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Combined Cycle MW control
Is the MW output of the steam turbine from a combined cycle plant controlled? Or is it just a result of the load(s) of the gas turbines?

Having previously worked with coal fired power plants, am trying to learn something new. So, this question might seem a little obvious to most.

Was a little surprised to understand that after the combined cyclone plant gets to higher load points, the steam turbine operates in, basically, valves wide open mode. The steam turbine then produces as much power as results from the exhaust gas temperature and flow exiting the gas turbine and entering the HRSG.

While the exhaust gas temperature exiting the gas turbine is controlled, that control action does little to regulate the actual MW produced by the generator connected to the steam turbine. Instead it primarily ensures that the steam temperature leaving the HRSG will be satisfactory (e.g. 1005 F.)

So, my question is: does the operator of a combined cycle plant attempt to achieve a particular Net MW production (e.g. Gas Turbine(s) + Steam Turbine(s)), or does he/she more or less park the gas turbine at a specified load point and the Net MW is whatever it is?

Put another way, how does the operation of a combined cycle plant interact with the System Operator for the entire grid? Will the combined cycle operator receive a request for a particular MW generation and if so, how will it be achieved?

Thanks for your help.

In a combined cycle plant the load is controlled by the gas turbines. The steam turbine is operated on inlet pressure control mode with a pressure setpoint that will keep the valves essentially wide open. This provides the maximum thermal efficiency.

For a multi-shaft configuration (multiple gas turbines/HRSG's feeding a single steam turbine), if running at reduced load, once the desired load is below 50% of rated, is more efficient to run with 1 gas turbine shut down than to run with both GT's at low loads. You may not be permitted to run the GT's at very low loads if you are on gas fuel with a DLN combustion system.

Look at the "hockey stick" performance curves for the plant.

In my plant (GT CC), the block MW is controlled from DCS. All 3 HMIs are connected to the DCS and when the grid operator sends total block setpoint, the DCS first would push the GT load until the ST start to pick up the load. Once the ST starts to pick up load and the total block setpoint is reached, the GT load would be varied in order to achieve maximum power possible from the ST. All this are done by DCS by updating the speed reference to the GT HMIs. The ST on the other hand is kept on IPC (inlet pressure control).

Hope this helps ;)

New is almost always good.

Most of the time.

In general.


I think it depends on the design of the plant and its components and the power sale agreement (sometimes called the "purchased power" agreement) in place. Most steam turbines in newer combined cycle power plants I have visited or worked in are operated as "followers", meaning the steam turbine-generator power output follows the amount of steam being generated by the gas turbine exhaust, just as you described. After start-up/warm-up, the control valves are fully opened and the power output is a function of the steam flow which is a function of the gas turbine exhaust. This is to maximize the power output of the steam turbine under near rated load conditions. And most gas turbines need to be at or near rated load to produce minimal exhaust emissions.

If there are controlled extractions, that can change the way the turbine is operated, and sometimes precludes this following mode of operation.

I believe that plants with a single-shaft unit, with both steam and gas turbines driving a single generator all coupled together on a single shaft, the steam turbine control valves are usually operated as you describe, again unless there are controlled extractions.

At the combined cycle power plants I'm familiar with the exported power is controlled by modulating the gas turbine output using an export or tie-line watt-hour meter as the feedback, which takes into account the gas- and steam turbine outputs.

And, the agreement with the grid operator can also have a very large impact in how the plant is operated. Some plants must not generate anything less than a certain amount, and if they do they are penalized. Any power in excess of that amount is sometimes paid for, sometimes not, and sometimes it's paid for at a different rate. Sometimes, if they exceed a certain generation they are also penalized. Some contracts are very specific, and some are very generic, and some are open to interpretation.

So, every power plant is different and is designed and operated differently and can have widely varying power sale (purchased power) agreements. And people, who operate power plants, have different ideas about how their power plants should be operated to be the most efficient. And it's their understanding of efficiency and their power plant and the instrumentation available and the purchased power agreement that makes the plant truly "efficient".

And, again, because people are involved the operating decisions and policies and contract interpretations can be very, Very, VERY creative and "interesting."

What I understand from your question, there are two basic queries to be replied - the VWO operation of the Steam Turbine in Combined Cycle mode and operation of such steam turbines in a grid.

The difference of basic approach in Coal Based and Combined Cycle Power Plants is, the coal based power plant is operated as a base load power plant and the droop of such plant is maintained at 4% to 6% (and higher between 90% to 100% load) so as to ensure that in case of a frequency fluctuation they react minimum compared to 3% of a combined cycle plants, which generally act as peak load units and due to their low droop, are the first ones to react in a frequency fluctuation.

But the point is, once the inlet Governor valve is left under Turbine Control System, there are throttle losses, the magnitude of such losses depends on the type of governing system the turbine has - Nozzle governing or throttle governing.

That is why, today even the operators of large coal fired base load units of 800 MW to 1000 MW specifies control system with sliding pressure operation, which is nothing but VWO operation at near base load, where the output shall be governed by the Boiler Pressure. This way the throttle losses are avoided. This range is typically restricted between 90% to 100%.

In Combined Cycle plants, the steam turbine control has three modes of control - Inlet Pressure Control, Frequency control and Output Control. The output control is typically used for such steam turbines when operating in a grid as a base load power plant; grid administrators may or may not agree to allow such control by the operator.

Besides these three controls another facility is provided in such steam turbine control systems, which is the sliding pressure Operation. In a Combined Cycle Power Plant, there is no point in restricting such sliding pressure operation or VWO operation to near rated load as, unlike coal fired boiler, the Gas Turbine can ramp up very quickly from a part load to rated load. Also Further, for a islanded plant, it is easier to control the total output by the Gas Turbine Control system as explained by "otised".

Hope I could clarify.


can you please clarify the difference between inlet pressure and sliding pressure control mode?

I'm not so sure about it.

If I understand correctly, inlet pressure control mode means that pressure is maintained at desired setpoint by the turbine control system. In that case,control valves are opening and closing to maintain the constant pressure.

On the other hand, there is a sliding pressure mode, where turbine control valves are open 100% and boiler is responsible to maintain the pressure in front of the turbine.

Am I right here? If not, please clarify.