[Hamid]

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 mal-operation?

Topic

[Hamid]

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.

[Jagadish Subbaramaiah]

CBF protection is a breaker backup protection and provided locally, hnece it is alco called as LBB (LOCAL BREAKER BACKUP PROTECTION). Generally in India we adopt a setting of 200mA (SECONDARY) for all feeders and transformers with a time delay of 200msec (This includes Breaker opening time, Protection reset time, Trip relay rest time, Arc deionisation time and margin). For generators, current setting will be lower in the range of 50mA, time delay will be 200msec. I thought 200 msec time delay should be good enough to ensure stability of CBF. In the new CBF protections, retrip function is available which trips its own breaker again before initiaing tripping of backup breakers. Retrip time delay is set between 50-70msec. What are the currents ettings and time delay settings adopted in your application?

[Hamid]

The timing chart in Figure below reflects the timing sequence of 138 kV breakers in some substations. At Power Plants, the required critical clearing time was calcultated to be 10 cycles. The recommended total clearing time was 9 cycles as shown in Figure below. In order to achieve this total clearing time, it was necessary to purchase a breaker failure relay that had a short fault detector dropout time. The special BF relay is selected because its instruction manual reflected a dropout time of less than 1 ms.

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