Research On The Technology Of High Voltage Superposed AC And DC Transmission Based On Zigzag Transformer

The transmission capacity of the traditional way of AC transmission line is always below it line thermal limit capacity, which result in a lack of transmission capacity in transmission network, power flow congestion, etc. In addition, the increasing of the grid scale and the growing size of the grid structure, coupled with the conflicting interests involved in land resource is difficult to open up a new transmission line corridor in the original transmission structure. One feasible way to solve the issue above is to transform the existing AC transmission lines in limited, achieving the power transfer and meeting the needs of sustainable development and environmental protection issues. Therefore, a technical method for converting AC lines into AC and DC hybrid lines is studied in this paper, namely the high voltage superposed AC and DC transmission technology, the major work is as follows.1) Firstly, some relevant programs to enhance existing AC transmission line transmission capacity are discussed, and then we point out the bottlenecks issues that the traditional AC transmission capacity transmission line is difficult to access it heat limit capacity, and introduce the high voltage superposed AC and DC transmission that transformed existing AC transmission lines for a limited. The idea of the novel transmission method is introduced, and two kinds of main circuit topology of the transmission system are analyzed in this paper. The analysis of two different ways that achieved superposition and separation of AC and DC proves the superiority by using Zigzag transformer.2) The winding structure and phasor relationship of the Zigzag transformer is analyzed, three-sequence equivalent circuit model of the transformer is established, In this paper, the mechanism of using Zigzag transformer to achieve superposition and separation of AC and DC, and to avoid magnetic saturation are analyzed. By building the Zigzag transformer simulation model, simulation results show the feasibility and correctness of Zigzag transformer to avoid the DC bias.3) With the concept of not altering the AC line structure, the constraints of the voltage and current in the AC and DC are analyzed. The effect of superposition ratio in voltage and current to the total power ratio of the transmission system is studied, studying the characteristics for the maximum power transfer of the high voltage superposed AC and DC transmission, and the relationship between optimal power transmission angle and the transmission distance is obtained. Besides, the relevant electrical parameters which impact the power increasing percentage of transmission system is studied, and the relationship between the power increasing percentage and the transmission distance is obtained.4) From the perspective of engineering application, several key elements which need to make important selection in line transformation is analyzed. Studying the line conversion and insulate coordination on the high voltage superposed AC and DC transmission, the high efficiency to enhance the transmission capacity of the superposed technology by using power cables and the transmission results in different voltage transformation are obtained. The simulation model for high voltage superposed AC and DC transmission with Zigzag transformer is set up to verify the correctness and validity of the transmission characteristics.