MD-11 Flight Crew Operations Manual
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Electrical -
Description and Operation
Elec.10.3
No Break Power Transfer (NBPT) and Parallel Operation
A complete NBPT system is provided on the aircraft. Power transfers between
external power, APU generator, and IDG electrical power will normally occur
without a power interrupt. If the two power sources cannot be synchronized
closely, a break power transfer will occur.
Non-interrupted electrical power is supplied to the aircraft buses by momentarily
paralleling alternate electrical power sources before the power transfer occurs.
The power source with the highest priority (or selection) momentarily parallels
with the lower priority power source before the lower priority power source is
disconnected from the bus.
When the electrical power source is available and selected on, the power source
priority for the ac generator bus is as follows:
1. Associated IDG.
2. APU generator.
3. The ac tie bus (external power or IDG).
When the electrical power source is available and selected on, the power source
priority for the ac tie bus is as follows:
1. External power.
2. Any IDG.
For example, no break power transfer will occur automatically when the IDGs can
no longer power the buses after engine shutdown. If both APU and EXT power
are available, the APU has the higher priority and will supply electrical power to
the buses. On APU shutdown, external power, if available, will take over.
Momentary paralleling between IDG, APU generator, and external power is
completely automated through the GCUs and EPCU once the flight crew has
selected the respective power source on or off.
Except for dual land operation and external power on the tie bus, the IDGs will
normally operate in the parallel mode. Parallel operation of the IDGs is also
completely automated through the GCUs and the EPCU.
Generator Control Units (GCU)
The respective GCU (1, 2, 3, and APU) controls the following:
• GR 1, 2, and 3.
• BTR 1, 2, and 3.
• IDG disconnect (crew command).
• DC ties 1 and 3.
• NBPT from APU generator to external power.
• NBPT from APU generator to IDG.
• NBPT from IDG to EXT power across BTR 1, 2, and 3.
• NBPT from external power to IDG.
October 02, 2006
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