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MAINTENANCE MANUAL
(k) The GCU provides a BTB lockout protection function. The protection function prevents BTB closure if: 1) A differential fault exists on the associated generator
bus, or 2) A differential fault exists on the tie bus, or 3) An undervoltage and an overload condition exists on the
associated generator. (This indicates that a phase current has exceeded the overload threshold of 12.5-Kva for 8.0 to
8.5 seconds).
(l)
The GCU sends a lockout signal to the BTB, preventing the BTB from closing and connecting the main buses. A BTB lockout causes the amber ISLN light in the BUS TIE switch to come on. The BTB lockout can be removed by cycling the appropriate BUS TIE switch when the fault no longer exists.
(m)
If the GCB fails to open following a GCR trip, the associated BTB is opened and locked-out. This action prevents motoring of the IDG through power delivered (to the IDG) by a closed BTB. The BTB opens in .05 |.02-seconds. (The GCB should open in .02-seconds following a GCR trip.)
(n)
Each GCU contains a watchdog circuit. If a GCU has an internal failure, the circuit opens the GCB and GCR. (The watchdog circuit in the APU opens the APU GCR and APB.)
(2) Differential Fault Sensing (Fig. 2, and 3)
(a)
The differential protection current transformers (DPCT's) provide current signals to the generator control units (GCU's), and the bus power control unit (BPCU). The BPCU, by means of four DPCT assemblies, monitors the tie bus. (This is the area between the two bus tie breakers (BTB's), the auxiliary power breaker (APB), and the external power contactor (EPC).) A tie bus fault occurs when a 28 to 35 amp differential current is sensed by the four DPCT's. The left and right GCU's, by means of four DPCT assemblies each, monitor their respective buses for a differential current of 28 to 35 amps. The APU GCU, by means of two DPCT assemblies, monitors its buses for a differential current of 28 to 35 amps.
(b)
When a differential fault is detected, a flag is set in the appropriate BPCU and/or GCU's. The BPCU and GCU's then communicate via the serial link to isolate the fault to a large zone. Tripping of various breakers (generator field control relay (GCR), generator circuit breaker (GCB), BTB, APB, EPC) then isolates the fault to a smaller zone.
(c)
Isolation of a fault may trip the GCB's, APB, and/or the BTB's. If a GCB or APB trips (opens), the amber OFF light in the corresponding GEN CONT switch comes on. (The GEN CONT switch is on pilots' overhead panel P5.) There will also be a level C message on the EICAS display: L (or R, or APU) GEN OFF. If a BTB trips, the amber ISLN light in the corresponding BUS TIE switch (P5) comes on. There will be a level C message on the EICAS display: L (or R) BUS ISOLATED. Tripping of the EPC causes the ON light in the EXT PWR switch (P5) to go off.
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
24-23-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
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