GE B90 Instruction Manual

Also see for B90: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

5-162 B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUALGROUPED ELEMENTS CHAPTER 5: SETTINGS55.6.4.2 Stage 1: InitiationThe breaker failure scheme is initiated by a FlexLogic operand representing the protection trip signal initially sent to thebreaker. This signal does not include other breaker commands that are not indicative of a fault in the protected zone. Theinitiating signal should be sealed-in if primary fault detection can reset before the fault is cleared. The seal-in is supervisedby current level, so it resets when the fault is cleared. If desired, an incomplete sequence seal-in reset can be implementedby using the initiating operand to also initiate a FlexLogic timer set longer than any breaker failure timer, whose outputoperand is selected to block the breaker failure scheme.Breaker failure scheme can be initiated either directly or with current level supervision. It is particularly important in anyapplication to decide if a current-supervised initiate is to be used. The use of a current supervised initiate results in thebreaker failure element not being initiate for a breaker that has very little or no current flowing through it, which may bethe case for transformer faults or ring bus where unequal current division between ring bus circuit breakers can delay thefailed breaker clearing time until the healthy ring breaker has operated. For those situations where it is required tomaintain breaker fail coverage for fault levels below the BF1 AMP SUPV PICKUP setting, a current supervised initiate shouldnot be used. This feature should be used for those situations where coordinating margins can be reduced when high speedreclosing is used. If this choice is made the current supervision pickup level must be lower than the minimum fault currentflowing through the breaker.Immediately after scheme initiation it sends again a trip signal to the breaker (re-trip). This is intended to reduce thepossibility of widespread tripping that results from a declaration of a failed breaker.5.6.4.3 Stage 2: Determination of a breaker failure conditionImproved flexibility is achieved by providing three timers or stages (Timers 1, 2, and 3) as shown in the Breaker Failure Logicdiagram. Each stage is individually enabled and asserts its individual operand.Only a breaker auxiliary contact (fast-operating) or current supervision (if enabled) can reset Timer 1 (early stage). If Timer1 expires, it implies that current is still flowing or the breaker (position status) is still closed; that is, the breaker failed toopen. The Timer 2 (main stage) logic is not supervised by breaker status, only by current. An output is issued if current isdetected after the Timer 2 delay interval. This stage detects the condition where a breaker opens mechanically but fails tointerrupt fault current.The HISET and LOSET settings discriminate between the pre- and post-tripping current variation for breakers that employopening resistors. If the current level is between HISET and LOSET, a time delay (BF x LOSET TIME DELAY) is added. Forcurrents above the HISET value, the BFxx LOSET TIME DELAY timer is bypassed.The BF x LOSET TIME DELAY is based on the time that breaker resistor limited current can flow through the circuit after thebreaker interruption time. Both current detectors provide fast operating time for small multiples of the pickup setting.The Timer 3 (slow stage) is supervised by breaker status (auxiliary contact) and control switch status, the latter indicating ifthe breaker is in/out of service. This mode is typically used during maintenance. There is no current level check for thisstage since this is associated with small currents. Timer 3 is typically set with a longer time.5.6.4.4 Stage 3: OutputThe outputs from the scheme are:• FlexLogic operands that report on the operation of portions of the scheme• FlexLogic operand used to re-trip the protected breaker• FlexLogic operands that initiate tripping required to clear the faulted zone. The trip output can be sealed-in for anadjustable period.• Target message indicating a failed breaker has been declared• Illumination of the face plate TRIP LEDThe Breaker Failure can be used with single-pole tripping schemes if the HISET and LOSET current elements are setbelow the lowest expected fault current and above the highest expected load current.