Influence Of Inhomogeneous Geoelectric Structure On Neutral Current Of Transformer

With the rapid development of HVAC/DC hybrid transmission system,it is imperative to evaluate and control the DC bias effect of power grid.The neutral current fluctuation of transformers caused by the current field of ground electrodes is an important index to evaluate the level of DC bias.At present,the neutral current fluctuation of transformers caused by the current field propagation in layered soil is mainly considered.However,the propagation process of the current field of ground electrodes is seriously affected by the soil electrical inhomogeneity,so it is necessary to use the threedimensional electrical structure to accurately calculate the current fluctuation at the neutral point.The Green’s function is a commonly used tool for calculating the propagation of current field in layered electrical structures,but it is difficult to be solved in three-dimensional electrical media.The field-circuit coupling method is the main method to calculate the neutral current fluctuation caused by the current field of the transformers,but this method has the problem of forming the mutual resistance of earth in three-dimensional soil.In this paper,a "three-dimensional resistor network" is proposed,which solves the problems of "difficulty in solving Green’s function" and "difficulty in forming the mutual resistance of earth".The rationality and accuracy of the three-dimensional resistor network method in analyzing the ground electrode current field are verified in earth surface potential comparison.And the correctness of the neutral current calculation model is verified in the IEEE standard example.On this basis,three dimensional soil models containing the water-bearing fault and other geoelectric structures are established,and the influence of different geoelectric structures and their parameters on the current field and the neutral current values is analyzed.Specific work contents and conclusions are as follows:(1)Aiming at the ground electrode current field generated by HVDC transmission under three-dimensional soil conditions,a three-dimensional resistor network method is established to analyze the current field in soil with arbitrary distribution of resistivity.The ESP distribution generated by different kinds of ground electrodes and the earth model is calculated and analyzed.The amplitude and relative error of ESP is contrasted among different methods.The feasibility and correctness of the three-dimensional resistor network method are verified.(2)Aiming at the problem that it is difficult to form the mutual resistance in threedimensional soil condition,the calculation model of transformer neutral current is established,the field-circuit coupling method is deduced based on the model,and the "mutual resistance" is calculated using three-dimensional resistor network method.Taking the grid structure of the IEEE standard case of 20 bus nodes as an example,its transformer neutral current calculation model including substations and transmission lines is built,and the direct current of each phase of transmission lines and transformer windings is obtained.The analysis shows that the relative error between the results presented in this paper and the standard results is generally less than 1%.The accuracy of the neutral current calculation model and the field-circuit coupling method are verified.(3)The effect of the three dimensional geoelectric structures on the ground electrode current field is modelled and analyzed.When the resistivity of geoelectric anomaly bodies and water-bearing faults is low or high,respectively,the ESP near the ground electrode will decrease or increase and the water-bearing faults under a substation will slow down the potential attenuation trend of the substation and its surroundings.The vertical fracture structure with high resistivity has great influence on the ESP and its attenuation trend.To summarize the above influence,the site selection of ground electrode or substation should consider the area with low soil resistivity,and consider the influence of the three dimensional geoelectric structures on the attenuation rate of earth surface potential.(4)The influence of three dimensional geoelectric structures on the neutral current of transformer is modelled and analyzed.The neutral current changes differently when the electrical anomalies are low or high resistivity,respectively.The influence of the resistivity of water-bearing faults on the neutral current is similar to that of the anomaly body.The fracture structure has little effect on the results of the neutral current.The influence of water-bearing faults models with different resistivities on the direct current of the grid structure of IEEE test case with 20 bus nodes is also analyzed.The above conclusions provide a new reference for the evaluation and treatment of DC bias,namely,the neutral current of the substation should be monitored and evaluated in seasons,in order to prevent the seasonal variation of geoelectric resistivity from affecting the treatment effect of DC bias.