The Study On Key Technology For Three-level Converter Based On LCL Filter

The neutral point clamp three-level converter has been one of the research focus topics in the field of high-voltage and capacity electrical energy. It can effectively improve capacity and voltage pressure levels of converters, reduce the output voltage harmonic and switching losses. Some key technologies for three-level converter based on LCL filter have been studied in this paper.Firstly, the topology and working principle of three-level converter based on LCL filter are introduced. The mathematical model is established in the three-phase ABC stationary coordinate system and the mathematical model in dq coordinates is obtained through coordinate transformation.Secondly, two kinds of modulation strategies of three-level converter are studied. For SPWM modulation, its difficulty of application in digital system has been pointed out. To solve this problem, an improved SPWM method has been given. For SVPWM modulation, after a detailed introduction, some improved methods is introduced included 60° coordinate transformation and two-level zoning method. Both strategies have been simulated and comparative analyzed in Matlab/Simulink.Then the neutral point balance problem and the reason for the neutral point voltage fluctuation are analyzed in detail. On this basis, neutral point balancing methods based on mid-point potential balancing factor is discussed. Otherwise, this paper proposes a hybrid three-level neutral-point balance control strategy used for the simplified three-level SVPWM. Simulation results verified that the algorithm can achieve neutral-point balance control of a full frequency range.This paper proposes a parameter design of LCL filter for three-level grid-connected converter based on SVPWM. Simulation results showed that the parameters designed by the method are smaller and more reasonable than traditional methods。 In this paper, quasi proportional resonant controller is applied in the control tragedy of three-level grid-connected converter. And the grid-connected control is simple and efficient.A control system which have DSP(MC56F84789) and CPLD(EPM1270) as the control center of the system has been constructed through theoretical study. The hardware schematics and software design process in DSP and CPLD are introduced.