The running indication on the DCS was missing while the machine was running - this was the complaint. Technician went and found a relay that receives a signal through CB auxiliary did not energize. He racked out the CB and applied "CRC lectra Clean" to clean the LV plug and socket and then racked in into the service.

The motor was started but no running signal was observed in DCS - the motor was stopped from an RCU at field as it could not be stopped from DCS. This time the technician racked out the breaker and put into the test position and tested it all appeared to be ok. He then racked in into service and the motor was started - it ran for half a minute and flashover occurred and as a result the upstream protection tripped. The primary side of the transformer has two cables of 3 core 185sqm terminated on it. One cable sustained some burning. The transformer had some oil leakage at the relief pressure valve.

The cable was tested IR at 5000V DC and the reading are 400GOHMS. The transformer was tested and the IR value is 2.9G OHMS. Al tests necessary wire carried and transformer is declared ok - oil tests- DGA, BD, RATIO, CORE TEST THE WHOLE LOT. The question what could have caused such a flashover on CB? Is there any body who can help

 

I don't have any good answers for you, but it sure has the making of a nightmare for me based on what you have described from a safety perspective.

--
Bob

 

Rashid.. my understanding of your problem is that:

1) A plant motor is supplied power from the 33 kV bus, presumably via a "captive transformer" whose Voltage Ratio is 33kV to some lower MV value? Or,

2) Are you talking about a 3.3 kV motor and breaker? Or,

3) Some arrangement other than described above?

Regards, Phil Corso

 

A motor running on such a high voltage usually take some time to start up, if it was still in the start up time it could have opened in this time which will mean it was under a far heavier load than normal.

Do some reading up on "current chopping"

 

Was this motor fitted with a "ZORC" Medium Voltage Surge Protector?

 

Rashid, assuming that your flash over occurred at the CB truck contacts and not within the CB itself, it seems that you had two faults actually which may both lead to the same cause. Firstly you had a missing signal from your CB, suspecting a faulty auxiliary contact, and then your CB flashed over.

The above leads me to think that your circuit breaker was not racking properly in its place. So when you were repeatedly making ON the CB, the starting and then running currents of the motor were passing through a plug and socket which were not completely home, resulting in overheating of the said contacts leading to the flashover.

Another possible cause could be that the repeated racking and unracking of the CB could have damaged one of the plug/socket connections again leading to an improper connection between CB truck and panel contacts.

One other point, you stated that the one of the cables on the primary side of the transformer sustained some burning. Is this cable somehow directly associated with the flashed CB? Could it be that (assuming that the cable is connected to the CB that failed) that the burning was caused by the heat generated from an improper connection as described above rather than by the flashover proper? Could it be that the cable termination itself was faulty (possible loose connection) resulting in the overheating of a bushing leading to the flashover?

The above are some points to look for in trying to determine the cause of your flashover.

Your technician should count himself lucky. If the CB flashed while he was racking it in, you would have an additional problem on your lap!

>Was this motor fitted with a "ZORC"
>Medium Voltage Surge Protector?

 

>>Was this motor fitted with a "ZORC"
>>Medium Voltage Surge Protector?

No the motor is not fitted with the ''ZORC''. The racking in of the breaker had already been completed and the door panel closed and the breaker was in service position ready for start. The door which was opened is the LV panel door. Please note that the CB contacts were not affected at all, both sets- the ones which go into the busbar and the ones which go into the cables side. The motor is fed via a delta/star connection - the transformer secondary star point is earthed through a resistor. All the three phases of the CB closed properly as the current flowing through the phases were all equal - this is recorded by protection relay. The circuit breaker has an interlock mechanism for which it will not closed if not properly racked in. The burning sign of the cable termination at the primary side of the transformer is due to a lose connection of copper screen strip to earth in one of the phases. This cable is connected to the CB in question. There are two cables running in parallel from the 33kV CB to the transformer which steps down from 33kV to 11kV. I suspsect there might be a loss of vacuum in one of the bottles. Do you have any experience of a case of loss of vacuum in CB?

 

Rasfid... see Control.com Thread #
http//control.com/thread/1026247457 "Flashing of Vacuum Breaker"

Regards, Phil Corso

 

Rashid, your comments re the location of the fault noted. Phil's pointer to failure of vacuum bottles will help. However I have one other small question. You are stating that the burning of the cable occurred on a cable that is AFTER the circuit breaker (at least that is what I am understanding) i.e. after the location of the fault. Again assuming that the burning was caused by the fault, how come you had such a high current passing through that copper screen? Did the fault current going to switchboard chassis (from the presumed failed bottle) try to pass (part or full) through this screen burning it, instead of passing through the switchboard chassis' proper earth connection? How is the cable screen connected at the transformer end, i.e. is it connected to ground also? If screen is connected to earth at both ends I expect some fault current to pass, but if the current was that high, then the switchboard's main earth needs to be checked.

Do you have busbar protection systems on this switchboard? If yes what type, and did they operate?

 

No there is no busbar protection and that is why we lost power on the whole board instead of only losing the section that had flashover (the cubicle that is connected to the busbar). The copper screen is earthed on both ends of the cables. I believe there was current that flew into the earthing of the switchgear body through the copper screen. This current must have come as a result of flushover between the phases and the body of the switchgear. The main earth of the switchgear was confirmed to be ok. Please note that the location where the burning of the cable occurred is at the downstream of the CB (this is where the two cables are terminated at the transformer primary side)

 

Rashid... your explanation of screen earthed at both ends caught my eye.

1) How is system neutral earthed?

2) What method is used for earth-fault detection?

(a) Residually connected Phase CTs

(b) Zero-sequence (window-type) CTs?

Phil Corso

 

we have faced same problem as like you, but after that we normalized that problem. so plz try to open the both isolator and again try to close and check its indication on a tag for closing. and try to close the breaker from local or from remote. i hope u will be success

 

Hi Phil,

The secondary side of the transformer is earthed through a neutral resistor. The primary side has a residually connected CTs to pick up the imbalance.

 

Rashid... I believe your first-step should be to confirm the type of fault that occurred!

Regards, Phil Corso