Research On Resonant Modular High-ratio DC Converter

As the key devices of high voltage DC transmission and distribution system,DC converter with high voltage ratio has become one of hot research topics.With this background,this thesis studies a resonant modular high-ratio DC converter(RMHRDC).Featured with modular structure,the RMHRDC consists of stack of half-bridge sub-modules(HBSMs),resonant inductor,parallel output inductor,and a rectifier bridge.The employment of cascade HBSM structure makes RMHRDC easy to be expanded.By controlling the number of HBSMs switching-in or-off,the RMHRDC can achieve high voltage ratio and multi-voltage ratio,and the sub-module capacitor voltage has self-balancing capability.By adopting the resonance principle,RMHRDC shows soft switching characteristics,and it further reduces switching losses.Then,this thesis studies the working principle of RMHRDC.The voltage ratio of RMHRDC,the sub-module capacitor voltage and the conversion of AC and DC energy inside the circuit are analyzed in detail.With simulation model,the high voltage ratio of RMHRDC,self-balance of sub-module voltage and soft-switching characteristics are simulated and verified.Secondly,besides analysis of the main circuit parameters and losses of RMHRDC,the influence of parasitic parameters on the output characteristics of RMHRDC are also revealed in detail.Thirdly,the most serious short circuit fault is selected for detailed studying.Based on mathematical calculation of fault current and analysis of influencing factors,the fault isolation control method after RMHRDC short-circuit fault are proposed.The correctness of the analytical formula and the feasibility of fault isolation control are verified by simulation.Finally,a low-power laboratory prototype was design,constructed and tested.The experimental results are consistent with the theoretical analysis and simulation results,which further verified the correctness and feasibility of the RMHRDC studied in this thesis.