Residual Voltage on Motor Bus

[bdn2004]

I'm reading an automatic switching procedure that will tie two 4160V motor busses together with an automatic relay on the the tie breaker. Each bus has (4) four 1000 Hp motors connected to it. Each bus is fed by it's own 10MVA, 15kV -4160V three phase transformer. It's in a main-tie-main configuration.

For example, this situation would occur if a primary feeder breaker to one of the 10MVA transformer's tripped.

It says...
The system will allow the tie breaker to close provided that the disconnected bus voltage is within 20 degrees of the powered bus voltage and the transfer occurs within a 10 cycle window.

Here's my question...where does that bus voltage on the disconnected bus come from? How long will that voltage be on the bus? Do the spinning motors that just lost their power source become a generator and supply a voltage into the bus? What is the voltage? Any info about this would be helpful.

 

[waross]

The motors will become induction generators on the loss of supply voltage.
After that, it depends. Do the motors trip of the bus when the supply fails? No problem. If the motors stay connected, what type of loads do the drive? Motors that take a long time to coast down will generate for quite awhile. Motors with power factor correction capacitors will generate longer. The motors may circulate current between each other and will generate longer.
I am concerned with your spec. Any voltage on the disconnected bus will be from the regeneration of the motors. Immediately that the power fails, the regenerated frequency from the motors will fall to match the motor speed or less.
That is, on a 1760 RPM motor the frequency will drop to 58.7 Hz. If there is a load on the regenerated voltage slip may drop the frequency further. Possibly 57.3 Hz. The frequency and the voltage will drop as the motor slows down.
Trying to find a 40 degree (20 degrees plus/minus) window within 10 cycles of different frequencies, one of which is decaying seems like a recipe for disaster.
Possibly someone found a specification for a different application and misapplied it here. I would disconnect the motors from the bus on a supply failure and use zero volts as a safety check.
Now if the motors are still online, they wil

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

 

[electricpete]

Do the spinning motors that just lost their power source become a generator and supply a voltage into the bus?

Yes. The rotor current continues to flow and continues to create flux which induces voltage in the stator winding. The equivalent circuit for that rotor current includes the elements in series: R2, X2, Xm. The angle of that total impedance determines rate of decay of rotor current and approx rate of decay of the generated voltage in the short term before the machine begines to slow down. As the machine slows down, the voltage decays even faster than predicted above.

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[davidbeach]

There are relays available for this specific application. Use one and it will close into the oncoming source if it can make a good safe transition, otherwise it will wait for complete collapse before closing the incoming breaker. Without that specialized relay I'd not try anything other than a outage of a few seconds.

 

[slavag]

Hi.
In additional to David's post.
You have found information about HBT ( high speed bus transfer) systems.
Few companies make such relays.
You have four option for the transfer:
1. Fast transfer.
2. 1-st considience
3. Residual voltage
4. Long time command.
Try: Beckwith, and ABB-SUE3000.
Best Regards.
Slava

 

[rcw retired EE]

Check the Beckwith website. They have some good papers explaining what happens on a fast transfer and what the limiting voltages should be based on typical motor standards.

Bob Wilson