Motor Differential Protection
Motor Differential Protection
(OP)
Our team is discussing the need for motor differential protection on a 3000 HP motor, 4.16 kV. There is high resistance grounding, 10 A, which is important since a stator ground fault, is low current, and can be cleared quickly by the MPR (motor protection relay), without any time advantage of differential protection. Likewise, high level or solid 3L, LL faults and unbalance can be cleared quickly.
What we are left with appears to be low current LL stator winding fault, with a high impedance in the fault circuit, if that is possible. Does this sound correct?
What we are left with appears to be low current LL stator winding fault, with a high impedance in the fault circuit, if that is possible. Does this sound correct?





RE: Motor Differential Protection
Don't look at the cost of protection compared to the cost of the motor, look at it as an insurance policy for your process.
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If we learn from our mistakes I'm getting a great education!
RE: Motor Differential Protection
RE: Motor Differential Protection
I'd definitely want a stator diff relay on a motor that size, and with the (relatively) low cost of a multifunction relay these days it's an even easier decision.
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If we learn from our mistakes I'm getting a great education!
RE: Motor Differential Protection
Using the core balance scheme on six-lead motors, though, may present some problems with long leads and low-saturation CT's.
old field guy
RE: Motor Differential Protection
RE: Motor Differential Protection
The analysis of the original post makes sense to me. In theory a ground relay should be able to detect anything that a differential relay detects except a phase to phase fault ( at what sensitivity it can be detected may bear further discussion... but seems to me a core balance CT might be more sensitive to ground fault than differential CT).. Phase to phase fault without phase to ground seems very unlikely....and if it should occur it would surely be in the endwinding... no core to worry about there.
So I agree it's an intriguing question. Here's what IEEE 37.96-2000 says on the subject:
So... the decision on whether or not differential protection is required seems to depend on rating of motor compared to transformer!?! Is it related to short circuit characteristics of the supply? I doubt it. Related to some kind of selecting coordination of motor tripping compared to transformer tripping? Doens't quite seem right. I have to admit I can't quite make sense of it at the moment.
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RE: Motor Differential Protection
"Where the starting current value approaches the fault current (motor kVA greater than half of the transformer rating), differential relays should be used."
So, maybe they are worried that the instantaneous overcurrent cannot be set tight enough and a phase to phase fault might occur at some level below the instantaneous overcurrent trip?
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RE: Motor Differential Protection
Scotty - They turn around motors after full rewind in one week in UK ?
Muthu
www.edison.co.in
RE: Motor Differential Protection
Muthu
www.edison.co.in
RE: Motor Differential Protection
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If we learn from our mistakes I'm getting a great education!
RE: Motor Differential Protection
Muthu
www.edison.co.in
RE: Motor Differential Protection
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If we learn from our mistakes I'm getting a great education!
RE: Motor Differential Protection
Wish I had clients like you. :)
Muthu
www.edison.co.in
RE: Motor Differential Protection
(I dont' doubt there is a scenario since as I stated above there is conventional wisdom... but the scenario is not clear to me).
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RE: Motor Differential Protection
To make it a little more satisfying for myself I add the following: Perhaps adding differential provides redundancy and diversity with other protection which increases the reliability and quickness of response. I could buy that, since response of relays to faults may not be 100% predictable.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Motor Differential Protection
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If we learn from our mistakes I'm getting a great education!
RE: Motor Differential Protection
I think the philosophy of differential is from days prior to the above, where it was also either solid ground or low resistance grounded system, and old style relays. I think the damaged stators some repair shops report are from these types of systems.
All I can see, concurring with electricpete, is redundancy. Relays do fail. I saw in a generation plant, a failure, but they had redundant relays from different manufacturers.
But if the relay fails in my case, maybe even that core balance diff out at the motor will go to a failed relay.
A better alternative may be to use a redundant relay at the switchgear.