直流输电系统变压器中性点电流分布的影响因素

Influential Factors on Distribution of DC Currents Flowing Through Neutrals of Transformers

直流输电系统单极大地回路运行时,直流电流流过变压器中性点,影响了变压器的运行。该电流的分布受到入地电流、土壤电阻率、变电站接地电阻、变压器等效直流电阻、AC系统网络拓扑和直流电阻参数等因素的影响。为研究这些因素的影响,将土壤模型和交流电网模型结合起来,建立了一个求解变压器中性点直流电流的模型来仿真比较500 kV电网实际数据。计算表明考虑海洋影响后,由于海水的低电阻率,造成处于距离接地极较远而靠近海边的厂站变压器中性点会流过较大的直流电流,而交流电网的拓扑结构造成处于相同地理位置的两变电站主变中性点流过的直流电流不相同。仿真结果解释了部分厂站变压器中性点流过较大直流电流的现象。

During monopolar operation of HVDC systems with ground return, a large of DC currents injected into ground will enter AC power system through neutrals of transformers and influence the status of transformers. The value of DC currents flowing through neutrals of transformers are related with many factors, such as currents of ground electrode, the soil resistivity, the configuration of ground electrode, equivalent DC resistance of transform- ers, DC resistance of power transmission lines, and AC power net topology. This paper constructs an AC power net model combined with a soil model. According to the southern 500kV AC power grid in China, DC current distribution is calculated in case of the land model and considering the sea＇s influence. Simulation results show there are some differences between model considering the sea＇s influence and pure earth soil model in calculation of the DC currents flowing through the neutrals of transformers. Because the resistivity of sea water is lower than soil, the potential of soil surface near sea decreases rapidly and is close to zero. More DC current flows through neutral of transformer which is far away from HVDC electrode and near sea. The topology of AC power net causes the different DC currents flowing through neutrals of transformers at the same site. The different connections of transmission lines lead to various equivalent DC resistance in the power network. The results explain the differences of dc currents flowing through neutrals in two nuclear power plants, they are almost at the same site.