Analytical Modeling of a Square-Wave-Controlled Cascaded Multilevel STATCOM

The aim of this paper is to present an analytical, state-space model of an indirect, voltage-controlled cascaded-type multilevel static synchronous compensator (STATCOM) with ldquosquare wave control.rdquo The multilevel converter model is segmented into a dynamic and static part in order to accurately represent all internal feedback connections. Each voltage component is analyzed in detail and described mathematically by an averaged expression with an equivalent capacitance. The STATCOM model is linearized and linked with a DQ frame AC system model and the controller model, and implemented in MATLAB. The controller gains are selected by analyzing the root locus of the analytical model to give optimum responses. The validity and accuracy of the proposed model are verified against non-linear digital simulation PSCAD/EMTDC in the time and frequency domain. The model is very accurate in the subsynchronous range, and it is adequate for most control design applications and practical stability issues below 100 Hz. Furthermore, the developed model can be used for multilevel cascaded converters which exchange real power.