Induction Generator Reduced Voltage Starting

[3winding]

The local utility will not accept the voltage sag produced by a small (750kW) induction generator being connected to their system. The generator breaker is closed after the unit is rotated to synchronous speed by the prime mover. The current inrush is therefore primarily composed of VARS to establish the magnetic field. One option as a method of mitigating this problem is using reduced voltage starting to limit the inrush and resulting voltage sag. The question is how to size the transformer and what configuration/transition of a bypass contactor should be used. Of course economics is an issue, so the smallest possible size transformer is the goal. Does anyone have any good references on this subject?

 

[jbartos]

Suggestion: Visit

http://www.puc.state.tx.us/electric/business/dg/dgmanual.pdf

where the Utility suggests hardware for starting the induction generator as an induction motor.

 

[jplinn]

We have several induction generators. We performed a test that measured inrush current on the generator verses unit speed at breaker closing. We found that the lowest inrush occurred at around 105% rated speed.

We performed the test by placing a resistor in the CT circuit and recording the current wave form during inrush. Also we recorded machine speed, voltage and breaker position. The test were performed at about 95%, 98% 100% 102% and 105% rated speed.

Perhaps you can test your unit in a similar way to determine the best closing point and decrease your inrush,

jpl.

 

[Rodmcm]

An option you have is to include a capacitor bank, this, on excitation will provide staring vars and reduce the inrush and hence voltage drop on the system, also it will assit the utility as they will not have to provide Vars to your generator, however, if the cap and generator become isolated in an overspeed condition then the resulting voltage increase can burnout the generator. We include a breaker on the capacitor to open in event of overvoltage.
I like jplinns' idea aswell, I must try it.

 

[jraef]

pintalo,
What transformer are you refering to? Do you plan to use an Autotransformer type RV starter?

We have provided solid state starters for exactly that application and they work perfectly. A bypass contactor is configured in simple paralell to the SCRs and closes once the SCRs are at full conduction.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Question to previous posting: Are those solid-state starters soft starters?

 

[jbartos]

Suggestion: When it comes to generator (motor during starting) and capacitors, there will be some oscillation present. Once motor starting of the generator approaches to the synchronous speed, there will still be some oscillations left. Concern is how AVR will tolerate the oscillation.

 

[Rodmcm]

There is no AVR on an induction machine.

 

[jbartos]

Comment on the previous posting: Agree, there is no synchronous generator type AVR. However, there is voltage limit range accomplished by the load limitation. Visit

http://www.qsl.net/ns8o/Induction_Generator.html

for:
Yes, but... are there problems? Well, there is no active voltage regulation, but keeping it within a tested load rating can keep it within any voltage parameters that you set. I feel that a voltage range between 105 and 126 volts is perfectly reasonable.

 

[Rodmcm]

Sorry Jb but I don't understand what the author means by this, the voltage you get on the terminals is the line voltage, not from the set, what does he mean?