Management of electrical power

Citation

(2000), "Management of electrical power", Aircraft Engineering and Aerospace Technology, Vol. 72 No. 6. https://doi.org/10.1108/aeat.2000.12772faf.003

Management of electrical power

Management of electrical power

Keywords: Aircraft, Electricity, Power

The various facilities provided by a manufacturer to deal with the distribution of electrical power throughout an aircraft and the evolving technologies of succeeding types can be appreciated by considering Boeing's expertise in this field. When the 757 first flew, it embodied a combination of well-tried features and the latest advances at that time.

The electrical power network on this aircraft (Figure 1) consists of two 115/200 volt, three-phase, 400Hz generating and distributing systems. The normal in-flight power sources for the electrical power system are two oil-spray-cooled brushless 90kVA integrated drive generators (IDG). These are lightweight compact units in which the generator and drive are side-by-side within a single housing. Another 90kVA generator is mounted on the auxiliary power unit (APU) for operation on the ground or in the air. The APU can be started with its own battery or with an optional transformer-rectifier.

Figure 1 Boeing 757 electrical power system - normal flight configuration

Crew actions are minimised by automatic start-up, load transfer, load shedding and return on line features. Essential loads are automatically transferred to backup power (24 VDC battery and 400Hz static inverter) when primary power fails. Main and standby systems are normally isolated to be compatible with requirements. Primary AC loads are connected to two main AC buses. It is possible to interconnect the main AC buses when power is supplied:

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  by only one generator;

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  from the APU generator; or

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  from an external power source.

When the two main generators are operating and one generator system fails, essential loads are automatically connected to the operating generator. Automatic protection is provided for overvoltage, undervoltage, overload, open phase, differential fault, overfrequency and underfrequency.

The 28 VDC power supply is through two unregulated transformer rectifier (T-R) units. The DC normally operates isolated; however, failure of an AC bus or a T-R unit will cause an automatic bus tie to occur. DC bus tie status is shown on the EICAS (Engine Indicating and Crew Alerting System) on the flight deck. A nickel-cadmium battery is installed to provide standby DC power for selected ground loads and in-flight loads when the primary sources of power are inoperative. A single phase 115 V 400Hz standby static inverter converts battery power into AC power for the AC standby loads. The standby loads are automatically transferred to the standby source (battery and inverter) when primary power fails. A selector switch permits overriding the automatic functions to a manual On or Off mode. The standby system supplies the AC and DC load requirements for a number of minimum essential equipments for a period of 30 minutes.

There is an external power receptacle located in the nose gear wheelwell. A three-phase 115-200 V, 400Hz and 28VDC power is supplied to each galley installation. The AC galley power supply is automatically reduced or interrupted for electrical power system conditions that would otherwise overload a generator channel. Two convenience outlets located in the passenger compartment supply 115 V 400 Hz single-phase power. A similar power outlet is located in the main equipment rack and in the flight compartment for test equipment. Power for aircraft servicing and ground handling is supplied from the ground service transfer bus (ground and flight loads) and the ground handling bus (ground loads only). It is possible to supply these buses from the APU generator or an external power source without energising the main aircraft buses.

Distribution control and protection

A further step forward is provided by the Smiths Industries Electronic Load Management System (ELMS) for the Boeing 777. Electrical power from the primary, backup and standby generating sources is monitored and controlled by this system which is also capable of absorbing other aircraft utilities functions (Figure 2). The ELMS controls multiple electrical power sources and effects the control, management and distribution of electrical power, as well as the switching of electrical loads throughout the aircraft.

In this aircraft, as well as performing essential functions for all levels of power supply, the system embodies a reduction in the weight and volume of equipment compared to previous practice, as well as improved diagnostics and easier maintenance using line replaceable units (LRUs). Installation costs are reduced, as well as the ability to integrate and test the system prior to installation.

Figure 2 ELMS role in Boeing 777 aircraft electrical power system

There is an engine-driven generator on each engine producing 125kVA, as well as one on the APU and an engine-driven backup generator. Batteries and Ram Air Turbine (RAT) generators are other sources of power, the latter being deployed automatically upon the loss of the engine-driven generators. ELMS also controls the power for the electrical and electromechanical equipment throughout the aircraft, as well as the monitoring function and linking of the various systems and units.

In the aircraft's electronics bay are situated the seven control panels of the system. Three are power panels, three power management panels and one controls ground handling and services distribution. The power panels are designed to supply units requiring higher loads and the power management panels for those with lesser needs. All connections are via ARINC 629 databus, for example the power management panels via this connection perform controlling logic for switching power to various systems, all of which have been previously selected. A system performance monitoring capability is also present, as well as BITE (built-in test equipment) with reports transmitted via the ARINC 629 databus.

An additional function concerns load shedding in which the system continually compares available power and current power demand, and then optimises power distribution under both the situations of normal starting procedure and that of fault conditions. A prioritizing of load is thus effected while also preventing an overload of power sources. Power sources can thus be sized, which means that low-priority loads do not have to be discarded, but can still be supplied when the situation is such that sufficient spare power becomes available.

Details of this process include the feature that when shedding becomes necessary, the system reviews the current power demand and the selected services and commences the process that selects a number of these that may be shed. This is in priority of operational requirement, so that their absence will not compromise the basic operational needs of the aircraft. Power loading will be brought back as the situation becomes more stable.

With this system, savings in many areas have been accomplished, not least in the reduction of complexity in wiring and also in flight crew workload. The built-in test facilities referred to enable simplified monitoring procedures compared to previous systems and can be viewed as a further substantial advance.

Integrated power

More recently, the integrated electrical power system (IEPS) installed on the Boeing 717 was the first of its kind to use a new approach which significantly reduces the number of components, containing far fewer wires, current transformers and other LRUs that need attention (Figure 3). Reliability and maintainability are prime driving forces, with minimising of alerts and failure indications and elimination of "nuisance" messages.

Problems related to current transformers are particularly difficult to find and the IEPS on the 717 has only one discrete transformer and protection is provided for the external power feeders. The integrated drive generators (IDG) and auxiliary power unit (APU) feeder current transformers are integrated into the power conversion distribution units, which simplifies the architecture, and reduces the time required for final assembly and troubleshooting.

Figure 3 Boeing 717 integrated system

The architecture is simplified and numbers just nine components, and the electrical power system 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. This is an improvement over previous practice, in that system separation and segregation are improved. Reliability and maintenance considerations dictate the selection of a low-maintenance nickel-cadmium battery with enough capacity for one hour of emergency operation.

The four primary LRUs are power conversion distribution units (PCDU), electrical power control unit (EPCU), power relays, and the integrated drive generators. The PCDUs combine the generator control unit, transformer rectifier unit, and primary AC/DC distribution system into a single LRU, which is interchangeable in any of three locations. This means that a live spare LRU is available during normal operation. A quick disconnect method for the power feeders leading to and from the PCDUs reduces removal and replacement time. All components and associated wiring integrated in the PCDUs are tested by the manufacturer and do not require verification during final assembly, thus reducing the scope of the on-aircraft test procedure.

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-lane configuration. It also has a push-button switch that initiates a return-to-service test. A MIL-STD-1553 databus connects the EPCU and PCDUs. An ELEC FAULT indication on the flight deck alerts the flight and maintenance crews to the need to inspect the PCDUs/EPCU local indicators. System configuration is available through the electrics page in the system display.

The power relays are derivatives of those on the 777, with the relays (generator and bus tie) considered separate LRUs and 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 and are mounted onto the PCDUs to simplify the electrical power centre. This installation also eliminates the need to remove the forward attendant seat to replace failed power relays.

The integrated drive generators (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 APU generator is a derivative of that on the MD-80 and was modified to incorporate a three-phase-phase permanent magnet generator similar to the one installed with the IDGs.

Overhead control panel functionality is similar to previous aircraft and was designed to minimize the need for additional crew training. The whole electrical power system is not connected to the central fault display system. LED indicators feature on the PCDU and EPCU front panels.