Switched capacitor converters:a new approach for high power applications

Abstract

High-power, high-voltage and high voltage-conversion ratio DC-DC converters are an enabling technology for offshore DC grids of the future. These converters are required to interface between offshore wind farms and an offshore DC grid and a key design issue is the size and weight of the converter, which significantly impacts the cost of the associated off-shore platform. In addition to this application, some rural communities, particularly in Canada, Australia and South Africa,which are located far away from the electrical power generators, can take the advantages of this technology by tapping into existing HVDC transmission line using a high voltage-conversion ratio DC-DC converter. The work described in this thesis is an investigation as to how such DC-DC converters may be realised for these applications. First a review of existing DC-DC converters was carried out to assess their suitability for the target applications. A classification of DC-DC converters into Direct and Indirect converters was proposed in this work based on the manner in which the energy is transferred from the input to the output terminal of the converter. Direct DC-DC converters, particularly Switched Capacitor(SC) converters are more promising for high-voltage, high-power and high voltage-conversion ratio applications, since the converter can interface between the low-voltage and the high-voltage terminals using low-voltage and low-power power electronic modules. Existing SC topologies were examined to identify the most promising candidate circuits for the target applications. Four SC synthesis techniques were proposed in order to derive new SC circuits from existing topologies. A new 2-Leg Ladder, modular 2-Leg Ladder and bi-pole 2-Leg Ladder were devised, which had significant benefits in terms of size and weight when compared with existing circuits. A scaled power 1 kW converter was built in the laboratory in order to validate the analysis and compare the performance of the new 2-Leg ladder circuit against a conventional Ladder circuit, where it was shown that the new circuit had higher efficiency, smaller size and lower output voltage ripple than the Ladder converter.