Control strategies for mutually commutated converter systems without cycloconverter turn-off capability

Mutually commutated converter systems consisting of a voltage source converter and a cycloconverter connected by a medium-frequency transformer allow bidirectional AC/DC conversion as well as voltage transformation and isolation by the transformer. However, such converter systems require an extra power conversion stage compared to conventional converter systems, which may result in low system efficiency despite the soft-switching commutation scheme. In order to reduce the power losses in the cycloconverter, it is desirable to utilize fast thyristors instead of IGBTs. As a consequence, the cycloconverter valves lack turn-off capability because a thyristor can not be turned off by its gate. This paper describes and evaluates the most common control strategies in order to control a thyristor-based cycloconverter in a mutually commutated converter system. The behaviour of the cycloconverter depends strongly on the chosen control strategy when one of the output currents approaches zero, which may inherently affect the harmonic properties of the output voltages and currents. Experimental results from a 20 kVA prototype show that it is possible to successfully operate a mutually commutated converter system without turn-off capability.