Voltage sag causing motor relay to trip

[philly]

I have been made aware of a problem recently where a voltage dip/sag at a facility is causing the motor overload relay to trop on overcurrent. I'm still waiting on more details but it appears that when the voltage dip occurs and the motor is running, the motor current increases and trips the motor protective relay on the 51 time delay function. The motor is a 4.16kV motor which I believe is a 200hp motor but again still waiting on more details.
ve been asked whether or not the relay setting can be adjusted in order to ride through this nusiance tripping. The relay pickup is set a 1.0X FLA b/c it was not known whether or not the motor had a S.F. greater than 1.

I'm thinking it is possible to adjust the relays time delay portion of the overload curve but I'm not sure if this will be enough to solve the problem or even the right solution Raising the time delay (while keeping it below the thermal damage point of the motor) will provide a longer tripping time for an increased current and may allow the motor to ride through but I"m not sure if this is the correct solution. The voltage dip seems to be short in duration from what I'm told, but if it lasts long in duration say several seconds, I'm not sure that adjusting the time delay would have any affect.

I'm curious if others have ran into this situation before and what they have done.

Thanks

 

[ATSman]

Check Coordination Study

Check Coordination Study

I have been made aware of a problem recently where a voltage dip/sag at a facility is causing the motor overload relay to trop on overcurrent. I'm still waiting on more details but it appears that when the voltage dip occurs and the motor is running, the motor current increases and trips the motor protective relay on the 51 time delay function. The motor is a 4.16kV motor which I believe is a 200hp motor but again still waiting on more details.
ve been asked whether or not the relay setting can be adjusted in order to ride through this nusiance tripping. The relay pickup is set a 1.0X FLA b/c it was not known whether or not the motor had a S.F. greater than 1.

I'm thinking it is possible to adjust the relays time delay portion of the overload curve but I'm not sure if this will be enough to solve the problem or even the right solution Raising the time delay (while keeping it below the thermal damage point of the motor) will provide a longer tripping time for an increased current and may allow the motor to ride through but I"m not sure if this is the correct solution. The voltage dip seems to be short in duration from what I'm told, but if it lasts long in duration say several seconds, I'm not sure that adjusting the time delay would have any affect.

I'm curious if others have ran into this situation before and what they have done.

Thanks

Sounds to me that this question should be posed to the EEOR(elec engineer of record) whether he be in house or a consulting engineer responsible for the coordination study of the plants electrical distribution system. He will need to review the current coordination study and compare it with the 50/51 protective relay pickup/ time delay settings. Their could be some wiggle room in the setting adjustment that would allow the motor to overide these momentary voltage sags but without the proper knowledge and for someone to just change settings could cause upstream circuits to trip or worse - motor damage.:slaphead:

 

[jim dungar]

What type of time curve is associated with the 51 function? A typical overload relay would provide Class 10 protection (10secs at 6X FLA). Of course it would be ideal to have all of the motor parameters, in particular# of starts, length of start and thermal time constants.

 

[Jraef]

A typical OL curve is I^2t, so if a sag is causing the trip, it must be of significant duration in which case I don't see an increase in trip time as being a valid solution. If as you say you can plot it out against a thermal damage curve for that specific motor it's worth a try, but my gut feeling is that all you will accomplish is an increased risk of motor damage.

The real issue to me would be to investigate the cause of the sag. Sags of long enough duration to cause something like this are doing other damage as well.

 

[Phil Corso]

Philly... what is the acceleration (runup) time, and what is the driven-machine?
Regards, Phil Corso

 

[templdl]

Sounds to me that this question should be posed to the EEOR(elec engineer of record) whether he be in house or a consulting engineer responsible for the coordination study of the plants electrical distribution system. He will need to review the current coordination study and compare it with the 50/51 protective relay pickup/ time delay settings. Their could be some wiggle room in the setting adjustment that would allow the motor to overide these momentary voltage sags but without the proper knowledge and for someone to just change settings could cause upstream circuits to trip or worse - motor damage.:slaphead:

Interesting that dip/sag remains to be ambiguous. Nothing was said about to what extent the the dip/sag was and as such how can the issue be addressed without knowing the extent. Providing some values would go a long way when addressing this issue, what would be allowed and what value presents potential equipment damage.

 

[ATSman]

Power Quality Survey

Power Quality Survey

Interesting that dip/sag remains to be ambiguous. Nothing was said about to what extent the the dip/sag was and as such how can the issue be addressed without knowing the extent. Providing some values would go a long way when addressing this issue, what would be allowed and what value presents potential equipment damage.

Thanks and I agree. That was the part I left out of my comments but was not successful in adding by my editing attemps. A power quality survey should be ordered using a good line disturbance analyzer to determine what templdl outlined and evaluated before an intelligent discussion can continue.