Induction Generator Frequency Stabilization

[jonbritt]

Does anybody have an explanation of why an induction generator running above synchronous speed doesn't force the system (or grid) to speed up. Frequency alters the speed of an induction motor, so if you speed it up to spin at lets say from 1800HZ - 1900HZ, how come the frequency of the system doesn't speed up? Lets say you had a big enough windmill that could produce more power than the grid could provide, and then got that spinning faster than synchronous,... would that scenario ramp up the grid frequency?

As far as self-excited induction generators, their output frequency is dependent upon rotor speed.

Is it just that the grid is so large, it is just regulating the frequency? What happens in a huge windmill farm where they are producing MW of power? Doesn't that try to force frequency up on the grid? Or am I looking at this all wrong. I understand that the torque is produced by slip or differential frequency from rotor to stator. For a wind generator that is experiencing high winds... this would result in more torque... or a larger slip... thus I would think it would be trying to speed up the grid frequency to reduce the amount of slip?

 

[ggunn]

Is it just that the grid is so large, it is just regulating the frequency? What happens in a huge windmill farm where they are producing MW of power? Doesn't that try to force frequency up on the grid? Or am I looking at this all wrong. I understand that the torque is produced by slip or differential frequency from rotor to stator. For a wind generator that is experiencing high winds... this would result in more torque... or a larger slip... thus I would think it would be trying to speed up the grid frequency to reduce the amount of slip?

I am no Large Wind expert, but my assumption is that the current from a wind generator passes through a DC step and gets synchronously inverted to AC for injection into the grid.

 

[zbang]

Not a generator expert....
If you have a generator connected directly to the grid it will spin at the grid speed, but by adjusting the excitation (if not self-excited) and energy in, you can adjust the energy out and IIRC the power factor. As far as I know, all wind-powered generating systems have an extra conversion step as ggunn mentioned.

 

[jonbritt]

Just to clarify I am not talking about a small scale rectified and inverted wind generator but rather generation in means of direct prime mover induction generation where the generator is synced with grid power initially

 

[GeorgeB]

I'm not pretending to be an expert, but IIRC, an induction MOTOR driven above synchronous speed pushes power back into the line at line frequency. The torque required to achieve the speed will be, AIUI, exactly the same as the torque loading the motor at that slip.

I may not have said that well; if a 4 pole 60 Hz 10 HP (7.5 kW) motor runs fully loaded at 1780 rpm, generating 7.5 kW will require/result in it spinning 1820 rpm. If I have sufficient driving torque to make it spin at 2000 rpm, I don't know what would happen, but my GUESS is that not much power would go back into the supply.

I would think a "torque control means" would be necessary using current as a feedback.

 

[ggunn]

Just to clarify I am not talking about a small scale rectified and inverted wind generator...

Neither was I.

 

[Carultch]

Does anybody have an explanation of why an induction generator running above synchronous speed doesn't force the system (or grid) to speed up. Frequency alters the speed of an induction motor, so if you speed it up to spin at lets say from 1800HZ - 1900HZ, how come the frequency of the system doesn't speed up? Lets say you had a big enough windmill that could produce more power than the grid could provide, and then got that spinning faster than synchronous,... would that scenario ramp up the grid frequency?

As far as self-excited induction generators, their output frequency is dependent upon rotor speed.

Is it just that the grid is so large, it is just regulating the frequency? What happens in a huge windmill farm where they are producing MW of power? Doesn't that try to force frequency up on the grid? Or am I looking at this all wrong. I understand that the torque is produced by slip or differential frequency from rotor to stator. For a wind generator that is experiencing high winds... this would result in more torque... or a larger slip... thus I would think it would be trying to speed up the grid frequency to reduce the amount of slip?

Generators for wind systems are a lot different than conventional electric generators, because they generate DC using either commutators or rectifiers, and have additional electronics to convert this back to AC at a frequency independent of the turbine speed.

Conventional electric generators, used with sources of power that are more easily controlled, are rigidly constrained from the mechanical rotation rate to the electrical frequency.

 

[electrofelon]

As I understand it, an induction motor/generator doesnt have to spin at a synchronous speed. The magnetic field is sort of "dragged along" with the rotor. Also I think it is basically a case of the grid being so large that you would be hard pressed to mess with the frequency even if you had a large supply that "didnt care" about maintaining 60 hz.