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6.6kv VCB

01/13/2017 2:43 PM

Hello everyone, I am unable to find the cause of problem. Here is the scenario. A 500kW, 6.6kv HT motor is getting supply from two vcb, one for forward motor rotation and other for reverse rotation. Inside these vcb's, there are HT fuses to protect the motor. Control circuit is designed in such a way that both of these vcb's can't be switched at the same time.

Now, the problem is one of these power fuses from both the vcb is getting burned, interestingly these fuses belong to the same phase. To track down the problem, I cut the vcb pickup supply from one vcb. But the problem is still the same.

It is sure that the problem is in power circuit not the control circuit. I request you all for valuable suggestions. Thanks

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#1

Re: 6.6kv VCB

01/13/2017 3:50 PM

You need to meg out the windings to check the insulation.....

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#2
In reply to #1

Re: 6.6kv VCB

01/13/2017 3:58 PM

Yes, megger the motor windings. Do not attempt to megger the supply side, though, or it is very sparky and smoke will be emitted.

Fuses blow for one and only one reason - excessive current.

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#3

Re: 6.6kv VCB

01/13/2017 4:29 PM

A fault is occurring to earth on the motor side of the VCBs.

The fault could be in the switchgear enclosure, the cables or busbars to the motors, the motor terminal box, or the motor itself.

You have not written if fuse blow occurs, instantly, every time you close a VCB or if it happens some time after closure.

An HV fault which can blow a fuse must leave black marks or melted metal where it occurs [could be hidden in a cable]. You must isolate the VCBs after opening, using the off-load isolator between them and the supply. Then discharge all the motor side with test voltmeter, followed by bonding each phase to earth with earthing leads, before visual examination of all items. Be aware that a voltage can re-appear on insulated items when an earth is removed, due to "dielectric polarisation".

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#4
In reply to #3

Re: 6.6kv VCB

01/13/2017 4:48 PM

Well-stated sir! Good points on working with these more safely.

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#5

Re: 6.6kv VCB

01/13/2017 9:24 PM

Vacuum circuit breakers used as motor controllers are well known to cause a phenomenon called "chopping current" which causes a severe voltage spike that can damage motor winding insulation. Here is an excerpt from the paper linked below:

  • Therefore if the current chop is .9 of an amp and the surge impedance is 3,000 ohm’s, the voltage transient will equal 2700 volts on top of the RMS system voltage whether it be 4160 or 5KV. However if the current chop is 5 amps times surge impedance of 3,000 ohms, then the voltage transient can equal 15KV on top of the RMS supply voltage.

Chopping current is exacerbated by the small distances used in vacuum bottles, but the bottles used in vacuum CONTACTORS are specially designed to mitigate it. Not so with vacuum CIRCUIT BREAKERS, hence vacuum CIRCUIT BREAKERS should not be used in lieu of vacuum CONTACTORS except in cases where the motors are run continuously and almost never shut down, or where there is no alternative. In the latter case, there are other (expensive*) precautions that should be taken then to help mitigate the effects of chopping current..

http://www.joslynclark.com.cn/up_files/myupload/current%20chopping-white%20paper(databinuser-2012-6-18-18-00-46).pdf

So most likely you have an insulation failure in the motor winding somewhere as others pointed out, caused by having used VCBs instead of contactors. A simple phase-to-earth megger test will NOT detect that if it is a phase-to-phase short or a turn-to-turn short in the windings of one phase, which from the description is a likely suspect.

*expensive compared to just using a contactor...

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#6

Re: 6.6kv VCB

01/14/2017 12:30 AM

Thanks a lot everyone for valuable inputs. I am attaching this pic of the circuit for more clarity on the issue.

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#8
In reply to #6

Re: 6.6kv VCB

01/14/2017 3:11 AM

Thanks for further information.

The Circuit code VC suggests you have Vacuum Contactors - not Vacuum Circuit Breakers VCB - true??

"Red" notes on sketch very difficult to read. First one not intelligible.

Second red note appears to be "This failure happened only upon starting motor then happened in running".

This would suggest progressive damage to insulation, which first broke down only with switching voltage spikes but developed to failure at normal voltage.

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#9
In reply to #8

Re: 6.6kv VCB

01/14/2017 3:47 AM

Yes that's vacuum contactor not breaker, my mistake. Here I am attaching the actual drawing of equipment. The first red note points the fuses which are getting burned.

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#12
In reply to #9

Re: 6.6kv VCB

01/14/2017 4:12 PM

Thanks for the circuit diagram.

It appears there are at least overcurrent protection relays, one per phase, fed by the "protection" CTs, and a residual relay in their common return, essentially "earth fault".

Can you confirm no protection relays have operated?

You have not mentioned such operation. It is possible a fuse is quicker operating, but it would be as well to check the relays are working at the required settings.

You may give fuse rating, CT ratios and relay settings if you wish.

You do appear to have surge arrestors at VC load side and at motor. It would help if you gave type numbers for devices SA1 and RC1 and any data on their serial plates.

My feeling is that you should carry out a close physical examination of all motor side HV gear as suggested, concentrating on the faulty phase. A mirror to view behind items is necessary.

Also use an HV insulation test set [first check the motor maker's recommendation for type and maximum voltage to be applied] first on everything of faulty phase, then splitting connections to narrow problem area, if high leakage found.

I wonder if you have a condensation problem. Usually, motors and other enclosures have heaters, which should be on when the motor is shut down. I have several memories of satisfactory & proven line cubicles and bus ducts which gave flash-over trouble because the heaters were not working, or not everywhere they should have been. Localised heating, even by the sun on an enclosure, can circulate moist air to (relatively) cold parts where vapour condenses.

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#15
In reply to #9

Re: 6.6kv VCB

02/09/2017 12:15 AM

Have you mega your motor winding insulation? If the motor winding insulation is conform no problem then you may want to check or replace your surge arrestor. It seem that your red phase fuse blown off for the forward and reverse VCB and both of these fuse, are link to the Red Phase surge arrestor. The other ways is to interchange both of the surge arrestor position and monitor again if the same phase fuse blown off again. From there you can able to detect where is the faults come from.

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#16
In reply to #15

Re: 6.6kv VCB

02/09/2017 12:30 AM

Thanks a lot for valuable suggestion. I have placed two separate surge arresters for both contactor. The system is under test since past 10 days. Till now it's working fine. We have set the testing time for a month. I will update this thread after testing. Hope it will work. Thanks you all once again.

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#7

Re: 6.6kv VCB

01/14/2017 12:42 AM

6.6 kV system is usually ungrounded.

If the fuse is suitable for inrush current at motor starting-including capacitor presence- if any- then a single short-circuit phase-to-ground does not produce sufficient current in order to blow the fuse. Has to be another point grounded on another phase which stay undetected for long time. It is weird one single phase is blowing out always since the current has to be the same in both phases. That means the second grounded point has to be on the supply side where a higher current protection device is provided [ or delayed circuit breaker switch, not fuse].

I agree the short-circuit in the motor circuitry could be on the cable termination or even on motor insulator. Opening the terminal box could show if it is an insulator damaged.

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#10

Re: 6.6kv VCB

01/14/2017 2:02 PM

If the phenomenon happens only rarely and at start only in my opinion, the fuse is undersized. You have to go up or change the fuse to a time-delay type. NEC indicates[ for up to 1000 V motor] 300% the motor rated for non-delay type or 175% if it is delay type fuse. However you may go up if it is not sufficient for starting current.

According to Schneider Electric Cahier technique no. 128 Design and use of MV current-limiting fuses Fig.19 a motor of 6.6 kV 500 kW of Istart=6*140=840 A has to be provided with 200 A fuse in order to withstand this current about 10 sec.

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#11
In reply to #10

Re: 6.6kv VCB

01/14/2017 2:07 PM

Is there a chance of wrong selection of surge arrester? Currently we are using RC type surge arrester.

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#14
In reply to #10

Re: 6.6kv VCB

01/17/2017 12:41 PM

Sorry. It is difficult a little to read the chart digits. However, if it is increased enough then

the current of 500 kW /6.6 kV induction motor it is only about 50 A [may be 54 A calculated] and

for 10 sec start time the fuse has to be 125 A rated.[NEC is about 100 A].

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#13

Re: 6.6kv VCB

01/16/2017 10:00 AM

I'm unfamiliar with many IEC symbols, but are those surge arresters or are they capacitors? It almost looks as though you have surge caps at the motor and PFC caps below the contactors. When you have multiple capacitors in a circuit, you run a risk of resonance. Even if they are both surge arrestors and those are RC networks for that purpose, the same thing can happen. I see no reason for having them in there twice no matter what.

That aside, the fact that your fuse is blowing on the same phase from the motor perspective points to an issue in that side of the circuit, like a pin hole leak in a motor lead conductor or a turn to turn short in a winding.

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