Research On Control Strategy Of Vienna Rectifier And Its Parallel System

As a power supply equipment for electric vehicles,the DC charging system is an important support for the long-term and stable development of the new energy vehicles(NEVs)industry.It plays a key role in realizing China’s dual carbon goals and constructing a green civilization.The DC charging system consists of two stages,the front-stage Vienna rectifier and the poststage DC/DC converter.In which,the Vienna rectifier connecting the AC power grid and the post-stage DC/DC converter is the core component,and its performance directly affects the power quality and reliability of the entire DC charging system.Although the Vienna rectifier control methods have been widely studied by many scholars,some problems are remained and need to be solved.For a single Vienna rectifier,the traditional control methods face the challenges of poor DC output voltage anti-interference and high AC input current harmonics.For the parallel system of Vienna rectifiers,the zero-sequence circulating current(ZSCC)and current zero crossing distortion are inevitable,which degrades the AC current quality and even causes the equipment to break down.The above issues restrict the application of the Vienna rectifier in DC charging systems.Thus,this thesis takes the Vienna rectifier as the research object,and carries out a series of studies including double closed-loop control strategy for single Vienna rectifier and ZSCC suppression and current zero crossing distortion elimination strategy for multiple parallel Vienna rectifiers.With these proposed control strategies,the system low harmonic and high reliability operation are realized.The main research contents and innovations are listed as follows.Firstly,the topology and operating principle of the Vienna rectifier are analyzed,and the mathematical models and equivalent circuits are derived in both stationary coordinate systems and synchronous rotating coordinate systems.Based on the above analyses,the mechanism of the current zero crossing distortion of the Vienna rectifier is studied.Then the current zero crossing distortion elimination methods are given,which are based on space vector modulation and carrier-based pulse width modulation.These researches lay a theoretical basis for the subsequent control strategy designing of the Vienna rectifier.Secondly,stable DC output voltage and high-quality AC input current are the primary control objectives of the Vienna rectifier.This thesis proposes a dual closed-loop control strategy to achieve the above goals.In the outer voltage loop,a sliding mode controller based on an improved variable power convergence law is designed.The control method improves the robustness and anti-interference of the DC voltage in case of load disturbance,which maintains the stability of the DC output voltage.In the inner current loop,an unweighted coefficient double vector fast finite control set model predictive control(FCS-MPC)method is designed to effectively reduce the harmonics of AC input current and achieve neutral point voltage balancing.Thirdly,parallel operation of the Vienna rectifiers is an ideal solution to increase system power rating.However,the parallel operation leads to the ZSCC problems,inevitably.In addition,the current zero crossing distortion of the Vienna rectifier and the ZSCC influence each other,which results in severe AC current distortions and even threatens the reliability of the system.Therefore,this thesis takes the parallel system of two Vienna rectifiers as an example and analyzes the generation mechanism of ZSCC and its interaction with current zero crossing distortion.Based on the above analyses,an improved fixed-time&reactive power injection control strategy is proposed.In this control strategy,the ZSCC is suppressed with the improved fixed-time controller,and the current zero crossing distortion is eliminated with the precise reactive power injection.By using this proposed method,the ZSCC suppression and current zero crossing distortion elimination are realized simultaneously.With the proposed method,the AC current quality is improved and reliable system operation is ensured.Finally,an experimental prototype of a three-level Vienna rectifier is built.The hardware design principle and scheme are analyzed.The main circuit,the driver circuit,and the voltage and current sampling circuit are designed in this thesis.Based on the experimental prototype,the dual closed-loop control strategy and improved fixed-time&reactive power injection control strategy are tested,respectively.The experimental results verify the effectiveness of the proposed control strategies.