Micrologic 5.0 P Main breaker Trip 1

[adityalagudu]

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.

 

[dpc]

The photo shows a date in the display of 2005??

 

[Parchie]

An old problem revisited?
Isd is short for short circuit protection. The breaker trips because the circuit experienced a short circuit. What made you think a change in the UV trip setting changes things?

 

[mparenteau]

Isd is a short time pickup on ML trip units.

Mike

 

[adityalagudu]

The timing on the picture was wrong, as it was set improperly. This event happened on new gear, installed recently.

We don't want to trip the breaker on UV, so currently thinking to change to Alarm only. We thought UV can happen when there is a trip in the upstream line and excessive load on one transformer.

Attached are the short-time pick-up and tripping delay settings from the SE manual. Looking to know recommendations on these settings for the Main breaker.

 

[dpc]

The Isd trip does not have anything to do with undervoltage I don't believe.

 

[Kiribanda]

I am in the same opinion with dpc. Isd has nothing to do with UV. It is short time setting.
Was there a fault at downstream?

 

[FreddyNurk]

If there's a concern with proper function of the Micrologic unit (i.e. why did it pickup 7.5kA and trip for an external event) then maintenance should be done to check that the unit and breaker are operating correctly.

Changing the UV setting won't help for your case, providing as you say, it hasn't detected a UV event. There's no point in trying to adjust a setting to make it less sensitive to an event it didn't detect.
I'd be more concerned with finding out what caused 7.5kA to flow through the breaker, or why Ia, Ib & Ic all read zero but Isd picked up 7.5kA.

EDMS Australia

 

[RRaghunath]

It is possible the LV incomer trip happened after the 25kV fault was cleared and the LV voltage restored.
Could it be that the blip in 25kV system resulted in three phase voltage dip in LV system. When the 25kV fault was cleared and LV bus voltage was restored, all the motors connected to busbar started drawing large currents (could even be equal to starting currents). Such a scenario might cause operation of Isd.
How long did the 25kV fault last and what was the type of fault?
If true, you may have protection coordination issue there.

 

[waross]

Good suggestion RRaghunath. Worth looking into.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

Thanks Bill!

 

[Hill Mark]

as RRaghunath mentioned earlier, you are very likely to be a problem of protections coordinating for them
values and how the trip was.