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6.6kV Motor Earth Fault

01/30/2016 2:27 PM

Good evening all. I have a 6.6kV, 580kW, Westinghouse / Teco, single speed motor, tripping on earth fault. The windings are connected with an internal star point, not terminal box links. The protection relay is a GE Multilin Motor Manager 5. Earth Fault trip setting is 6A for 0.2 Seconds. At the motor, with the feed cables disconnected, PI results of 7 are achieved at 5kV DC. However, when attempting to commence an AC test, between the windings & earth, As soon as I try to apply the voltage, I have immediate leakage current which eventually trips the set where I preset the trip level to. 16 to 20 mA. I am using a 30kV capable tester. I cannot get any voltage applied with this set, & my target was initially 5kV for 1 minute. To fill in more detail, the motor has just been extremely expensively rewound, due to an earth fault. A 10kV Insulation Resistance test was carried out with a different Megger, by the repair contractor, & we experienced good results of several Gig Ohms after 1 minute. A second flash test was carried out with a different 5kV AC test set, by the repair contractor, & we experienced repeatable trips at 3.6kV. So, the protection relay agrees with 2 AC test sets, that there is an earth fault. And, the 2 IR / PI testers ( Meggers ) agree there's no fault. I know I am comparing AC to DC testing, but why are the Meggers showing nothing of concern. They should surely indicate a low reading in the presence of an earth fault ? When the motor was fitted yesterday, I did test run it for DOR purposes, uncoupled, & it accelerated up to speed, but i stopped it only a few seconds later. The relay was happy at this time. No fault evident. Operations ran the motor up last night & it immediately tripped on protection. This has "done my head in", & the motor repair contractors are equally bemused. The motor is heading back to the repair shop, where it will be Baker tested. Can anyone offer any advice ? Thanks in advance. CTB

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#1

Re: 6.6kV Motor Earth Fault

01/30/2016 2:51 PM

"the motor has just been extremely expensively rewound, due to an earth fault"

If you keep doing what you always did, you'll keep getting what you always got.

Hire a competent electrician.

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#3
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Re: 6.6kV Motor Earth Fault

01/30/2016 5:34 PM

Vulgar/Rude/Improper Behavior: This post was deleted because it did not adhere to the behavioral policies of the site. Please review Section 14 of the CR4 Site FAQ and the CR4 Rules of Conduct.

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Re: 6.6kV Motor Earth Fault

01/30/2016 6:10 PM

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#5
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Re: 6.6kV Motor Earth Fault

01/31/2016 2:28 AM

Reported as offensive

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#6
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Re: 6.6kV Motor Earth Fault

01/31/2016 9:25 AM

Off topic?

Since when has reporting foul language that would get a life time ban on any other forum deserved what amounts to a reprimand?

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#7
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Re: 6.6kV Motor Earth Fault

01/31/2016 11:38 AM

Thanks.

No good deed goes unpunished.

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#2

Re: 6.6kV Motor Earth Fault

01/30/2016 4:49 PM

Sounds like your fault protection is faulty....

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#8

Re: 6.6kV Motor Earth Fault

01/31/2016 3:54 PM

You have done the cable as well, haven't you?

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#9
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Re: 6.6kV Motor Earth Fault

02/10/2016 1:26 PM

Interesting difference in results at DC and AC voltage test coils of your motor. Even more interesting is behavior of earth fault relays at idle and under load motor which leads to a bad connection of current transformers or faulty relay .Stay on the line and tell us what happens to your motor .

Good luck !

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#10
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Re: 6.6kV Motor Earth Fault

02/11/2016 1:33 AM

Sometimes high voltage motors have surge capacitors at connecting plate tied to windings and they can be a good reason for good results in DC test and occurrence of leak current for AC testing .

Good luck

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