Research On Neutral Point Potential Based On Diode Clamped Three-Level Inverter

With the continuous development of power electronic technology and computer technology, more attention paid to the piont of energy conservation and emission reduction, the application of WVF has been widely popularized. But in high-power field, the application of VVVF is restricted with the tolerance of the voltage and current of the power electronic devices. Three-level topology structure solved this problem, in which diode clamped three-level topology structure is most widely used. But neutral point potential fluctuations which caused by three-level structure is urgent problem. Therefore, the problem of neutral point potential fluctuations in three-level inverter which based on the asynchronous motor vector control system is deeply studied in this thesis.It is that neutral point current is the essence of neutral point potential fluctuation analyzed in this thesis, and neutral point current mathematical model is established. According to the mathematical model of neutral point current, the limitation of SVPWM algorithm is analyzed in detail. Then the improved virtual space vector algorithm is proposed, and the algorithm is deeply researched and analyzed.Firstly, the topology structure of diode clamped three-level inverter and the working principle of main circuit are analyzed in this thesis. Then the 27 kinds of working states of three-level inverter are obtained, and the switch transient change of the same bridge arm is analyzed. Space vector diagram is obtained by 27 kinds of working states. Through the impaction of switch state on neutral point potential, the fact that the essence of neutral point potential fluctuations is neutral point current is analyzed, and neutral point current mathematical model is established. Then the corresponding neutral point current under different working conditions is analyzed. Then, on the basis of the impaction of switch state on neutral point potential, the impaction of neutral point potential offset on space vector synthesis is analyzed.Secondly, the judgment of the sector, the choice of space vector, the calculation of working time and the selection of switching sequence about the three-level SVPWM algorithm are detailedly analyzed in this thesis. Then the relationship among neutral point current of each small sectors, modulation, phase angle and power factor angle are analyzed by the formula of neutral point current and the formula of three phase current instantaneous value. Then, the limitation of SVPWM to neutral point potential control is analyzed. Thus the improved virtual space vector algorithm is proposed, and the judgment of the sector, the choice of vector, the calculation of working time and the switching sequence about algorithm are detailedy given.Thirdly, several mathematical models of asynchronous motor are discussed in this thesis. The basic principle of vector control are discussed in detail, and the model of stator voltage decoupling, current loop, speed loop and indirect calculation of rotor flux in the vector control system are designed.At the end of this thesis, the control system based on the above theory analysis is analyzed in the Matlab/Simulink software platform, and the simulation results show that the control system has good dynamic response and steady-state accuracy. In terms of neutral point current control system simulation, simulation results show the proposed control algorithm can effectively solve the problem of neutral point potential and the proposed scheme is correct and feasible.