Influence And Restraining Measures Of DC Bias In UHVDC Project

UHV DC transmission is characterized by huge transmission capacity, long transmission distance, low line loss, and can optimize energy resource allocation in a wider range. As a result, UHVDC transmission has a significant advantage and great prospects for development. When monopolar ground operation mode happens, larger DC current injects the DC grounding, resulting in ground potential change around DC grounding electrode, causing inflow of large DC current into AC transformer neutral ground, affecting normal and stable operation of transformers.This paper sets Jiangsu-Ximeng HVDC transmission project which is being planned as an example and discusses the DC bias phenomenon which will occur during monopolar ground operation after the completion and its restraining method. The key elements are as follows:Setting AC ground network as the border, the DC bias calculation is devided into two parts, including ground and underground part. For the ground portion, the paper finishes a complete model of the DC current flow path through the establishment of the main transformer DC model, substation DC system model as well as AC and DC grounding. The underground section uses simulation software CDEGS to get DC grounding potential distribution, uses a direct calculation method and Field-Circuit alternative and iterative way which takes ground shift caused by DC current flowing in AC grounding into account. Then this part calculates, predictes and analyzes the results separately. The results of calculations indicate that DC bias of 500kV substation area is serious and needs treatment.Considering the predicted circumstances as well as advantages and disadvantages of many DC bias control measures, this paper chooses the suppression method of low resistors setting in the main transformer neutral points, and optimizes it by (discrete) multi-objective particle swarm optimization algorithm. In order to optimize the targets of the sum of limited DC bias current and the sum of the resistance, this paper sets artificial decision function, then obtains optimal inhibition of DC bias current limiting resistors, based on the configuration of resistors in the entire network. This part then using discrete multi-objective particle swarm optimization algorithm, setting limited DC bias current resistor size and configuration as the optimization target, in order to analyze the effect situation for the whole network optimization (sensitivity) based on optimization results,discussing the economic performance of DC bias control measures.Previous work provides a theoretical measure for DC bias current which will happen after the completion of HVDC transmission projects.