The protection system false operation

[m.s.j]

In an extra high voltage substation, the output overhead lines protected by standard protection system.
That protection system contains high quality main and backup distance protection, sync/auto reclosing relays and separated static relay that specified to line circuit breaker which is named "CBF protection".
The CBF protective relay has connected in series to same current transformer core that connected to distance relay. Also because the extra high voltage power system stability requirements, the relays setting have done in tightly coordination systems. But the designed protection system sometimes couldn't operate correctly.
Sometimes all circuit breakers that connected to one of two substation bus bar (double bus bar system) are tripped due to one sever short circuit that occurred in one of output lines without any bus bar protection activity.
According to more investigation and data recording analyses, the line distance protection had operated correctly (less than 30 ms) and it had opened relevant circuit breaker in time. Also the CBF protective relay had operated too.

How can you explain the reason of that protection system mol operation?


REGARDS
MSJ

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Creative thinking is enjoyable,Then think about your surrounding things and other thought products. http://electrical-riddles.com

 

[dlgoff]

"Sometimes all circuit breakers that connected to one of two substation bus bar (double bus bar system) are tripped due to one sever short circuit that occurred in one of output lines without any bus bar protection activity."
I am not sure I follow totally but couldn't this sever short cause a transit pulse to trip the other breakers?

 

[m.s.j]

I am not sure I follow totally but couldn't this sever short cause a transit pulse to trip the other breakers?

For drawing and CBF logic diagram refer to attached file.

BEST REGARDS
MSJ

 

[m.s.j]

Is existence of CT secondary current possible without any its primary current?
Yes it is possible in transient duty. That current named decay or subsidence current.
The subsidence current is the current that flows through a CT burden after the line breaker opens.
As you know, the circuit breaker failure over current relay (50BF) should be fast pickup and dropout relay. The subsidence current may affect the dropout time of breaker failure over current element. If the 50BF element is picked up the breaker failure time delay, other breakers must trip to isolate the failed breaker.
CT subsidence current keep the 50BF element asserted longer than necessary and may contribute to a false breaker failure operation in tightly coordinated systems.
Nowadays some digital base protective relays are able to detect CT subsidence current.


BEST REGARDS
MSJ


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Creative thinking is enjoyable,Then think about your surrounding things and other thought products. http://electrical-riddles.com