On soft-switching isolated AC/DC converters without auxiliary circuit

Abstract: The thesis is concerned with a class of power electronic converter systems that permit bidirectional AC/DC conversion with galvanic separation of the AC and DC sides by a transformer operating at elevated frequency. The systems consist of a capacitively snubbered voltage source converter and a cycloconverter, separated by the mentioned transformer. The use of a medium-frequency transformer permits considerable reduction of the transformer weight and size. Both systems with single-phase and three-phase AC terminal are conceivable.It is shown that the converter concept is compatible with soft switching of the semiconductor valves, which considerably reduces the switching losses and the stress on the valves. This can be achieved in all points of operation without any need for auxiliary valves dedicated to the commutation process. A resonant mode of commutation, which is vital for extending the soft-switching operating region also to low load conditions, is described in detail.The demand for soft switching imposes certain important constraints on the possibilities for pulse width modulation of the AC side voltage. These tend to result in low-order harmonics in the output voltages that are not negligible. The impact of the constraints on the resulting pulse patterns is analysed by analytical derivation of the harmonic spectra. This way the low-order harmonics were found to originate from sidebands around multiples of the switching frequency. Furthermore, advanced modulation schemes, both for single-phase and three-phase systems are described and analysed. These schemes allow for improved harmonic performance and overall loss reduction. The improvement in harmonic performance is most pronounced at low modulation index.A down-scaled prototype converter with a rated power of 40 kVA was designed and built up. The prototype can be configured for operation with single-phase or three-phase AC terminal. In the thesis special emphasis is put on the implementation of the commutation and modulation methods in the digital control system. Experimental results from the prototype verify the intended operating principles. Measurements made on the prototype indicate an overall efficiency of up to 92%.The envisaged applications are found in the high power range, where the advantages of soft switching are most significant. Two examples of possible applications are outlined: line side power conversion in rail vehicles and systems for energy capture from offshore wind power parks.