Research On Neutral Point Clamped Three-level Inverter For Medium-High Voltage Converter

With the development of electric drive technology, especially the increasingly processof power electronics technology, the middle-high voltage frequency control system which isconsidered as large-capacity transmission technology has been widely used. As the core partof high capacity and middle-high voltage frequency converter, multilevel inverter has beenextensive researched in recent years. In a variety of control strategy, space vector pulse widthmodulation (SVPWM) become the focus of attention, with the advantages of optimize theoutput voltage waveform, large modulation ratio, high utilization of the bus voltage andeasily digital realization.This paper firstly introduces the status of middle-high voltage frequency converter,classification and topology of multi-level inverter. Take diode-clamped three-level inverteras the research object, the control strategy and related problems are analyzed and studied.First, the topology and principle of three-level inverter and SVPWM are analyzed. Thetraditional SVPWM has two different types, which are table lookup and reference voltagedecomposition. SVPWM of table lookup is clear and easy to accept. Reference voltagedecomposition type is simplified from table lookup type. Both algorithms contain a numberof trigonometric calculations, which can cause large calculated amount. Then this paperproposes a SVPWM based on60degrees coordinate system. In the algorithm, the spacevector is decomposed into this coordinate to simplify the calculation, improve efficiency.Second, the inherent problem of imbalance in the neutral point in neutral point clampedthree-level inverter is deeply researched. Meanwhile, it puts forwards solving methods fromtwo aspects of hardware and software.Then, this paper verifies the feasibility of the two modulation methods, which are tablelookup type and60degrees coordinate SVPWM, through MATLAB simulation. Furthermore, a simulation platform of diode-clamped three-level middle-high voltage frequencycontrol system is build to verify the efficiency of the control algorithm of three-level and thefeasibility of method to solve problem of imbalance in the neutral point.Finally, with DSP2407as the core, the hardware circuit of SVPWM speed controlsystem is designed simply to research the control algorithm through experiment.