Research On Evaluation And Mitigation Of Transformer DC-bias Currents In Urban Power Grid Systems

With the networked and large-scale development of subway systems,the phenomenon of subway stray current flowing into urban power grid frequently occurs.At the same time,the complex operating conditions of subway trains and the urban power grid topology bring new challenges to the analysis,evaluation,and mitigation of the transformer DC-bias current,which are shown as follows: 1)As the source of the transformer DC bias,the spatial and temporal distribution characteristics of the stray current in subway lines are unknown.2)The path of subway stray current flowing into the urban power grid is unknown,which leads to the difficulty in analyzing the transformer DC-bias current.3)The complex and variable propagation path of subway stray current in the urban power grid aggravates the mitigation difficulty of the transformer DC-bias current.Based on this,this thesis focuses on the analysis,evaluation,and mitigation methods of the transformer DC-bias current of urban power grid,and focuses on solving problems such as modeling of the subway stray current,calculation of the transformer DC-bias current,quantitative evaluation,and optimization mitigation.The main work of the thesis can be summarized as follows:(1)Aiming at the problems of the uncertain temporal and spatial distribution of subway stray current,a modeling method of the temporal and spatial distribution of subway stray current based on the long subway line is studied.The proposed method can reduce model complexity,improve the efficiency of the simulation while ensuring the accuracy of the model calculation.Based on this,considering the dynamic changes of train position and traction current,a simulation method for the temporal and spatial distributions of stray current in the subway are proposed.The temporal and spatial distribution characteristics of stray current in the subway are analyzed under the condition of train dynamic operation,and the influence of parameters such as rail transition resistance and soil resistivity on the temporal and spatial distribution of stray current is discussed.The distribution characteristics of the source of transformer DC bias of urban power grid are clarified,which provides a model basis for further calculating and analyzing the transformer DC-bias current.(2)Under the staggered distribution background of the subway and urban power grid,through the analysis of the subway and urban power grid electrical connections and topological structure,the double paths of subway stray current flowing into the urban power grid are given.On this basis,considering the geographic information and electrical equipment parameters of subway and urban power grid,a spatial and electrical coupling modeling method of the subway and urban power grid is proposed.Using the proposed method,the coupling model of the subway and urban power grid is established,and the transformer DC-bias current of the urban power grid is calculated.The correctness of the proposed model is verified by comparing the simulation results of transformer DC-bias current with the field measured data.It provides a model basis for further analysis and quantitative evaluation of transformer DC-bias current characteristics.(3)Aiming at the problem that the various characteristics of the transformer DC-bias current are unclear under the dynamic running condition of subway trains,a dynamic characteristic analysis method of the transformer DC-bias current based on correlation analysis is proposed.The correlation between rail potential and transformer DC-bias current is analyzed,and the negative correlation between rail potential and transformer DC-bias current is revealed.The correctness and universality of the negative correlation are verified by theoretical analysis and simulation experiments.Based on the coupling model of subway and urban power grid,the dynamic variation characteristics of transformer DC-bias current are discussed under the condition of train dynamic operation,and the alternating positive and negative and periodic variation characteristics of the transformer DC-bias current are given.(4)Aiming at the problem that it is difficult to quantitatively evaluate the degree of transformer DC bias caused by the random fluctuation of subway load,using the Monte Carlo simulation method to model the uncertainty of subway load,a probability evaluation method of transformer DC bias considering the randomness and uncertainty of subway load is proposed.The probability evaluation indexes of transformer DC bias are given,which provide a theoretical basis for the quantitative evaluation of transformer DC bias.The application of this method in the evaluation of transformer DC bias of urban power grids is discussed.The severity of transformer DC bias and the overall level of transformer DC bias of urban power grids are quantitatively evaluated,which provides a reference for the further optimization and mitigation of transformer DC bias of urban power grids.(5)Aiming at the problems of high cost and low efficiency of the existing mitigation method for transformer DC bias,an optimal mitigation method for transformer DC bias based on the genetic algorithm is studied.Based on the effect of the transformer neutral point capacitor isolation device on the transformer DC-bias current,an optimization model is established to minimize the maximum value of the transformer DC bias current and the capacitor isolation device.The optimization calculation method of the installation position of the capacitor isolation device based on the genetic algorithm is presented.By embedding Monte Carlo sampling of subway load into the calculation process,the optimization mitigation method of transformer DC bias of urban power grid is presented.Compared with the existing mitigation method,the number of capacitor isolation devices installed in the proposed method is less,and the investment and operation cost are effectively saved.Extensive analysis and simulation studies have been performed in this thesis to illustrate and verify the above methods.The results confirm that,compared with the existing methods,these proposed methods have obvious advantages in analyzing,evaluating,and mitigating the transformer DC bias of the urban power grid.The thesis provides a theoretical basis for promoting the progress of the quantitative evaluation method and optimal mitigation strategy of transformer DC bias.