Influences of voltage variations on electric power architectures for hybrid electric aircraft

Abstract

Hybrid electric and universally electric aircraft seem to be one possible option to fulfil ambitious future emission and noise reduction targets forced by the European Commission with the Strategic Research and Innovation Agenda and the NASA with the NASA N+3 goals. The overall vehicular efficiency of such concepts can be considerably improved. A key element is the design of the electric power train system with regard to efficiency and also mass. Based on a battery-powered direct current (DC) system architecture, the efficiency impact of different design voltages has been investigated in this paper. The main components within a DC electric architecture are power electronics such as converters and inverters, transmission cables, protection devices and cooling systems. Especially, the power electronics show the highest sensitivity when choosing a design system voltage. Two main losses occur within those components, conduction losses and switching losses. While conduction losses decrease with an increase in the design voltage, switching losses normally increase. Therefore, for a required design power there will be a trade-off between conduction and switching losses. For that purpose, two different architecture design philosophies were investigated to compare the system efficiencies at different design voltages. The first architecture is the constant system voltage (CSV) architecture, which compensates a decreasing output voltage of a battery, and a variable system voltage (VSV) architecture, which does not compensate this voltage drop. It has been identified that for an electric power train system delivering a constant power of 6000 kW, the VSV shows the best efficiency. The optimum system voltage of the VSV architecture is near the operating voltage of the electric motor, while the optimum system voltage of the CSV architecture was identified at higher voltages. These results will serve as baseline for the identification of the best voltage level for a mass- and efficiency-optimized system.