Research On The Kev Technologies Of Modular Multilevel Medium-to-Low Voltage DC Transformer

The DC power system has shown many advantages with large-scale utilization of renewable energies, change of load structures, and fast development of power conversion techonologies. At present, the critical issues in the DC network such as grid stability, circuit protection, grid structure and so on, have been widely studied both in acdamics and in industries. And several pilot projects on the DC grid have been reported. In order to realize an efficient interconnection between different grids and an intelligent management of electric energy, the DC transformer has attracted a lot of research interests, and becomes one important topic in the field of electrical technologies. Therefore, the key issues in the DC transformer research are investigated in this dissertation.Compared with silicon IGBTs, silicon carbide (SiC) MOSFETs have a lower switching loss and a faster switching speed. When applied in DC transformer converters, the isolation frequency can be beyond the audio frequency range to avoid the acoustic noise. The DC tranformer performance will also be dramatically improved. In order to use SiC MOSFET intrinsic diodes as freewheeling diodes in power converters to reduce the cost and package complexity, the reverse recovery characteristics of the intrinsic diode are systematically evaluated from four aspects in terms of turn-off voltage, turn-off current, current commutating slope (di/dt) and junction temperature. Meanwhile, the characteristics of a SiC Schottky barrier diode and two kinds of silicion p-i-n diodes are also tested to show the performance of the intrinsic diode.Till now, many efforts have been carried out on the investigation of the DC transformer and its applications. Limited by the voltage rating of power semiconductors, many high frequency transformers are usually used to realize galvanic isolation and voltage matching between different grids in the present solutions, leading to high manufacturing cost and large volume of the whole converter.In order to overcome these issues, an isolated bidirectional modular multilevel DC/DC converter is proposed for DC transformer applications in this dissertation, which realizes an efficient connection between the medium dc voltage and low dc voltage. Several low voltage submodules are cadcaded to form the high voltage switching valves in the proposed structure, thus low voltage rating power devices can be adopted. And a simple high frequency transformer is used for galvanic isolation and voltage conversion. Meanwhile, the modular structure helps cost reduction and mass production, and the voltage modulation is flexible. Since only one transformer is used, the volume and manufacturing cost of the transformer can be reduced. Based on the proposed structure, a dual phase shifted control strategy is proposed for output power regulation and voltage balancing control. The output power is regulated by the phase shift angle between the primary side voltage and the secondary side voltage, which can fully utilize the energy stored in the leakage inductance of the main transformer. As a result, the power devices are soft switched, and the switching losses are greatly reduced. The submodule voltage balance is realized by a simpe phase shift control between different submodule output voltages in each arm. A fast voltage balancing process is achieved without increasing the switching frequency. A prototype based on SiC MOSFETs is constructed to verify the proposed circuit topology and the control strategy.In order to simplify the voltage balancing control, the voltage gain of the proposed converter with dual phase shift control should be less than a critical value, which limits further circuit optimization. Thus a modified voltage balancing method is proposed. The output power is still regulated by the phase shifted angle between the primary side voltage and the secondary side voltage to achieve a high efficiency. But the duty cycle of the submodule output voltages is modified from 50% to adjust the charging/discharging state of the floating capacitor in each submodule. With this control method, the voltage gain limitation is not limited, and the submodule voltages are well balanced.Based on the proposed single phase structure, a multi-phase modular multilevel DC/DC converter is proposed, and a three phase structure is analyzed and discussed. The detailed comparisons between the three phase and single phase converter are presented. In the proposed multi-phase converter, the current stress of the power devices is reduced; the output current ripple is low, resulting in a small filter; the circulating power is reduced; and dv/dt also decreases. With the multi-phase structure, the performance of the converter is further improved.