Fault Current-Division Factor of Substation Grounding Grid in Seasonal Frozen Soil

The determination of the maximum ground fault current, which has a close relation to the maximum hazardous step-and-touch voltage, is essential for the safety of a substation. Seasonal frozen soil changes the grounding resistances of the substation grounding grids and the tower footing devices, which seasonally influences the maximum grid current. This paper first introduces the method to calculate the fault current-division factor, and then discusses the influence of the seasonal frozen soil layer on it by using the proposed method. Based on the cases of two substations connected by the transmission lines and without neutrals or other connection paths, the influence of several factors on the fault current-division factor, including the thickness of seasonal frozen soil, and the number of transmission lines with overhead ground wire connected to the substation grounding grid, is analyzed. It is reasonable to consider the influence of the seasonal frozen soil on the fault current-division factor in grounding grid design. According to the analysis, adding vertical grounding rods to the grounding grid and the tower footing grounding device can almost eliminate this seasonal influence.

I. Introduction

The electrical properties of a substation grounding system, such as grounding resistance, step voltage, and touch voltage, are related to soil characteristics. The soil characteristics change in different seasons. During freezing seasons, the resistivity of soil increases to several tens of times of that in other seasons, and is measured in the range of 1000 to 15000 m. In North China, the thickness of the frozen soil is from 1 to 2 m, but in Northeast China, it can reach about 4 to 6 m. The frozen soil strongly affects the safety of the grounding system as discussed in [1]–[3]. The grounding resistances of the substation grounding grids and the tower footing devices increase in the frozen soil. As a result, the fault current-division factor may change, which would affect the design of the grounding system. A discussion about the influence of the frozen soil on the optimal design of the grounding system can be seen in [4].