Site hosted by Build your free website today!

Designing The Boeing 777 Electrical Power System

by Carl B. Tenning

The Boeing 777 is a large twin engine jet transport designed for regional and extended range service. The primary electrical system is a 3-phase, 4-wire, constant frequency 400 Hz, 115/200 Volt system, which has been the industry standard on commercial jet transports since the Boeing 707 (and maybe before, but that's as far back as I go). The neutral is connected to the airframe so that single phase circuits are supplied with a single wire, using the airframe as the return circuit. There is also a 28 Volt direct current system with the negative pole connected to airframe, such that 28 VDC services are also supplied with a single wire using the airframe as the return circuit. The 28 VDC power is derived from the AC power system by transformer-rectifiers.

The primary in-flight electrical source on the 777 is a 120 kVA constant speed two-pole brushless alternator. This represents a 33% increase in capacity over the next largest electrical source on previous Boeing commercial jet transports, the 747-400, 757 and 767. The alternator in these machines is integrated with an axial gear differential/hydraulic constant speed drive. The speed on the 120 kVA machine maintains an average 24,000 rpm plus or minus about 1%. This is currently the lightest weight technology in kVA per pound for supplying a 400 Hz constant frequency system, actually weighing just slightly less than the 40 kVA drive/alternator on the earlier Model 727. Minimizing weight has been achieved partly by increasing the alternator speed, partly by improved packaging and partly by switching from air cooling to oil cooling. The progression on Boeing airplanes has been from air-cooled 6000 rpm machines on the 707, 727, 737-100/200/300/400's and 8000 rpm machines on the 747-100/200, to oil-cooled 12,000 rpm machines on the 767, 757 and 740-400, and ultimately to 24,000 rpm machines on the 777. The 24,000 rpm constant speed Integrated Drive Generator, or IDG as it is referred to, was a mature technology also in service on the McDonnell Douglas MD-11 and Airbus A330.

The system on the Boeing 777 is configured with one IDG per engine, driven directly from the engine gearbox. There are two main electrical distribution buses, a left main bus and a right main bus, each normally supplied independently by their respective IDG. There is a third main alternator driven from an auxiliary gas turbine engine (called the auxiliary power unit or APU for short) located in the tail of the airplane, which is also rated at 120 kVA. Since the APU runs at a constant speed, this 24,000 rpm alternator is a direct drive, i.e., not an IDG. It is, however, essentially the same alternator used in the IDG. While the APU alternator is primarily a ground power source for the airplane, it can also be operated in flight to replace the loss of one engine driven IDG.

The 777 also incorporates a second power generation system, called the Backup System. This is a 25 kVA system intended to support essential services in the unlikely event of loss of all main alternators. A variable speed direct drive alternator was selected for this system to minimize the size of the engine installation. There is one backup alternator per engine directly driven from the engine gearbox at approximately 14,000 to 28,000 rpm. The output power of this alternator, at a frequency of 933 Hz to 1867 Hz, is converted to a constant 400 Hz by an electronic converter in the fuselage. Also incorporated within this machine are two permanent magnet alternators which are used as isolated independent supplies for the electronic flight controls system.

Flight instruments on the 777 are primarily powered by 28 Volts DC. Direct current power is supplied by three-phase transformer-rectifier units. There are four such units each rated at 120 Amperes. The choice of using 28 VDC instruments facilitates powering these instruments from battery power when necessary. Flight instruments and flight controls can also be powered from a Ram Air Turbine driven alternator which deploys automatically for loss of all engine driven generator power.

Distribution from such a redundancy of sources is arranged such that the maximum services can be maintained in the event a loss of any source. Depending on what sources are lost, services are supplied in proportion to the importance of the service. The most important services, such as flight controls, can be supplied from any of the electrical sources. Flight instruments are supported by all but the permanent-magnet generators, which are reserved exclusively for flight controls. Other important services, such as fuel boost pumps, cabin temperature controls, and the pilots window heat, can be supplied from either of the three main generators or the two backup generators. Services of lesser importance are supplied solely from the main generators. Non-essential services, such as lavatory water heaters and galleys, are automatically shed, if necessary, when there are less than two main sources.

Each new design is an evolutionary process, analyzing the problems or weaknesses of previous designs and trying to improve on them. The Boeing 777 preliminary design first analyzed the total electrical load requirements based on the size of the aircraft: galley service requirements, the anticipated motor loads for various fans, fuel pumps, electrically driven hydraulic pumps, various heater and lighting loads, instrument power, and unique to the 777, the flight controls electric load. The busing configuration originally followed that of the Model 767 electrical system, but with the increase in capacity needed for the larger airplane. Except for the addition of a second external power connection, the main AC system architecture is essentially the same as the 767. Some simplification of the electrical controls were made to maximize reliability. To reduce power interruptions, the 777 was designed to do no-break power transfers between external power, APU power and engine power. The configuration of the backup power system was revised to minimize the number of switching contactors. Instrument power distribution became different when the decision was made to select mainly 28 VDC powered instruments instead if single phase 115 VAC powered instruments. A completely new requirement for the 777 was the flight controls power. The system architecture, number of generators, number of control units and other devices were selected by the Boeing design team.

Specifications were then prepared for each system and component. Potential suppliers were invited to make proposals. The Boeing team then evaluated the proposals and selected the winning suppliers.

As the designs of the selected suppliers progressed, Boeing engineers reviewed details for compatibility between systems. This was the first program where extensive use was made of system simulation during the design process.

For each of its commercial jet transport programs Boeing has assembled a test rig of the complete electrical power generation system using actual aircraft equipment. Testing is conducted for system development, FAA certification and service readiness. The types of testing include (1) all combinations of power transfers between sources, (2) voltage regulation under various loading, (3) the response to various faults and failures to verify the functioning of system protective functions, and (4) system indication in the flight deck and maintenance indications. Testing for the 777 was done in the newest Boeing test facility, the Integrated Aircraft Systems Laboratory (IASL) in Seattle. The larger generators of the Boeing 777 required new higher capacity drive stands. This laboratory has six drive stands, each with a capacity of 800 horsepower (1500 horsepower peak), three of which were in use by the 777 program.

Carl Tenning joined Boeing upon graduation from the University of Washington in June 1958. Assignments have included design, research and testing of electrical systems and equipment for the Models KC-135, 707, 727, 737, Boeing SST, the 757, 747-400 and preliminary design for the 7J7. In March 1967 he was appointed an FAA Designated Engineering Representative for the Boeing Company. From January 1990 until retirement in July 1995, he was the design supervisor of the Boeing 777 electrical power generating system.