Under Voltage Trip Relay

[hanksmith]

Currently working on the design of a power system for a small cement plant, the main incoming breakers are going to use under voltage trip relays with a 0.5 to 3 second time delay.

The reason for the time delay is the maintenance guys do not want to reset all the breakers every time there is a lightning strike that causes a very short power outage. The utility already told me that they try once to automatically reclose at 2 seconds.

My question comes with up with what happens when the voltage does not go to 0 and is really just a sag. Will the motors get damaged if the time delay is set to 3 seconds and the relay is set to trip at a 20% drop in voltage. Motors could continue to run at 70%, 60% or possibly a lower voltage for 3 seconds before the UVT trips, will damage occur? Plant will have some across the line high torque loads along with some large VFDs.

Thanks

 

[slavag]

Hi Hanksmith.
I always ask in this case: For what you need UV function on the main breaker. Are you have some back-up generator?
You need protect your motors, every motor or one UV relay per bus, as you want.
In additional, MCC contactors will opened by UV in about 0.7Un.
Best Regards.
Slava

 

[dpc]

I agree with Slava. NEMA requires motor contactors to stay pulled in down to 85% of control power voltage, I believe. Below that, they may drop out. At 70% voltage, nearly all of them will have dropped out. At 60%, they will be all be off.

The motors will generally not be damaged by short term voltage sags. Damage can occur if maintained ON/OFF controls are used and the motors try to start back up immediately after a short power blip. The motors should stay off until things settle down and power is restored and stable.

The VFDs will likely shut themselves down before the UV device trips the main breakers.

 

[waross]

Large motors with high inertia loads may be damaged by being quickly re-energized by re-closers. When they loose power they become induction generators. On re-energization the odds are that they will not be in sync. The motor current may exceed locked rotor current.

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

 

[hanksmith]

I didn't even think of the starters pulling out, I will contact the MCC vendor to see if they have information as to the pull out voltage of the starters.

The PLC will have to be programed to keep loads de-energized until the power system is stable after a bump and start loads on a prioritized basis.

Thanks