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717 Integrated Electrical Power System

The 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.

The integrated 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).

The 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

  1. Fewer wires, current transformers, and other LRUs.
  2. Integrated wiring and discrete components.
  3. Collocated LRUs.

1. 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 contacts.

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.

2. 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 also deleted.

3. COLLOCATED LRUS
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 LRUs are

  • Power conversion distribution units.
  • Electrical power control unit.
  • Power relays.
  • Integrated drive generators.

Power conversion distribution units.
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).

All components 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.

Electrical 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. 4).

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).

Power relays.
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.

Integrated drive generators.
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.

Functionality 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 crew training.

The electrical power system is not connected to the central fault display system. The PCDU and EPCU front panels include LED indicators.

SUMMARY

Integrating 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.

GARY PERKET
TEAM LEADER
ELECTRICAL POWER INTEGRATED PRODUCT TEAM
BOEING LONG BEACH DIVISION

HECTOR SILBERMAN
SENIOR PRINCIPAL ENGINEER SCIENTIST
ELECTRICAL POWER INTEGRATED PRODUCT TEAM
BOEING LONG BEACH DIVISION


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