Novel measurement system for grounding impedance of substation

In order to measure the grounding impedance of a grounding system more precisely and easily, two key problems of the measurement system: the location of the voltage electrode and the elimination of the noise and interference must be conquered. The new theoretical analysis and measurement system of the grounding impedance based on short-current lead wire and a swept frequency ac source is proposed. The compensating position of the voltage electrode to obtain the real grounding impedance is evaluated by analyzing the soil structure, the grounding system under test, and the position of the current electrode. This new system measures the ground impedance at several frequencies other than power frequency, then interpolates them to acquire the ground impedance at the power frequency. The mutual inductance between the potential and current wires is analyzed and eliminated. The industrial computer-based grounding impedance measurement system is developed and the analysis software is programmed. A field test was performed and compared with the traditional method in the substation and the novel system worked well.

I. Introduction

The substation grounding impedance should be regularly measured to evaluate the availability and safety of the grounding system. Traditional measurements are based on the fall-of-potential method. Generally, in this method, large ac current is injected into the grounding system under test and the voltage between the grounding system, and the voltage electrode is measured. A current electrode is often placed far away from the ground system to decrease the measurement error; therefore, a long electrode wire is needed. Long wires lead to more interference. Moreover, in the case of voltage electrode wire being placed in parallel with the current lead wire, the longer the current electrode lead wire is, the higher the inductive voltage between the voltage and current lead wires is, which makes the entire measure process very difficult [1].