The paper presents a study on the effects of double earth faults in Medium Voltage (MV) networks, in presence of electrical interconnection of the earthing systems of the secondary substations.In urban areas, indeed, the earth electrodes of the secondary substations are interconnected and, in many cases, they are also connected to the earth electrodes of the primary stations, in order to realize quasi-equipotential surfaces, intrinsically safe against electric shocks, named Global Earthing Systems (GES).The concept of GES has been introduced in the technical literature ten years ago in the Document HD 637 S1 by CENELEC, with the aim of providing a solution to the problems of designing and verifying the effectiveness of the earth electrodes of the secondary substations located inside urban areas. In these areas, usually, fault currents are higher and it is more complicated to execute large earthing systems and to execute earth resistance measurements. Unfortunately, the concept of GES has not been adequately exploited, primarily due to the absence of precise indications on how to recognize a GES among all the interconnected, but not necessarily intrinsically safe, large earthing systems. The study of these systems has been done in the past by using practical methods or touch and step voltages measurements and, first of all, with regards only to single line to earth faults.Nevertheless in very old MV network, expecially in those managed with unearthed neutral points, single line to earth faults can evolve into double earth faults, characterized by very high fault currents (about the 80% of the three-phase fault currents).Therefore such faults can significantly influence the earth potential rise distribution both at the fault location and also close to the other unfaulted substations.Moreover the thermal effects of the double earth fault current on the earth electrodes and on the metal sheaths of the MV cables can be very damaging.In the paper, a model for studying double earth faults is presented and many different fault situations are examined varying geometrical and electrical parameters like:-the earth resistances of the secondary substations’ earth electrodes;-the number of interconnected earth electrodes;-the fault location;-the distance between the substations;-the effect of the presence of the earth electrode of the primary station in the interconnection of earthing systems.Aim of the paper is to find general conclusions on the effects of double earth faults in presence of interconnected earthing systems in order to give a contribution to the definition of a general and easily applicable procedure for the identification of GES.
|Numero di pagine||0|
|Stato di pubblicazione||Published - 2009|
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