Firing angle optimisation for chain cell converters

Firing angles for chain cell multi-level converters can be optimised to reduce waveform distortion. Optimisation techniques have previously been developed that calculate the required firing angles, but these techniques fall short of accurate optimisation due to over-simplification of the problem. Established optimisation routines often only work on a subset of possible converter modes, hence optimal solutions are not produced for all converter conditions. The optimisation routines also do not model the finite capacitance in each cell element, assuming that the capacitance is so large that it may be treated as infinite. This paper develops equations that model the behaviour of a k-cell chain cell converter in a detailed manner. The converter model handles all operation modes and takes finite capacitance into account. New optimisation algorithms are developed that accurately find the required firing angles for a k-cell converter. Specific examples are given for 3 cell and 5 cell converter designs. The subject area covered is relevant to both the design and control of chain cell converters