Dear All,

Im really feel very much thankful to all of you guys for contributing your valuable knowledge to support and learn the newbees like me.

I'm new in power generation system & have worked with gas turbines. I want to know what are the difference between Gas turbine and Steam turbine control philosophy, AVR control and generator transmission. is there Isoch/droop control in STG also? please elaborate.

 

Saif,

Welcome.

When considering AC power generation, there is the generator and its prime mover. The prime mover supplies torque to the generator, which converts the torque to amperes (which are transmitted via wires to motors and other devices which convert the amperes into torque or other useful work). So, regardless of the type of prime mover all of the same principles still apply and must be satisfied: speed (frequency) control, synchronization, AVR control, Droop/Isochronous, etc.

Many steam turbines in combined-cycle power plants are simply "follower" units, meaning their control valves are driven full open after synchronization and they turbine converts whatever steam flows into the turbine as gas turbine load to torque applied to the generator. These steam turbines aren't really capable of Isochronous speed control, or even of droop speed control (the gas turbines are much larger than the steam turbines).

So, depending on the application, from an AC power generation perspective the control philosophy is basically the same--since there's virtually no difference between generators driven by steam turbines or gas turbines (or hydro turbines or reciprocating engines--all generator prime movers are simply torque producers).

Hope this helps!

 

> Many steam turbines in combined-cycle power plants are simply "follower" units,

I would like to thank CSA for providing a simple explanation to help me understand.

When u said steam turbines in combined-cycle power plants are simply follower units, then how the load sharing occurs between the two generators.

 

In a combined cycle power plant the "load sharing" between the gas turbine generator and the steam turbine generator is not controlled - it exists as a result of the thermodynamics. The gas turbine will provide roughly 2/3 of the power and the steam turbine 1/3 of the power. The overall load of the combined cycle unit is controlled by the gas turbine load control - EXCEPT when there is supplemental firing of the HRSG. There may well be a plant load control in the plant DCS to adjust the gas turbine load based on system load demand, especially when there are multiple combined cycle units at the plant. Also note that there are combined cycle configurations where the gas turbine, steam turbine and generator are all on one shaft. The unfired energy split between the gas turbine and steam turbine will still be as I stated above.

With supplemental firing, the load control in the plant DCS will control the supplemental firing when the load demand exceeds the capability of the unfired plant.

 

In a combined cycle power plant providing power to a larger grid the speed of the two generators is a function of the grid frequency. Both generators--and their prime movers--will operate at speeds directly proportional to grid frequency when synchronized to the grid. (Neither generator is large enough to change grid frequency appreciably--usually.)

And the steam turbine is usually no more than about half the size of the gas turbine (unless the HRSG has supplemental firing or there is more than one GT/HRSG supplying steam to the steam turbine). So the steam turbine can't effectively change the speed of the gas turbine (again since they are synchronized to the grid).

Steam production is a function of gas turbine load--and exhaust temperature and flow. Steam pressure also varies with exhaust temperature and flow, so there's really no need to control the incoming steam flow or even pressure to the steam turbine, so the steam turbine control valves are usually just driven to full open once the steam turbine has been synchronized and is above some minimum load. As steam flow and pressure changes with gas turbine load and exhaust temperature and flow the steam turbine load will "follow" the gas turbine load--up or down.

Hope this helps!!!