| It is of great significance for China to develop electric vehicles in the face of environmental pollution and oil and gas shortage.Power battery is the energy source of electric vehicle.Fast and safe DC charging system is an essential infrastructure for the promotion and application of electric vehicle.The 380V distribution adopted by the traditional DC charging system is the secondary distribution obtained by reducing the voltage of 6kV～35kV medium voltage through the power frequency transformer.The voltage is subject to multi-level transformation,and the energy loss is serious.Moreover,the power frequency transformer has a huge volume and high cost,which is more and more difficult to meet the needs of high efficiency and high power density of power electronic equipment.In recent years,as a medium voltage primary distribution scheme,the medium voltage direct mounted charging system has received extensive attention and attention from the charging market at home and abroad.Its input end is directly connected to the medium voltage power system without power frequency transformer,which has the outstanding advantages of high output power,high conversion efficiency,low cost of transformation and distribution,multi charging port output and so on.Medium voltage direct mounted charging system has become an important development direction of fast charging technology in the future.After investigation,a 10KV medium voltage direct mounted charging system with twostage topological structure of "cascade rectifier+high frequency isolated DC/DC converter"is adopted in this paper.Among them,the former rectifier not only directly interacts with the medium voltage distribution,but also outputs a stable DC voltage to the latter stage,which is the core and key of the system.Cascade NPC rectifier is widely used as cascade topology because of its strong withstand voltage capability.However,its related technology and theory are not perfect.There are still some problems,such as large switching loss,poor power quality at AC side,voltage instability under load disturbance and so on.For the above problems,in this article takeing the cascade NPC rectifier as the research object,puts forward the highperformance modulation and control strategy,and realizes the performance requirements of the system,such as "high efficiency,low harmonic,high immunity and fast response".The main research results are as follows:Firstly,the working principle and cascade system structure of NPC rectifier are introduced.Based on single module topology and cascade multi module topology,the general mathematical model based on switching function is deduced respectively.With instantaneous power theory,their mathematical model in two-phase dynamic dq coordinate system is further deduced.Secondly,aiming at the problems of low power transmission efficiency and network side harmonic pollution,an efficient weight factor based hybrid SVPWM method(WFBH-SVPWM)is proposed.WFBH-SVPWM Accurately calculate the peak current ripple of the power grid and divide the action interval based on the weight factor,which overcomes the mutual restriction between low power loss and high power quality of cascade NPC rectifier,and provides theoretical guidance and technical support for green and reliable power consumption of medium voltage direct mounted charging system.Thirdly,aiming at the problems of voltage instability and slow dynamic response under load disturbance,a command filter backstepping control(CFBC)strategy based on command filter is proposed.By introducing a new command filter,the fast and accurate estimation of the derivative of the current reference signal is realized,and the operation burden of the controller is reduced.The proposed control strategy realizes the fast response,high disturbance rejection and stable control of the output of the cascade NPC rectifier with system parameter disturbance.Finally,the simulation model based on Simulink and the 6kW experimental platform based on dSPACE software and hardware working platform are built.The validity and feasibility of the controler and modulation method proposed in this paper are proved by simulation and experiment. |
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