Earth Fault in 6.6KV Motor

What does the motor manufacturer have to say on this topic?

i mentioned the motor manufacture "ASI Robicon"

What type of starting control & protection are you using for this motor?

How did the phone call with them go?

http://www.macraesbluebook.com/search/company.cfm?company=346513

Come back here after the phone call, and discuss, then.

How sad and frustrating!

Blocked by a gatekeeper most likely.....

Keep trying I guess...

There may be a back door in. You could pretend (or maybe not) to be shopping for a new motor which may get you through to a pre-sales engineer and a productive conversation could ensue. Sometimes you have to be a little devious to get satisfaction.

It is impossible for us to diagnose your motor's problem solely from a verbal description without being able to actually inspect and test the motor. You need to get a qualified electrical testing contractor to come test and evaluate your motor.

4 megohms isn't great insulation resistance, but it isn't bad enough to cause 42 amps of ground current. There must be someplace where the insulation has been compromised or damaged, which (now that the rotor is removed) is not reading in the megger test. Moisture in the insulation may be the cause of the 4 megohms, but it isn't necessarily the cause of your EF trip. Heaters should help bring the moisture down, and a low voltage current source has been used in the past to heat windings to drive water out of insulation.

However, you need to have a detailed inspection of the insulation done to find where the fault is. Not only should you have a full set of megger tests done (phase-to-phase and phase-to-ground), but also winding resistance tests to see if you have shorted turns in one or more of the windings. A hipot test may cause the fault to show itself again (at the cost of having somewhat more damage to repair). It's there somewhere, but you need to have someone qualified investigate.

The question is, did they use a Megger or an Ohmmeter?

I haven't seen either mentioned up till you posted.....

Like PeterT said, 4MΩ,6.6kV and 42A doesn't "add up". R=V/I.....you're looking for something under 1kΩ

Did you check the rotor windings?

OK. It would be useful to know how the original poster got on with a telephone call to their technical helpline.

check the system of the earth fault . the insulation resistance 4Mohm is not enough.

I'm neither an electrical or motor engineer but, you say you have carried out insulation resistance tests on the windings, but what windings? Presumably you tested the stator windings but the windings of the rotor? If they are shorted internally to earth then the motor will be a transformer with shorted secondary windings. ie draw loads of current through the primary windings and have no output. If the stator is good then I suggest the rotor is faulty and earthing through the bearings. Is there any signs of burning in the bearing surfaces or lubricant? If you have been able to check both sets of windings then you could still have a dynamic fault on the rotor causing it to short when rotating. Just a thought from somebody with no related knowledge.

You have stated that the relay is set for 4.0.A Instantaneous E/f current detected is 42 A.How did u find this.The relay acts if set correctly .When u check the winding you have not found any heating marks?IR value appears good.Have tried again to renenergise the motor?It looks as if the relay has acted and prevented a damage.We presume you are referring to a cage rotor when u say induction motor not a wound one.Try starting the motor without the load.Could have been maltripping due to a spike in voltage.But make sure by doing IR value ,high voltage test and PI value before doing a no load running.

Dear emadshaaban, as you said motor is 2.6MW 6.6KV induction type.I would like to bring your attention on the ground and insulation test.The intrument you are using that may be not sufficient for testing the insulation of motor,because you are using very high voltage (6.6KV).So you need much more voltage of same voltage you are using for motor input supply,then you can check the insulation resistance,I think you know that insulation may get failure when high volatage is applied ,then at least we should test the insulation at more than operation voltage.

thanking you.

and all the best.

Good point even though it is obvious.

If OP could provide a summary of his test procedures, instruments used and how the values quoted were obtained it may shed more light.

You only need one "aha moment" to get to the bottom of this.

High voltage withstand test is different than the Insulation resistance (IR) measurement. In fact high voltage withstand test is considered successful only when after high voltage withstand test, Insulation resistance measurement (Polarization Index i.e. PI) is acceptable. IR can be measured with suitable megger depending on the rated voltage of the equipment under test. For a 6.6 KV machine, 500 or 1000 volt megger is good enough. In fact for a good machine , IR is independent of value of the voltage. Higher voltage megger gives lower value of IR if the surface of the test equipment is contaminated resulting in surfacing tracking. As far high voltage with stand test is concerned, relevant IEC or national standards specify the value of the test voltage for new machine. For repaired machine it is up to the owner and the repair agency to test the machine at mutually agreed voltage.

If you have the correct test instrument for testing 6.6KV insulation my question is Have you tested the cables supplying the motor to make sure there is not a fault in the cables?

How would that give an earth fault at the motor? An earth fault in the cable will be making things happen at the distribution board/motor controller.

As you stated about the earth fault current measurement of 42A when the motor was connected & operating, compared with the insulation test result 4 megohms after it was disconnected.

"4MΩ,6.6kV and 42A doesn't "add up". R=V/I.....you're looking for something under 1kΩ"

The OP doesn't state where the earth fault current was measured, Isn't the earth fault current monitored/ detected at the distribution board/motor controller while the motor is in operation? I assumed that would be the case & I thought there could possibly be more than 1 problem that caused the 42A earth fault current & that could have been a problem with the cable.

The protection relay is in the distribution board or wherever but the CT talking to the relay is wrapped around the motor's earth strap.

Can the OP confirm this please. Is it different?

where is the CT wrapped around the earth strap going to the motor. Would it be in the motor terminal box or would it be in the distribution board where the earth strap originates from ????

If it's in the terminal box of the motor it is definitely not the cable but if the CT is in the distribution board where the earth strap originates from, it could be sensing a fault in the cable but I could be wrong. It was just a thought of why the calculations of the fault current & the insulation resistance do not work out as you & peterT had stated previously

...and you might be right too! No wrong answers until the OP provides additional information.

Actually OP has been pretty good so far. If he could paint clearer word pictures with more detail it would help the helpers.

yes it would be great for the OP to comment on our discussion

It is futile on our part to talk to a wall. It will be best for all of us to wait for feedback from the OP.

Dear all

many thanks for your time and advices .

first i wonaa to confirme that the mentioned motor is Squirrel cage induction motor not wounded .

second we tried to use Hipot testing ,where it goes to failure in 5sec with 14Kv.

also today we cleaned the winding using ISO probanol,so we ge inprovmet and the winding insulation gives 7Mohm.

now we use injection current to dry the winding but we did not get any improvments

When you suspect earth fault cleaning may not be of any use .Are u using the motor with a V/f drive? what is the load?

Please see #5

"we use direct online starting protected by Sepam 1000 relay"

This should have churned out lot of useful forensic data on the fault condition.

Why is the poster beating in the bush?

Non-realisation of the proverbial 'bird in the hand' would be one possibility...

Thank u sir.I missed it.Thanks for pointing out.

Dear sir

i after i get improvment in insulation resistance by using solvent cleaning up t0 12Mohm.we connect welding machine in series with the winding but >>>

after 10min, i did not get any heatup in winding and i heard sounds (tic--tic) comes out from winding and insulation test gives 0 hom.

1) why the winding cann't be heated up (winding resistance PH 2 PH is Okay)?

2) what the causes that make the winding to decrease ?

SOS

What is the output of welding machine AC or DC? Please stop all troubleshooting work forthwith. Please seek professional help at your place expediously.

I think you just burnt out the motor winding(s).........

why?? i thing it poor insulation was that cause to break we i inject motor rated current

Please see your post #34 where-in, you had got insulation of 12 M ohm before your testing of motor. Welding machine can deliver over 100 + DC Amps, depending on its setting which is fatal for AC windings. 12 M ohm is not poor insulation.

Didn't get warm? "Tic tic" sounds like heating somewhere even if it is spot boiling of the varnish!)Was the ground isolated or connected to one of the welder terminals?

Did you start off on low current and slowly wind it up under observation?

How many DCamps did you measure during this exercise? (or if is it an AC welder then the ACamps?)

What was the winding continuity resistance post (and before) injection?

It would be nice to know what really happened inside. Will you be sticking your nose in at the motor repair facility?

Sorry for all the questions......

I think it was a very expensive experiment I hope they don't garnish his wages to pay for his mistake. He didn't know the difference between applying AC or DC to the winding so I doubt he has the knowledge to give you any accurate information.

I suppose we'll find out if the cable supplying the motor has any faults when he hooks up the rewound motor & tries to start it, maybe we'll get another post asking for help. If it does have maybe it'll fry the cable & he'll have an exciting tale to tell.

He has sent the motor for rewind, he says, so I guess it will be in the hands of an expert now. However, connecting a welder to the windings would surely only burn a short circuit. The output voltage of the welder will be well below the rated voltage of the motor and would therefore push negligible amperage through the windings unless there was a fault. As I mentioned in my first reply, I am no engineer or electrician but unless I'm missing something here, why are assorted posters saying things like "you just burnt out your motor"? I am at a loss to know what testing with a welder would achieve though.

42 amps is significantly well above the allowable insulation leakage limit at machine rated voltage.

Insulation leakage is normally measured in microamps not amps.

Based on your numbers you have a far greater fault conditon(s) than just wet insulation.

The recorded fault current of 42 amps indicates your impedance to earth ground was around 157 ohms when the GF protection initiated.

The motor winding conductors could be shorted to ground inside the stator slot(s) and would in no way be visible by physical inspection.

What value of voltage did you perform the test at?

DC or AC test set?

The amount of fault current is directly proportional to the magnitude of test voltage applied and the type.

A result of 4 Megs @ 500 VAC is quite different than 4 Megs @ 7KVDC

You really need to obtain the OEM engineering requirements for testing the machine and follow them closely.

To me it sounds like you are unsure about how to administer/execute a proper, safe MV test on your equipment.

For safety reasons and to obtain the best analysis of your problem you would be wise to seek qualified, professional help from a reputable motor repair service and/or the manufacturer.

If you will accompany the qualified representative during the test, ask questions, and record/remember the steps you can learn a great deal about proper, safe test methods and eventually take care of the equipment testing on you own.

Lastly;

Using a low voltage souce such as a welding machine can be a viable method of drying out a machine winding.

The current must be very carefully controlled otherwise you will suffer damage to the winding insulation from overheating/exceeding the insulation temperature rating.

The welding stinger would be connected to one phase conductor and the ground lead would be conected to a different phase conductor then rotated periodically to the next combination at timed intervals.

You must monitor the motor winding temperature closely or suffer the consequences.

After all discussions question still remains as to how it was decided 42amps E/f current passed through.If Instantaneous relay set for 4.2 amps has acted then let through current cannot be of that magnitude. Considering the rating and eff of 95% for such motors the normal operating current at Full load is around 239A.As long as the heating current was less than this (orDC equivalent) he should not have had any problem.Some info is missing

Combined with proper procedures and consideration I reckon a welder is just fine for supplying current for heating and drying windings. DC or AC.

As long as the operative "Combined with proper procedures and consideration" is followed, it is accepted. The question here is, was it so in this case?

Hard to say without input from OP.

My impression is that he is reasonably competent. Competent enough to learn anyway.

Dear all

interestedly i read all of your replies,thanks for ever one tried to give me a help.

regarding to your questions?

1-i measured the earth fault current by multifunction protection relay (Sepam 1000+)

2- during using of welding machine for dry. the winding temperature never increase although we i measured current flow 260 and the winding resistance was 2ohm for 3 winding .why?

DC sees only resistance of 2 ohms. This current of 260 A can melt a 7mm steel rod instantly. It will not give you any time to observe any thing & immediately it goes open circuit.

260A through 2Ω. There was 30kW of action happening somewhere....

Dead short in the windings is what that looks like. But where is the heat? Are you sure about the measurements? When it failed permanently did the current drop to zero?