Research On Pulse Optimization And Control Of Five-Level Reinjection Current Source Converter

For traditional 12-pulse converter,when the DC-side current is six-times the specific triangular waveform of the fundamental frequency,the AC-side current harmonic can be completely eliminated.In actual engineering,the linear multi-level waveform approximation can be used to approximate the ideal reinjection current waveform.In this thesis,a Five-Level Reinjection Current Source Converter(FLR-CSC)system is constructed by using a linear five-level reinjection current instead of an ideal reinjection current waveform.This thesis has done the following research on some key issues of the FLR-CSC system:Firstly,the principle of harmonic elimination of the traditional 12-pulse rectifier in the ideal reinjection current is analyzed in detail.Based on this,the current harmonic characteristics and DC-side voltage characteristics of the FLR-CSC are analyzed,and the simulation verification is completed.Secondly,an optimization method of FLR-CSC reinjection pulse combination method is proposed.The influence of non-ideal reinjection current on AC-side current harmonie distortion(THD)of FLR-CSC system is obtained by harmonic analysis method.By summarizing the principle of reinjection pulse combination,the voltage and current distribution of the reinjection circuit under the reinjection pulse are analyzed,and the harmonic characteristics of the actual five-level reinjection current under different reinjection pulse combination modes are obtained.An optimal reinjection pulse combination method is obtained by programming the 72 reinjection pulse combination methods.Aiming at the closed-loop control problem of FLR-CSC system,the closed-loop small-signal model is established to analyze the influence of system parameters on the closed-loop stability,adjustment time and overshoot of the system.Aiming at the closed-loop control problem of FLR-CSC system,the closed-loop small-signal model is established to analyze the influence of system parameters on the closed-loop stability,adjustment time and overshoot of the system.In addition,the FLR-CSC system provides a 7.5°zero current interval for the pre-stage main-bridge thyristor commutation due to the presence of the five-level reinjection circuit,which allows the FLR-CSC system to implement the fourth quadrant commutation and get commutated condition.Based on this,the double-bridge series closed-loop power decoupling control is studied and verified by simulation.This thesis builds an experimental prototype platform to verify the above theory and simulation results.The experimental results show that the harmonie elimination effect and voltage characteristies of the FLR-CSC system are consistent with the theoretical analysis,and the optimization effect of the reinjection pulse combination method is obvious.The DC-side current closed-loop control of the FLR-CSC system and the closed-loop power decoupling control of the DFLR-CSC system are effective.