mode operation or FSNL operation; if there are other generators connected together in island mode, and which other generator is being operated in Isoch mode).