Analysis On The Temperature Characteristic Of Soil And Its Effect To The DC Grounding Electrode Heating

The safe operation of DC grounding electrode is an important base for maintain theentire DC system safety and reliability, and the soil properties of location for thegrounding electrode is the one of the main factors influencing the electrode performance.With the rapid development of UHVDC project in our country, the rated current isincreasing. In the condition of the system operates in the monopole ground return mode,the current travels into the earth through the grounding electrode is equal to the ratedcurrent, leading to more serious temperature rise to the electrode and its neighboringsoil, and inconvenience for electrode design and maintenance. In addition, it’s moredifficult to find an appropriate place for electrode because of the problems mentionedbefore. The common grounding electrode alleviate the pressure of site selection, but thismode may cause more current flow through the electrode, resulting in the electrodeover-temperature and even burned, then threaten the grounding system security.Therefore, no matter from the demand for practical engineering applications, or analysisof the ground electrode temperature rise process, further research is indispensable.This paper selects three typical textures of the soil samples from the receiving endof the HVDC system to conduct a simulation experiment. Controlling the temperaturerange and measurement interval, soil resistivity measured by voltammetry at pre-settemperature, and soil weight recorded for the water content calculation. Then, thequantitative expression of the water content, soil resistivity with the temperature riseobtained from the experiment results analysis and fitting, combined with the thermalparameters of the classic formula deducing the relation between soil thermal parametersand temperature rise, in order to make a basis for studying the process of the groundingelectrode temperature rise.In order to consider the influence of the temperature rise to the soil parametersaround the electrode, and the temperature rise process variation after the changes of soilparameters, a finite-element model which takes electric-thermal coupling effect intoaccount presented to further analysis of grounding electrode temperature rise in thispaper. In this work, the problem between open boundaries of earth environment with thelimited area are processed by introducing a spatial transformation; the problem aboutthe convergence of the equation caused by the marked rise in parameters between soiland electrode are solved by the weak form of finite element method. From the above way, COMSOL finite element simulation software is chosen to modeling, and the modelshould meet the requirements by model skill settings.The common structures of the grounding electrode are simulated in the paper basedon two calculation model, traditional (without considering the soil variation with thetemperature rise) model and electric-thermal coupling model. By comparing thesimulation results, analyze the differences between the two models and the superiorityof the coupling model. In addition, especially for the cirque electrode in UHVDC,simulation model established for calculating a long time operation in the monopoleground return mode, with the simulation results, the paper proposes to optimize ascheme for the maximum temperature the grounding electrode should controlled with inorder to keep the UHVDC operation safety and reliability.Above all, the temperature characteristic of soil is an important factor to influencethe grounding electrode temperature rise. Therefore, more valuable parameters forproject will be provided after considering this factor to ensure the entire the DCtransmission system in long-time operation safety.