Temperature Rise Characteristics Of DC Deep Well Grounding Electrodes And Development Of Simulation Software

At present,most of the DC grounding electrodes in China adopt the ring configuration with horizontal arrangement where the electrode site covers a large area.With the rapid development of the HVDC transmission,system capacity and rated current are increasing,and they lead to difficulty for site selection and eminent domain.As deep well grounding electrode needs less land,it can be built in narrow terrains or uneven areas,which greatly reduces land use and cost.However,due to serious terminal effect,the temperature rise has become an important factor restricting the development of deep well grounding electrodes,in the meantime,theoretical analysis is also insufficient.Therefore,it is urgent and necessary to study the temperature characteristics of deep well grounding electrodes so as to provide reference for design and application.Bade on finite element method(FEM),this paper establish a coupling model of current and temperature field of deep well grounding electrodes by which the transient and steady-state temperature fields of electrode are calculated.Results show that the transient and steady-state temperature distributions are very different and the time required to reach the steady state is extremely long(several years).Considering the time of monopole operation is limited in deep well grounding electrodes,it's more meaningful to study the transient process of temperature rise.Secondly,by changing the structure parameters of a single electrode,and the arrangement types of several electrodes in parallel,the influence on the temperature rise of deep well grounding electrodes are discussed in detail by simulation.The results show that the maximum temperature rise rate of the electrode decreases with number of sub-electrodes,electrode length,coke thickness,and electrode interval.When current is injected into the top of electrode,considering the resistance of steel and accumulation effect,current concentrated at the injection point,the joint result of conductor heating and end effect may cause the temperature rise on the top of electrode higher than the bottom.The temperature rise at the top of electrode increases with buried depth,but it is more susceptible to the soil structure when considering layered soils.The steel pipe changes the leakage current density distribution of the feeder rod,but it has less effect on the temperature rise.The maximum temperature rise is almost independent of the number of electrode segments,injection points,and their locations.A vertical electrode was used in the temperature rise tests in uniform and in two-layer soil.The test current flows into a steel drum from the electrode to simulate the actual current distribution,and the upper layer of soil with higher resistivity was simulated by sand.It was found that,compared with the homogeneous soil,the temperature rise at the lower end of the electrode in the two-layer soil was significantly higher,while that of the top end is lower.Due to the drying of soil,the voltage and temperature of electrode rises rapidly in later period.While putting a layer of coke around the electrode,the temperature rise reduced significantly.Based on the parameters of soil samples,the temperature rise of the test device was calculated.The results are in good agreement with the measured values,and the accuracy of the simulation model has been verified.Finally,based on ANSYS and the established electro-thermal coupling model,a software has been developed to calculate the temperature rise of the deep well grounding electrodes.The software can analyze the steady-state and transient temperature field of deep well grounding electrodes under different arrangements and multi-layer soil models.