Calculation And Suppression Of Magnetic Bias Current Caused By Grounding Current Through DC Earthing Electrode

High voltage direct current(HVDC)transmission system has been widely used in the inter-regional interconnection of power grids.Under monopole grounded return operation mode,the grounding current will increase the surrounding surface potential,and generate the direct current(DC)in the neutral-grounded alternating current(AC)transformer,causing DC bias and threatening the safe and stable operation of the AC grid.Therefore,it is of theoretical and engineering significance to study the bias magnetic current of power grid caused by the ground current of the DC grounding electrode under different conditions and its suppression measures.The main content and research results of this paper are as follows:Firstly,the finite element method(FEM)is applied to the calculation of earth surface potential(ESP),Considering the size of earth electrode is much smaller than that of the ground,the earth electrode is equivalent to the point source,and two calculation methods of grounding resistance are compared between formula and parametrized programming in ANSYS software.Based on the calculation results of the grounding resistance,the surface potential distribution of the typical horizontal layered earth model is calculated,then the effects of soil resistivity,buried depth of electrode and amplitude of grounding current on the distribution of ground potential are analyzed.Secondly,according to the flow characteristics of the bias current in the AC power grid,the DC equivalent model of the power grid is established considering factors such as transformer type,transmission line and substation,and the solution method of bias magnetic current based on the node voltage method is presented.Taking the Gansu Power Grid as an example,the bias current in the Gansu AC power grid caused by the grounding current of the earthing electrode at the sending end of the Qishao ±800kV ultra high voltage direct current(UHVDC)transmission project is calculated.Thirdly,considering the mutual influence of different voltage levels on the biasing current of power grid,and taking into account the variation of grid parameters such as the magnitude of grounding current,line resistance and substation ground resistance,the influence rules of different factors on the bias magnetic current are calculated and analyzed.And compared with the measured data,the effectiveness of the network equivalent model and the nodal voltage method to calculate the bias current are verified.Finally,aiming at the problems of capacitor blocking device to manage the bias magnetic current,a single-point and multi-point control scheme using small resistance is proposed.By analyzing the influence of different governance schemes on the distribution of magnetic bias current,the governance effects of various schemes are compared,The research methods and conclusions of this paper can be used as a reference for the prevention and treatment of transformer biasing current and the configuration of the suppression devices.