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Circuit Breaker Fail

Contents

1 INTRODUCTION ....................................................................................................................................2

2 RELAY SETTINGS ................................................................................................................................3

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Circuit Breaker Fail

1 INTRODUCTION

The purpose of circuit breaker fail protection is to respond to the situation of a stuck circuit breaker whichfails to clear the fault current for which the protection system has issued a trip signal. General causes ofthis failure are due to the electro-mechanical assembly and complexity of modern circuit breakers whichemploy hydraulic, pneumatic and mechanical operation.Circuit breaker fail protection is initiated from the Ohmega protection trip signal and also from a statusinput driven by an external protection. The Ohmega also includes a blocking feature initiated by a statusinput.The protection is aimed at high voltage plant protection systems giving high-speed operation and systemsecurity to minimize faults remaining uncleared.The relay continuously monitors the line currents from which the duration of any fault current can bedetermined. The Circuit Breaker Fail protection provides two time delays; a re-trip trip signal for re-tripping the failed circuit breaker; and a backtrip signal used to trip another breaker(s) in the systemproviding total fault protection. The re-trip output provides the opportunity to re-energise the failed circuitbreaker trip coil through separate trip circuits. The time delay OFF setting removes the led indication anddisables that function.Trip decisions made at the end of either of the circuit breaker fail time delays, depending upon phaseovercurrent elements which indicate whether the circuit breaker has failed to trip.

The circuit breaker fail logic corresponds to the conventional relay scheme where timers for re-trip andbacktrip are initiated by the trip output or external input. The timer will drop off if the initiation stimulus isremoved. At the end of the time periods the current is checked. Both the auxiliary and the characteristicenergising quantities are required throughout the time delay period for an output signal to be given. Ifeither of these signals disappear before the completion of the time delay, the function will resetpreventing an output. This logic determines that marginal fault conditions could remain on the system if abreaker failed to trip. Specific applications should be reviewed to establish the required settings toprovide system security.The circuit breaker fail algorithm enables high speed and consistent operation; and high speed resetwith minimal time delay overshoot avoids unnecessary trip decisions.Each phase current is continuously processed over a rolling cycle of samples and the rms contentdetermined. The resultant current level is used to determine trip logic to the circuit breaker fail algorithm.Each phase overcurrent detection is identical.

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Circuit Breaker Fail

2 RELAY SETTINGS

CBF- Allows the CBF function to be enabled or disabled

CBF Setting (0.05-2, 0.2xIn) basic overcurrent setting used as check that line is live.

CBF ReTrip Delay (0-60000, 80ms) This time delay applies after trip initiation until the CBFReTrip output is activated

CBF BackTrip Delay (0-60000, 120ms) This time delay applies after trip initiation until theCBF BackTrip output is activated

Status InputsCBF External Trip allows allocation of a status input(s) to this function.

CBF Inhibit- allows allocation of a status input(s) to this function.