Research On Control Method Of Three-level Inverter And Its Application In Vector Control

Recently, multi-level inverters have been widely studied in high power level application with high voltage output. As one new type of converter, multi-level converter provides high power output through changing circuit topology, without step up and step down transformer, and equalization circuit. The waveform of the line-to-line voltages is composed of multi-voltage levels, which leads to lower THD and lower switching voltage stress. So this technique has been widely concerned in applications such as medium voltage drive and flexible ac transmission system. The three-level inverter and its control strategy are studied in this paper.The topologies of multilevel inverter and control methods are summarized. Moreover, their advantages, disadvantages and application fields are also analyzed and compared. The structure and principle of three-level NPC inverter are analyzed in detail.PWM control technique is one of the key techniques for multi-level inverter .It is proposed in the same time with multi-level inverter topology. The PWM technique not only determines the performance of inverter, but also affects the quality of the output voltage waveform, the loss and the efficiency of the system directly. Proper PWM control method ensures operation with high performance and high efficiency. There are two main types of PWM control method, which are carrier-based PWM method and SVPWM method.SVPWM control schemes for the three-level NPC inverter are discussed in detail. Based on analyzing the control principle and technique characteristic of tradition SVPWM, one fast SVM algorithm based on 60°coordinate are introduced, which can get the three nearest vectors around the reference vector and their dwelling time easily. The reasons of neutral-point voltage unbalance and the methods of solving this problem are discussed.Two vector control schemes of an induction machine using three-level neutral point clamped (NPC) inverter are studied. One is zero-sequence component imposed three-level vector control system of induction machine. The other is three-level inverter vector control system based on SVPWM algorithm. In the former system, the direct rotor-flux-oriented control of induction machine supplied by a voltage-source inverter with fast current control is employed. The switching frequency optimal PWM (SFOPWM) method is used for the control of a three-level inverter with imposing zero-sequence component. In the latter system, The inverter is controlled by SVPWM algorithm based on 60°coordinate. Results from computer simulation show the good performance of the three-level inverter fed drive with vector control.In the end, three-level inverter experiment equipments are designed, which conclude main circuit of three-level inverter, DSP2812 control board, drive and protect circuit and auxiliary power. The program of control system is programmed, and the control strategy is tested by doing experiments.