Gas Turbine 9FA Machine Response During Plant Blackout Condition

Dear all,

We are having 351 MW CCPP having plant configuration like GT 9FA (225 MW) + HRSG + Steam Turbine (130 MW). Actually, we recently faced plant black out condition.

During plant blackout, GT Speed was increased up to 3252 RPM (54.2 Hz) & again it was decreased the speed up to 2820 RPM (47 Hz) during same incident GT Generator circuit breaker is closed condition.

However, GT was tripped on "Loss compressor discharge bias" when GT speed reached < 2820 RPM (47 Hz) because, as per logic GT Compressor bleed valve should be open when GT Speed is less than 2820 RPM(47 Hz).

The above said GT Speed increasing & decreasing was happened during between 70 secs & GT Load was between 7-10 MW.

We also checked the Electrical logic - Generator GCB i.e. > 52.5 Hz & <47.5 Hz trip logic is not active.

I want technical information regarding,

1. "Gas Turbine 9 FA machine response during plant Blackout condition".
2. How Droop & Isochronous condition will work?
3. During blackout when Gen GCB is Closed & GT frequency > 52.5 Hz or Gt Frequency <47.5 Hz - Can we Open HVCB to GT Houseload /Island condition. what will happen if we modify the said logic.
4. Please Suggest Electrical or Control side logic modification required to survive GT Houseload /Island condition.

Thank You very much in advance.

Hello,

In my thinking and understanding, a plant black-out means there is NO electricity in the plant, that the grid supply is unavailable (the "HVCB" (High-Voltage Circuit Breaker??) open) is open and except for emergency lighting and emergency power (for Emer. L.O. Pumps and DCS, etc.) there is no power. So, I'm unclear how a plant can be blacked-out and the auxiliaries still running with the HVCB still closed.

Are you saying the grid supply was unavailable even though the "HVCB" (what is usually called a "tie-line breaker" because it connects (ties) the plant to the grid) was still closed and there was no power available from the grid?

>1. "Gas Turbine 9 FA machine response during plant Blackout
>condition".

That would depend on how the plant was intended to be operated when there was no power available from the grid--how the high- and medium voltage circuits were designed and how the turbine control system was configured.

>2. How Droop & Isochronous condition will work?

I'm not going to go into Droop or Isochronous Speed Control in any depth (they've been covered many, Many, MANY times before on control.com), but GENERALLY if a plant was intended/designed to be able to supply the plant loads (lights, motors, etc.) when there is no power available from the grid there is some signal that is passed to the turbine control system which tells the turbine control to switch from Droop- to Isochronous Speed Control. The turbine(s) are usually in Droop Speed Control when synchronized to the grid, because that's the governor mode that allows multiple generators and their prime movers to all be synchronized together and powering loads on a grid. When separated from the grid (such as when the tie-line breaker (the HVCB??) is open, the gas turbine will switch to Isochronous Speed Control Mode, which is the governor mode that attempts to maintain a constant frequency (speed) by adjusting the fuel flow-rate as load changes to hold a constant speed (frequency).

In a condition such as yours where the ST is also producing power (you didn't say if it was or wasn't), since it's smaller than the GT the GT turbine control system will do its best to control frequency (speed) if the GT is in Isochronous Speed Control and the ST is NOT in Isochronous Speed Control. (Only ONE turbine on a grid or a islanded group of turbines can technically be in Isochronous Speed Control; if multiple turbines (prime movers) are in Isochronous Speed Control they will fight each other for control of speed/frequency and usually that ends in wildly fluctuating speeds/frequencies (usually at very fast rates of change) and eventually in a blackout such as I described.)

Also in a condition such as yours, the ST load has to be reduced very quickly, and that's sometimes very difficult to do unless the ST is tripped or the steam can be bypassed to the condenser. The ST in a CCPP is a "follower"--since the steam production is a function of the GT exhaust which is a function of the GT load, the ST can only produce the power which is available to it and since it's smaller than the GT it can't really control speed/frequency.

>3. During blackout when Gen GCB is Closed & GT frequency >
>52.5 Hz or Gt Frequency <47.5 Hz - Can we Open HVCB to GT
>Houseload /Island condition. what will happen if we modify
>the said logic.

Again, your "blackout" condition is not clear, as usually, when there is a blackout the tie-line breaker (the HVCB??) is usually open or opened because you <b>DO NOT</b> want the grid power to suddenly return while the GT and/or ST CBs are closed and producing power AND the tie-line breaker (the HVCB??) is closed--because that could result in very big sparks and possible (likely) mechanical damage to the generators, couplings and prime movers, not to mention the switchyard and "HVCB."

>4. Please Suggest Electrical or Control side logic
>modification required to survive GT Houseload /Island
>condition.

Totally impossible without being able to see the plant one-line diagrams and understanding how the plant was designed and constructed.

When there is a "blackout" (which I'm going to define for this paragraph as a loss of AC (Alternating Current) from the grid), if it is desired for the plant to continue to run it should be fully isolated from the grid (the tie-line breaker (the HVCB??) should be opened to prevent possible problems when grid power is restored). The GT should automatically switch to Isochronous Speed Control Mode, and the ST should either be tripped off-line or very quickly unloaded. 7-10 MW of load is very small in comparison to the rating of a GE-design Frame 9FA heavy duty gas turbine, and it may be somewhat difficult for the unit to maintain a stable 50.0 Hz, but it should be possible, as long as the ST isn't contributing more than 7-10 MW (really it should only be contributing the smallest amount possible, or tripped off-line).

You have to consider the ST in this condition, and what to do with the steam being produced when the plant is islanded from the grid (when there is no power from the grid). That's important, especially when the house load is so small.

Hope this helps!