Grounding system impedance: Range of frequency linearity

The measurement of grounding system impedance of in-service power facilities is a very complex issue since the power system frequency interference and harmonic frequencies may adversely affect the measurement. Recently, to overcome this problem the Frequency Shift Method (FSM) has been proposed. FSM requires that two of three test frequencies must be above power system frequency (in our case 50 Hz). However, due to the nonlinear dependence of the grounding system impedance versus frequency, the choice of test frequencies is not arbitrary. In this paper, this issue is experimentally examined based on the analysis of grounding system impedance linearity range. For this purpose, the grounding system impedance, for a sample of 16 substations (HV and MV), is examined. We find that the highest test frequency should be ≤80 Hz.