High Performance DC-AC Conversion Techniques for the More Electric Aircraft

Abstract

To reduce emissions and maintenance cost, bulky mechanical actuators and systems in aircrafts are expected to be replaced by lightweight electric or hybrid solutions requiring power electronic converters. One requirement is for a DC-AC converter to synthesize an AC source to power some of the AC loads, and the power density of this converter would be a key design criterion. The main objective of this research is to investigate techniques to improve the power density of this converter, such as through improved design techniques and the use of new wide band-gap semiconductor technologies. It may also be possible to apply these innovations to other DC-AC converters requiring high performance.The thesis identifies the semiconductor losses and the weight of the AC filter as two key design aspects that significantly affect the power density of DC-AC converters. Further reductions in filter weight are hindered by the switching capability of traditional Si semiconductor devices. However new wide band-gap devices such as SiC have lower losses and are capable of switching at high frequencies, making them a candidate technology for applications targeting power density.Firstly, the operation and the loss mechanisms of three widely used DC-AC converter topologies are analysed, which quantifies the reduction in semiconductor losses using SiC devices. Then analytical loss models for three converter topologies are derived, and after applying device datasheet values in the loss models, the SiC two-level converter has been identified as the most suitable converter. The SiC two-level converter gives a 3 % increase in efficiency at a switching frequency of 50 kHz, compared to the same topology switched at 20 kHz with Si IGBTs. Secondly, design constraints for the filter are identified and incorporated into a set of equations that allow calculation of a minimum weight LC filter. Thirdly, models developed for the LC filter are extended to the LCLC filter and the interleaved LCLC filter, which results in a 70 % reduction in filter weight compared to the LC filter. Finally a 10 kW, 100 A single-phase prototype using SiC devices is constructed to demonstrate and validate the analysis and the design methods. The converter achieved a gravimetric power density of 2.9 kW/kg and a volumetric power density 5.3 kW/litre with an efficiency of 95.1 % at 10 kW.

Date of Award	1 Aug 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Rebecca Todd (Supervisor) & Andrew Forsyth (Supervisor)