Modulation Strategies for Mutually Commutated Isolated Three-Phase Converter Systems

Converter structures consisting of a voltage source converter and a cycloconverter connected by a medium frequency transformer have several attractive properties. Bidirectional AC/DC conversion as well as voltage transformation and isolation by the MF transformer can be achieved. Furthermore, by consistently commutating the two converters in alternation it is possible to achieve soft switching for all semiconductor devices. However the demand for soft switching imposes certain restrictions concerning the modulation of the converter system that may adversely affect the harmonic properties of the AC side voltage. Unlike conventional VSC modulation the modulation of the studied converters is heavily dependent on the load angle. Various possible modulation schemes are described in detail. In particular a new space vector-based modulation method derived by adapting a method originally developed for quasi-resonant DC link converters is presented. The studied modulation schemes are compared and evaluated in terms of the output voltage harmonic distortion and the RMS transformer current. The comparison shows that the presented SVM-based method gives superior performance in almost all operating points. An exception is operation at load angles near plusmnpi/2 rad and intermediate modulation depth. Under these circumstances the harmonic properties deteriorate due to an increased production of low frequency harmonics