State feedback control strategy and voltage balancing scheme for a transformer-less STATic synchronous COMpensator based on cascaded H-bridge converter

In this study, a transformer-less STATic synchronous COMpensator (STATCOM) based on cascaded H-bridge (CHB) multilevel converter with floating DC links is presented. This study introduces a control strategy based on state feedback control algorithm in order to supply reactive power to the grid and to compensate the total losses in the internal H-bridge cells. Using this strategy, the converter stability and the fast dynamic response are guaranteed. Moreover, a voltage balancing strategy is proposed, which not only regulates the DC-link voltage of H-bridge cells to the reference value, but also evenly distributes total reactive power among the converter cells, even when there is difference between the losses of H-bridge cells. A 21-level CHB-based STATCOM is simulated in PSCAD/EMTDC environment to evaluate the performance and effectiveness of the new control strategy. Furthermore, the validity of new method is verified on a down-scaled 7-level CHB prototype.