substation earthing design

8/3/2019 Substation Earthing Design

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NEEC National Electrical Engineering ConsultancyDesign Management Construction

6 14 Bringelly Road www.NeecGroup.com

Kingswood, Australia [email protected]

NSW, 2747 Phone: 0415 77 55 75

ACN: 132586675 ABN: 86132586675

Substation Earthing Design

One of the main reasons behind the earthing design is to achieve a safe environment in the vicinity

of HV for people and workers. The hazard can jeopardise two categories of people:

The public that can be affected by the step and touch voltage. Workers who can be affected by the step and touch voltage as well as the earth potential rise

(EPR) zone.

The step and touch voltage can be determined from the two equations 1 and 2, these twoequations are calculated using the resistance from a 50 Kg person That is used when assessing the

public access area. Equations 3 and 4 calculate step and touch voltage using 70Kg body weight, this

calculation can be used in restricted areas within the site.

t

CV ss

touch

174.0116+= (1)

t

CV ss

step

696.0116+= (2)

t

CV sstouch

236.0157+= (3)

t

CV ss

touch

942.0157+= (4)

09.02

109.0

1

+

=

s

s

s

h

C

(5)

Where

Cs is the derating factor relating to

surface layer thickness and resistivity

s is the top surface layer

t is the primary clearance time

The EPR can be calculated by applying Ohms law and using the grid resistance and the maximum

fault current of the supplied system.


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NSW, 2747 Phone: 0415 77 55 75

ACN: 132586675 ABN: 86132586675

A. Earthing designEarthing design is divided into several sections, first is the gathering of informations. In order for

an earthing design to be effective and to address all the required areas including the financial aspect

of the project, it is important to make sure all the following informations are available and not

limited to:

Site layout

Maximum fault current

Clearance time

Back up time

Location of the feeding station

Near-by utility

Communication network in the area

Feeding characteristics

Any special considerations

Using the measured soil resistivity, it is now possible to determine the safety level by using

equations 1to 4.

Designing the grid using the site layout drawing is to avoid the clashes with the foundation of the

building. The grid is usually composed of a few vertical electrodes connected together through a

mesh of horizontal conductors.

The layout of the grid is completed by computing the grid resistance and the maximum fault

current to determine the maximum EPR and mesh voltage (the mesh voltage will be the maximum

possible touch voltage onsite). If the result is within the safety limit, the earthing design is

considered acceptable; otherwise it needs some modifications for the designed system. These

modifications can be achieved by dropping the resistance of the grid to an acceptable value where

the drop voltage is less than the safety limit.

The feeder characteristics can also be taken into consideration. The earth wire and the cable screen

can be very effective in the fault current return path. If the cable bonded at both end it will create a

return path to the source for the fault current, usually around 80% of the fault current will find its

way back to the source using the earth wire or the screen of the cable. Thus using this method is a

must to study the impact under any fault at the supply station. A separate study must be undertaken

to calculate the fault current into the ground and into the earth system.

B. Earth Grid CalculationAfter the determination of the soil resistivity test it is possible to determine the grid resistance

using hand calculations or computer simulation using software such CDEGS. The grid can be


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NSW, 2747 Phone: 0415 77 55 75

ACN: 132586675 ABN: 86132586675

varied in shape and contents depending on the job nature; the aim of the grid design is to achieve a

low resistance path to accompany the fault current without exceeding the drop voltage safe limit.

The grid can consist of a vertical electrode in the ground; the resistance of this electrode can be

calculated using equation 6:

= 1

8ln

2 d

L

LR

g

(6)

Where

L is the buried length of the electrode in metersD the diameter of the electrode in meters

If one electrode could not achieve the required resistance level, placing few electrodes in parallel

will help in reducing the grid resistance, equation 7 shows the resistance for the grid formed with

few electrodes in parallel.

= 1

2ln

b

L

LR

(7)

WhereL is the buried length of the electrode

b equivalent radius off the electrode at the surface

( )5.02

2

25.0

)4( shS

dhsSb

+=

=

(8)

Where:

d is the diameter of the electrode

h buried depth

s distance between 2 parallel electrodeS distance from one electrode to the image of the other in meters

The earth grid could also consist of mesh grid buried at a depth of 0.5 meters or more, the mesh

could consist of multiple horizontal conductors buried at the required depth. Equation 9 shows the

resistance of this mesh:

= 1

)(

4ln

5.0dh

L

LR

(9)

Whereh is the buried depth


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NSW, 2747 Phone: 0415 77 55 75

ACN: 132586675 ABN: 86132586675

L length of the electrode

d diameter of the electrode

The paper is supported by the following publication:

M Nassereddine 'How to design an effective earthing system to ensure the safety of the people July-ACTEA 2009

substation earthing design

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