400kV disconnection sequence

[stratford]

Dear all. On iste we have a three winding step-up transformer connceted to the grid of 400kV via a disconnector and a circuit breaker. The disconnector is between Step-Up transformer and circuit breaker. When we de-energize from the 400kV grid we are instructed to open first the circuit breaker and then open the disconnector in a period of 10 seconds maximum after the opening of the circuit breaker. I cant quite understand this logic. Can you help me on that?

 

[cuky2000]

In contrast to circuit breakers, standard disconnect switches are designed with very limited or none interrupting capability. Therefore, circuit breakers should be open first and let drain the trap charges current stored in lines and other part of the system.

The reconnection sequence should be in reverse mode, first open ground switches (if any), close phase switches and then close circuit breakers to connect the system. This should be consider even for energize unloaded transformers and transmission lines.

Even thought some switches may be equipped with attachment such as arching horn, Quick-break whip device or vacuum interrupters to provides limited load-break capabilities the primarily function of disconnect switch is for visual isolation of energized parts.

See the enclosed link video of a 115 kV disconnect switches equipped with some of the attachment described above:

http://video.google.com/videoplay?docid=-2696382918978422965&q=disconnect+switch

 

[stratford]

Dear all, my main concern is not if i open first the breaker and then the disconnector, i know that. My concern is when i open the circuit breaker if i have to open immediately the disconnector (in a period of 10 sec) as per manufacturer recommendation.