Three-level Inverter Algorithm Optimization And The Neutral-Point Voltage Balancing Control

With the rapid development of the industrial system, the low voltage and low power equipment cannot meet the needs of social production, which is becoming gradually high voltage and high power. At present, China is accelerating the reform of the industrial system. Low efficiency and high pollution power devices have been unable to meet the needs of industrial and domestic products and the devices is transforming to high-voltage power. The research of high voltage and high-power multilevel inverter is becoming gradually the focus in the modern power electronic technology. Multi-level inverters are used in Frequency Adjustable Speed System, City Electric Locomotive, Active Filter of Power System, Reactive Power Compensation and Flexible Transmission and Distribution Systems because of the advantages of high dielectric strength, high equivalent switch frequency, small common mode voltage.First, the thesis discusses the development trend of three-level inverter, while the three-level structure based on structure, Elaborating various aspects of the situation and status of the topological structure and development, discussing the present technical difficulties of the medium voltage transmission system, and given more common products of the medium voltage transmission in current market. On this basis, firstly described its basic operating principle, and establishing a mathematical model of diode clamped three-level inverter.Second, researching the traditional three-level SVPWM algorithm then selecting the three-level SVPWM as basic algorithm of this article, and at the same time studying the two-level SVPWM algorithm, due to the high complexity of the three-level SVPWM algorithm, so selecting the simplified three-two-three level conversion strategy, and thus presents a new switching sequence algorithm model, the new switching series not only meet the three conditions but meting switching sequence as defining, the new switching sequence also can reduce the complexity of the traditional three-level SVPWM algorithm. When adopting the new switching sequence under matlab/simulink environment, the simulation results show that the output of line voltage and phase voltage waveform of the inverter output satisfied the traditional wave, and the also has low harmonic distortion, proving the new optimization of sequencesFinally, contrary to the inherent neutral-point volatility of the three-level diode clamped converter topology, this paper propose two neutral-point control scheme, the first by setting vectors to change the role of small vectors time program, proposing a time compensation voltage segment policy, which can be used both for SPWM and SVPWM method, this policy is simple and practical, that can be used for actual production. The second strategy is dedicated to the traditional SVPWM method based on the neutral-point of the transformer switch sequence control strategy. The simulation results show that two strategy both can effectively carry out the neutral-point control, have greatly reduced the neutral-point fluctuations, and significantly reduce total harmonic distortion, so the traditional method has been greatly improved.