EARTHING DESIGNE

8/19/2019 EARTHING DESIGNE

1/17

EARTHING AND

CONSIDERATIONS FOR ITS

DESIGNPresenter : Dr. J. K. Arora

•

An earthing system is the total set of measures usedto connect an electrically conductive part of the

power system to earth.

• A well-designed earthing system ensures correct operation

of protective devices on occurrence of earth faults orlightening strikes, safety of equipment and personnel and

prevents build-up of electrostatic charges or occurrence of

dangerous induced voltages on equipments and structures.


2/17

ROLES OF EARTHING SYSTEM

i) Correct operation of the electric network, leading

to good power quality through:• Low zero sequence impedance for return of

unbalanced fraction of three-phase ac

• Rapid and unambiguous identification of fault

conditions for efficient relay and fuse coordination

ii) To ensure electrical safety for exposed humans

and animals by:

• Fast identification of faults, leading to reducedfault duration

• Limiting touch and step voltages and resulting

body currents to safe values


3/17

iii) To protect against lightning by:

• Limiting potential differences across electrical

insulation on stricken equipment• Providing low-impedance paths for dissipation

of stroke current energy

iv) Providing low-impedance paths for dissipation

of stroke current energy

EARTH ELECTRODEA conductor or group of bare conductors in

intimate contact with, and providing an electrical

connection to earth

Simple Electrode or combination of simple electrodes


4/17

Figure 1. Current dissipation through soil fromlong vertical rod electrode


5/17

IMPORTANT PARAMETERS

Efficacy of an earth electrode determined from

i) earth resistance of the electrodeii) the dangerous voltages namely maximum value of

step voltage and touch voltage

iii) transferred potential

Earth resistance - A function of i) Soil resistivity, ii)electrode geometry defined by shape, dimensions

and layout of earth conductors forming the

electrode, iii) its depth of burial, and pattern of

current dissipation in earth around the electrodeDangerous voltages depend on aforementioned factors

and magnitude of current that flows between grid

electrode and the surrounding soil


6/17

Earth Electrodes & Earth Resistance

• Metallic rods buried vertically into earth orhorizontal bars or strips or a combination of both

• Earth resistance not due to conductors but due to

flow of current across the soil• Current flow between earth electrode near fault

to ground electrodes towards sources of current

• Current flow assumed to be flowing acrosshemispheres of increasing surface areas

• Resistivity of conductor a few micro-ohm-m,that of soil a few ohm-m to hundreds of ohm-m


7/17

• Resistance of conductors of ground electrode is

negligible

• Resistivity of mass of soil important; at a distancewhich is about 5-10 times the extent of electrode, cross-

sectional area of current path is so large that additional

resistance of soil beyond it is very small

• Size of electrode is important; ground resistanceinversely related to size. Same material concentrated in

a small volume has larger ground resistance than when

spread over a larger area

• If soil is rocky or sandy, good option to encase the

conductors in cement concrete for good contact w soil


8/17

PRINCIPAL DESIGN DATAi) resistivity of the soil, ii) size and shape of the

area over which grid electrode is to beinstalled, iii) magnitude of single line to earth

fault current and grid current.

Besides these, the other data needed is i)material of earth conductors and type of joints,

ii) duration of fault current, iii) special

considerations of corrosion and minimum size

of earth conductors if any, iv) shock duration,

v) resistivity of gravel layer and its depth, and

vi) type of fencing/boundary wall.


9/17

RESISTIVITY OF SOIL

• Soil resistivity measurement is Wenner four-

probe method• Dependent on moisture content of soil, seasonal

variation near surface, but constant in deeper soil

• Earth formed from layers of different materials• Soil modification may work mostly for simple

electrodes. For a grid laid in concrete slab of say

1:3 cement:sand ratio (av. Resistivity 150 Ω-m) ,

grid at most acts like a plate electrode

• Based on interpretation of measurements,

uniform or two-layer soil model may be adopted


10/17

i) Earth resistance R G (Ω) of the electrode in

uniform soil of resistivity ρ Ω-m, for a grid electrode

of area = A m2 is approximated by Laurent’s formula

above

ii) Increase in length of horizontally buried earthconductors by placing them closely has little affect

on earth resistance

iii) ‘ A’ is whole of contiguous area of gridiv) Substations with low resistances are not an

indication of safe design, nor is a substation with a

high resistance necessarily an indication of an unsafe

desi n

A4R

G


11/17

EARTH FAULT CURRENT AND

GRID CURRENT

• Only faults that cause fault current to flow intoearth to be considered

• It determines magnitudes of EPR, step voltage

and touch voltage• For a single line to earth fault

)]XXX( j)R R R R 3/[( 021021f I0 = E

If = 3 I0

Standardized values of current such as 65 kA, or 40 kA, or

31.5 kAbased on three-phase current breaking capacity of

circuit breakers


12/17

Electrode Conductor • must be able to carry the current which is to flow into

soil for the required duration of time• inefficient joint will have its own resistance causing

heating and damage at the joint

• Corrosion is dependant on presence of moisture andsalts in the soil; is a function of soil resistivity

• Use of Mild Steel avoids galvanic action with other

underground utilities, which are mostly of steel

• Area of MS conductor in mm2, tf seconds, and I amps3

f c10tKIA


13/17

Table II. Corrosion allowance for steel earth conductors

S.

No.

Resisti

vity

(Ω– m)

Class (corrosive)

of soil%

Thickness

mil mm

1 Up to

25

Corrosive &

Severely Corrosive

30 180 4.5

0

2 > 25 <

100

Mildly &

Moderately

Corrosive

15 90 2.2

5

3 > 100 Very Mildly

Corrosive

10 30 0.7

5


14/17

DURATION OF FAULT CURRENT AND

SHOCK DURATION

• fault duration tf for should be the maximum possiblefault clearing time including back up

• 1 second for stations using solid state or digital relaysand 3-second for stations using electromagnetic relays

• operating time of protective relays and circuit breaker isused as shock duration ts for personnel safety againstshock

• ts

taken 0.5 s for stations using digital relays and 1 s forstations using electromagnetic relays

• appropriate value applicable keeping in view the relayand circuit breaker operating times to br chosen


15/17

RESISTIVITY OF GRAVEL LAYER AND

ITS DEPTH

• Gravel of resistivity s & thickness hs spread overareas of swyd for several reasons; one of which isincrease in earth resistance of foot and themaximum permissible values of E

step

and Etouch• Resistance modification factor is

• Instead of gravel concrete slab

09.0h2

)1(09.0

Cs

s

s

At some stations a slab of cement concrete is being tried

below gravel to prevent growth of weeds/grass. Resistivity

of gravel is important to calculate Cs.

Alternate

expressions for Cs available


16/17

FENCING/BOUNDARY WALL

Touch voltage maximum near fence. Conductor spacing

may have to be reduced to make it safe

Touch voltage maximum between fence and a point 1

meter away outside itin area without gravel

Alternately, boundary wall topped with fence may be

considered

SOFTWARE IN EARTHING DESIGNMany formulas currently in use obtained empirically from

computer simulation results

Several limitations on use of formulas

When the conductors are unequally spaced or when thesoil model is two-la er or more use of software mandator


17/17

132 kVSystem bus

132/33 kV transformers 132/33 kV transformers

33 kV lines

33 kV substation

11 kV line

System generator

Three-phase fault level on 132 kV bus is 31.5 kASingle line to earth fault on 33 kV bus at the distrib. s/s,

fault current = 1819 A. This current returns to the grid

station through earth, the grid current is thus 1819 A.

SLG fault on 11 kV bus at the distrib. s/s, fault current= 4617 A. No current returns to the grid station through

earth and the grid current is zero

SLG fault on 11 kV line very near to the distrib. s/s,

fault current = 2616.4 A. grid current = 2616.4 A

Documents

EARTHING DESIGNE