Research On Three Level Inverter SVPWM Control Strategy Based On DSP

Currently, the neutral point clamp (NPC) three-level inverter is constantly one of the research highlights in the field of the middle and high voltage and large capacity for electricity transformation, it can effectively enhance the voltage withstand level for the system capacity of the inverter, lower the harmonic of output voltage and switching loss. Among those diverse control strategies of three-level inverter, Space Vector Pulse Width Modulation (SVPWM) has the advantages of being high busbar voltage utilization, big modulation ratio, apt to digital realization method and feasible for neutral point voltage control. However, there are still some problems existing in the three-level inverter like the simplification of three-level control, harmonic control of the inverter with nonlinear loads, and neutral point potential balance of three-level inverter. Given the problems mentioned above, this dissertation conducts research for each of them.To begin with, the basic theory and concrete implementation of SVPWM control for two-level inverter are studied, and thus the SVPWM control for three-level inverter is able to be simplified accounting for the combination of those common points between the two-level inverter and the three-level inverter. Afterwards, the mathematical models in ABC, dq and αβ frame for three-phase inverter are established. Based on the Equivalent Transformation theory, the transform equations between different frames are derived. Then, it is deeply analyzed that nonlinear loads cause the inverter output waveform distortion, and the operating principle in company with the parameter design are also studied.On account of the analysis of inverter and rectifier load, the dual-loop decoupling PI controller is designed in dq frame, and the Quasi-PR controller is designed in ap frame. While proceeding the parameter design of the dual-loop PI controller, the parameters are optimized on the basis of the cross-over frequency both in inner and outer loops. The Quasi-PR controller has a proportional regulator of inner current loop to boost the current track and speed. And a multi-resonance Quasi-PR regulator is adapted in the outer voltage loop to achieve high performance. In allusion to the rule that two vectors corresponding to the same switching state have opposite effects on the capacity voltage on the DC side, the neutral point’s potential fluctuation of three-level inverter has been efficiently controlled by changing the duty time of negative and positive tiny vectors, applying delay-loop control and accuracy control.Finally, the simulation model of inverter under the two control systems are built in MATLAB/Simulink based on the mathematical models of inverter, plus with the design of two controllers, and the two control systems performance of inverter are compared under the nonlinear load. The simulation result shows that using the dual-loop PI controller can increase the bandwidth of the outer loop, diminish the THD (Total Harmonic Distortion) of the output voltage, and also the Quasi-PR controller with multi-resonance has better performance than decoupling PI controller. The simulation outcomes of two different approaches to the neutral point potential balance--delay-loop control and accuracy control--proof that the latter one possesses better effectiveness.