Voltage Sensorless Control For Single-phase CHB Converter Under Distorted Grid Conditions

With the development of power electronic devices and the increasing demand for electrical energy,Pulse Width Modulation(PWM)converters play a key role in power conversion.In order to realize medium-high-voltage and high-power power conversion,cascaded H-bridge(CHB)converter has been widely used due to its good power quality,high reliability and modularity.These typical applications include power transmission,distributed power generation,power quality management,motor drive,etc.For a single-phase grid-connected converter,the control strategy is a key factor that determines the performance of the equipment,which ensures the steady-state accuracy,dynamic response,anti-interference ability and the reliability.Therefore,it is valuable to make reseach into current control strategies for single-phase CHB converter,aiming to simplify the structure and improve the performance.Considering its topology characteristics and applications,the key technologies of CHB converter are firstly analyzed in this thesis.Then,a voltage-sensorless control mehod is applied to improve the reliability and reduce the hardware cost.The emphasis lies on the accurate reconstruction of the grid voltage or virtual flux signal.Aiming at the problem that the existing sensorless methods have poor performance under distorted grid conditions,this thesis proposes a new flux observer with enhanced harmonic filtering ability,which can adapt to complex operating conditions.Thus,the fundamental virtual flux signal is obtained for synchronization.On this basis,a phase-locked loopless control method is deduced in detail,with which the phase-locked loop and coordinate transformation are omitted,avoiding the complicated parameter design and improving the dynamic performance.On the other hand,it is incomplete to filter harmonics only in the synchronization link of control system.Therefore,based on the impedance model,the influence of the background harmonics on the output current has been studied,then different solutions are analyzed and compared.In order to improve the harmonic impedance of the system,the harmonic peaks of the current loop is effectively increased by the proposed current controller.Consequently,the proposed strategy realizes both tracking of fundamental current reference signal and rejection of grid harmonics interference.Meanwhile,it has no impact on dynamic response of the control system,achieving fast and smooth dynamics process.In addition,in order to ensure the stable operation of CHB under various working conditions,the principle and realization of the DC voltage balance strategy are analyzed.Finally,based on the MATLAB/Simulink simulation software and the built experimental prototype,the control strategy proposed in this thesis is sufficiently verified.The simulation and experimental results show that the control strategy proposed in this thesis has excellent performance,and the comparative analysis verify its superiority.