The use of electronic power distribution systems (EPDSs) allows for a wider range of topologies for an aircraft power system than is easily possible by the use of traditional circuit breakers. Weight and cost savings, plus increases in safety and reliability, have for some time made EPDSs the logical choice for large commercial aircraft, and smaller aircraft would also benefit, says Astronics.
An EPDS also allows for much more flexibility in the design of the aircraft power distribution scheme. Several topologies for 28-V dc and 120-V ac systems can be employed that make the power system more damage-tolerant.
Another advantage of EPDS is that load shedding can be accomplished on a circuit-by-circuit basis, eliminating the need for special “shed” busses and associated hardware.
Basic topologies for EPDSs allow the breakers to be located near the loads for maximum wire weight savings. There is a variety of power sources and emergency power schemes that can be combined in various ways to suit an individual aircraft. Usually a few thermal breakers and fuses will remain for redundant power to flight-critical systems.
In example topologies described below, control of the EPDS originates with the avionics system through power system controllers called system control interface units (SCIUs). There are other options as well, suited to many different aircraft, that have no need to communicate with the avionics system.
A two-zone EPDS is the simplest topology. It replicates the design of a conventional system, except for the key fact that the electronic circuit breaker units (ECBUs) are located near the loads. This basic topology can be easily modified with additional bus feeds and contactors to provide more versatility in the case of a bus fault or physical damage to the aircraft.
Modified two-zone EPDSs use a traditional left/right topology, but with the additional capability of being able to disconnect any ECBU from the bus. This topology uses the same number of ECBUs and sources as the simple two-zone system. It is in the process of being certified in a Part 25 aircraft.
Two-zone systems allow the ECBUs to be located throughout the aircraft to take maximum advantage of wiring savings. Any damage to the feed to an individual ECBU can be detected and the feed disconnected, but that ECBU would then be inoperative.
As its name implies, in four-zone EPDS topologies, aircraft power is split into four zones, with contactors to route power between the power sources and the zones. One or more ECBUs are placed in each zone, physically close to the loads. This topology, certified and flying in a Part 23 aircraft, allows for the isolation of any zone in the event of damage to the aircraft. It also allows the power sources to be automatically switched in the event of the loss of a source. This basic design can be modified with additional bus contactors to provide even more flexibility.
A dual-feed EPDS topology combines the previous two into an extremely fault-tolerant system. Each ECBU is fed by two sources, and the SCIU computer can select which source feeds which ECBU.
Damage to any one bus or system component will have no effect on any ECBU input feed. This system is ideal for airframes where structural damage may occur, but it uses more wiring and contactors than the other topologies.
EPDS systems are more than a drop-in replacement for conventional breakers, as they can perform control and switching functions as well as protect the wiring. To control the EPDS, several options are available.
An avionics interface option ties the SCIUs directly into the avionics system and has display screens hosted by the avionics system for control and status monitoring. This method produces the cleanest cockpit but suffers from the need to develop and certify the display software.
A second option combines the SCIU with an illuminated switch panel for control and monitoring purposes. No interface to the avionics is required, but one can be provided if required for monitoring or caution/warning announcements. Various lighting, display, and input options are available.
Another option is a dedicated display that can be used in place of the avionics displays. All three techniques can be combined in various ways to produce a custom system unique to any particular aircraft.
This article is based on SAE technical paper 2009-01-3122 by Fred Potter of Astronics AES.