Investigation of earthing resistance of substations

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Dr. Ladányi József
Department of Electric Power Engineering

The most important tasks of electrical safety of power systems are to keep the touch and step voltages and the earth potential rise under a certain level at substations and transmission line towers. Therefore, proper establishment of the earthing grid of substation or transmission line is essential, which also means low enough resistance to earth of the grid. There are several methods to measure resistance to earth, but the

Fall-of-Potential Method is one of the most popular.

My thesis has three fundamental purposes. The first is that to introduce the referred terms and rules through international and national literatures. The second purpose is that to review the measuring methods of resistance to earth, including the problems, factors affecting the error of the measurement and practical suggestions how to handle these. And the third, to present utilizing simulations how does the placing of the auxiliary earthing probe and voltage probe affect the accuracy of the Fall-of-Potential measurement, especially if these probes are not satisfying the criteria of the minimal distance from the earthing grid (it is frequent because of external circumstances).

In my simulations I investigate how does the 61.8% rule corresponds to reality, which claims if we locate the probes in line, to measure the exact EPR we must place the voltage probe exactly at a 61.8% distance from the injection point compared to the auxiliary earthing probe. Furthermore I give information about the error of the measurement if we place the voltage probe at a 50%, 61,8%, and on the opposite site of the injection point at 100% distance, and also about the interval’s size around the ideal location of the voltage probe, where the error of the measurement is less than 5%. I examine all of these factors according to the auxiliary earthing probe distance, in case of two earthing grid type, three different soil structures and more directions.

The specialized literatures does not include such detailed investigations, subsequently the results of these simulations can significantly help the colleagues’ work in reference to resistance to earth measurements.


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