Low power factor tripping

[emviana]

Hello.
In our industrial plant, we have some problems during an induction motor starting. I'd like to talk about ideas to solve this problem. The electrical diagram is attached.

Describing the problem:

Two generators (A and B) connected in two load bars (A and B) that connect to a synchronizing bus. We have at same bus a synchronous motor (SM) and induction motor (IM). With generator A and the synchronous motor (SM) in operation and generator B out, the energization of IM causes SM tripping by low power factor. I suspect that SC limiters are "blocking" reactive power flow form generator A to IM, and all reactive power is being supplied by SM. This fact causes low power factor since the ratio Q/P changes on synchronous motor. I was thinking to operate SM in a overexcited mode and then start the induction motor. I'm not secure about this. Is it correct? How can I start the induction motor without tripping the synchronous motor? We couldn't try to start IM with generator B in operation.

Thanks a lot.

 

[dpc]

I think you need to review the need for tripping on low power factor and possibly eliminate this trip or change the setting. As an alternative, the low power factor trip could be temporarily blocked during motor starting.

The induction motor is going to draw what it draws during starting. I suppose you could add some capacitors to improve power factor during starting, but I'd look into getting rid of the low power factor trip. True loss of field protection should allow operation anywhere within the generator capability curve.

It's also possible that the motor is simply too large to start on one generator.



David Castor

http://www.cvoes.com

 

[emviana]

Thanks, dpc.
Actually, we have tried to change SM to a overexcited mode, but it didn't work. I think that SC limiter impedance is high, and so IM drains from a lower impedance source, in this case, the synchronous motor. In this way, i'm not sure if operation with generators A and B will solve the problem.

 

[waross]

As dpc says, either inhibit the low PF trip or add a time delay. Even if there is a good reason for the low PF trip, a low PF for the time it takes to start a motor should not be a problem.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[emviana]

Thanks waross.
Reactive power demand at induction motor starting is about 15 MVar (0.2 PF estimated at starting). I'm not sure if SM can generate 15MVar. PF protection is set with 0.7 inductive with 0.2 seconds. It was adjusted in compliance with IEEE C37.96-2000 - IEEE Guide for AC Motor Protection. I'll search for synchronous motor datasheet.