GE Gas Turbine behaviour during House load and Islanding Conditions

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Thread Starter

Hari

This is with reference to a Combined cycle power plant with two GE Gas Turbines, one steam turbine and the customer is the grid. There were two isolated incidents and the speed variation behaviour in the both cases were different

Issue-1- House load Condition
Both the outgoing lines breakers opened and Gas Turbines continued to operate with breaker in closed condition sharing between them a house load of 6 MW. Frequency mantained at 50 Hz. Selection remained on droop and preselect load (With about 65MW load set point).
On attempting for further loading ( manual speed/ load raise command ),GTGs generator frequency did not cross 50.8Hz.( This operation was attempted thinking machine have off loaded unaware about Line breakers tripping).
On attempting to lower the speed (manual speed/ load raise command), GTGs speed could not be lowered than 3000rpm. (This was done to synchronise the Generator directly with the line.)

Issue -2 Islanding Condition
Some other day the plant came into islanding mode. It was running at a load of 214MW and
suddenly load dropped to 168MW because the extended substation to which lines are supplying got isolated from Grid.
Here again assuming a load throw off, tried to increase load.Already frequency has increased due to load throw off. The action has increased the frequency to trip level of 52.0 Hz. This has
resulted in tripping of the units.

My point is how the machines frequency behaved differently in both the cases and I am looking for possible answers for the below questions- 1)Why the first case the frequency did not raise beyond 50.8Hz where as in
second case frequency rose beyond trip level?
2)Why in the first case frequency did not go below 50 Hz?
3)Is the Mark-V control system which was causing this kind of behaviour?

There is no input of Line breaker status to MarK V system.

Hari
 
It's virtually--no pun intended--impossible to say why your units are behaving as you described WITHOUT being able to examine their configuration and the CSPs (Control Sequence Programs) of the Mk Vs. It would also be important to examine the "one-line" diagram for the electrical power distribution

You have made no mention of what the steam turbine-generator did during these excursions; it is presumed it is connected to the same grid and house load as the GTGs (gas turbine-generators)....

Further, are there some signals which are passed between a remote control system or a local plant control system (such as a DCS) that can change the operating mode from Droop Governor Mode to Isochronous Governor Mode?

In the first case, something HAD to tell the Mk Vs to lower TNR (Turbine Speed Reference) when the tie-line breakers opened--otherwise the frequency would have been MUCH greater than rated just like in the second situation you cited. You didn't say what the load on the turbines (including the steam turbine) was before the tie-line breakers opened.

Do your units have Isochronous Governor mode? If so, there may be a limit on the upper- and lower turbine speed setpoints.

The second case still says that something told the units to lower TNR in the first case because they DIDN'T drop their TNR when the extended substation dropped off, which is what caused frequency to raise in the second case. The breakers which isolated the plant from the grid in the first case were seemingly closed in the second case.

The Mk V will only do what it's programmed to do, and these behaviors apparently have been programmed. Have you examined the design documents used to build the plant? Many times the designers write detailed descriptions of operation which are used to program and configure the various control systems in the plant. Usually, these descriptions are provided in the plant operation manual(s). Have you consulted them?

markvguy
 
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