AC GENERATION -APU GENERATOR AND IDG POWER SYSTEMS -GENERAL DESCRIPTION
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777 AIRCRAFT MAINTENANCE MANUAL
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AC GENERATION -BACKUP GENERATOR POWER SYSTEM -GENERAL DESCRIPTION
General
The backup power system supplies 115v ac, 400 Hz power to the left, right, or both transfer buses. The transfer buses usually get power from the main ac buses. If a main ac bus loses power, the backup power system supplies power to the related transfer bus.
The backup power system also supplies power during autoland operations.
The backup power system has two backup generators and a backup generator converter. The system is independent of other airplane systems. There is no time limit for backup power system operation.
Backup Generator Power
The backup converter gets power from only one ot the backup generators at any time. The converter changes the variable frequency power to 115v, 400 Hz ac power and sends it to the transfer buses. Power goes through the converter circuit breakers (CCB).
Only one backup generator supplies power at a time. Normally, the left backup generator supplies power to the left transfer bus and the right backup generator supplies power to the right transfer bus. If the left and right transfer buses need power, the right backup generator supplies the power if it is available.
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The backup generator converter also controls, monitors, and protects the backup power system. The converter gets an input from each backup generator (BACKUP GEN) switch. When the switch is in, the converter controls the related CCB and transfer bus breaker (TBB) automatically. When the switch is out, the converter opens the CCB, closes the TBB, and trips the field of the generator. The converter causes the OFF light to come on for each switch for any of these conditions:
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Switch is out
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Backup generator field relay opens because of a fault
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Engine fire switch is pulled out
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Engine is shutdown.
Training Information Point
The backup converter defaults to the right backup generator. If the right backup generator is not available or faulty, the converter uses the left backup generator.
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AC GENERATION -BACKUP GENERATOR POWER SYSTEM -GENERAL DESCRIPTION
No-Break Power Transfers
The backup generator converter controls the CCB and TBB to do no-break power transfers to the transfer buses. When a main ac bus loses power, the BTBs close to permit the opposite main ac bus to supply power to the main ac bus that lost power and its related transfer bus. The backup generator power system then momentarily connects the main ac bus power and the backup generator converter power on the transfer bus so there is no interruption in power. The backup generator converter electronically matches the power on the transfer bus to do a no-break power transfer.
Engine Start Test
After an engine starts, the backup generator supplies power to its transfer bus for approximately 15 seconds. This makes sure the system operates before each flight.
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AC GENERATION -BACKUP GENERATOR POWER SYSTEM -GENERAL DESCRIPTION
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AC GENERATION -COMPONENT LOCATIONS -GENERATORS
General
The ac generation system has these five generators:
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IDG (2)
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Backup generator (2)
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APU generator.
IDG
There is one IDG on each engine. The IDG attaches to the aft face of the gearbox.
Each IDG supplies a maximum of 120 kva of power.
Backup Generator
There is one backup generator on each engine. The backup generator attaches to the forward face of the gearbox.
Each backup generator supplies a maximum of 20 kva of power.
APU Generator
The APU generator is in the APU compartment. It attaches to the APU gearbox.
The APU generator supplies a maximum of 120 kva of power.
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AC GENERATION -COMPONENT LOCATIONS -GENERATORS
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AC GENERATION -MAIN EQUIPMENT CENTER -COMPONENT LOCATIONS
General
These ac generation system components are in the main equipment center:
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