Say, I have a GTG running alone in droop & it is loaded to 25%,50% and 100% values. What will fall - the speed or terminal voltage or both?

Thanks,
Ivan

 

If by gas turbine-generator operating "alone" you mean the unit is not being operated in parallel with any other generator and prime mover and the GTG is supplying a load independent of any other generator and prime mover. then the only way to increase the load is to start a motor or turn on a light or start a computer.

You can't load a generator which is independently supplying a load by increasing the torque being produced by the prime mover--the only thing that will happen is that the frequency of the system (generator and load) will increase. It doesn't matter if the prime mover is being operated in Droop Speed Control mode or Isochronous Speed Control Mode--if it's supplying a load independent of any other generator and prime mover, an operator cannot increase the load from the prime mover's control system. The load is the sum of the lights and motors and computers connected to the generator.

So, unless you have some way of controlling the amount of motors and lights and computers connected to the generator and gas turbine you can't change the load of the unit regardless of the mode of operation of the prime mover control system..

Now, let's suppose that the gas turbine-generator is independently supplying a load that you can control, such as a variable load bank. And let's further suppose the gas turbine is being operated in Droop Speed Control mode. If the unit is operating at rated speed with zero load and you start increasing the load to 25% of the rating of the gas turbine output, if the unit has a 4% droop setting, the speed will decrease to 99%.

If you then increase the load to 50% of the rating of the gas turbine output, the speed will decrease to 98%.

If you then increase the load to 75% of the rating of the gas turbine output, the speed will decrease to 97%.

If you then increase the load to 100% of the rating of the gas turbine output, the speed will decrease to 96%.

The generator exciter voltage regulator type, mode and setting will determine what the generator terminal voltage does. Most exciter regulators being operated in AC or Automatic Voltage Control will try to maintain generator terminal voltage at the AC Voltage Regulator setpoint. The type of load (resistive, inductive, capacitive) will also have an affect on the generator terminal voltage of a generator independently supplying a load, so the exact effect the load will have on the generator terminal voltage will depend on the type of load. (Most load banks are just simple resistive devices.)

 

Oh, in the above example, the speed will decrease if the operator does nothing.

In the example above, the operator could make an adjustment to the turbine control system to return the speed (and hence the frequency) back to rated at each load, up to 100% of rated.

And, that's another thing that wasn't made clear in the first post: When the speed decreases below rated, the frequency will decrease proportionally. Frequency is directly proportional to speed.

Voltage is also proportional to speed if excitation is held constant, but an Automatic, or AC, exciter regulator can increase the excitation to try to maintain rated terminal voltage.