Calculation And Experimental Research On Temperature Characteristic Of DC Grounding Electrode

Because of the development of UHV DC transmission, the DC grounding electrodebecomes more and morecomplex. The current flowing through the DC grounding electrode is increasing by the higher voltage level and more common and the tempturerise of grounding electrode will be more serious. The soil parameter considered as aconstant value in traditionaltemperature rise calculation, but in fact, the soil parameters are very sensitive to changes in temperature and will change bytemperature rise, thereby affecting the temperature of electrode and the nearbysoil. Thispaper established a model of DC ground electrode temperature field considering changes in soil parameters and carry out two types ofDC grounding electrode temperature rise testincluding vertical grounding electrode and horizontal grounding electrode in1000kV UHV AC test base. The main research works are as follows:(1) Acoupling calculation model of DC grounding electrode current field and temperature field is established, which is based on the results of current fieldcalculation. Thetransient process and steady-state temperature distribution in grounding electrodeare calculated. In this paper, the finite element (FE)-infinite element (IFE) coupling method is proposed to calculate current field, whose coupling method simulates far-field effect withinfinite element method and simulates near-field effect withfinite elementmethod. The comparison results indicate that the amount of subdivision is decreased with FE-IFE coupling method than that with FE method, and it reduced the error caused by artificially defined truncation boundary at infinity with FE method.(2) Temperature rise of DC grounding electrode is calculated numerically in miscellaneous soils. In practical cases, parameters vary with different typeofsoil. In the numerical calculation, considering the influence of soil parameters, the calculation results of temperature rise show:with higher soil heat conductivity, the steady-state temperature of DC ground electrode is lower, with more uniform temperature distribution; with smaller ratio of soil thermal capacity and thermal conductivity, it has smaller thermal time constancy. Therefore, it’s better to select places with high thermal capacity and thermal conductivity to set the DC grounding electrode.(3) The influence of soil resistivity on temperature rise of DC grounding electrode is researched with soil of different type. The soil resistivity has a significant influence on temperature rise of DC grounding electrode. The current diffusion tends to be more non-uniform with higher soil resistivity, resulting in a more non-uniform steady-state distribution of temperature. In practical cases, soil resistivity is a function of temperature. And it increases rapidly when the temperature is higher than some critical value. In this case, the temperature increases exponentially with the soil resistivity. Considering this case in the calculation, it’s proposed that the variation of soil resistivity should be considered in the design of ground electrode and the temperature should not exceed65℃.(4) Temperature rise tests based on field similarity theory were conducted for vertical ground electrode. DC current was applied to the ground electrode and the temperature rise and current diffusion? of ground electrode were measured by thermocouple and hall current sensors. With the influence of end effect, the disperse current density is gradually increasesfrom head to toe, and disperse current density in the bottom of10cmis far greater thanthe average disperse current density of the whole electrode. Closer to the bottom of the electrode, temperature rise is earlier, rising rate is higher,amplitude is larger, With the passage of time, the temperature rising rate decreased and the largest temperature rise in the bottom ofelectrode.(5) Simulation tests based on field similarity theory were conducted further to study temperature rise of horizontal ground electrode. Single linear grounding electrode was applied, and the temperature rise and current diffusion of ground electrode were studied. There’s a significant end effect on the ground electrode, resulting in an symmetrical current divergence distribution that the current divergence decreased from two ends to the middle of the electrode; the temperature distribution of grounding electrode is high at both ends, low in the middle.At the beginning of the test, temperature rise is faster and it is gradually slow downwith the increase of current load time. A special phenomenon occurred in tests that the temperature curve at the injecting point tended to be upturn, was mainly caused by the evaporation of water with high temperature.(6)It’s fitted soil resistivity and determined soil thermal parameters oftest location. Tests’ results show that the soil resistivity rises with the temperature rise and the temperature rise curve upturns at the injecting point. Soil thermal parametersvary with temperature was determinedbased on the measured data and loaded into simulation.Soil resistance of test soil was fitted according to Campbell empirical equation of soil electric conductivity and a fitted equation is given out in this paper. And the simulation results with the fitted equation are much closer to practical tests.