3 CIRCUIT BREAKER FAIL IMPLEMENTATIONCircuit Breaker Failure Protection is implemented in the CB FAIL column of the relevant settings group.3.1 CIRCUIT BREAKER FAIL TIMERSThe circuit breaker failure protection incorporates two timers, CB Fail 1 Timer and CB Fail 2 Timer, allowingconfiguration for the following scenarios:● Simple CBF, where only CB Fail 1 Timer is enabled. For any protection trip, the CB Fail 1 Timer is started, andnormally reset when the circuit breaker opens to isolate the fault. If breaker opening is not detected, the CBFail 1 Timer times out and closes an output contact assigned to breaker fail (using the programmablescheme logic). This contact is used to back-trip upstream switchgear, generally tripping all infeedsconnected to the same busbar section.● A retripping scheme, plus delayed back-tripping. Here, CB Fail 1 Timer is used to issue a trip command to asecond trip circuit of the same circuit breaker. This requires the circuit breaker to have duplicate circuitbreaker trip coils. This mechanism is known as retripping. If retripping fails to open the circuit breaker, aback-trip may be issued following an additional time delay. The back-trip uses CB Fail 2 Timer, which wasalso started at the instant of the initial protection element trip.You can configure the CBF elements CB Fail 1 Timer and CBF Fail 2 Timer to operate for trips triggered byprotection elements within the device. Alternatively you can use an external protection trip by allocating one of theopto-inputs to the External Trip DDB signal in the PSL.You can reset the CBF from a breaker open indication (from the pole dead logic) or from a protection reset. In thesecases resetting is only allowed if the undercurrent elements have also been reset. The resetting mechanism isdetermined by the settings Volt Prot Reset and Ext Prot Reset.The resetting options are summarised in the following table:Initiation (Menu Selectable) CB Fail Timer Reset MechanismCurrent based protection The resetting mechanism is fixed (e.g. 50/51/46/21/87)IA< operates AND IB< operates AND IC< operates AND IN< operatesSensitive Earth Fault element The resetting mechanism is fixed.ISEF< OperatesNon-current based protection (e.g. 27/59/81/32L)Three options are available:● All I< and IN< elements operate● Protection element reset AND all I< and IN< elements operate● CB open (all 3 poles) AND all I< and IN< elements operateExternal protectionThree options are available.● All I< and IN< elements operate● External trip reset AND all I< and IN< elements operate● CB open (all 3 poles) AND all I< and IN< elements operate3.2 ZERO CROSSING DETECTIONWhen there is a fault and the circuit breaker interrupts the CT primary current, the flux in the CT core decays to aresidual level. This decaying flux introduces a decaying DC current in the CT secondary circuit known assubsidence current. The closer the CT is to its saturation point, the higher the subsidence current.The time constant of this subsidence current depends on the CT secondary circuit time constant and it is generallylong. If the protection clears the fault, the CB Fail function should reset fast to avoid maloperation due to thesubsidence current. To compensate for this the device includes a zero-crossing detection algorithm, which ensuresthat the CB Fail re-trip and back-trip signals are not asserted while subsidence current is flowing. If all the sampleswithin half a cycle are greater than or smaller than 0 A (10 mS for a 50 Hz system), then zero crossing detection isasserted, thereby blocking the operation of the CB Fail function. The zero-crossing detection algorithm is usedP14x Chapter 8 - CB Fail ProtectionP14xEd1-TM-EN-1 185
