Motor Under Voltage Protection Nuisance tripping

[iphapppi]

Hi All
we have fuel oil supply pump motor rated 90 kW to supply fuel to main boiler. this motor is supplied from grid, and unfortunately the grid is often faulted by lightning strike. after the fault happened, the voltage is decrease for a long time and recover again.But our motor have already trip because under voltage protection. the setting of under voltage is 0,75Un and time delay 9s.
I want to ask whether i can increase the time delay setting or simply block the protection so the motor will not trip because of this condition and what is the best practice for motor under voltage protection setting? because if the motor trip interlock to boiler trip system because of MFT(Main Fuel Trip) and it cost a lot to re- firing the boiler again. Thanks

 

[waross]

There are four issues with under voltage.
1> Overheating.
2> Not enough torque to drive the load, which results in faster overheating.
3> When starting, not enough torque to bring the motor up to speed.
4> Drop-out or failure to pull in of contactors and relays.
Increasing the time setting is advice that we cannot give you without more information. If the motor is lightly loaded it may be acceptable. If the motor is heavily loaded you may trade a low voltage trip for an over temperature trip.
BUT
At 75% and 9 seconds I am surprised that relays and/or contactors are not dropping out.
You may be better served with an auto-transformer and contactors to automatically boost the voltage to the motor in times of low grid voltage.

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

 

[iphapppi]

Hi Waross,
Sorry for my lack of knowledge on electrical power system, what do you mean by "relays and/or contactors are not dropping out" Is It unproper to set under voltage as low as 75% ? because I later found that our Medium Voltage Motor even set on lower value at 54% with 9s time delay.

 

[davidbeach]

"Proper" is what ever works for you. If you can get those voltages for those times, you are obviously using a battery backed control power system, either DC from batteries or AC from a UPS. Using control power from CPTs in the buckets you won't get anywhere near that kind of ride through.

 

[waross]

To add to David's post; Many control relays and contactors will drop out if the voltage to the coil drops to about 75% or 80% of rated voltage. If your motors don't stall or overheat I guess those settings may work for you. In addition to the independent control power your motors may be over-sized for the load. That will give you a thermal cushion to avoid excess heating.

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

 

[iphapppi]

What is the definition of over sized motor? I have checked the operation condition, running current is about 77% of I rating. Can we say it is over sized?

 

[waross]

Yes.

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