直流输电接地极对地表电位分布的影响

Analysis of Effects of DC Earthed Pole on Earth Surface Potential Distributions

为消除直流输电的单极运行方式给我国交流电网的安全运行带来的威胁,采用有限元法分析了直流输电单极运行时接地极附近的地表电位分布特征,并通过简化的大地电阻率数学模型,定性分析了地形、大地各层电阻率、接地极设计和多个接地极共同作用等对地表电位分布的影响。因大地导电条件复杂,传统方法很难计算出有实际意义的可能流过变压器中性点的电流值,故根据等效阻抗的概念和逆向求解的思路,提出一种计算流过变压器中性点直流电流的新方法。

Some transformers have been found vibrating and noising in China Southern Power Grid and East China Grid while HVDC systems is operating in ground return mode, this phenomenon is not only bad to transformer itself, but also does harm to the whole power system. Distributions of Earth Surface Potential （ ESP） and DC; current flowing in AC system are very important for understanding and eliminating the danger caused by HVDC system. In this paper, Finite Element Method is used to analyze distributions of ESP near the DC earthed pole when the HVDC; power transmission systems is operating in ground return mode. By utilizing simplified earth resistivity model, several factors that may affect distributions of ESP are taken into consideration, such as landform, reslstivities of different earth layers, DC earthed pole designs, several poles working together. Furthermore, due to the extremely complex electric conditions of earth and interactions between earth and power grids, ESP calculated based on the simplified earth resistivity model are not applicable to evaluate the degrees of the DC currents flowing through the earthed points of AC transformers accurately. So a new method to evaluate the degrees of dc currents is mentioned. According to the new method, equivalent impedances between DC earthed pole and transformers earthed points are used to evaluate the distributions＇ of dc currents. Therefore, distributions of dc currents would be calculated without considering complex earth conditions. And the equivalent impedances can be worked out if adequate monitoring devices are installed on the transformers＇ earthed points, which are used to detect dc currents flowing in power grids.