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MV Motor ground fault protection setting

MV Motor ground fault protection setting

MV Motor ground fault protection setting

A bit of confusion I need clarification on.  We have 5 kv starters with Multilin motor protectors that have ground fault protection. There are two CT inputs, one a 1 Amp input for a 2000/1 ratio CT and another for a 5 Amp CT input (donut type).  Given a typical HP range in our plant of 800 to 1250 HP, how do I determine what is the best CT arrangement and primary current trip setting to protect my motors from significant damage.  We have resistance grounded 5 kv busses if that is significant to setting.

RE: MV Motor ground fault protection setting

I am not sure of the actual relay you have, so my comments apply to the 239 relay, as referenced by Jbartos.

The method of system grounding at your plant is very significant.  The ground fault protection system that should be installed will depend on how much ground fault current is available. So, what sort of grounding system have you got and what is the available ground fault current?

If you have high resistance grounding then you will have very low levels of ground fault current available (and the fault current will essentially be the same irrespective of where the earth fault is).  For this, you should use the GE core balance CT, which has a ratio of 50:0.025A.

If you have a system with low resistance grounding that limits the ground fault current to 400A then you can use a 5A core balance CT and a setting of 10% of the motor full load current would be reasonable. In this case, you could use a core balance CT of 100:5A or 200:5A ratio.  If you can not get a 200:5A CT, then you could possibly use a 100:1A core balance CT, but in programming the relay, you would need to programme the ground fault CT ratio as being 500:5A and the ground fault setting appropriately.

You do not have to use a separate core balance CT if you can live with a ground fault setting that is 20% of the phase CT.  For this, you connect the residual current from the phase CT's into the earth fault CT input.

What sort of ground fault setting do the other motors in your plant have?  Are they all the same, irrespective of motor size or are they a fixed percentage of the motor full load current?  It may pay to follow what is already being done (provided it is OK!).

Hope this helps.


RE: MV Motor ground fault protection setting


Go with Jbartos and bigamp suggestion, I use multilin relays also, almost 128 relays (all types from 469,489,750,760), and in my experience GE Multilin has good Product support, if stiil stuck try to call them.

RE: MV Motor ground fault protection setting

For low resistance grounded systems (400-1200 Amps ground fault), we typically use a 50:5 zero sequence CT and set the Multilin relay to trip at 0.2 x CT (10 amps primary) with 20 ms delay.  This usually works with no problem, but occassionally we run into one that must be set with slightly higher pickup and/or time delay to get past the inrush transients.

RE: MV Motor ground fault protection setting

To add one comment to jwerthman's suggestion, a long-standing precuation is to route the phase conductors symmetrically and tightly bundled through the CT core to represent a truly accurate zero-sequence current.  This increases security of the relay function while not sacrificing realiability.

RE: MV Motor ground fault protection setting

Good comment busbar.  That can definitely be a problem, particularly with sensitive settings.

RE: MV Motor ground fault protection setting

Suggestion: The ground current sensing CT has to be placed over the conductors that are not shielded or over the motor feeder cable that is not shielded, since the shielding prevents the CT to sense the common mode current in conductors/cable flowing into ground.

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