Theoretical framework for minimizing converter losses and harmonics in a multilevel STATCOM

In this paper, an analytical framework for minimizing losses and harmonics is presented for the cascaded-type-based multilevel static synchronous compensator (STATCOM) with "square-wave control." The focus lays on low losses, low-voltage total harmonic distortion (THD), and low dc voltage ripple. All of the system parameters are studied, including buffer reactance, de capacitance, transformer ratio, and transformer leakage inductance. The equivalent capacitance concept is used to represent the complex structure of the multilevel converter. It enables studies of the internal STATCOM resonances which have a significant influence on triplen harmonics and losses. The interactions between system parameter are investigated and analytical formulae for optimal parameters are derived. Detailed PSCAD digital simulations are used to confirm the analytical formulae. The summarized design steps are offered that are recommended to obtain optimum STATCOM system parameters.