Comparative analysis of grounding systems formed by feeders in one case with uninsulated and in the other case with insulated metallic sheath

摘要

It is well known that the metal-sheathed cables essentially affect the potentials appearing during a ground fault in substations located in typical urban surroundings. This fact was recognized early in the case of cables with uninsulated metallic sheath (the electrical protection). The metallic sheaths of these cables are in direct and continuous contact with earth, and in accordance with that these cables act as long grounding electrodes. As a consequence the underground cable lines belonging to the MV distribution network supplied by one HV/MV substation spontaneously form one very large grounding system around this substation. However, instead of this type of cable, constructed mainly as a three-core cable, cables with insulated metallic sheaths, constructed mainly as single-core cables are more and more applied in contemporary MV networks. The cables of this type do not act as grounding electrodes and the following quite logical question arise: how the cable of this type will influence the general conditions for solving the grounding problem of distribution substations, especially of those working in HV networks with directly grounded neutral(s)? The analysis performed in this paper shows that the grounding systems formed by this type of cables also manifest excellent grounding characteristics. The explanation for this statement, which may appear paradoxical at first sight, is in the following facts. The grounding systems formed through this type of cables cover urban areas which are many times larger than in the case when the metallic sheath is uninsulated. Because of that mutual interference between different parts of such grounding system in dissipating the ground fault current into the surrounding earth (the so-called proximity effect) practically do not exist.