Application of Electromagnetic Field Theory to Measure Correct Grounding System Impedance: A Parametric Analysis

Measurement of the grounding system impedance of a substation or power plant is often required immediately after construction, in order to verify that the design calculations correctly predict the performance of the system. Years later, new measurements are sometimes required to check that the performance of the grounding system has not deteriorated. This type of measurement, however, which is typically carried out using the fall-of-potential method, is plagued by a number of potential problems: conductive coupling between the grid under test and the remote current return electrode, especially for soil structures with low resistivity over high; inductive coupling between current and voltage test leads; inductive coupling between test leads and grounding grid conductors; inductive coupling between test leads and power line static or neutral wires; additional grounding provided by power line static and neutral wires, which lowers the apparent impedance of the grounding grid. This paper presents a methodology for measuring ground impedances that minimizes the effects of inductive coupling, conductive coupling, and power line grounding. A parametric analysis is carried out to illustrate how measurement error changes as a function of grid dimensions, soil structure, test electrode locations, test lead separation distance, locations of test lead connections to the grounding grid, and test signal frequency used. It is found that accurate ground impedances can be obtained by interpreting the test data using electromagnetic field models that determine the expected error as a function of test signal frequency.