Overexcitation of induction motor/PFC combination 1

[KJvR]

Hi,

I am currently designing a MV switchboard controling 3x500kW motors. PFC is done by connecting capacitors directly onto the 3.3kV busbar. Worst case scenario will be at least one stage (286kVAr) with one 500kW running with a trip on the incomer/11kV feeder. No load current of the motor is 30A and over voltage would therefore occur with a power outage to the board (Assume a high inertia load).

I have simulate this condition using ATP. When a pure capacitor is connected to the motor, the expected voltage rise is obtain. However, the PFC bank consist of large inrush current limmiting reactors (22.6mH) in serie with the capacitor. If I model this, the voltage colapse to almost zero at point of power loss (for a few hundred milliseconds) before the motor speed drops dramatically after which the voltage recover to an over voltage situation and the speed that slowly decrease from this point (as expected). I have two questions and would appreciate any help:
(a) Is the ATP motor model correct in that the voltage should colapse and why would this happen? My knowledge on induction motor generation is poor and I can't work it out or find literature on this colapse.
(b) Can the sudden reduction in motor speed cause mechanical damage to the motor?

KJvR

 

[rbulsara]

Should not the motor controller/breaker open upon a power loss? The coasting motor should not be back feeding the caps connected to the bus as you describe.

On the other hand, PFCs are often connected to motor terminals (or load side of the controller), where your question may be more pertinent.

 

[Laplacian]

http://www.mastercontrols.com/EngInfo/Articles/Nailen/TA_RNail.htm

 

[Marke]

Hello KJvR

You are correct, there is a potential danger if the supply breaker opens and leaves the capacitors effectively connected across the motor terminals. If there is other load connectd, the danger is lessened. The major problem is when there is only the motor operating.

With the motor operating, it is not possible to rely on other circuitry on the ouptu side of the breaker detecting the opened breaker based on voltage alone.

As you know that the capacitors could cause a problem if the supply is lost, and if the supply is lost you will have no current flowing in the feeder, you could use both an electrical interlock on the feeder breaker to drop out the capacitors plus an current detection relay on the feeders to provide an additional interlock. If the current in the feeders is too low, you do not want the capacitors connected as it could cause a supply resonance problem and/or could indicate an open feeder.

The detuning reactors will not eliminate the resonance between the motor and the correction capacitors, just shift the frequency slightly.
The real danger occurs when the motor slows down and runs through resonance between the motor reactance and the capacitor reactance.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[waross]

It appears that you are correcting the power factor to 100%.
How does your simulation look if you reduce the ammount of correction to 90% when only one motor is operating? Is this feasible with your expected operating conditions?
respectfully

 

[KJvR]

Thanks Laplacian for the link. Still working through it but I start to understand the consepts and dangers. I have changed the values of the capacitor and reactors. I think Mark is correct in that the motor stator coils, cable, reactor and capacitor caused a resonant frequency just below 50 Hz. When the system looses power and the speed reduce to that resonant frequency, it breaks it very hard. I am still not sure exactly what happen inside the motor.

Waros, the client requested a system with two capacitor stages while the board have three motors. They also requested the stages to be the same size as used on the rest of the plant therefore the problem. However, I have reduce the capacitor bank for a 90% P.F under 25% load conditions. No overexcitation, just a long magnetising time constant. However, if all three motors are running the PF is well below target.

 

[Skogsgurra]

What happens inside the motor is simply that the rotor flux continues to rotate with the rotor and generate a three-phase voltage across the disconnected phases.

If there are capacitors connected, they will supply the required reactive power to keep rotor flux up. This will go on as long as the machine is rotating. It means going through a range of frequencies - some of them sometimes happen to resonate with circuit elements in the system.

Gunnar Englund

http://www.gke.org