Capacitor Pre-Charge Method for Back-to-Back Seven-Level Hybrid Clamped Converter Without Extra Power Supply

Multilevel converters with floating capacitors generally require precharging to build up the voltage on floating capacitors and dc-link before the normal operation. In particular, precharge is challenging for converters with overvoltage concerns under initial zero capacitor voltages. As a promising topology, seven-level hybrid clamped (7L-HC) converters also require precharge of capacitors and shall avoid high voltage stress during the precharge. Considering the high cost of deploying extra dedicated precharge circuits in a medium voltage system, the precharging method without relying on dedicated power supplies is favored. In this article, specific switching states and their corresponding selection schemes are designed to precharge floating capacitors of back-to-back 7L-HC converters. In this process, the inrush current and voltage stress can always be limited in the safe range. This method only needs series and shunt resistors typically already exist in practical converter circuits, resulting in low cost and ease of implementation. The design method of the resistances is also provided. Both simulation and experimental results validate the feasibility of this method.