Could someone explain whether variable speed with a single-shaft gas turbine is permitted while the generator is producing power?

One publication I read indicated that the shaft speed must be constant to provide the proper generator frequency. Another article said that generator load can be used to control the shaft speed if desired during normal operation. Both referred to single-shaft gas turbines in which the generator is connected to the gas turbine shaft.

Could someone please give some advice as to whether the generator can compensate for changing shaft speed and still match the grid frequency?

My background has mainly involved mechanical/chemical aspects of gas turbines and less so the electrical aspects.

 

F = (P * N) / 120

F = Frequency

P = Number of poles of generator

N = RPM of generator rotor

On a single shaft gas turbine (even one connected via a reduction/load gear) driving a synchronous generator, the speed of the gas turbine shaft is a function of the frequency of the grid to which the generator is paralleled.

The "key" here is 'synchronous.' Every generator connected to the grid is running at a speed that is proportional to the frequency of the grid. No one generator can run any faster or slower than the above formula states. They are all <b>synchronized</b> together. The magnetic forces of the generator rotor field keeps the rotor locked in synchronism with the AC frequency which is on the stator.

Any attempt to increase the speed by increasing the fuel results in increased torque being applied to the generator rotor, which converts the torque into amps.

If the fuel being burned in the gas turbine is exactly equal to that required to keep the turbine/generator shaft spinning at the same frequency as the grid (defined by the above equation), then there will be no power available from the generator.

If the fuel is less than required to keep the turbine/generator shaft spinning at synchronous speed, the generator will become a motor, driving the gas turbine (called "reverse power").

One has to be really careful when reading about droop speed control and loading of prime movers. Many of the authors are speaking completely theoretically, and not talking about real world conditions of "infinite" grids.

Hope this helps!

 

CSA,

when you said: "If the fuel is less than required to keep the turbine/generator shaft spinning at synchronous speed, the generator will become a motor, driving the gas turbine (called "reverse power"). does it mean that turbine will start spinning in the other direction?

Can reverse power damage the turbine? If so, why?
Thank you

 

The only difference between a motor and a generator is whether the device is producing torque from amps (a motor) or converting torque into amps (a generator). Motors and generators are electrically very similar, it's just that there aren't many motors as big as most generators. Most generators are synchronous machines, and most motors are induction machines, though many very large motors tended to be synchronous machines in years past. For all intents and purposes, a motor can be a generator (if the device it's driving produces torque instead of requiring it), and a generator can become a motor if the torque being produced by the generator's prime mover is reduced to the point that it's not providing any torque input to the generator.

A generator is a device for converting torque into amps, which can be transmitted long distances over wires, and then re-converted back into torque through a motor.

To reverse the direction of rotation of a three-phase electric motor, one has to swap two of the phase leads. When a prime mover stops producing torque and the generator is still connected to the grid the "rotation" of the stator leads isn't reversed.

Many motor-operated devices employ something called regenerative- or dynamic braking, whereby the load is slowed or stopped by using the motor as a generator (this usually requires a variable frequency electronic "drive" and controls). Many hybrid cars use regenerative braking to charge the batteries when the car is slowing or being "braked" to a stop. The directions of rotations don't change, just whether or not the rotating electrical device is producing torque or converting torque into amps.

What can damamge a prime mover (a turbine, or a reciprocating engine) is when the energy input to the prime mover is cut off and the generator is not disconnected from the grid. Imagine what would happen to a reciprocating engine if it were being rotated by it's generator under reverse power! The aftermath is not a very pretty sight. Steam turbines can also be catastrophically damaged if "motorized" by their generator under reverse power. Instead of the buckets being "driven" by steam flow, they are trying to draw steam into the turbine and the buckets aren't manufactured to withstand forces in the opposite direction. Also, without steam flow to "cool" the buckets (yes, cool) the buckets will generate their own heat as they are spun by the generator under reverse power and suffer damage from overheating due to "windage".

Single shaft heavy duty gas turbines, on the other hand, have this great big torque-consuming thing at the other end of the shaft, the axial compressor. When a gas turbine is "motorized" by it's generator under reverse power the generator is driving the compressor in the same direction, forcing air through the compressor just as if the turbine were driving the compressor. However, since the compressor consumes as much as two-thirds of the energy produced by the turbine, if the turbine isn't produing enough torque, the generator can become a great big "load" on the grid if allowed to operate for very long.

That's why there are reverse power relays on generators; not to protect the generators (they don't care whether they are converting torque or producing torque), but to protect the prime movers from the effects of being driven by the generator acting as a motor under reverse power.