Research On Key Technologies Of Parallel Grid-connected Converter

With the rapid development of modern society and the advancement of industrialization,the demand of energy is also growing.Renewable energy sources such as solar energy and wind energy have inexhaustible and non-polluting advantages,which have aroused widespread concern.As a medium for converting the energy stored by new energy sources such as solar energy and wind energy into electric energy with the same frequency and phase,the grid-connected converter has also attracted the attention of experts and scholars at home and abroad,and has achieved rapid development.With the continuous upgrade of the power level of the grid-connected system,the requirements for the capacity of the grid-connected converter are becoming more and more stricts.Due to the limitation of current stress and voltage stress of power components,the capacity of a single grid-connected converter is limited,and in many cases,the requirements of the grid-connected system cannot be met.At this time,the parallel operation mode of t he grid-connected converter is proposed.Compared with a single grid-connected converter,the parallel grid-connected converter has the advantages of convenient installation and maintenance,high reliability and low cost.In practical applications,it gradually replaces the status of a single large-capacity grid-connected converter.This paper starts with the development background,research significance and application prospects of grid-connected converters.Firstly,it reviews the research status of grid-connected converters at home and abroad,points out the problems existing in parallel operation of grid-connected converters,summarize the current major solutions.Then,the parallel type grid-connected converter system is introduced.The working principle of the converter is introduced.The mathematical model of the grid-connected converter is established.Then,starting from the related problems of a single grid-connected converter,introduced the software platform design of the system.When designing the vector control strategy based on grid voltage orientation,a grid voltage separation algorithm is proposed,which can extract various components among many components,and proposes a forgetting filtering algorithm suitable for grid voltage,which can effectively suppress the influence of sampling error.Two common phase-locked loops are introduced,and the design of the main parameters of the voltage outer loop and current inner loop is described.A current controller with fast dynamic response is also introduced,and improvements are proposed to improve its control performance.The causes of the dead zone during the modulation of the converter,the influence of the action and the compensation algorithm are analyzed.The definition and mathematical model of the circulation are given on the topology of the parallel grid-connected converter.Two kinds of mainstream modulation methods are introduced,and the characteristics of the circulating variation of the parallel converter system under these two modulat ion modes are analyzed in detail.The carrier period is taken as the minimum unit,and the graph of variation of the circulation in the two modulation modes is given.Aiming at the low frequency circulating current caused by different common mode voltage injection,a hybrid modulation strategy is proposed.For the high frequency circulating current caused by the inconsistent carrier period,a line carrier period synchronization algorithm is proposed,which effectively suppresses the circulating current between the parallel converter systems.Finally,a parallel grid-connected converter physical platform was built in the laboratory to verify the effectiveness and feasibility of the proposed suppression strategy.