Research On Control Strategies Of Three-level Inverter

During recent years, multi-level inverter has been widely researched in high power level application with high voltage output. Power energy with characteristic of high capacity and high quality can be achieved by this type of inverter, in which relatively small-capability and low-voltage switches are adopted. So this technique has been widely concentrated in such application as medium-high voltage transducer and Flexible AC Transmission SystemBeing the basis of further research, status quo of multilevel technique is summarized in chapter 1, and five interrelating technologies, which are respective high speed DSP, power switch, optical isolation and communication skills, distributed power supply skills, heat management technology and influence development of the multi-level, are specified as well. In chapter 2, four basic topologies of multilevel and several control strategies are analyzed in detail and compared. In chapter 3, the hardware configuration of three level inverter and control flat of dual-DSP are discussed.In chapter 4, the principle of the three-level SVPWM is specified and consequently, the cause of unbalance of the neutral-point voltage is detailedly analyzed. The affects on the neutral point voltage of four types of voltage space vectors are debated qualitatively and quantificationally. A method of generating the equivalent modulating waveform of SVPWM is put forward. On the basis of freedom degree control and switch frequency optimization, five types of bus clamped SVPWM are proposed, correspondingly, the switch sequences and application are analyzed. Finally, a SVPWM control strategy, which can balance the neutral point voltage by changing the control factor on the basis of judging the DC Voltage of two capacitors and the directions of three phase output current, is presented. In chapter 5, achievement of the SVPWM driving signal by using the tool of SIMULINK and DSP is discussed in detail. The operation characteristic of bus clamped SVPWM, which has no change in some phase switching status during one period and the total switching times can be reduced by 33% compared with usual SVPWM technique. In addition, different types of CSVPWM results in different affects on neutral point voltage, which affords a method to solve the problem of neutral-point voltage unbalance. The strategy of changing control factor is illustrated as an example to solve the problem of the unbalance of the neutral-point voltage. Both experimental and simulation results verify the availability of this control strategy.In chapter 6, the principle of two typical SHEPWM methods is discussed and two methods for solving the nonlinear transcending equation are discussed and compared in detail. The results of simulation and experiment verify the validity of the two methods and characteristic of SHEPWM control strategy.With the development of the further research on multi-level technology and the development of DSP skills, high-capacity and high-voltage switch skills, optical communication and isolation technology, and etc, the multi-level inverter will be further widely used.