Micrologic 5.0 P Main breaker Trip

[adityalagudu3291]

One of our Micrologic 5.0 P main breaker has been tripped due to Isd = 7500A, due to a blip in the 25KV high voltage side.

We are thinking about adjusting the Under-voltage trip settings but there isn't any history of UV trip on the trip unit.

We are trying to identify the root cause and come up with any preventive actions from happening again, if anyone had previous experience with these trip units, any input will be helpful.

 

[ron]

In case you didn't know, the Isd value is the short time pickup setting for the trip unit. Short time pickup is somewhat in the faster acting area of the time current coordination curve (usually between 6 and 30 electrical cycles on the Y axis), although not as fast as the instantaneous pickup.

The 5.0P does have an undervoltage alarm that can be enabled to trip a load shed function on the breaker, but you might want to do some troubleshooting first to understand the root cause before establishing a value. Maybe just alarms initially.

 

[xptpcrewx]

One of our Micrologic 5.0 P main breaker has been tripped due to Isd = 7500A, due to a blip in the 25KV high voltage side.

We are thinking about adjusting the Under-voltage trip settings but there isn't any history of UV trip on the trip unit.

We are trying to identify the root cause and come up with any preventive actions from happening again, if anyone had previous experience with these trip units, any input will be helpful.

I am going to take a wild guess and say there is probably a surge arrestor on that bus. If there is, check the counter. One possibility with a surge arrestor is if the MV system experiences a surge, the surge would naturally get transferred to the LV equipment, which would cause the surge arrestor to fire, momentarily drawing a high surge current and consequently trip the breaker. Note: In this case, you wouldn't find evidence of an actual fault or damage given the surge arrestor did its job and clamped the overvoltage excursion.

Not sure what undervoltage has to do with this. With a large surge current as described above (and as indicated by the 7.5-KA event), the system voltage would naturally sag, so an undervoltage condition might be what is happening after the fact.

 

[paulengr]

I am going to take a wild guess and say there is probably a surge arrestor on that bus. If there is, check the counter. One possibility with a surge arrestor is if the MV system experiences a surge, the surge would naturally get transferred to the LV equipment, which would cause the surge arrestor to fire, momentarily drawing a high surge current and consequently trip the breaker. Note: In this case, you wouldn't find evidence of an actual fault or damage given the surge arrestor did its job and clamped the overvoltage excursion.

Not sure what undervoltage has to do with this. With a large surge current as described above (and as indicated by the 7.5-KA event), the system voltage would naturally sag, so an undervoltage condition might be what is happening after the fact.

An SiC surge arrester will do this but MOV and surge cap behavior is different.

It can also be a sag or an outright single phasing condition. Any large motors will suddenly increase current draw as they stall out from a voltage sag. Common reasons for such a sag include faults (shorts) as well as sectionalizers and reclosers particularly if they are used as ties or bus transfers that operate single phase instead of ganged operation. Also a faulty AVR (can be intermittent) can cause issues. This behavior is fine with single phase loads but not 3 phase. The best solution often is either look at current imbalance, stall, or under voltage or both on large motors and trip them offline if you’d rather not lose the distribution breaker. Good quality microprocessor based trip units are under $300-400 now so there is little excuse not to use one on the larger rotating loads if you don’t have the capability already. Plus any motor over about 200 HP is thermally limited by the rotor at stall so simple thermal overload relays are really inadequate.