Active length of the cable lines with an uinsulated metal sheath from the standpoint of grounding effects conference EUROCON2009

It is well known that the MV paper-insulated cables lines act as long grounding electrodes, or inject fault current directly into the surrounding soil. As a consequence of that the MV lines performed of this type of cables spontaneously form a very large and complex grounding system around the HV/MV distribution substations. The area encompassed by this type of grounding system is much larger than occupied by the HV/MV substation itself. Because of that these grounding systems manifest excellent performances including a very low value of the impedance to ground. The problem with this type of grounding systems appears only at the design stage of the HV/MV substation, when safety condition inside and in the vicinity of this substation should be estimated. This paper provides improvements in accuracy to the only existing practical method developed for the analysis of this type of grounding systems.

I INTRODUCTION

The grounding system of the HV/MV distribution substations is not strictly limited on a priori defined area. They are formed freely and spontaneously so that they encompass various underground metal structures including those, which only in a trivial rate, because of great distance from the HV/MV substation, perform grounding function. Thus, it can be said that the space limits of the grounding system formed by this way are practically identical with the space limits of the potential field produced by the grounding system it-self. Certainly, the area encompassed by this way is the largest possible one. For the particular design characteristics of the implemented MV cables, it depends only on the soil reistivity of the encompassed urban area [1]. As a consequence, the grounding systems spontaneously formed around the HV/MV substations manifest very desirable performances. However, the problem rises when this favorable fact should be confirmed by calculations.