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Topic Title: 6.6kV motor testing following protection trip on Earth Fault
Topic Summary: Contradicting test results ?
Created On: 30 January 2016 08:23 PM
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 30 January 2016 08:23 PM
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LU56DEN

Posts: 4
Joined: 30 January 2016

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.
 31 January 2016 03:36 PM
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williamjohn

Posts: 194
Joined: 22 November 2010

3.6 kV rms ac gives 5.09 kV peak. Theoretically the fault may withstand 5 kv dc Megger test but breakdown at 3.6 kV rms ac.
 31 January 2016 04:56 PM
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williamjohn

Posts: 194
Joined: 22 November 2010

Further thought. What is the neutral earthing arrangement of your 6.6 kV supply. If the neutral is not solidly earthed, then your motor protection relay may be tripping on the capacitive current to earth with no fault anywhere.
 31 January 2016 06:43 PM
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LU56DEN

Posts: 4
Joined: 30 January 2016

Thank you for your reply williamjohn.
The motor windings also passed a 10kV Megger IR test - >3 G ohm after a minute.
The 6.6kV neutral earthing is via a NER to limit fault current.
However, the protection relay which tripped on earth fault ( 24 A recorded ) almost certainly did so correctly, as we can't get the motor to pass an AC test between windings & earth.
Incidentally, as the connection bus bars, in the motor terminal box, are supported by insulators, I tested all of these today & easily achieved 10kV AC without any leakage current.
Testing straight to the outcoming flexes of the motor winding, the same results are achieved as yesterday - earth fault.
:-(
 01 February 2016 12:30 PM
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williamjohn

Posts: 194
Joined: 22 November 2010

24 amps is not capacitive current.
If the windings are not well braced they can bend onto a fault during the high starting current and withstand insulation tests at other times. However in this case your ac insulation test would not show a fault.
 01 February 2016 05:38 PM
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williamjohn

Posts: 194
Joined: 22 November 2010

Further thought. The NE resistor on the 6.6 kV supply indicates that you have your own generators at 6.6 kV. Depending on the purchase spec, they may generate with a large third harmonic content. The third harmonic currents in each phase add and the third harmonic capacitive current to earth may be appreciable.
This is not normally a problem as a delta/star transformation kills the third harmonic component. However it can be a problem at generated voltage. I have measured over 20 amps capactive current on an 11 kV system supplied by our own generators but in this case there were several miles of 11 kV cable.

Can you measure the frequency of your earth fault current. If it is third harmonic, then you may have found the reason for the current to earth.
Best wishes
John
 01 February 2016 09:45 PM
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LU56DEN

Posts: 4
Joined: 30 January 2016

Yes williamjohn.
We are a generator.
Our UATs ( 15.75kV to 6.6kV ) are fed from a tee off between the GCBs & the GSU transformers.
We are aware of spikes on the entire system, right down to domestic building supplies, & it's known to be caused by excitation thyristors.
Other sites with the same machines by the same OEM have identical waveforms...
I digress... We think the motor circuit feeder protection relay operated correctly due to a genuine fault. Please don't loose sight that my dilemma is that I am testing a motor stator, at the motor terminal box, whilst disconnected from its incoming supply cables. Passes DC PI & IR testing up to 10kV DC. Fails AC testing tripping the test set at 3.6kV. I am at the shop tomorrow, following the motor for post mortem. Will keep you posted of findings,
which should, I hope include results from Baker testing ( surge testing )
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