Research On The Several Key Technology Of Grid-Connected Inverter Based On Two-phase Stationary Coordinates

With the rapid development of the power electronics technology and the modern control theory, the three-phase voltage grid-connected converter with adjustable power factor, bidirectional energy flow, quick and stable dc side voltage mediation ability and dc side don’t need a lot of supporting capacitors is widely used in solar energy, wind energy generation, static var compensator, active power filter, unified power flow control, high-voltage direct current transmission field.The performance of control system depends on the key technology of grid-connected converter. especialiy when three-phase grid voltage are unbalanced, or when three-phase grid voltage are disturbed by external factors, control system cannot output three-phase sinusoidal current, low frequency harmonic occur, resulting in an increase of the degree of distortion of the grid voltage, seriously equipment and other loads are damaged, so the research of key technology of converter control is necessary.This paper first to T three-level converter topology is analyzed, the switch model are established under the three-phase static coordinate system, two-phase static coordinate and two phase rotating coordinate system, thus, T three-level converter of the relationship between various physical quantities and principle of T three level grid-connected converter are mastered. at the same time, under the traditional double PI control, principle of T three-level converter are analyzed. it is verified by experiments.Secondly, in this paper phase-locked loop of grid-connected converter are analyzed under two phase static alpha beta coordinate.based on the existing main phase-locked loop method, the article focuses on the proposed detection method of grid voltage and their micro-componental,by solving the corresponding system of equations, the fundamental component amplitude and phase information of positive and negative sequence can be obtained.Because of differential operation would be resulting in greater error.then using the AC forgotten integration algorithm.so the signal can be separated into a smoother grid voltage by the filter.Then, the mathematical model of PWM rectifier based on deadbeat control (DBC) under the two-phase stationary αβ frame and its discrete method are introduced. A predictive current weighted correction method is proposed, which is based on "two step prediction" of the reference current. This method can not only deal with the problem of delay compensation because of the sampling and digital processing delay, but also reduce the sampling error to improve the system stability. Finally, the experimental platform of PWM rectifier is built in the laboratory, and the feasibility of the proposed control method is verified.Finally, The instantaneous power model under αβ frame is described under unbalanced grid voltage in this paper, which combines the separated positive and negative sequence components with instantaneous power model to derive the reference currents of three different control objectives under αβ frame. Inner current loop adopts deadbeat control, so positive and negative sequence components are not required to be separated from currents under unbalanced grid voltages. Finally, the experimental setup of three-phase grid-connected converter is built in the laboratory, and then the effectiveness of this method is verified by experiments.At the same time also on T tri-level grid inverter control of some common problems are studied:(1) commutation mode of T three-level converter and dead zone compensation is analyzed; (2) reasons of neutral voltage unbalance and control method of T three-level (3) A modulation method with reduced switching losses and considering the neutral point voltage balance.