Control and Design of a High Voltage Solid State Transformer and its Integration with Renewable Energy Resources and Microgrid System

Solid State Transformer (SST) has been regarded as one of the most emerging technologies in the power distribution system. It has the advantages of low volume, low weight, fault isolation, and potential additional functions, such as voltage regulation, harmonic filtering, reactive power compensation, and etc. However, the involvement of large number of power devices makes the control of SST a challenge. In addition, the high voltage and high power hardware design of the SST is not easy, certain design methodology needs to be developed. Furthermore, the cost of the SST is much higher than the traditional transformer, how to justify this cost gap is also of great importance. In this dissertation, a systematic literature review is conducted for the development of SST in the future distribution system. The key components essential for the SST are reviewed and different techniques are compared. It is pointed out that the SST consists of multilevel/modular power converter structure with advanced power devices and magnetic materials achieves best performance in terms of the size and efficiency in high voltage operation condition. While the challenges, including control architecture and design methodology, need to be addressed. In addition, the potential markets for SST need to be identified for possible commercialization of the technology. In this research work, a three-stage modularized type SST based on Si IGBT is selected as the research target aiming at developing advanced control technologies, design methodology, and application areas of the SST. The first part of the dissertation focuses on the analysis, control, and design of the presented three-stage SST topology. First of all, the voltage and current sharing issues of the presented SST topology are analyzed and addressed. Firstly, a new control structure and design methodology is proposed for balancing the voltage of the rectifier stage by using the feedback regulation. This controller minimizes the coupling effect between the voltage balance controller and the original system controller. Therefore, the design of the original system controller can be as easy as the two-level converter system, and the two controllers will not interact with each other. Secondly, the modulation based voltage balance method is also explored with extremely fast voltage balance response. The design of the control system can also be regarded as a two-level converter system and the voltage balance is achieved by choosing the most suitable switching pairs of the H-bridges. Thirdly, a current sensor-less current balance controller is proposed for the parallel operated DC/DC stage. This method does not need any additional current sensors and can achieve the power sharing among converters of DC/DC stage well. Fourthly, a 3.6kV-120V/10kVA SST hardware prototype is designed and demonstrated for the smart grid application. The proposed control methods in chapter 3 and chapter 5 are adopted in this high voltage SST prototype. Various tests are conducted to verify the key characters of the presented SST topology compared with the traditional transformer. The second part of the dissertation focuses the on the advanced application of presented SST in the future renewable energy and microgrid systems. Firstly, a family of SST interfaced wind energy systems are proposed with the integrated functions of active power transfer, reactive power compensation, and voltage conversion. The proposed wind energy system can effectively replace the traditional transformers and reactive power compensation devices, therefore a highly compact and integrated system can be obtained and the cost of the SST can be better justified. Secondly, a SST interfaced microgrid system and its centralized power management strategy are proposed. The presented microgrid system can access the distribution system without bulky transformers and can manage both the AC and DC grid simultaneously, operating like an AC/DC hybrid microgrid. In this condition, SST plays as an energy router, benefiting the future residential systems. All the technologies proposed in this work are original and provide value information for further promotion and commercialization of the SST concept.