0.5Amps Earth Leakage current on 4.7MW motor 1

[kabos]

We are commissioning a new 4.7MW (6.6kV) induction motor. When the motor was tested on no load it was found that there is a 0.5 Amps current as Io. All connections checked & meggered (>9 GOhm), but nothing was found. Still it is showing 0.5Amps Io. Can anyone please advise from where this leakage can be & how dangerous is it to couple the motor & run?
Thanks in-advance
Kabos

 

[dccap]

Dear Mr. Kabos,
First of all where are u measuring this current? Is this reading taken from the relay. Hope connection must be residual for the earth fault element. In this condition there are chaces of some curent in earthfault element due to mismatch of the CTs. Tell about the problem in detail?

 

[electricpete]

I agree it sounds like dccap has identified the likely cause of your reading as ct mismatch in a residual-connected ground circuit. Your FLA is probably in neighborhood of 400A and no-load current ~ 80A (ct primary)? Did you measure at the relay and then multiply by the ct ratio to give primary current? If so 0.5A is less than 1%.

You might want to take a look at your protective relay settings to see how far you are from a trip.

If you want some extra peace of mind you can take a megger reading.

 

[kabos]

Thanks dccap & electricpete, The value 0.5 amps is measured via CSH 200 core balance CT connected to a digital relay. These CT's measure directly the earth fault current. So the reading is the actual value of leakage cuurent.

As for an additional megger reading, we did...& the value is good >9 Giga.

 

[electricpete]

Some quick thoughts on core balance ct:
Ideally it only cares about the current that passes within the window. But I have heard they are actually sensitive to placement of cables within the window. Cable forced against edge may not read quite as high as cable in center.

Shield currents can be a consideration also.

If you were continously passing 0.5A, I am pretty sure there would be some residual effect which would show up on the megger. So I suspect the explanation will lie in careful review of the ct.

I'm sure there are others here that can provide better advice on this. Also the power engineering forum can provide some comments.

 

[dpc]

As electricpete mentioned, the physical installation should be checked out. The cables should pass through the CT at right angles and as near the center of the opening as possible.

If the CT is on the incoming cables, proper handling of the cable shield wires is critical. Review the relay instructions for recommendations. In general, the CT must be placed over a region of the cable where the shield has been removed, or the shield must pass through the CT twice.

Also, since the output of the CT is fairly low, twisted shielded cable on the secondary is always a good idea.

Good luck.

 

[busbar]

To add to other wise advice—realize also the significant difference between digital-display resolution and analog accuracy. In this case, a major portion of the reading may be ‘way-in-the-mud’; id est, noise.