Research On The Common-mode Voltage Cancellation Techniques In Five-level Inverter

With miniaturization, multi-function and high-performance of inverters, especially the total digitalization of their control scheme,the flexibility and adaptability of inverters are improved continuously. Therefore, inverters are used in modern industry more widely. Now, almost all inverters use PWM (Pulse Width Modulation) control technologies.However, inverters make great contribution to industry and agriculture, they bring noticeable negative influence at the same time. With the development of modern power device, theirs switching frequency increase at very fast speed. Voltage source PWM inverters will generate high frequency common-mode voltage, which induces high shaft voltage, leads to bearing current, and results in premature bearing failure. High-speed switching power electronic devices generate strong EMI (Electro-Magnetic Interference). Conducted EMI and radiated EMI will disturb other control systems or electronic equipment, even result in malfunction. Long cable between PWM inverters and motors leads to over-voltage at motor terminals, which not only worsens the above adverse effects, but also stresses on winding insulations. These adverse effects decrease reliability, increase failures, but always were ignored by users. However, the damages due to the adverse effects may be larger than the cost of the AC variable frequency drive system. There, investigating the adverse effects of PWM inverters and their corresponding counter measures have important significance.In order to suppress the adverse effects and improve the reliability, inverter output common-mode voltage and its correlative suppression techniques of AC variable frequency drive system are investigated in this thesis. Common-mode voltage generated by voltage source PWM inverter and the essences of its adverse effects are investigated, and common-mode voltage reducing methods are also presented. PWM technology for common-mode voltage reduction in five-level inverter is investigated whereas common-mode voltage generated by the two-level inverter can only be reduced by an additional filter. The article analyses the basic principle and control regulation of five-level inverter, proposes the SPWM control strategy for five-level inverter, and presents the analysis of the frequency spectrum of the technology.