Induction Motor Drives Fed By Cascaded Neutral Point Clamped Inverter

Recently, cascaded neutral point clamped (NPC) inverter is more suitable for medium voltage high power application, and it becomes a hot research subject. This topology of multilevel inverter is a preferred candidate in the medium voltage high power equipment. In this dissertation, the problem which is interrelated with the topology of cascade neutral point clamped inverter with clamped-diode is investigated in detail:1. First of all, conventional topology is generalized, consequently, the topology of cascade neutral point clamped inverter with clamped-diode is specified, and furthermore, both operating principle and mathematical model are presented.2. Considering characters of three-level H-bridge structure with clamped-diode each phase, three carrier modulation schemes based on cascaded carriers and phase shifted consideration are presented.3. A new effective SPWM control scheme for the cascaded NPC inverter is proposed, in which a pulse encode and pulse rotation are fulfilled for all modulation indices. The four modulation schemes are correspondingly carried out, the frequency doubling modulation method and pulse rotation modulation method are good at ensuring voltage balance of branch voltage and series capacitors in each module, and the voltage of all modules. But the pulse rotation modulation method is more suitable, because it uses less system resource than other modulation methods.4. The internal voltage drop of the inverter, due to the cascade structure of many IGBT-diode modules' series connection, is analyzed, which cause distorted phase voltage and currents at low speeds when the frequency and output voltage are low. The current closed-loop control compensates the distortion of phase voltages and currents.5. The pulse rotation control strategy is introduced into vector controlled induction motor system fed by a cascaded neutral point clamped (NPC) inverter. Then the current closed-loop system based on the vector control is designed by MATLAB/Simulink. The emulation results valid the new control strategy and control system.