separation of mv and lv earthing systems in electrical distribution substations_2 column

7/26/2019 Separation of MV and LV Earthing Systems in Electrical Distribution Substations_2 Column

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Separation of MV and LV Earthing Systems

in Electrical Distribution Substations:Comparison of Standards

S. D. Buba1 !. ". !an #hmad1 M. $. #.

#b %adir1 C. &omes1 '. 'asni1 M. (sman)

1. Department of Electrical and

Electronic Engineering

"aculty of Engineering

*ni+ersiti ,utra Malaysia--// Serdang Selangor Malaysia.

). Department of Electrical ,o0er

Engineering

College of Engineering*ni+ersiti enaga 2asional

-/// %a3ang Selangor Malaysia

Abstract

Integration and separation of medium voltage(MV) and low voltage (LV) earthing systems in

electrical distribution substations has been a topicof debate in recent times. Proponents ofintegration of MVLV earthing systems base theirargument on the resistance of the earth grid andthe magnitude of the earth potential rise (!P"). Ifthe earth grid resistance is less than #$ and thee%pected !P" is not more than &''V the earthingsystems should be integrated otherwise theyshould be physically separated. pponents of


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integration of MVLV earthing systems on theother hand base their argument on the fact that itis impossible to maintain an earth grid resistanceof less than #$ throughout a given period due toseasonal variation of soil resistivity and alsoimpossible to control the !P" within safe limitsdue to its dependence on magnitude of faultcurrent as such the earthing systems should bephysically separated. *urrently there is nospecific recommended value for the physicalseparation distance between MV and LV earthingsystems in available literature. +his paper reviewsapplicable earthing standards to compare therecommended distances with regard to separationof MVLV earthing systems.

%ey0ords: earthing systems, integrated earthing,separated earthing, transferred earth potential

Introduction

Distribution substations are the most common

facilities in electric po0er systems that pro+ide the

distribution circuits 0hich directly supply electricityto consumers. hey are typically located near the

neighbourhoods they supply. Depending on the type

of e4uipment used substations could be categori5ed

as air insulated outdoor air insulated indoor gasinsulated outdoor gas insulated indoor or hybrid

technology types. Voltage le+els at distribution

substation +ary bet0een countries but typical

distribution +oltages used in Malaysia are and116V. 7n this paper air insulated outdoor distribution

substation operating at 8116V 9MV and /.-6V9LV is considered.

7n the e+ent of an earth fault such as single lineto earth 9SLE double line to earth 9DLE or three

lines to earth 9LE in a distribution net0or6 +ery

high fault current flo0s through the earthing grid

causing earth potential rise 9E,;. he E,; relati+eto a distant point on the ground is highest at the point

0here the current enters the earthing system and

declines 0ith distance from the source. E,; is a

ma3or concern in the design and operationof electrical substationsbecause of its ha5ard to

people and e4uipment. 7n se+ere cases of earth fault

the potential gradient 9+oltage drop 0ith distancemay be so high that a person in the +icinity of anearthed facility could be in3ured due to step or touch

potential. his transferred potential could also be

ha5ardous to people and e4uipment outside the

substation and may be propagated to the LV supplynet0or6 through the neutral system earthing. 7n

addition any conducting ob3ect connected to the

substation earthing such as telephone 0ires rails

fences or metallic piping may also be energi5ed to

the earth potential in the substation /


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separation of mv and lv earthing systems in electrical distribution substations_2 column

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