Failed 15 kV Vacuum Circuit Breaker

Ronald,

from the details you have given, I am inclined to think that there was some looseness between the contact rose of the circuit breaker and the fixed contact of the switchgear assembly. This resulted in the overheating of the said contact, and the ensuing heat damaging melting the adjacent assemblies. In such a situation the actual current passing through the circuit breaker remains the same and so the protective device will detect the failure, until an insulation part somewhere in the assembly fails leading to an earth fault. Fortunately it seems that you just managed to get to the point where things started failing, but before the final catastrophic flash-over.

Suggest two things, especially if you have similar circuit breakers in your plant:

1. check the internal resistance of the circuit breaker, obviously with the CB racked out but in closed position. Compare the readings you get with manufacturer specification.

2. insert dummy fixed contacts (which should be an exact replica of the fixed contact you have in the switchgear assembly) in the circuit breaker roses, and check again CB resistance. Compare with manufacturer specification. In the event that there is weakening of the rose springs this test should indicate a higher than spec resistance.

As a final check, ensure that the CB racks in parallel and in line with the fixed contacts, and that there is no lateral stress on the rose. This will also increase the contact resistance.
 
Hi Jojo,

I inspected the phase A fixed contact (behind the shutter) inside the switchgear but found no damaged whatsoever except for a little burn mark on the inside surface of the round epoxy supports. The CTs installed around the fixed contact has no damage also. It's rather very clean in there (no soot marks).

On the other hand, the VCB contact rose (fingers?) had signs of overheating (greyish discoloration).

I disregarded this angle (loose contact between the VCB contact fingers and switchgear stationary contact) because I think it's insignificant.

Are you saying that the heating started there and radiated through the lower primary bus of the VCB and then overheated the sliding contacts, lower contact block of the bottle, strut kit, etc.?

Regards,

Ronald
 
Hi Ronald,

I am interpreting your second paragraph in your original post as indicating a severe overheating resulting in melting of CB internal components. Obviously the causes of overheating could be all along the path from the busbar side fixed contact to the circuit side fixed contact. The failed CBs I have seen practically all had loose springs on fingers as the cause of failure. But there could be other causes also.

Overheating can start anywhere along this path where there are loose connections. These could be caused either by weakened springs in fingers, loosened bolts, faulty contacts, etc. So you need to inspect this path and see where the main source of the overheat was, basically which component is mostly damaged.

My above comments assume that all protection systems were fully functional, and that the start of things was not a failed insulation component within the CB resulting in a continuous earth fault current proceeding to burning. So with this assumption the main cause of overheating would be a loose connection along the said path, which is not detected by protection systems.
 
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