Impact of underground cables on ground impedance measurement implementing FSM concept in FOP measuring scheme

Ground impedance measurement in medium voltage substations, designed to operate in a predominantly cable network, is a complex research task. Namely, the inability to consider all the factors that can affect the measured value, in a situation where one deenergized cable line is used to form the Fall-of-Potential (FOP) measurement scheme, is directly reflected to an adequate assessment of safety risks. This paper proposes an alternative measurement method so called Frequency Shift Method (FSM), which inherently guarantees "better" measurement than any of the conventional methods. By providing insight into the behavior of the active and reactive ground impedance components, and at the same time taking into account the topological structure of the existing cable network, the application of the FSM concept in the FOP measurement scheme points out to alternative paths of test current distribution, and in that way pointing to the illogicality of the obtained measurement results. Experimental verification was conducted under real operating conditions, and can serve as a guide for solving similar problems in practice.