Study On Cascaded Power Electronic Transformer Based Microgrids In Buildings

Building microgrid, an emerging comprehensive power distribution system for constructions, is integrated with distributed generations, energy storage devices, As a significant part of the development of building microgrids, power electronic transformers can not only provide diversified AC/DC interfaces, but also possess the functionalities of power flow control, harmonic suppression and system protection. Therefore, this thesis is going to provide a new scheme which aims to construct the building microgrids based on cascaded power electronic transformers.To start with, a feasible cascaded power electronic transformer is designed in accordance with an actual distribution network in a building. The transformer consists of three main parts, including the input stage, the isolation stage and the output stage. Among them, a cascaded H-bridge rectifier is set up as a grid-connected converter for power control and energy conversion on the input stage; a bidirectional power converter with resonance soft switch is used and the DC/DC transformation units are assembled in input-series-output-parallel (ISOP) mode, for the purpose of voltage transformation and electrical isolation between the primary and secondary side on the isolation stage; and on the output stage, a three-phase fully-controlled inverter is utilized to convert the low-voltage direct current into the alternating current of power frequency for electric consumers. Meanwhile, the power electronic transformer provides DC interfaces on the low-voltage DC side for integration of the distributed generations as well as the DC loads.Secondly, the researcher analyzes the feasibility and necessity of DC distribution scheme in the distribution systems of buildings with the consideration of the current situations of building microgrid and the advantages of DC distribution network. In addition, the structures of today's DC distribution network and corresponding voltage levels are described, and a power distribution scheme for building microgrids based on cascaded power electronic transformers is presented. By introducing the PV generation system and battery energy storage device, the maximum power tracking control method of PV generations and the control strategy of energy storing devices are explained in details. Followed by the explicit explanation, the energy management and the coordination control strategy of the proposed building microgrid based on cascaded power electronic transformers are stated under the PV connection operation and islanded operation.At last, a building microgrid network based on cascaded power electronic transformers is constructed in MATLAB/SIMULINK for stability investigations under the PV connection operation and islanded operation. Moreover, with the issue of harmonic pollution caused by nonlinear loads, three-phase imbalance and voltage sags, an application of power electronic transformer on power quality improvement in building microgrids is investigated.