Adaptive VSG Control And Optimized Modulation Strategy Of Three-level Inverter In Micro-grid

As the connection between renewable energy and local loads or power grid,the interface inverter in micro-grid has the advantages of flexible operation and excellent reliability,which has become a research central issue both in theory and engineering.The control method of the interface inverter is an important factor affecting its performance.Although it has been widely concerned,its ability in aspects of control strategy and modulation strategy can be roomed for improvement,which causing the outstanding control characteristics of the inverter itself can not been fully utilized.In this paper,the interface inverter with T-type three-level topology is adopted.The research is conducted aiming at the issues existing in aspects of the control strategy and modulation strategy.The specific arrangements are as follows:Firstly,the topology of the inverter is introduced,and the research statuses of inverter underlying control strategy and modulation strategy in micro-grid are summarized.The working principle and commutation process of the T-type three-level inverter is analyzed,various working states are introduced and its mathematical model is deduced,which lay the foundation for the design of controller and the optimization of modulation strategy.Secondly,aiming at the disadvantages of lacking of characteristics such as"inertia" and "damping" in the power electronics micro-grid in the off-grid state and the frequency stability is easily damaged when it is disturbed,the virtual synchronous generator control strategy of inverter in micro-grid is studied based on the working principle of synchronous generator.The functions of various parameters in the control system are obtained by the ways of analysing the rotor motion equation of the synchronous generator.A improved control strategy of virtual synchronous generator is proposed.The control strategy can improve the frequency stability of the micro-grid through the changing virtual coefficients under the situation of different frequency offsets and frequency change rates.Thirdly,the modulation strategy is studied.The equivalence between the carrier-based pulse width modulation strategy and the space vector pulse width modulation strategy is introduced,and the DC voltage utilization rate of various modulation strategies is clarified.The exsiting problems and its causes of T-type three-level inverter in micro-grid from aspect of modulation strategy are studied.To improve the efficiency and waveform quality of the system,in order of priority reducing switching frequency,ensuring the neutral-point voltage balance and improving the common-mode voltage,an optimized modulation strategy is studied based on hybrid discontinuous pulse width modulation strategy and variable carrier.Finally,the model of T-type three-level inverter in micro-grid is established by Matlab/Simulink,control strategy and modulation strategy is combined to use in the model,the control method in this paper is verified by simulation experiment.