Research On Control Method Of Three-phase Three-level NPC Converter System

Using renewable energy such as wind energy,solar energy,tidal energy and biomass energy to replace traditional fossil energy is a good way to solve environmental pollution and energy crises.However,the randomness,volatility,and intermittent nature of renewable energy have caused a series of power quality problems,and even affected the safety and stability of the power system.The three-phase three-level Neutral Point Clamped(NPC)converter has the advantages of less harmonic content,low switching device loss,and large output capacity.Using the three-phase three-level NPC converter as the connection hub between the renewable energy system and the power grid can improve power quality,reduce the harmonic pollution of renewable energy to the power grid,and improve the power factor.The three-phase three-level NPC converter is a multivariable,strongly coupled nonlinear system.The unique sliding mode of sliding mode control makes the controlled system have good robustness and excellent response characteristics.This paper studies the control method of three-phase three-level NPC converter based on sliding mode control.By analyzing the basic principles of the three-phase three-level NPC converter and its working mode at different levels,the mathematical models of the three-phase static coordinate system and the two-phase rotating coordinate system are obtained,which provides a basis for the research of control strategy.Aiming at the problems of modulation and neutral point imbalance of three-phase threelevel NPC converter,the modulation strategy based on Space Vector Pulse Width Modulation(SVPWM)is analyzed from three aspects: sector judgment,action time calculation,and action sequence design.A SVPWM modulation method of gh coordinate system that can reduce the amount of calculation is studied.The reason for the unbalance of the midpoint potential of the two capacitors on the DC side is analyzed.The imbalance of the midpoint potential is controlled by the control method of positive and negative small vectors,and the modulation and control methods are simulated and verified.Aiming at the robustness of the three-phase three-level NPC converter,the study is based on sliding mode control.First of all,in the traditional control algorithm,complete the feedforward decoupling of active and reactive currents,and design a double closed-loop control method based on the DC bus voltage and the AC side inductor current to ensure the stable output of the three-phase three-level NPC converter.Secondly,based on the approximate structure of the Radial Basis Function(RBF)network,an adaptive sliding mode control method for the current inner loop is designed to improve the control accuracy of the system.Thirdly,based on the variable speed exponential approaching law,the voltage outer loop sliding mode control method is designed to weaken the system chattering and improve the robustness.The Lyapunov stability analysis of the controlled system is carried out.Finally,through numerical simulation and comparison with the feedforward decoupling control method,the reliability of the proposed method is verified.The experiment sets up a three-phase three-level NPC converter experimental platform,using DSP TMS320F28377 as the core processor,designing circuit device parameters,and then using this platform to conduct experiments on the feedforward decoupling control method and sliding mode control method.The results of the experiment show the feasibility and effectiveness of sliding mode control in practical applications.