Steam Turbine Operated in Pressure/Power Control Mode

[LittleMan88]

Hopefully the following questions can be answered by those who have knowledge/experience in steam turbine operation, especially when considering combined cycle power plants.

What would be the scenario(s) when a steam turbine is manipulated to operate in a pressure control mode, and in a power control mode?
What are their advantages and disadvantages? Are they contradictory to each other?

Can the steam turbine switch mode from one to another frequently? Under what circumstances the change of mode would take place?

If the unit/plant is connected to the national grid that has power sale agreement with the grid system operator, how would the different modes affect both the grid and plant against compliances (e.g., dropping <= 4% ~ 5%, maintaining frequency 60Hz)? How would the term PFR (Primary Frequency Response) fit in the subject? How would the situation change when a gas turbine comes in to play?

Are there any books or materials you would suggest reading about this topic?

Your valuable explanations and feedbacks are very much appreciated!

 

[crshears]

Hello 88,

I can't speak to all of your questions, but I can speak to at least some...

Why stick purely to one or the other?

Sure, pressure following mode ensures the steam turbine only draws as much steam as is available, no more, no less; power control mode can be set to follow any parameter you choose, but unless there are multiple loads / sinks for the steam, the plant imbalance will play havoc with steam pressure and cause major boiler / HRSG behaviour issues.

I'm no controls engineer, and the more learned colleagues here may have better insights, but based on operating experience I offer the following thoughts...

Might it be possible to select two or more proportional control modes, depending on present operating state? During plant start-up, steam turbines are typically bypassed until satisfactory vacuum can be drawn and proper condenser operation can be established, so manual adjustment of parameters is more or less a given, with steam fed to the turbine at the flows, temperatures and pressures needed for run-up, synchronization, and turbine warm-up [taking differential expansion into account].

In my humble view, once the first gas turbine, its HRSG and the steam turbine are in stable operation, pressure control of the steam turbine should predominate, with frequency control active but serving a secondary role. This is so because with the gas turbine set at a speed droop of, say, 4%, during system disturbances the gas turbine output may deviate significantly from schedule, with HRSG steam production following suit...during such situations, the frequency following portion of the steam turbine control will make a prompt adjustment to the steam flow, and the pressure following portion will act to do the necessary pressure trimming.

In a two-over-one or three-over-one combined cycle plant, as the second and/or third gas turbines are placed in operation, the pressure/frequency weighting ratio may require adjustment so that the following behaviours are well-balanced to the plant characteristics prevailing at the time.

The AHJ [authority having jurisdiction] over the grid may well have a standard or regulation stipulating the minimum acceptable speed droop of the steam turbine; you'd want to talk to them.

Hope this helps.

BTW this might better have been posted in the Power Generating Facility Engineering forum...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Then there are the non condensing turbines that are used with recovery boilers at pulp mills.
The generator driven by the turbine supplied part of the plant load.
It's been a while and I don't remember the exact pressures but as I remember, the steam output of the recovery boiler was close to 1000 PSIG. All of the steam output was fed into a turbine. A valve tapped off some steam from the turbine at around 125 PSIG for use in the plant. The control of this valve was downstream pressure of the 125 PSIG system. The remainder of the steam fed the second section of the turbine and exited to the low pressure heating system at around 15 PSIG.
In the event that there was not enough 125 PSIG steam exiting the turbine to supply the 125 PSIG steam demands some of the input to the turbine was diverted to a desuperheater which made up the balance of the 125 PSIG steam demand.
There was a second desuperheater to make up the demand of the 15 PSIG system.
The primary purpose of the recovery boiler was to recover both chemicals and heat energy from the black liquor. The steam produced depended on the quantity and quality of the black liquor fed into the fire box.
The secondary purpose of the recovery boiler was to supply 125 PSIG steam and 15 PSIG steam for the plant.
Power generation was a by product.
Power generation was a means to extract the energy in excess of that required for the plant steam systems.
Basically the difference between the boiler output and the plant steam demand was used to generate electrical energy to supply part of the plants electrical energy consumption.
This is another type of control where the steam output is what it is and the electrical output is what it is. The control is the pressure of the 125 PSIG system and the 15 PSIG system.


Bill
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"Why not the best?"
Jimmy Carter