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Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/01/2020 2:23 AM

Dear Experts,

what may be the reason of spark at short circuit contactor contacts when we try to start the motor and with the drive command motor trip immediately on short circuit fault while spark observed at contacts of short circuit contactor. Motor stator and rotor Insulation resistance measured and found good in G-ohms, cables also checked for IR and found good. liquid starter is at Rmax position in start and Short circuit contactor is open.

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#1

Re: Short Circuit Contactor of Liquid Starter spark. motor rated 8 MW , 13.8KV

12/01/2020 4:18 AM

I guess it's possible the wires are shorted leading to the motor...

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#2

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/01/2020 11:39 AM

Not sure what you mean by “short circuit contactor”.

The “shorting” contactor of a wound rotor motor should NEVER be used in starting the motor from a standstill, the current will be enormous and likely exceed the capability of the contactor. You should ALWAYS be starting the motor with resistance, in your case through the liquid, then the shorting contactor is only used as the final final step in shorting out the resistance AFTER the motor is accelerated as far as the resistance will allow AND YOU NEED FULL SPEED from the motor. If you are engaging the shorting contactor earlier, you are damaging it.

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#3
In reply to #1

Re: Short Circuit Contactor of Liquid Starter spark. motor rated 8 MW , 13.8KV

12/01/2020 7:42 PM

"AOIP starters for slipring motors, also named electrolytic starters or Liquid Resistance Starters (LRS), use mobile electrolytic resistances. Each starter is made of 3 tanks (one per phase) filled with conductive liquid named electrolyte (water mixed with salt, usually Sodium Carbonate) and two immerged electrodes."

The points on the shorting contactor might be welded together, or the contactor is damaged possibly from overheating and is not opening fully...

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#4

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/02/2020 5:16 AM

It is not possible to carry out fault-finding without being able to see and measure the equipment is question.

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#5

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/02/2020 8:02 AM

Thanks a lot for all gentlemen for their expert advise. Actually contactor was faulty and by replacing it, problem resolved.

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#6
In reply to #5

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/02/2020 3:46 PM

So the next question is, why did that contactor fail? As I said, if you are closing it too soon, you will damage the replacement one too.

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#7
In reply to #5

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/03/2020 2:49 AM

Prevention of recurrence hasn't been addressed.

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#8

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/03/2020 5:09 AM

Dear PWSlack and JRaef

The root cause of spark at contactor was that contacts which should be remain open on startup were short circuited due to dielectric strength lost in vacuum contacts chamber. it wasn't that it was closing too early rather it was giving close contact all the time. now after replacing with spare one it is working fine.

thanks a lot all of you for your technical advise.

regards,

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#9
In reply to #6

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/03/2020 5:20 AM

JRaef: Contactor is getting close command after start time (35 sec) which is set in PLC and after getting the contact feedback of Rmin limit switch. which means starter has reached to the last step. in the issue which we observed, contactor contacts were shortened due to loss of dielectric strength hence even if the starter was at Rmax which is the 1st position, and resistance is there in the circuit but due to faulty contactor, resistance was bein bypassed and motor was drawing very high current resulting in spark and Instt. overcurrent message on protection relay.

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#10
In reply to #8

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/03/2020 7:01 AM

Prevention of recurrence still hasn't been addressed.

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#11
In reply to #10

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/03/2020 8:53 AM

Ok please guide what actions should be taken to avoid repitition.

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#12
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Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/03/2020 1:17 PM

There should be regular maintenance testing of the contacts with a log kept as an approach to failure can be recognized and corrected before complete failure occurs...preventative maintenance = less downtime and safer environment

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#13
In reply to #11

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/05/2020 12:56 PM

You must be able to test the operation of the resistor and short-circuit contactor without switching on the voltage at the motor stator terminals . This should always be done when standing for a long time or if there is any doubt about the correctness of motor starting equipment. In addition, the overcurrent protection would have to react in the event of a fault in the short-circuit contactor jamming and start of the motor with a short-circuited rotor.

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#14

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/06/2020 5:15 PM

Assuming from your description of the failure that this is a vacuum contactor (and at 13.8kV I would hope so). Loss of vacuum in the contactor is generally a matter of either a failure of the bellows (leaking) or a blowout due to having operated on a fault. A blowout in this application seems unlikely, so a failure of the bellows would be my guess.

Are you using a vacuum CONTACTOR, or a vacuum BREAKER that is being used as a contactor? The difference is that the vacuum bottles used in breakers are often not the same as those used in contactors, because breakers are not expected to open and close as much as a contactor will. So if it is a vacuum breaker, that might be the reason for a premature failure.

Or... it could just be old and worn out. They do have a finite life.

Here is some reading on the differences.

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#15
In reply to #14

Re: Short Circuit Contactor of Liquid Starter Spark, Motor Rated 8 MW, 13.8 KV

12/10/2020 12:56 AM

it is vacuum contactor and not CB.

yes the issue may happen due to elapsed life time of equipment.

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JRaef (3); Panta SD (1); PWSlack (3); SolarEagle (3); Tahir Majeed (5)

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