Research On The Topologies And Control Strategies Of Current Source Multilevel Converters

With the development of the superconducting magnetic energy storage (SMES) technology, superconducting inductors can be used as higher efficiency energy storage elements, the multilevel current source inverters (CSI's) will be very attractive in high power applications due to their advantages such as controlling the output current more directly and conveniently, faster dynamic response, easy regeneration capability, and implicit short-circuit protection. This dissertation presents some new topologies and control strategies of multilevel CSI's, including following aspects.Based on switching function, mathematical models of three-level CSI and the combinational multilevel CSI have been analyzed in abc stationary coordinate and dq rotated coordinate, which are the basis of control system design. The application of the duality principle in deriving multilevel CSI topologies are discussed.A new kind of direct type single-phase multilevel current source inverter topology is proposed in this paper. The topology is made up of a DC-bus current changing circuit and a single-phase inverter. Including very few switching valves and current sharing inductors, the structure of this type of inverter is very simple. A 5-level CSI and a 7-level CSI using the new topology are discussed in detail. The switching strategy that can ensure equal current division among the branches is presented. The waveforms of output current of the two inverters under capacitive load and inductive load are provided. The method of low-order harmonic reduction by varying the output level widths is proposed. This inverter is verified by the simulation and experimental results.A new type of three-phase direct type multilevel CSI topology is presented. The new multilevel CSI's synthesize the staircase current wave from several levels of balance inductor currents. The structure of the new inverter is very simple. The current balance of sharing inductors can be achieved without close-loop control. A three-phase 6-level inverter and a three-phase 12-level inverter using the new topology are discussed in detail. The waveforms of output current and current-sharing inductors current of the two inverters under capacitive load and inductive load are provided. The proposition is verified by the simulation results and experimental results. Compared with traditional multiple CSI's, the new topologies have many advantages.A new type of split-control three-phase 5-level CSI is proposed in this dissertation. The inverter is decoupled by the neutral line of the loads. Each phase of the inverter operates separately so that multilevel PWM technique can be applied. The digital current source multicarrier phase opposition disposition (POD) PWM signal generating technique is developed. The operation principle of the three-phase 5-levelCSI using the new topology is introduced by analyzing one phase cell. An experimental prototype of a three-phase 5-level CSI has been built to practise the proposition in the paper.Several PWM techniques applicable to combinational multilevel CSI are studied in this dissertation. Carrier Phase-shifted SPWM technique is introduced. Multicarrier PWM method is discussed and applied to the new single-phse 5-level CSI. Some basic points about SVPWM method applied to a three-phase combinational 5-level are proposed.