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to ON or, ON to OFF and APU is running and VFSGs
are not available.
Break
787 Flight Crew Operations Manual
Electrical -
System Description
6.20.4 Boeing Proprietary
AC Buses
Main AC Buses
The four 235 Vac buses are the main AC buses, and are designated L1, L2, R1,
and R2. These buses directly support large loads, including those loads defined in
various electrical operating modes. They also distribute power to all remaining
airplane buses.
Each 235 Vac bus is powered by a dedicated VFSG. They are connected in a
ring-tie bus loop, which allows two alternate power paths to each bus if its primary
VFSG is unavailable. The ring-tie buses are designated Left Tie Bus (LTB) and
Right Tie Bus (RTB).
When the ASGs are powered, the LTB is powered from the left ASG, and the RTB
is powered from the right ASG. During ground operations with ASG power only,
the LTB includes L2 and R1 buses, and the RTB includes L1 and R2 buses.
When the VFSGs are powered (and the ASGs are not), the LTB is powered by the
L2 235 Vac bus, and the RTB is powered by the R2 235 Vac bus.
The ring-tie buses also allow the APU with its associated two generators to assist
the VFSGs during times when the Wing Ice Protection System (WIPS) is needed
and the APU is operating. When the APU is running, WIPS is powered from the
LTB and RTB. When the APU is off, WIPS is powered from the L2 and R2 buses.
During ground operation with engines OFF, the 235 Vac buses can be powered
from the ASGs, or by the 115 Vac forward external power sources through the
ATUs.
115 Vac Buses
There are two 115 Vac buses, designated left (L) and right (R). When VFSGs and
/ or ASGs are on-line, the 115 Vac buses are powered from the Autotransformer
Units (ATU). When both forward external power supplies are powering the
airplane, power is supplied directly to the respective left and right 115 Vac buses.
The majority of the loads on the 115 Vac buses are cabin systems, lighting,
miscellaneous ECS loads, galley power, and those loads defined in various
electrical operating modes.
DC Electrical Systems
The DC electrical system powers the airplane DC buses. There is also a flight
control DC electrical system, which is independent from airplane main electrical
system.
787 Flight Crew Operations Manual
Electrical -
System Description
Boeing Proprietary 6.20.5
DC Buses
Main DC Buses
The two main DC buses are powered from dedicated Transformer Rectifier Units
(TRU), and normally operate isolated from each other. One can be powered by the
other if its normal TRU power source is not available, though some non-essential
loads are likely shed.
These buses are multi-function 28 Vdc buses powering cabin systems, avionics,
miscellaneous system controllers, and loads defined in various electrical operating
modes.
±270 Vdc Buses
There are four ±270 Vdc buses, each powered by a dedicated Autotransformer
Rectifier Unit (ATRU). These buses operate isolated from each other, and cannot
be tied together if one bus loses its normal power source. The airplanes use of
variable frequency power requires large motor loads to be powered by motor
controllers. These buses provide that power. This high voltage DC power is not
distributed anywhere else in the airplane.
The motor controlled loads include the hydraulic system Electric Motor Pumps
(EMPs), Environmental Control System (ECS) compressors / fans, engine / APU
starting, and the Nitrogen Generation System (NGS).
Flight Instrument Buses
The captain and first officer each have one flight instrument bus, which are
powered from dedicated TRUs. The buses normally operate isolated from each
other. One can be powered by the other if its normal TRU power source is not
available, though some non-essential loads are likely shed.
Flight instrument buses are multi-function 28 Vdc buses powering instruments,
and those loads as defined in various electrical operating modes.
Flight Control DC Electrical System
The flight control DC electrical system consists of four dedicated 28 Vdc power
supplies for the Primary Flight Computers (PFC) and Actuator Control
Electronics (ACE), designated C1, C2, L, and R. These power supplies are
independent from the main aircraft electrical system, and also independent from
each other. This ensures that failures within a single supply do no affect the other
supplies.
Permanent Magnet Generators (PMG) housed within each VFSG are the primary
source of power for the power supplies. Backup sources for the PMGs are the 28
Vdc aircraft electrical system, main battery, and dedicated 10 minute batteries.
　
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