(6) Source Overload (Galleys and Utility Buses Shed)
(a)
Source overload shedding is controlled by the BPCU. The BPCU monitors the load supplied by both IDGs, the APU generator, and external power. Source overload shedding occurs when: 1) The airplane is on the ground or inflight, and 2) any source (L IDG, R IDG, APU, or EXT PWR) is loaded,
a) above 90 kva for 4.5 minutes, or b) above 112.5 kva for 4 seconds.
(b)
When the conditions above are met, the galley and utility buses supplied from the overloaded source are shed. The UBR(s) and galley ELCU(s) associated with the overloaded source de-energize. (The right main ac bus has two galley ELCUs. The left main ac bus has one.) The associated amber OFF light in the UTILITY BUS switch comes on, and the L (or R) UTIL BUS OFF message appears on the EICAS display. (This is a level C EICAS message.) The 90-kva level corresponds to about a "1.00" AC LOAD indication on the ELEC/HYD maintenance page of EICAS. The 112-kva level corresponds to about a "1.25" AC LOAD on EICAS. The BPCU initiates utility and galley bus shedding by removing grounds to the ELCUs and UBRs.
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(c)
If one source is supplying both main ac buses at the time of the overload, both utility buses and all galley buses will be shed. If a separate source is supplying each main ac bus, only the utility and galley buses supplied by the overloaded source are affected by shedding. During normal two generator operation, source loading will not exceed 90 kva.
(d)
If one IDG is supplying power to both main ac buses, GCU protection circuits act to prevent loss of this remaining IDG due to a passive failure. (An example of a passive IDG failure is the presence of open diodes in the IDG rotor circuits.) Detection of a passive failure of the remaining IDG causes the corresponding bus tie breaker (BTB) to trip open. Therefore, all load transferred to the one operating IDG is dropped. The load is reduced to a value equal to or less than that carried before the failure of the other IDG. This prevents the combination of 1) a passive IDG failure, and 2) a high load, from depressing system voltage and tripping the remaining IDG off line.
(e)
Resetting a source overload shed is done by cycling the UTILITY BUS switch controlling the shed utility and galley buses. If the overload continues, the buses will again be shed after the source overload time delay.
(7) Single Source Operation
(a)
Load shedding occurs when single generator operation occurs inflight, or during the takeoff roll. The takeoff mode is sensed by the advanced position of both thrust levers. The takoff mode load shed is needed to assure adequate generator voltage for hydraulic pump starting. This is needed for braking in the event of a refused takeoff due to engine failure.
(b)
All galley and utility buses are immediately shed if: 1) One source (L IDG, R IDG, or APU) must supply electrical
power to both main ac buses, and 2) the airplane is inflight, or 3) both thrust levers are advanced on the ground.
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(c)
When the conditions above are met, the BPCU de-energizes all UBRs and galley ELCUs. The galley and utility buses have no power. The amber OFF light in the UTILITY BUS switches come on, and the L and R UTIL BUS OFF messages appear on the EICAS display. (These are level C EICAS messages.) The BPCU does not allow the utility or galley buses to energize as long as the above conditions continue. (The BPCU initiates utility and galley bus shedding by removing grounds to the UBRs and ELCUs.)
(d)
If a second source (either IDG or APU generator) comes on line to supply main ac bus power, the utility and galley buses are automatically reset. This reset is accomplished by relay and interlock circuits external to the BPCU. With only one source operating, both bus tie breakers (BTBs) are closed. When the second source comes on line, at least one BTB opens, to prevent parallel source operation. This provides 28 volt dc to the UTILITY/GLY PWR FLT RESET relay. This one shot relay energizes the L and R UTIL BUS RESET relays for 1.5 seocnds. This relay action has the same effect as manually cycling the UTILITY BUS switches. Power to the utility and galley buses is restored.
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