Research On Modulation Algorithms And Control Strategies Of Cascaded Inverter

In recent years, owing to the development of power electronics andautomatic control technology, and the demand of higher power equipmentin industry, cascaded multilevel inverters have become widely used in theapplications with medium/high voltage/power ratings. Compared withconventional two-level/three-level inverters, cascaded multilevel inverterhas the advantages of larger number of output voltage levels, lower totalharmonic distortion of output voltage, the capability to realizelow-frequency staircase modulation, less voltage stress for powersemiconductor devices, less switching loss, etc.Firstly, this dissertation introduces various topologies of single-phasemultilevel inverters, including diode-clamping, flying-capacitor andcascaded multilevel inverter, and compares and analyzes these topologieswith respects of the number of dc-link sources, voltage levels and powersemiconductor devices, bi-directional power flow, the isolation betweeninput and output ports, efficiency, etc. The dissertation proposes a novelthree-phase cascaded multilevel inverter topology with single dc-linksource and input-output isolation, and introduces its operating principles.As to the modulation strategy, this dissertation essentially studies avariety of low-frequency staircase modulation strategies, includingequal-area principle, minimum area difference method, selective harmonicelimination approach, and minimum total harmonic distortion method. Forthe minimum total harmonic distortion method, by taking the advantage ofKKT conditions optimization algorithm, the dissertation derives themethodology to calculate the conducting angles of each voltage level under the conditions of both equal voltage level difference and unequal voltagelevel difference. Furthermore, the dissertation proposes the strategy toimprove the dc-voltage utilization ratio and reduce algorithm complexity,and finally conducts the related simulation research.Considering closed-loop control of inverters, by building thesimplified circuits and mathematical models of the three-phasesingle-source cascaded inverter, the dissertation studies its doubleclosed-loop PI control strategy. In order to overcome the problems ofstability and phase lag of PI control, this dissertation proposes a controlstrategy based on Lyapunov stability, and eliminates the steady-state errorwith reference model adaptive control. These control strategies arecompared and analyzed with simulation platforms.