SOFT START ON GENERATOR USED AS TURNING GEAR

[xxjohnh]

Some vendors now offer a variable speed drive to use for acceleration and turning gear for Gas Turbines instead of starter motors.. Since synchronous motors are not inherently self starting, use an embedded squirrel cage, does any one have an explanation how this is applied to use the generator as a synchronous motor.

 

[xxjohnh]

It seems that VFDs can start synscronous motors from standstill. Not sure about the special case of Generator Solid State Starting devices. Anyone doing it?

 

[Skogsgurra]

Yes. Have started synchronous (no amortisseur winding) motor with VFD. Since a VFD can easily output 0 Hz, there is no problem in starting from standstill. "My" motor was in the 10 MW range, but I see no principal problem in doing it with larger units.

Our problem was to keep pole angle stable during acceleration (no rotor position feedback). Solved by going from vector to torque operation.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[xxjohnh]

What waveform do get at 0 Hz, DC? How do they get starting torque? I have been trying to find a technical explanation of how this works.Do you have the field connected?

 

[ScottyUK]

The LCI output does not go right down to zero speed (DC) but it generates an output frequency which is sufficiently low that rotor, with the field energised, is able to follow the rotating stator field. There are huge torque pulsations as the rotor poles slip past the rotating stator field until the rotor breaks away and begins to rotate synchronously.


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If we learn from our mistakes I'm getting a great education!

 

[GTstartup]

The problem with LCIs is they cannot load commutate from zero speed. The commutation is "forced" so that the generator is spinning at enough speed to create generator voltage feedback. ABB calls this "pulse mode" on their LCIs. I assume the same applies to the Megatrol The turning gear is not involved, in fact it is turned OFF before starting otherwise the spinning up of the generator could damage the barring piston.

 

[Skogsgurra]

xx: I wasn't using an LCI. It was a standard PWM inverter. Zero frequency is just a combination of DC outputs that depends on the angle where the systems happens to be resting at.
Yes, excitation must be on so that the stator field has something to 'pull'. Then start rotating the three-phase system (equals increasing frequency from zero) and take it from there. No sweat - if you can keep pole angle stable.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[xxjohnh]

Thanks, an old dog can learn new tricks.

 

[Skogsgurra]

Fine. But do not forget the old tricks!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...