Research On The Fundamental Techniques Of Medium-High Voltage Converter

Multilevel converters have been becoming increasingly popular in recent years in medium-high voltage ac drive systems and FACTS equipments, mainly due to their high voltage-capability, high output quality, low switching losses and low EMC concerns. The key technologies of multilevel converters such as topology, control strategy and the design of isolated driving power supplies of the semiconductor devices included are researched in this dissertation.A composite cascade multilevel converter composed of a main converter and an accessorial one is presented which is well suited for application in high voltage situations with high performance requirements. A hybrid diode-clamped multilevel converter topology is obtained by extending the topology of a five-level hybrid diode-clamped converter and the new converter has fewer clamping diodes compared to the classic one with the same number of output voltage levels. A new kind of multilevel converter composed of a H-bridge and several bidirectional switches is presented which has a series of advantages when used in low voltage situations with high performance requirements.Generalized SHEPWM technique applicable to multilevel voltage inverter is studied in this dissertation. A method for establishing nonlinear equations directly and simply according to the quarter symmetric waveform for Multilevel Selected Harmonic Elimination is proposed. Convergence of the nonlinear equations is improved by a new method of obtaining initial values using Multi-carrier PWM method. Simulations of SHEPWM technique applicable to multilevel voltage inverter using POWERSIM 4.0 are carried out. Experiment result of a diode-clamped three-level voltage type inverter composed of MOSFETs controlled by a two-DSP (TMS320LF2407A) control flat-form is obtained. Simulation and experiment results proved the advantages of the SHEPWM based method.A new kind of power supply with multiple isolated outputs used for gating semiconductor devices is developed. The main parts of the power supply include a DC current source, a bridge-type inverter, a series of output transformers and shunt voltage regulators. A single turn primary winding is used in output transformers. The isolation voltage among all outputs and input can be considerable high if the primary winding is made by high-voltage cable. The number and locations of the transformers can be arranged quite easily to meet the requirements of the main system. Compared with other power supply used in the same purpose, the proposal one has the advantage in size, weight, cost, efficiency and reliability. With the increase of voltage level and number of devices in the multilevel converter, the proposed power supply will be more advantageous.