Startup Control to Eliminate Inrush Current for Star-Connected Cascaded H-Bridge STATCOM

Soft startup is essential to the operation of the star-connected cascaded H-bridge (CHB) STATCOM. The conventional startup stage involves only uncontrolled rectifier process in which the dc voltages are charged by inserting startup resistors to limit the inrush current. Nevertheless, another inrush current is induced at the beginning of normal operation due to the low charged dc voltages during the startup stage. In this article, to eliminate this inrush current, a novel startup control is proposed for further charging the dc voltages after the uncontrolled rectifier process. During this controlled charging process, the startup resistors are not bypassed so as to smoothen the dc voltages and limit the inrush current. In this scenario, the overall dc voltage control with constant charging current is proposed by establishing a relationship between the overall dc voltage and the positive sequence modulation index. Besides, the cluster dc voltage balancing control based on the negative sequence voltage injection is proposed by redistributing the active power of the STATCOM. Moreover, the charging current is designed to reduce the active power of the startup resistors and increase the charging speed. Due to the flexibility of the startup control, it can be applied to any voltage-source-based ac/dc grid-connected converter. Finally, the performance of the proposed control is verified by the simulation and experimental results.