Question on Large Motor Under-voltage Protection

Boerg

Member
What does everyone typically see/set for under-voltage pickup and time delays on large motor buses?

I work for a utility and assess the stability of the electric transmission system. We have models for large industrial/commercial loads with 3PH motors. Part of the model is the under-voltage protection elements. Reading the IEEE standard on motor protection, it says to set pickups and time delays to coordinate with utility protection (allowing remote faults to clear) as well as the most severe local motor start.
 

jim dungar

Moderator
Staff member
Rarely do I apply motor undervoltage protection which has been coordinated with the utility.
It is really customer, utility, and service voltage dependent. Typically large motors, especially those fed by VFDs, have individual UV protection, so my service entrance relays are used to prevent closing the main onto a utility bus with low voltage. I can't remember the last time I tripped the service offline, except when some type of source throwover scheme was employed.
 

topgone

Senior Member
What does everyone typically see/set for under-voltage pickup and time delays on large motor buses?

I work for a utility and assess the stability of the electric transmission system. We have models for large industrial/commercial loads with 3PH motors. Part of the model is the under-voltage protection elements. Reading the IEEE standard on motor protection, it says to set pickups and time delays to coordinate with utility protection (allowing remote faults to clear) as well as the most severe local motor start.
Why do you want to install an under voltage protection on a "big motor"? Starting big motors will always go hand in hand with voltage dips! Did I miss anything?
 

Boerg

Member
Why do you want to install an under voltage protection on a "big motor"? Starting big motors will always go hand in hand with voltage dips! Did I miss anything?
It is an additional bit of protection to automatically trip at a voltage when motor stalling is unavoidable. It backs up the overload and overcurrent protection but should trip faster with severely depressed voltage. Per IEEE, UV protection must coordinate with starting as well as utility short-circuit protection.

Typically it’s applied on the bus so to trip all motors, even the small ones.


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Boerg

Member
Rarely do I apply motor undervoltage protection which has been coordinated with the utility.
It is really customer, utility, and service voltage dependent. Typically large motors, especially those fed by VFDs, have individual UV protection, so my service entrance relays are used to prevent closing the main onto a utility bus with low voltage. I can't remember the last time I tripped the service offline, except when some type of source throwover scheme was employed.
Thanks for the info. I suppose VFDs could be a large component since I would guess they have similar specifications. I’ll try to look at their datasheets and see if I can glean how they are implementing UV protection.

Do you have any idea what the VFD penetration would be for an average facility(United States I assume)? Obviously there’s a lot of variance here. I would say that power plants average ten or twenty percent.


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steve66

Senior Member
"Coordinate the motors under voltage protection with the utilities protection..." That's a new one for me, but I don't usually deal with utility stuff or large motors.

The first question to ask is which OCP should trip first? The utilities, or the downstream motor OCP?

With overcurrents or short circuits, that's usually a simple question to answer - the smallest or most downstream device that sees the fault current should normally trip first.

But with under voltage, how do we know if the problem is a locked motor, or if its the utility supply problem? In most cases I would assume that the utility supply is stiff enough that an undervoltage would not be caused by a downstream motor, it would more likely imply a problem in the utility supply itself.

To me, that implies that on an undervoltage, we would actually want to utility OCP to trip before the downstream motor OCP.
 
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