Control

We have a combine cycle power plant with two 25 MW and one 5 MW gas turbine and one 15 MW steam turbine. Our plant peak demand is 52 MW and we have 18 MW surplus available which is exported to grid. The problem is associated when in operation the network gets separated with the grid. As soon the separation occurs the frequencies get disturbed and its projected that the steam turbine will trip on overspeed. What are the control measures required to prevent the steam turbine from tripping.

A couple of questions first:

1. You did not indicate how your steam turbine power is controlled. Is your steam turbine running with constant steam supply pressure (i.e. governor valves are throttled when steam turbine is running lower than rated power output) or is the steam turbine running in sliding pressure mode (i.e. governor valves are always fully open, and the steam pressure is 'controlled' by the exhaust gases of the gas turbines?

2. How come your steam turbine trips out on overspeed, yet your gas turbines do not, when you get separated from the grid? Does the steam turbine have a lower overspeed protection setting? I assume that the steam turbine generator and the gas turbine generators are all connected to the same busbar, and so when you get separated from the grid, all generators on your combined cycle plant will overspeed at the same rate, given they are running in parallel.

3. How is the whole combined cycle controlled vis-a-vis frequency/load control. Are the gas turbines running in droop mode, or are they running at base load control irrespective of frequency?

4. Was your combined cycle plant designed to run in stand alone mode, or does it need to be connected to the grid to maintain good frequency/speed control?

5. When you get separated from the grid, do the gas turbines lower their power output automatically (either through their control systems, or throught the combined cycle control system), or do they remain at the same power output as before the separation from the grid?

6. What happens after the steam turbine trips out on overspeed?

One must note that:

1. when you get separated from the grid you will always and definitely get a frequency disturbance, unless the power you are generating from your combined cycle plant matches that of your connected load, which does not seem to be your case, given you are exporting 18MW.

2. Unless you act directly on the steam turbine governor valves, the steam turbine is slow to react to sudden frequency changes, given the thermal inertia of the boilers (especially if you are operating in sliding pressure mode), so your only way of returning 'fast' to your nominal frequency after the distrubance is to act directly on your gas turbine control systems, keeping in mind that the steam turbine will follow but with a time delay.

Hope this gives you an insight of your problem.

Responding to Khurram’s 24-Jan-08 (00:14) query... while control measures to overcome STG over-speeding is somewhat problematic, it should be relatively easy to determine if STG acceleration following a load-loss event will lead to a trip:

Based on the conditions you cite, ROT (Rule-of-Thumb) calculations indicate the STG will not trip! Note, I say "easy" with tongue-in-cheek because the outcome is dependent on what actual configuration, electrical parameters, and mechanical characteristics are, vs generic info I have!

If interested, please contact me!

Regards, Phil Corso ([email protected])

You are like to have a bypass stack in your system. We don’t have one; newer CCGT usually doesn't have any bypass stack.

My first questions:
1) Is/was this problem always existing?

2) Did you have any control system upgrade?

In general you should have a plant supervisory system, which should take care for load shedding and load sharing of your plant. If you don't have these systems, problems are inevitable!
You may try to tune the droop setting of the STG_GEN and GT-GEN. In general the droop setting is 4%. You may try to decrease the droop setting of the GT-GEN and increase it for the STG-GEN (f.i 6%).

Consider also changing the operating philosophy, such as selecting the STG-GEN in ISLAND mode.

Good Luck.