The protection system false operation

In order to make things clear, please answer what asked below.

QUOTE

In an extra high voltage substation, the output overhead lines protected by standard protection system.

Question: What are

-the voltage level?

-the substation configuration?

-the lenght of the protected lines?

-the short circuit current?

- the CTs ratios and accuracy classes? What is the saturation characteristics of the CTs?

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".

Question: What do you mean with "high quality main and backup distance protection" ?

Please remember that static relays have ben superseded by digital relays. Nevertheless, some of them still operates very well.

The CBF protective relay has connected in series to same current transformer core that connected to distance relay.

Question: Could you please better explain this sentence? It seems that something is lacking in the sentence.

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.

Question: Please explain what you understand by "operate correctly". Does the protection operate? Or it does not?

If it operates, does it according to manufacturer reccomendations?

Are the dc auxiliary services properly operating?

Who is the CBF manufacturer? Do you have a catalog of such protection?

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?

Question: Please note that a protection never fails to operate. Conversely, the application dictates the behavior of it, say, proper settings, proper system devices selection, and so on.

REGARDS

MSJ

Without seeing the coordination curves for all of the relaying, I can only hazard a guess. If CBF is a Circuit Breaker Failure relay, it will act to completely isolate the associated circuit breaker if it thinks the breaker failed to open when required. In order to isolate the breaker, the CBF would trip the next breaker upstream in every direction.

I suggest you verify the coordination and timing of the CBF. Also, test the relay itself several times, using various input currents to verify the relay curve.

I recently had a misoperation where an operational amplifier in a static relay failed intermittently. It would cause the relay to become extremely sensitive at random times, and was devilishly difficult to find. We completely disconnected and nearly disassembled a substation transformer before identifying the true cause.

Please assume all selected relays and their setting are correct.

BEST REGARDS

MSJ

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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