The potential for matrix conversion in marine electric propulsion systems
The growing interest in marine electric propulsion is driven by the need to improve ship performance. This thesis presents a review of existing marine electric propulsion technology and potential future variable speed drives. The matrix converter was found to be a promising alternative to existing technology and this thesis describes an investigation into its potential for marine electric propulsion. The matrix converter performs direct AC-AC conversion using a high switching frequency to produce high- quality converter waveforms. The investigations were carried out using a 10kVA laboratory matrix converter and a series of computer models. The laboratory matrix converter employed IGBT bi-directional switches operated by a custom-built stand-alone control system employing various Venturini algorithms. The completed system was then used to investigate the performance of the matrix converter, examine the operational effect of different switch sequences on loss and converter input and output waveform harmonics. Computer models of the various control algorithms were developed and verified against results produced by the laboratory matrix converter and further developed for more detailed investigation. An examination of switch commutation and the effect on loss is reported and this led to the development of a novel commutation strategy named 'Opti-Soft'. This strategy maximises 'natural' current commutation in the matrix converter and balances switching loss across devices without increasing total loss above existing switching strategies. The results from the computer models show the 'Opti-Soft' strategy to possess a similar harmonic characteristic to the conventional strategy allowing the use of existing filter designs. The design of a conceptual marine matrix converter drive and ancillary equipment is also considered and its performance investigated. The problems of reduced supply utilisation, topology selection, cooling and protection are examined. A novel mode of operation that takes advantage of the matrix converter's unrestricted output frequency is proposed. The conclusions discuss the findings of the study and relate the practical issues that will need to be overcome before the matrix converter can be considered for marine use.