High voltage DC supply for more electric aircraft

Abstract

The More Electric Aircraft concept aims to replace the hydraulic, pneumatic and mechanical power systems with electrical ones [1]. This has several advantages such as, increasing operational efficiency and reducing the weight and size of the system. Thus, with the MEA fuel is saved which for airlines it is important since this would allow them to be more competitive and this also helps them to meet more stringent requirements on emissions. Till now only two commercial aircrafts have used this concept these are the Airbus A380 and the Boeing 787 [5]. The MEA requires a much larger electrical power system than a conventional one, since it is servicing loads that were previously powered with a different power system. The electrical power system of a conventional airplane is usually less than 300kVA but that of an MEA is about 1MW. Inverter driven motors are used for energy efficiency and their accurate control but require a DC supply. To keep the weight of the electrical power system as low as possible HVDC is preferred because with higher voltage less current will pass through the cabling thus, thinner cables can be used [9]. The generating system of any airplane is AC thus, a converter is required to change AC to DC. The preferred solution is that of using an Autotransformer rectifier unit which is the main subject of this project, which mainly consists of an autotransformer and 6-bridge diode rectifiers. The advantage of using such system is its low complexity, high reliability , high system efficiency and low weight when used in airplanes due to the supply frequency of the aircraft being from about 360 to 800Hz thus allowing a smaller core of the autotransformer. There are several autotransformer configurations available such as the star, delta and polygon configurations. The characteristics of these configurations will be investigated and the most suitable option identified. An Autotransformer Rectifier Unit is a multipulse rectifier, with its number of pulses being a multiple of 6. When its pulse number is increased the DC output voltage becomes smoother due to the larger number of pulses. This also leads to reduced input current THD and lower harmonic injection in the aircraft electrical system.