Transient impedance of GIS grounding grid

The transient impedance of a grounding grid for a commercial 550 kV gas-insulated substation (GIS) was measured on site using steep front currents with rise time from 100 ns to 2 /spl mu/s. From the measured results, it was found that the transient impedance of the measured grid for those currents is simulated by a series circuit with an inductance 1 /spl mu/H and a resistance 3 /spl Omega/. Applying the obtained impedance to the lightning surge analysis, it is pointed out that the grounding condition at an arrester tank end has a remarkable influence on the protecting performance of a surge arrester. >