Optimized stair-case modulation for modular grid connected converters

Stair-case modulation is a simple direct-switching technique applied for modular multilevel converters to reduce the switching frequency of each module and hence minimize the switching losses. The technique could be optimized using a certain modulation index (M) for minimized THD of the output voltage and reduced number of modules; hence decreasing the cost and size of the converter. However if M changes, increased harmonics may result raising the need for an on-line technique that recalculates the switching angles for reduced voltage harmonics with varying M. An adaptive least mean square algorithm is adopted here for this purpose and compared with other techniques. It has proven to produce less voltage THD as compared to a previous on-line technique over the same range of modulation indexes.