Induction Generator Starting At Synchronous Speed

[3winding]

I am looking for information on the magnetizing inrush of an induction generator started at or just above synchronous speed. The local utility claims that this process will cause excessive voltage sag on the system. Their EMTP model shows an inrush of 5 plus times rated current when the breaker closes. Does anyone have any knowledge on this topic?

 

[WattsNew]

pintalo,

Just like any other induction machine, this device will pull Locked Rotor Current when the breaker is closed. LRC is 4-6 times FLA of the machine. You can use ETAP to determine the value of the voltage sag when induction machines are started. Hope this helps.

 

[3winding]

tanuj - The prime mover accelerates the generator to synchronous speed, then the breaker is closed to energize the generator, so locked rotor currents do not apply.

 

[twosockets]

pintabo,

I think you are partially correct. There would be no significant inrush only if the generator was turning at synchronous speed at the time the breaker is closed. It is far more likely that there would be a slight difference between the generator speed and the synchronous speed at the time of the breaker closing, thus a higher than normal inrush. The bottom line is that an induction generator is nothing more than an induction motor when being brought up to speed, if it is not being driven by the prime mover.

I believe that it is likely that the utility does not want the synchronous generator connected to their system. They probably do not like the fact that they have to supply VARS for excitation at no charge and, perhaps, pay higher than market rates for energy that they are forced to buy.

 

[poiuy]

This will happen even at synchronous speed if there is a phase difference between the voltage on the line and on the generator before connecting it to the line. Worse case scenario is at 180 degrees phase difference where you may have 2 times the peak voltage between a generator's phase and the corresponding phase in the system.
The best way to do it is to run the gen at a speed very close to synchronism and monitor the voltage difference between the corresponding phases and connect when this is minimal (ideally 0). If you have a slight difference in frequency between the generator and the line you will be able to connect even manually because the voltage difference will oscillate very slow from 0 to 2 x the peak voltage.
Also, although it's obvious, make sure you have the same phase sequence and voltage (rms) on both the line and the generator.

 

[twosockets]

poiuy,

I don't think so. Remember that this is an induction generator. Without the field excitation provided by the utility, the generator is deader than 4 O’clock. In other words, synchronizing as you described is irrelevant.

 

[3winding]

This is an induction generator, NOT SYNCHRONOUS. Very little voltage is present on the terminals of an induction generator before breaker closing. This is not an issue, nor is the fact it consumes VARS from the system. Power factor correction is applied after starting. The issue is how much inrush will be expected if the machine is run to synch speed and then the breaker is closed. I agree that the residual voltage on the terminals may have a small effect on inrush, although negligable. The inrush will be due to setting up the field in the rotor.

 

[DougMSOE]

Back about 10years ago I had the opportunity and the responcibility to design and start up just the thing you folks are talking about, so this is a real life experiment/experience.
The local utility voiced the same concerns, high current rush. When we brought the first unit on line (400KW), we did it at synchronous RPM only about 1.5 -2 times inrush of rated current.
The second unit about 10-20 RPM > sync RPM. This lead to about 4 times inrush, but the directional current meters said the current was out going, yes we had some caps in service.
The first unit had no caps but did have rotor at sync RPM. At that RPM value you are limited to only the stator circuit impedance. Look at the equivalent circuit. Above or below sync RPM, the rotor becomes involved.

 

[twosockets]

DougMSOE,

I would generally agree with your opinion/experience. I would submit that the 1.5-2 times inrush current when closing the breaker at synchronous speed was due to the fact that your "synchronous" was probably approximate.

You are correct in stating that the "current" (actually power flow direction) was outgoing when closing the breaker at higher than synchronous speeds. The inertia of the rotor would cause a large momentary surge in current until the rotor decelerated to a near synchronous RPM.

 

[poiuy]

Oops, my fault, the key word "induction" somehow escaped me...Sorry.

 

[pareshcvora]

To my understanding the induction generator operating at synchronous speed would equate to a transformer (as there would be no flux cutting) and hence the in rush current on switching on to the system would depend on the exact moment of closing in the sinusoidal voltage cycle of the supply. This would thus vary from rated magnetising current to several times magnetising current of the induction machine transformer.

For the speeds different from synchronous speeds some more thought needs to be applied considering both transfomrer and motor/generator functions.

 

[3winding]

DougMSOE,

I agree with your analysis. I'm not sure how you were reading the values of current? The inrush the utility is simulating with EMTP is in the order of 100 mS. My gen is 700 kW on a weak system (13MVA SC). I always bring the caps on after energization on these gens. Wouldn't starting the gen with the caps already connected increase my voltage sag problem because of charging required? I think closing the breaker as close to sync speed as possible is very important. The utility study as shows that the prime mover slows down at breaker closing from 1200 rpm to 1166 rpm for 130 mS. This would cause reverse power to flow and trip the unit as I have a reverse power relay set for 0 secs. I don't understand how magnetizing current (imaginary power) can put counter torques on the shaft of the prime mover?