Calculating Ground Resistance

8/9/2019 Calculating Ground Resistance

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2.2 Calculating Ground Resistance

The following formula (source: IEEE Std.142:1991) enable the resistance to ground tobe calculated.

here:


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! " resistance in #$ " resisti%it& in #.cmd " distances ' in cm

S " sace between ground rods

Multiple Ground Rods space factor will be as follows:

2.2.1 Calculating Ground Resistance for substations

Ideall& a ground s&stem should be as close to *ero resistance as ossible. +or mosttransmission and other larger substations the ground resistance should be about 1# orless. In smaller distribution substations the usuall& accetable range is from 1',#deending on local conditions. Estimation of the total resistance to remote ground isone of the first stes in determining the si*e and basic la&out of a grounding s&stem.

- minimum %alue of the substation grounding resistance in uniform soil can beestimated b& means of the formula of a circular metal late at *ero deth once thesoil resisti%it& has been determined.

se the following formula to estimate the minimum resistance that can be e/ected indesigning a grounding s&stem:


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here:

Rg =ground resistance in #.

=a%erage earth resisti%it& in #0m.

A " area occuied b& the ground grid in suare meters.

" .14

Ea!ple"1:

hat is the grid resistance of a s&stem if $ " 2,3 #0m and - " ,33 m2

#olution:

5alculating with the abo%e formula the results are as follows:

So !g " 1.67 #

8e/t an uer limit of the substation resisti%it& can be obtained b& adding a secondterm to the abo%e formula. The second term recogni*es the fact that the resistance ofan& actual grounding s&stem that consists of a number of conductors is higher thanthat of a solid metallic late. This difference will decrease with the increasing lengthof buried conductors aroaching 3 for infinite when the condition of a solid lateis reached. (IEEE'63)

To estimate the uer limit use the formula:

here:

Rg " ground resistance in #.

" a%erage earth resisti%it& in #0m. This measurement should be located on therints or the 1333 #0m can be used.

A " area occuied b& the ground grid in suare meters.


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$ " total buried length of conductors in meters.

" .14

%ote"1

se the abo%e formula to aro/imate the ground resistance of a s&stemand not as a relacement for actual ground measurements.

The total burial length is the combination of the hori*ontal and %ertical conductors inthe grid as well as the ground rods. can be calculated as:

here:

$C " total length of grid conductor (m)

$R" total length of ground rods (m)

- better aro/imation was determined to include the grid deth

here

& : deth of the grid (m)

This euations shows that a larger the area and the greater the total length of thegrounding conductor used would resulting a lower ground grid resistance.

'( )erif*ing Ground Grid Conductor +nstallation

our %erification of a grid s&stem starts with insections of the station la&out lanshowing all ma;or euiment and structures.

The area of the grounding s&stem is the single most imortant geometrical factor indetermining the resistances of the grid. arger grounded areas result in lower gridresistance and thus lower a/imum rosecti%e ground fault current assing between grounding s&stemand the bod& of earth


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2. ?uration of flow of this current (based on a duration of 1 second)

. Soil resisti%it& and the nature of the ground at the site.

%ote"2

It is not ossible to use the short'time current rating of the brea@ers orthree seconds for the first two of the abo%e arameters. E%en in areas oflow soil resisti%it& it would be difficult if not imossible to design anelectrode adeuate for such a dut&. It is therefore necessar& to determinethe ma/imum current and its duration of flow (1 second gi%en b& design)which the electrode must safel& transmit to or from the bod& of earth.

'.1 -esign Guidelines and Reuire!ents

- continuous conductor loo surrounds the erimeter to enclose as much area

as ractical. This ractice hels to a%oid high current concentration and hence high

gradients both in the grid area and near the ro;ecting cable ends. Enclosing more

area also reduces the resistance of the grounding grid.

ithin the loo the conductors la& in arallel lines and where ractical along

the structures or rows of euiment to ro%ide for short ground connections.

- t&ical grid s&stem for a substation ma& include 73 or 123 suare millimeters

(mm2) 8o. 403 or 203 -< bare coer conductors buried 16 inches (3., m) below

grade minimum saced 13 to 23 feet ( to A m) aart in a grid attern. -t cross'connections securel& bond the conductors together b& thermite welding bra*ing oraro%ed comression connectors.

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Calculating Ground Resistance