Influence of Ground Wire Shunting on Grounding Impedance Measurement in Substations

The shunt of test current from the ground wires, including overhead lightning conductors and outer sheath of cables, would seriously affect the accuracy of grounding impedance measurement. To solve this problem, we analyzed the shunt by simulative calculations. Firstly, we measured the current distribution in a grounding impedance measurement of a 220 kV substation. The results coincide with the simulative calculation, and this verifies the feasibility of the latter method. After calculating, we analyzed the main factors and their influencing patterns of ground wire shunting. Ground wire shunting could highly reach as 50% or more, even got worse with greater grounding impedance of substation, smaller transmission tower impedance, great number of outlets, and dominating single-layout type of transmission lines, and eventually more seriously affected the tests of grounding resistance. If the ground diverting is not taken into consideration, grounding resistance measurement will have great negative deviance. At last we proposed a solution to improve the accuracy of grounding impedance measurement: when it is easy to disconnect grounding wires, the grounding resistance measurement should be further corrected based on simulative calculations of ground current shunt.