Research On Modulation Strategy Of The Three Level Converter On Low Switching Frequency

High power converter is widely used in industrial production, which is an important part of the energy conservation plan in our country. Limited by the manufacture, power and some other factors, high power semiconductor devices have to work under low switching frequency(≤500Hz). Based on the neutral-point clamped three-level inverter and the three phase bridge controlled rectifier circuit, two low switching frequency space vector modulation strategies and a simplified four quadrant rectifier control strategy are researched.Firstly, as the traditional symmetrical regular sampling SVPWM’s output harmonic performance is poor, with controlling sampling frequency and switching frequency respectively, the asymmetric regular sampling SVPWM is researched through sampling frequency doubled to switching frequency. The analysis of double Fourier theory provides theoretical proof that asymmetry sampling modulation has more advantages over traditional SVPWM for some baseband harmonics and sideband harmonics eliminated. The output harmonic performance of ASVM and traditional SVPWM was compared through Matlab/Simulink simulation. The decreasing harmonic of ASVM under low frequency is analysed.Secondly, through analyzing characteristics of achieving synchronization and symmetries, the basic requirements, vertor switching rules, detailed implementation steps of three-level synchronization and symmetries SVPWM are researched to maintain voltage waves synchronization, three-phase symmetry, 1/2-period symmetry and 1/4-period symmetry with arbitrary number of pulses. It is proved that Line voltage excludes even harmonics and 3 or 3 times harmonics by Fourier analysis so that total harmonic distoration of voltage and current is lower. Deadzone compensation and overmodulation compensation strategy of synchronous symmetric space vector modulation are researched. The feasibility and validity of the modulation strategy is verified by simulation and the harmonic performance of synchronous symmetric space vector modulation, asynchronous SVPWM and SHEPWM with same number of pulses is compared.Thirdly, a simple modulation strategy of the four quadrant rectifier is researched, as IGBT is controlled a grid synchronous switch, switching frequency is just 50 Hz. The detailed circuit principle and commutation process of the grid side current are analysed. Harmonic characteristic of the grid current during rectification and regeneration is analysed by Fourier analysis. When it comes to 12 pulses rectifier circuit, the improved current harmonic is analysed. The establishment of rectifier model helps to define inductance and capacitance selection or reference value. Simulation results can prove the effectiveness of the modulation, analyse the influence of the line inductance to the rectifier and investigate grid current harmonic characteristic.Finally, based on three-level inverter and three phase bridge rectifier hardware experiment platform, DSP and FPGA algorithms are designed. The effectiveness of the modulation algorithms at different frequency and modulation index as well as the current harmonic characteristic of the rectifier are analysed.