Research On Start-Stop Process Analysis And The Control Strategy Of Modular Multilevel Converter

With the continuous national development of energy science and technology,the construction of AC/DC hybrid grid has gradually become an important part of the future electric power industry.Power conversion device based on Voltage Source Converter(VSC)is becoming the key equipment of AC/DC network interconnection and Modular Multilevel Converter(MMC)becomes one of the most promising converter topology with many advantages,such as low construction difficulty,high conversion efficiency,good waveform quality and strong fault handling ability.One feature of the MMC is that each phase unit contains a large number of submodule(SM)capacitors,in order to ensure the safe and normal operation of the converter,it is necessary to effectively control the capacitive energy.Therefore,before and after the steady-state operation,the control system requires to adopt the appropriate way to precharge and discharge these SM capacitors.Due to the complicated transient process of start-up and shut-down,suitable control method is especially significant to improve the start-up speed,enhance stability,inhibition of over-current phenomenon.In this paper,some researches have been studied including the starting and stopping process analysis of MMC,the precharging equivalent circuit and the open-loop precharging control strategy:(1)Based on the half-bridge MMC,this paper analyzes the two-stage startup and shut-down operations widely used in the existing papers and proposed the improved start-stop control strategy.According to the energy flow relationship of different units in the converter,a multi-level capacitive voltage dynamic balancing control is established.With the fully consideration of the influence of the AC power grid,the simulation models are built and tested in MATLAB and PSCAD.It proves that this method can guarantee the stable power exchange between MMC and AC grid while precharging and discharging submodules rapidly,and the correctness of the closed-loop control strategy is verified.(2)In order to avoid the influence of complex closed-loop control with by-pass current-limiting resistors on the control system security,the SM precharging equivalent model with current-limiting resistors is constructed.For the MMC in high voltage level,the online computation of real-time sorting and voltage balancing control is complicated because of the large number of SMs in each arm.Considering that the voltage difference of SMs does not affect the completion of the startup phase,a Boost precharging strategy is proposed for MMC with a large number of SMs.Taking advantage of the immutability of inductive current,the effect of simultaneously SMs precharging can be achieved by regularly bypassing and blocking SMs.The influence of carrier frequency and duty ratio on precharging efficiency and the setting method of corresponding control parameters are analyzed.Aiming at the low and middle voltage MMC with few SMs,this paper proposes a general open-loop precharging strategy with improved modulation.The arm equivalent model is established to replace the switching operation of different SMs,and the optimal switching function corresponding to the maximum charging efficiency is derived by considering the different precharging directions from AC or DC side.An improved PSC modulation and capacitor sort method,which is suitable for the drastic change of capacitor voltage,is constructed to realize the safe and efficient precharging control of MMC.The MMC precharging model is built in the simulation platform,and the effectiveness and safety of the proposed strategy are verified by the simulation results.(3)Based on theoretical derivation and analysis,a down-scaled MMC prototype is built under laboratory conditions.The concrete realization process of physical platform construction is introduced in detail,including hardware circuit and parameter design,software control program and signal logic processing.Two proposed open-loop precharging control strategies are tested on the prototype,and the high efficiency of capacitor precharging is guaranteed with the current-limiting resistors.The obtained experimental waveforms are consistent with the simulation results.The experimental results of precharging on the MMC platform verify the safety and effectiveness of the proposed methods.