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Steam Turbine Speed Controller
How Steam Turbine Speed Controller works?

Hallo Friends,

I am doing master thesis and working towards Steam turbine process simulation. As I am new in this field, want to know about basic Speed controller for Steam turbine.

Any one can help me out ?


Hello kuldeep,

there is so much to say about stem turbine controller. First of all, most modern controllers can work in either load or pressure mode.

Load mode means that steam turbine controller acts to turbine's control valves in order to maintain power output at desired value (setpoint). This mode is often called boiler following mode because pressure in front of a turbine oscillates and master boiler controller is responsible to keep steam pressure steadily as much as it can.

In pressure control mode, turbine controller keeps steam pressure at desired value (setpoint), while power output oscillates. This mode is called turbine following mode.

Load mode is often called Load/Speed mode, because when the generator is not synchronized to the power grid, controller acts as a speed controller. When unit is synchronized, it automatically becomes load controller.

When steam turbine controller works in load control mode, it can work in either adroop or isonchronous mode.

Droop control is classical proportional controller. This means that otput power can be somewhat different from the setpoint which depends on changing the frequency of the grid. This is also called a primary control (or regulation). When there is a mismatch between production and consumption, the grid frequency is changed. Because of this, turbine speed is also changed and is different from the setpoint. That is where proportional controller acts to restore the speed of a turbine and change load. That is the case whn actual output load can be different from the desired one.

On the other hand, an isochronous mode means that turbine controller acts as PI (proportional + integral) controller. This means that in this mode, turbine controller will always match actual power (load) to be equal to setpoint value no matter if there is a change in frequency.

There are few other people here that can contribute more to this.

Hi Mik,

Thanks a lot for help.

can you please explain me about multifunction Speed controller?

In my project, compressor is driven by steam Turbine. At start up, speed controller and mixer level control the steam flow to the Inlet of Turbine. After synchronizing Speed, Motor will drive whole crank and speed controller will switch over to Power controller.

Can you please explain me, how power controller will maintain turbine speed at synchronized speed?


Hello kuldeep,

When generator is not synchronized to the grid, steam turbine controller is in so called speed mode. That mode is essentially the same as power (load) mode. It is not called the load mode since , there is no load i.e. generator is not synchronized to the power grid. In this phase, if you add more steam, the turbine will rotate faster (speeding up), otherwise it will slow down if you reduce steam flow (close control valves). If you want to synchronize the generator it needs to rotate at speed that matches the grid frequency (for example if power grid frequency is 50 Hz and generator is two pole machine, it will rotate with 3000 rpm). So to maintain the desired speed control deviation is formed as difference between set point speed and actual speed. Depending on that control deviation the controller will be opening or closing the control valves.

When generator is synchronized to the power grid it will rotate with synchro speed i.e. it will rotate at exactly the same speed that matches the power grid frequency. In that case, if you add more steam, it will not start to rotate faster, but more power is going to be produced.

Power is equal to h* steam flow, where h is steam enthalpy. So if oyu add more steam, power will be greater.
This can be seen also from the other point of view. Power = T*speed, where T is torque. If you add more steam and the speed is constant then, Torque is increasing. Since generator is an electric machine that convert torques to the current, this will mean greater current flowing out of the generator. Total effect is increase in power.

I hope I explain the things technically correctly. I know there are more better experts here, like CSA, so hope he's going to put in his two cents.


You have explained things very well!

I'm personally having a difficult time understanding kuldeep's project. He's talking about a steam turbine driving a compressor and synchronous speed. Synchronous speed is something that usually refers to a generator-drive application, but I suppose it could be used for some compressor drive applications, as well.

Your explanation, Mik, is right on for a generator-drive application, and probably pretty close to accurate for a compressor-drive application, too!

Hi mike,
your explanation was correct for Power generation, but we are not going to generate power here. After synchronizing speed only to couple motor), customer wants to drive whole train by motor without passing more steam into turbine to maintain synchronizing speed. Because after synchronizing speed compressor's Antisurge control valves, Extraction valve of Steam turbine opens and train must run on synchronized speed.

My question was, how to couple motor after synchronization speed?

How should I calculate the power for speed?

Hello kuldeep,
you weren't specific about the actual application which was misleading for me.

I must admit that I don't fully understand your problem and your definition of synchronization. I'm not an expert for compressor application.

If you provide more details, I'm sure there is someone who will be able to help you.

It seems to me as if the project involves a compressor that can be driven either by a turbine or an electric motor. The problem involves changing over from turbine to electric and maintain drive to the compressor throughout.

If this is the case, one solution I have seen is to have centrifugal clutches between the drivers and the compressor. No specific control or synchronising scheme is necessary. The turbine is run at a speed slightly below motor running speed (if it's a standard induction motor it will not be i synchronism with the power supply anyway). On starting the motor, the motor clutch will engage and drive the compressor up to its (the motor) running speed. This should unload the turbine via normal governor action. The turbine clutch can then be disengaged and the turbine stopped.

Other ideas may be out there.


Sounds more like an induction-generator application!

Regards, Phil


What I understand is your compressor will be driven by Motor and for some reason you require to shift the drive to turbine or viz a viz. In this case you need to have arrangement of Train such that both Motor and Turbine are coupled with compressor on both end of compressor with Hydraulic coupling (Torque convertor). If Compressor is running with Motor and you need to change over to Turbine, (Turbine at Full speed) then charge the Hydraulic coupling (Torque convertor) gradually and simultaneously reduce the pressure of hydraulic coupling on motor side, Maintaining the pressure/Flow of compressor till Motor side torque convertor pressure becomes Zero, then you can stop the Motor and now compressor is on Turbine drive.

warm regards, CPPguy

Hi all,

from what I can gather, it seems that our friend has a compressor which is driven from one end by a steam turbine and driven by a synchronous motor from the other end.

I am assuming this because he needs to accelerate the compressor motor train to synchronous speed using the steam turbine and then synchronise the motor. Finally the whole train ends up being driven by the motor only as the steam turbine valves are opened/closed depending on which they are.

If this is the case, standard synchronisation facilities for synchronous machines are required on the motor.

My question is how is the steam turbine kept running but not providing motive power. From my knowledge this can only be done by keeping the turbine under vacuum conditions to eliminate windage losses in the steam turbine (similar to single shaft combined cycle gensets which are not equipped with SSS clutch when run in open cycle mode).


By manish jadav on 9 August, 2014 - 10:59 pm

thanks for reply. but my confusion is if you select ISCHO mode in steam turbine, it means the frequency is fix but load may vary as per plant load. but you wrote that frequency is change.