integrated electrical power system aboard the Boeing 717 is a first
in the commercial aviation industry. This simplified system greatly
reduces the number of current transformers traditionally found in
electrical systems. It also integrates wiring and discrete components
into three line replaceable units that are all located in the avionics
and electrical compartment instead of throughout the fuselage. These
reductions decrease the number of components and increase the ability
of maintenance personnel to quickly correct a system anomaly and
return the airplane to service sooner.
electrical power system (IEPS) installed on the Boeing 717 uses
a new approach to electrical systems, which traditionally require
time-consuming, point-to-point troubleshooting of wiring (fig.
1). This new approach also significantly reduces the number
of components in the system. The 717 IEPS contains far fewer wires,
current transformers, and other line replaceable units (LRU) that
need troubleshooting. While retaining 100 percent of the functionality
of traditional systems, the 717 IEPS simplifies electrical system
architecture, makes troubleshooting easier, and improves dispatch
reliability (table 1).
electrical system for the 717 improves upon the standard for developing
such systems. In the past, design was guided by the need to use
off-the-shelf components, keep development costs low, and incorporate
proven reliability. Improvements on the 717 were driven by additional
requirements, including the need for the electrical system to be
inexpensively built, highly reliable, easy to maintain, and a single
point of contact for all issues related to electrical power.
To achieve these
goals Boeing considered the following items during design of the
717 electrical system: Simplified systems architecture (fig.
2) to enhance reliability and maintainability, dispatch reliability,
maintenance cost, proven components and design concepts, other airplane
in-service experience and focus items to avoid repeating mistakes,
minimal number of alerts and failure indications, elimination of
nuisance fault and redundant indications, and minimal effect on
other aircraft systems. The resulting IEPS features
wires, current transformers, and other LRUs.
wiring and discrete components.
FEWER WIRES, CURRENT TRANSFORMERS, AND OTHER LRUS
Problems related to current transformers are difficult for maintenance
personnel to find and may require considerable time and knowledge
to solve once discovered. These problems can include loose wires,
reversed connections, and high resistance in coil-shorting relay
The 717 has only
one discrete current transformer, and it protects the external power
feeders. The generator (integrated drive generators [IDG] and auxiliary
power unit [APU]) feeder current transformers are integrated into
the power conversion distribution units (PCDU). This simplifies
the architecture and significantly reduces the time required for
final assembly and troubleshooting.
INTEGRATED WIRING AND DISCRETE COMPONENTS
By simplifying the architecture of the 717 IEPS to nine components,
Boeing was able to delete from the system or integrate into the
PCDU a total of 60 of the components found in a traditional system.
All 150 wires that traditionally interconnect these components were
Previous Douglas designs located all electrical controllers in the
forward left generator rack in the electrical/electronics bay of
the airplane. The electrical power system on the 717 is an improvement
over these designs in terms of system separation and segregation.
It is concentrated into four primary LRUs, all of which (except
for engine-mounted generators) are split between the forward left
and right radio racks in the avionics and electrical compartment
for faster access.
Boeing also considered
an additional LRU that would contain all the emergency power system
components, including battery charger and static inverter. However,
those components were kept discrete because it was not feasible
to incorporate them into the 717 IEPS. The resulting reliability
estimated for this LRU did not support the targets Boeing established
for 717 dispatch reliability and maintenance cost. The battery selected
is low-maintenance nickel-cadmium with enough capacity for one hour
of emergency operation.
The four primary
- Power conversion
power control unit.
- Power relays.
These combine the generator control unit, transformer rectifier
unit, and primary ac/dc distribution system into a single LRU. The
same LRU is interchangeable in any of three locations, meaning that
a live spare LRU is available during normal operation. To reduce
removal and replacement time, a quick-disconnect method for the
power feeders leading to and from the PCDUs was created (fig. 3).
and associated wiring integrated in the PCDUs are tested by the
manufacturer and no longer require verification during final assembly,
reducing the scope of the on-aircraft test procedure.
power control unit (EPCU).
The EPCU provides control and protection for external power. It
also supervises such functions as no-break power transfers, automatic
emergency power switching, and dual-land configuration. The EPCU
also has a pushbutton switch that initiates a return-to-service
test. A MIL-STD-1553 data bus connects the EPCU and PCDUs (fig.
An ELEC FAULT
indication in the flight deck alerts the flight and maintenance
crews of the need to inspect the PCDUs/EPCU local indicators. System
configuration is available through the electric page in the system
display (fig. 5).
The power relays are derivatives of those installed on the 777 and
were chosen for easy installation and removal. The relays (generator
and bus tie) are considered separate LRUs and are mounted to the
front face of the PCDUs. For the APU position, the relay installed
in the bus tie relay location functions as the main external power
relay. The power relays are all electrically held, which means the
coil is energized for as long as the relay needs to be closed. This
allows all relays to be the same to provide a hot spare. Electrically
held relays were also selected because they are simpler and more
reliable than the magnetically latched type.
The power relays
are mounted onto the PCDUs to simplify the electrical power center.
Installing the relays there also eliminates the need to remove the
forward attendant seat to replace failed power relays. The relays
were relocated on the 717 to solve this access problem, which was
present on previous Douglas twinjets.
The IDGs provide primary ac power and combine the constant speed
drive (CSD) and a generator in a single LRU. The CSD transmission
oil cools the generator. The IDG chosen for the 717 is lighter and
more reliable than the CSD and generator equipment installed as
separate LRUs in previous Douglas-designed models. Already proven
as a mature, reliable component on Fokker 50 airplanes, the IDG
installed on the 717 was modified only slightly to improve maintainability.
The APU generator
is a derivative of the MD-80 generator. It was modified to incorporate
a three-phase permanent magnet generator similar to the one installed
with the IDGs.
of the overhead control panel, shown in figure 6, was not changed
from the MD-80 and MD-90 designs to minimize the need for additional
power system is not connected to the central fault display system.
The PCDU and EPCU front panels include LED indicators.
the electrical power system on the 717 resulted in several
changes compared to previously designed systems. These include
significantly fewer components and LRUs, which decrease the
potential for component failure and the time required for
return to service. Specific examples include a reduced number
of components (to 9 from 60), the elimination of more than
150 wires, a weight savings of 46 lb (21 kg), and exceeded
dispatch reliability rate goals.
ELECTRICAL POWER INTEGRATED PRODUCT TEAM
BOEING LONG BEACH DIVISION
SENIOR PRINCIPAL ENGINEER SCIENTIST
ELECTRICAL POWER INTEGRATED PRODUCT TEAM
BOEING LONG BEACH DIVISION