Research On Voltage Balance, Clamping Scheme And Modulation Technique Of Multilevel Converters

Voltage-source multilevel converters (VSMC) are valuable and become researching hotspots for their multilevel output, low THD and the large potential applications in high-voltage and high-power fields. Although there are quite a few of the industrial applications of VSMC, the research work is far from stop due to its various categories and complicated structures. This thesis focuses on the carrier-based PWM methods suitable for the flying-capacitor multilevel topology (FCMT), the voltage balance problem of DC capacitors of conventional diode-clamping topology, the control of three-phase three-level rectifier (VIENNA) and the applications of hysteresis-current control in multilevel converters.According to the characteristics of PDPWM and PSPWM, combining the balance principle of FCMT, the carrier-rotated PWM method is built. The harmonics characteristics is derived essentially and quantatively by using dual FFT and proved in the three-level and five-level inverter simulation and three-level inverter experiment at high and low modulation indices. The results of derivation, simulation and experiment in three-level inverter show the same harmonics performance essentially for the carrier-rotated PWM method and PDPWM. For their phase voltage, most of the harmonic energy in the switching-frequency point and its side band concentrates at the switching frequency, and this harmonic component will be canceled in the line-line voltge. Therefore, the harmonics are easy to be filtered and the best harmonics performance achieved. Comparing with PSPWM, the carrier-rotated PWM method can not only increase the equivalent switching frequency and balance the voltage of the flying capacitors, but leads to much lower harmonics at the switching frequency of output line-line voltage because there is no phase shift between the carriers.This thesis also researches the possible applications of COPWM to FCMT and gets the conclusion that COPWM, which is suitable for diode-clamping topology, has no contribution to the voltage balance on flying capacitors. So it cann't be used directively to FCMT. A novel carrier-based PWM method with overlapping characteriscs and voltage balance capability is proposed. The three-level flying-capacitor inverter modulated by using the novel method proves the validity of the new carrier-based PWM method.The voltage balance of DC capacitors is the important issue in conventional diode-clamping topologies. In this thesis, a novel topology—three-phase four-leg hybrid-clamping multilevel topology is proposed. In this topology, there is an additional leg for clamping function based on conventional one. Compared with the state-in-art topology based on the hybrid-clamping technique, the four-leg one can significantly reduce the number of active switches in use and decouple the main switches and clamping switches completely, so the independent control achieved. All of the control methods of conventional topologies can be used in the four-leg topology without considering the voltage balance of capacitors, while this is not easy for other topologies based on the hybrid-clamping technique before. The validity of the four-leg topology is proven by simulations of the three-level and five-level inverter and experiments of the three-level inverter. It is one of the potential topology in the multilevel conversion.The similarity of a kind of single-phase three-level rectifier and the VIENNA rectifier (three-phase three-switch three-level rectifier) are compared together. Based on the comparison, the control scheme of this single-phase three-level rectifier is extended to three-phase counterpart. The control method is cost-effective, simple and reliable. The switches work at constant frequency and half value of the output voltage across each switch. The results of the simulation and experiment prove that the control scheme is well.On the basis of an active power filter system, the non-offset hysteresis current control method is used with the three-level flying-capacitor topology as main converter and the respondent performance is analyzed. The SVM-based hysteresis current control method for three-phase four-leg three-level converter is researched and the simulation carried out. This control method can partly reduce the switching times by narrowing the possible position of the voltage vector.Finally, the thesis makes a summary of the whole work and gives expectation for future research on the multilevel converters.