Research On Model Predictive Control Of Three-Level Inverter And Its Parallel System

In systems such as power energy consumption and renewable energy power generation,the inverter plays an important role in ensuring the smooth flow of energy and realizing high-quality electric energy.To meet the increasing requirement of power quality and efficiency,new inverter topologies and control methods are being developed.Due to the lower output harmonics and higher overall conversion efficiency,three-level inverters have become a hot topic in research and applications.Model predictive control(MPC)has been widely considered as the next generation of the most promising high-performance control method in the field of inverter control.However,there are still many problems when applying the MPC-based method to the three-level inverter.Therefore,this thesis takes the three-level inverter and its parallel system as the research object.Researches are performed including the performance optimization of direct power control(DPC)methods of a single three-level inverter,leakage current suppression for the photovoltaic system,common mode voltage(CMV)suppression considering the dead time effects,as well as the zero sequence circulating current(ZSCC)suppression of the three-level inverter parallel system based on the MPC method.It is hoped that MPC-based methods can be used to solve the above problems in the practical application of three-level inverter and its parallel system.The major work and innovations of the thesis are as follows:Firstly,this paper takes the three-level T-Type inverter(3LT~2I)as an example.The basic working principle of the 3LT~2I is analyzed and the prediction models for the current and the complex power are deduced,respectively.Besides,the basic principle of the finite control set model predictive control(FCS-MPC)is introduced.Two key techniques for the implementation of the FCS-MPC based method including the computational delay compensation and the extrapolation interpolation are analyzed.Simulation results are presented to verify the analysis above.Secondly,in view of the problem that the steady-state accuracy of the traditional DPC method is low,this paper proposes a new model predictive direct power control(MPDPC)method.A new predictive model is developed,in which the conjugate of the complex power is then chosen as the control variable.Based on the newly developed model,principles of the voltage vector(VV)selection,as well as the cost function design,are introduced in detail.The specific steps for the implementation of the proposed MPDPC algorithm are given.By replacing the switch vector table of the traditional DPC method with the accurate model of the inverter,the MPDPC method proposed in this paper can significantly improve the steady-state and dynamic performance of the inverter compared with the traditional DPC method.Comparative experiments verify the effectiveness of the proposed MPDPC method.Aiming at the problem of the leakage current in the system with non-isolated three-level photovoltaic inverters,the mathematical model of the leakage current is first derived and the fluctuations of the CMV is demonstrated to be the excitation source of the leakage current.Based on this conclusion,a new MPC-based method is proposed for the three-level inverters to maintain constant CMV by modifying the control set.So that the leakage current can be eliminated.The overall control block diagram of the photovoltaic system is presented and the validity of the proposed methods are verified by simulations and contrast experiments.To further realize the CMV suppression of the three-level inverter considering the dead time effects,the influence of the dead time effects on the CMV is further analyzed in detail on basis of the MPC-based method which realizes the constant CMV by modifying the control set.Based on the analysis,a novel MPC-based CMV suppression method which takes the dead time effects into account is proposed.The proposed method re-optimizes the control set based on the currently active vector to ensure that the CMV remains constant during the dead time state.Simulation and experimental results demonstrate that the CMV fluctuations caused by dead time effects can be effectively suppressed with proposed methods.Finally,due to the problem that the ZSCC appears in the parallel operating system with three-level inverters,an equivalent model of the ZSCC is developed by taking the parallel system where two 3LT~2Is are involved as an example.The mathematical model indicates both the differences of CMV and neutral point potentials(NPPs)between paralleled 3LT~2Is are the exciting sources of the ZSCC.On this basis,a novel MPC-based strategy is proposed to eliminate the ZSCC in parallel operating 3LT~2Is and the design method of related parameters in the cost function is given in detail.In the proposed ZSCC suppression strategy,an additional CMV constraint term is added to the cost function to maintain the CMV of each parallel inverter a constant value,so that the difference of CMVs between paralleled 3LT~2Is can be eliminated.Besides,an active NPP disturbance based ZSCC feedback control method is investigated to compensate the ZSCC by further modifying the cost function.This strategy can be directly applied to the parallel systems where more 3LT~2Is are involved.Simulation and experimental results demonstrate that the ZSCC between paralleled inverters can be effectively suppressed by the proposed strategy.