6.6kv VCB

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

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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

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].

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.