Gas Turbine Generator suddenly drops load and switches mode 2

[hayaku]

Hello there,

We have a 10 MW GTG operating in parallel with several more identical units. At the time of a thermocouple alarm (which was just acknowledged and reset) the unit dropped a few MW of load and switched operating modes from ISOCH to DROOP. Any ideas if these two things are indeed related?
The TC's measure the turbine inlet temperature which i believe, affects the maximum power output of the turbine.
Anyone dealt with generators behaving in this way and what were the likley causes of sudden load drops and/or switching modes?

Thank ye kindly

 

[Alex68]

The answer is YES!
Even if the generators are connected to the grid, a transient stability study could be important, above all in cases like this in which you have small inertia.
Please remember to bring your generators close to the stability limit (rotor angle close to 90°) before starting a disturbance: high voltage at the grid node, low voltage at the generators nodes.

 

[mc5w]

When using a wattmeter on the utility tie to control the speed setpoints of the generators, the change in plant load comes from the utility instaneously and then the wattmeter adjusts the speed setpoint hence power output of the local turbines. A small change of utility frequency comes from the droop characteristic of the local turbine governors.

In other words, the wattmeter puts the change in plant load onto the local turbines after about 0.1 to 10 seconds depending on how you have the feedback loop set up.

 

[EddyWirbelstrom]

Thanks mc5w.
Is my following understanding of your posting correct :-
When the local generators are in 'Load Control Mode', a step increase in plant MW load is instantaneously supplied by the Utility interconnector because the 'No-Load Frequency' setting of the local generators remains unchanged if there is no signal from the Interconnector wattmeter.
If it is desired to supply the load increase from the local generators, then a wattmeter signal from the Interconnector can be used to adjust the 'No-Load Frequency' setting of the local generators - gradually ( 0.1 to 10 sec ) to avoid stabilty problems.
I understand there would be a small momentary decrease in local generator frequency as the generator load is increased.

Question 1.
Why would there be a 'small change in Utility frequency' ?