Introduction to aircraft electrical power distribution systems

The trend in modern aircraft design is away from mechanical systems (hydraulics, pneumatics, etc.) and toward electrical components, or Aircraft Electrical Power Distribution Systems. There are several benefits of the modern design (particularly weight savings). However, as with any airplane design, no system can be fielded before it can be proven safe, reliable, and able to be maintained over the aircraft’s life. In this article we will provide an overview of aircraft power systems and some first level considerations when starting the design process (for a more in-depth discussion of MEA, you can read Lectromec’s Insights from Europe: The More Electrical Aircraft Conference article).

The main function of an electrical system is to generate, regulate, and distribute power throughout the airplane. Aircraft performance is directly connected with the reliability of electrical systems and subsystems. Generally, aircraft electrical systems utilize both AC and DC power.

AC POWER

The AC power is typically a three-phase wye generator at 115VAC using 400Hz. Use of 400Hz power has been a standard for decades as the power can be produced with smaller and lighter generators than 50/60Hz systems. Although the use of higher frequencies is not ideal for long distance power transmission (more sensitive to voltage drop), the benefit of the lighter system is ideal for aerospace applications.

The electrical generating capacity of the generator will vary depending on the application, but can be more than 200kVA per generator. Additional information about aircraft power systems can be found in MIL-STD-704: Aircraft Electric Power Characteristics.

DC POWER

When used for power applications (as oppose to signal), the most common DC voltage is 28VDC. Changes in airplane design and increased power consumption are pushing up these voltages (to learn about research on failure consequence of higher voltage power systems, you can read Lectromec’s 270VDC Arcing in Aerospace Wiring article).

System Complexity

The distribution systems used in modern aircraft create a complexity that impacts power system design, physical layout of components, wire routing, and wire selection. The benefit of these complex systems is that they can rout power around localized faults to maintain airworthiness. One such example among modern aircraft is the Boeing 787 Dreamliner. The Boeing 787 runs power from the generator to the electrical equipment bay, then distributes the power from various locations within the aircraft (to get a better understanding of wire risk levels, you can read Lectromec’s Understanding Wire Failure Risk Levels article).

Advantages of the New Aircraft Design (more complex electrical power systems)

• Decreased total usage of heavy gauge power feeders, which results in reduced weight
• Decreased total power distribution wiring
• Better electronic control of load throughout airplane

Disadvantages of the New Aircraft Design (more complex electrical power system)

• More vulnerable to electrical failure
• Significant increase in the need for power, which can be very dangerous if something goes wrong on the aircraft
• Increased probability of short circuit and loss of power, due to more electrical circuits
• Higher frequencies, which increase sensitivity to voltage drop and increase the likelihood of loss of power

Military handbook MIL-HDBK-516B provides several suggestions for increasing the benefits of designing aircraft power systems. These practices that can be implemented to reduce both the cost of future upgrades and the failure probability of power systems.

1. The Electrical Power systems (EPS) should be capable of supplying all electric power requirements for all modes of vehicle operation plus additional capacity to provide for growth loads.
2. The EPS shall provide protection to prevent unsuitable external power from being applied to the airplane.
3. The buses, conductors, and circuit breakers should be capable of handling the load growth provisions.
4. The buses and distribution circuits shall be configured so normal EPS operation loads receive power from the aircraft primary power source(s), ground power, or an auxiliary power source as applicable.
5. In the event of an EPS failure that reduces the amount of available power below total aircraft requirements; non-flight critical and/or pre-selected loads should be automatically disconnected as required.

Further Reading

Further reading on aircraft electrical power distribution systems:

• The next 15 years of aircraft power systems [here]
• Use of higher voltages [here]
• Finding the right size wire for the application [here]
• Pairing the correction size circuit protection [here]