Decaying Motor Voltage 2

[macmckim]

Does anyone have a good read on the subject of when we have a Main Bus A -Tie-Main Bus B system where one of the main “B” transformer failed and need to close my tie breaker to avoid all
the motors shutdown on bus “B”.
Is there a time cycle where I can catch motor before I will do damage to the shaft? Looking at 10 - 16 milliseconds for system to re-energize.
Motors are 800 to 200HP, 3600RPM, 4kV, - application are screw compressors.

thankyou

 

[ZeroSeq]

Very interesting topic. Are these sync or induction motors?

I also wouldn't be confident I would be able to restore voltage to the stator in under one cycle post fault/failure.
The tie breaker should be interlocked with the upstream transformer protection so that it does not close on the live bus. Therefore, lets do some quick math:

a. Transformer protection of any sort (87, 50/51, 63, etc.) initiates and sends trip signal to breaker = ~1 cycle
b. Transformer main breaker opens = ~3 cycle minimum
c. Tie breaker gets close signal from PLC and closes on dead bus = ~1 cycle minimum.

A minimum of 5 cycles have passed before you can restore voltage.

Whether or not this will have an adverse effect on the shaft depends on the time constant of the load.

 

[macmckim]

Zeroseq - The motors are SCIM , induction motors.

thanks for the comments. Even though SCIM are 3600RPM (low LRT, IE 60 to 70%), concern is if motors re-energize at a different phase angle,
that the motor may go into breakdown torque (BDT of 200%) and cause some shaft damage - motor or coupling or screw compressor shafting.

 

[ScottyUK]

Can you tolerate the increased fault levels of running with the tie closed and implement a directional protection scheme on the transformers, possibly supplemented with a unit scheme to give faster clearance of in-zone faults on the transformers?

 

[xnuke]

Search the internet for "motor bus transfer" and learn about the options available.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ScottyUK]

Alternative search term would be 'fast bus transfer'.

These schemes usually initiate closing of the oncoming source breaker before the offgoing breaker has fully cleared to minimise the time off supply. If the offgoing breaker fails to open then the results will probably be very disruptive to plant operation, especially if the new supply closes into an uncleared fault and subsequently trips.

 

[eeprom]

When the motors lose their source, they will still have a significant rotor field, for at least a few cycles, probably 5 or so. This of course depends of the R/L of the rotor. During the time of field decay if you close in with another source, the odds of the motor and the new bus being in phase is pretty small. The result will be a very loud thump, and the possibility of tripping. I have tried this with a VFD, trying to "catch" a running motor as it is dropped off of line voltage. This worked once I delayed the switchover to about 200 mS, not including the contact closure time.

EE

 

[waross]

concern is if motors re-energize at a different phase angle,
that the motor may go into breakdown torque (BDT of 200%)

For worst case out of phase closure, the current may approach 200% of normal BDT values. Transient torque may approach 200%² or 4 times normal BDT.

Bill
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