"Tie-Breaker" Switchgear and bridging momentary power loss

[CraigRochester]

I'm working on a project to evaluate the best way to sequence loads for power outages in a pharmaceutical production facility.

We have:
[ol][li]Two independent main feeds from the power company @ 13.8 KV[/li]
[li]"Tie Breaker" switchgear that switches all power to the active feed upon loss of one.
[/li]
[li]Step down transformers for 4.6KV(chillers),480 V, and other low voltage services.[/li]
[li]Backup generator for selected equipment[/li]
[li]UPS service to selected equipment: all control systems and local PLCs[/li]
[li]Control systems monitor the generator's automatic transfer switch(ATC) and initiate power loss response[/li]
[/ol]

Our current sequence of operations for responding to power loss is to initiate shutdown of all controlled equipment whenever the ATC output contacts for power loss close.

My question is: Can I bridge the brief(exact time being determined) period when the "Tie-Breaker" switchgear transfers without initiating a full shutdown sequence. If so, could you provide guidance on how to evaluate the systems' capability to experience a quick power loss.

Thanks,

Craig

 

[davidbeach]

If you can't transfer motor loads from the dead source to the live source within a few cycles, you will need to do one of two things. One, you must get rid of all the motor loads before closing into the new source as the motors will have slowed down and will be out of phase with the oncoming source; this can also be accomplished by waiting for 2-5 seconds for the voltage on the motors to collapse. Two, you could have a sync check relay with predictive closing so that you can close the tie breaker while the motors are in phase with the incoming source.

Making the sync check work, as well as the very fast transfer, probably means that you will need to drop one source for glitches that you might not otherwise consider worth separating just so that you can make the transfer before the motor voltage collapses too much.

Probably, shutting down the motors and restarting will be the least complex, most reliable.

 

[waross]

I agree with davidbeach.
You may want to look at Variable Frequency Drives with the capability to "pick up" a spinning motor.
Putting critical motors on VFDs with restart capability and allowing non critical motors time to slow down and restart may be an economical compromise.
Another option is a large Uninteruptible Power Supply.
There was some discussion recently on a combination standby generator and inertial UPS. Search this forum and the electric motors and motor control engineering forum.
respectfully

 

[CraigRochester]

Thank you David,

When you say "a few cycles", you mean a few 1/60ths of a second? Sorry to be dense.

Is the synchronization requirement applicable to all motors types and sizes? Our important motors are clean room fan motors. Typically ~50-80HP on VFDs. If we loose air flow, the room environments are "breached" from a QA standpoint.

 

[CraigRochester]

Thank you waross,

I looked at the manual for our FUJI drives and there is a "Restart after a momentary power failure".

I appreciate y'alls help in pointing me in the right direction. I'll get an EE to pick it up from here.

Craig

 

[davidbeach]

Yes, electrical cycles, 1/60th second at 60Hz. Drives are not motors from the standpoint of load transfer and are a good solution to the problem when the drive is capable of dealing with a momentary outage and picking the motor up where ever it is in phase/frequency relationship to the supply.

When directly connected, motors are motors for what is being discussed. Synchronous motors will maintain their field, and therefore their terminal voltage much longer than induction motors will, and induction motors with PF capacitors will maintain terminal voltage longer than induction motors without PF caps.

 

[CraigRochester]

David,

Thanks again for your help.

Much appreciated!

Craig