Different Layouts for Substation

The substation is the part of an electrical supply system which transmits the high voltage
from the generating substations to the local distribution networks. Between the generation
and distribution, the voltage may vary through several substations. The different types of
layout for substation are explained below in details.

Typical Radial Substation

In the radial substation, there is only one source of feeding the load as shown in the figure
below. This supply system is unreliable because of total blackout when source fails, or line
develops a fault. Such type of substation is used in distribution system particularly in the rural
area because of their unimportance.

Tapped-Substation
This supply is equally unreliable and insecure because there shall be total supply failure when
source or line become faulty.
LILo (Line In Line Out) Substation
In this substation, a long distribution line is brought in and brought out from newly created
substation as shown below. This scheme is bit expensive because of the need of an extra
layout. But it is more secure.

Interconnected Substation
This is the most preferred supply system which is safe secure and reliable.The outage of the
source and line does not effect power supply system because many other alternatives are
available.
SUBSTATION ELECTRICAL EARTHING
Definition: The process of transferring the immediate discharge of the electrical energy
directly to the earth by the help of the low resistance wire is known as the electrical
earthing. The electrical earthing is done by connecting the non-current carrying part of the
equipment or neutral of supply system to the ground.

Mostly, the galvanised iron is used for the earthing. The earthing provides the simple path
to the leakage current. The shortcircuit current of the equipment passes to the earth which
has zero potential. Thus, protects the system and equipment from damage.

Types of Electrical Earthing

The electrical equipment mainly consists of two non-current carrying parts. These parts are
neutral of the system or frame of the electrical equipment. From the earthing of these two
non-current carrying parts of the electrical system earthing can be classified into two types.

 Neutral Earthing
 Equipment Earthing.

Neutral Earthing

In neutral earthing, the neutral of the system is directly connected to earth by the help of the
GI wire. The neutral earthing is also called the system earthing. Such type of earthing is
mostly provided to the system which has star winding. For example, the neutral earthing is
provided in the generator, transformer, motor etc.

Equipment Earthing

Such type of earthing is provided to the electrical equipment. The non-current carrying part
of the equipment like their metallic frame is connected to the earth by the help of the
conducting wire. If any fault occurs in the apparatus, the short-circuit current to pass the earth
by the help of wire. Thus, protect the system from damage.

Importance of Earthing
The earthing is essential because of the following reasons

 The earthing protects the personnel from the shortcircuit current.

 The earthing provides the easiest path to the flow of shortcircuit current even after the
failure of the insulation.
 The earthing protects the apparatus and personnel from the high voltage surges and
lightning discharge.

Earthing can be done by electrically connecting the respective parts in the installation to some
system of electrical conductors or electrodes placed near the soil or below the ground level.
The earthing mat or electrode under the ground level have flat iron riser through which all the
non-current-carrying metallic parts of the equipment are connected.
 When the fault occurs the fault
current from the equipment flows through the earthing system to the earth and thereby protect
the equipment from the fault current. At the time of the fault, the earth mat conductors rise to
the voltage which is equal to the resistance of the earth mat multiplied by a ground fault.

The contacting assembly is called

earthing. The metallic conductors connecting the parts of the installation with the earthing are
called electrical connection. The earthing and the earthing connection together called the
earthing system.

Difference Between Grounding and Earthing

One of the major difference between the grounding and the earthing is that in grounding, the
current carrying part is connected to the ground whereas in earthing the non-current carrying
parts is connected to ground. The other differences between them are explained below in the
form of the comparison chart.

Comparison Chart
Basis For
Grounding Earthing
Comparison
The current carrying part is The body of the equipment is connected
Definition
connected to ground. to ground.
 Basis For
Grounding Earthing
Comparison
Between the equipment body and earth
Between the neutral of the
Location pit which is placed under the earth
equipment and ground
surface.

Symbol

Zero Potential Does not have Have

Protect the power system
Protection Protect the human from electric shock.
equipment.
Provide the return path to the It discharges the electrical energy to the
Application
current. earth.
Three (Solid, Resistance and Five (Pipe, Plate, Rod earthing, earthing
Types
Reactance grounding) through tap and strip earthing)
Color of wire Black Green
Use For balancing the unbalance load. For avoiding the electrical shock.
Neutral of generator and power The enclosure of the transformer,
Examples transformer is connected to generator, motor etc. are connected to
ground. the earth.

Definition of Grounding

In grounding, the current carrying parts are directly connected to the ground. The grounding
provides the return path for the leakage current and hence protect the power system
equipment from damage.

When the fault occurs in the equipment, the

current in all the three phases of the equipment become unbalance.The grounding discharges
the fault current to the ground and hence makes the system balance
The grounding has several advantages like it eliminates the surge voltage and also discharge
the over voltage to the ground. The grounding provides the great safety to the equipment and
improves the service reliability.

Definition of Earthing

The ‘earthing’ means the connection of non-current carrying part of the equipment to the
earth. When the fault occurs in the system, then the potential of the non-current part of the
equipment raises, and when any human or stray animal touch the body of the equipment, then
they may get shocked.

The earthing discharges the leakage current to

the earth and hence avoid the personnel from the electric shock. It also protects the equipment
from lightning strokes and provides the discharge path for the surge arrester, gap and other
devices.

The earthing is achieved by connecting the parts of the installation to the earth by using the
earth conductor or earth electrode in intimate contact with the soil placed with some distance
below the ground level.

Key Differences Between Grounding and Earthing

1. The earthing is defined as the connection of the non-current carrying part like the
body of the equipment or enclosure to earth. In grounding the current carrying part
like neutral of the transformer is directly connected to the ground.
2. For grounding, the black colour wire is used, and for earthing the green colour, the
wire is used.
3. The grounding balanced the unbalanced load whereas the earthing protect the
equipment and human from an electrical shock.
4. The grounding wire is placed between the neutral of the equipment and the earth
whereas in earthing the earth electrode is placed between the equipment body and the
earth pit which is placed under the ground.
5. In grounding the equipment is not physically connected to the ground, and the current
is not zero on the ground, whereas in earthing the system is physically connected to
the ground and it is at zero potential.
6. The grounding gives the path to an unwanted current and hence protects the electrical
equipment from damage, whereas the earthing decrease the high potential of electrical
equipment which is caused by a fault and thus protects the human body from the
electrical shock.
 7. The grounding is classified into three types. They are the solid grounding, resistance
grounding and reactance grounding. Earthing can be done in five ways.The different
methods of earthing are the pipe earthing, plate earthing, rod earthing, earthing
through tap and strip earthing.

Specifications for Earth Electrodes

1. The earthing electrode should not be placed near the building whose installation
system is earthed more than 1.5 m away.
2. The resistance of the earth wire should not be more than 1 ohm.
3. The wire use for electrode and circuit should be made up of the same material.
4. The electrodes should be placed in vertical position so that it can touch the layers of
the earth.

The size of the conductor should not be less than 2.6 mm2 or half of the wire used for
electrical wiring. Bare copper wire is used for earthing and grounding. Green 6 THHN
(Thermoplastic high heat neutral coating wire) and gauged copper wire of different sizes like
2,4,6,8 etc. are also used for earthing and grounding.

Methods of Earthing
There are several methods of earthing like wire or strip earthing, rod earthing, pipe earthing,
plate earthing or earthing through water mains. Most commonly used methods of earthing are
pipe earthing and plate earthing. These methods are explained below in details.

Earthing Mat

Earthing mat is made by joining the number of rods through copper conductors. It reduced
the overall grounding resistance. Such type of system helps in limiting the ground potential.
Earthing mat is mostly used in a placed where the large fault current is to be experienced.
While designing an earth mat, the following step is taken into consideration.

 In a fault condition, the voltage between the ground and the ground surface should not
be dangerous to a person who may touch the noncurrent-carrying conducting surface
of the electrical system.
 The uninterrupted fault current that may flow into the earthing mat should be large
enough to operate the protective relay. The resistance of the ground is low to allow
the fault current to flow through it.The resistance of the mat should not be of such a
magnitude as to permit the flow of fatal current in the live body.
 The design of grounding mat should be such that the step voltage should be less than
the permissible value which would depend on the resistivity of the soil and fault
required for isolating the faulty plant from the live system.
 Earthing Electrode

In this type of earthing any wire, rod, pipe, plate or a bundle of conductors, inserted in the
ground horizontally or vertically. In distributing systems, the earth electrode may consist of a
rod, about 1 meter in length and driven vertically into the ground. In generating substations,
grounding mat is used rather than individual rods.

Pipe Earthing

This is the most common and best system of earthing as compared to other systems suitable
for the same earth and moisture conditions. In this method the galvanized steel and perforated
pipe of approved length and diameter in place upright in a permanently wet soil, as shown
below. The size of the pipe depends upon the current to be carried and type of soil.

Normally, the size of the pipe uses

for earthing is of diameter 40 mm and 2.5 meters in length for ordinary soil or of greater
length in case of dry and rocky soil. The depth at which the pipe must be buried depends on
the moistures of the ground.
The pipe is placed at 3.75 meters. The bottom of the pipe is surrounded by small pieces of
coke or charcoal at a distance of about 15 cm. Alternate layers of coke and salt are used to
increase the effective area of the earth and to decrease the earth resistance respectively.

Another pipe of 19 mm diameter and minimum length 1.25 meters is connected at the top of
GI pipe through reducing socket.

During summer the moisture in the soil decreases, which causes an increase in earth
resistance. So a cement concrete work is done to keep the water arrangement accessible, and
in summer to have an effective earth, 3 or 4 buckets of water are put through the funnel
connected to 19 mm diameter pipe, which is further connected to GI pipe.

The earth wire either GI or a strip of GI wire of sufficient cross section to carry faulty current
safely is carried in a GI pipe of diameter 12 mm at a depth of about 60cm from the ground.

Plate Earthing

In Plate Earthing an earthing plate either of copper of dimension 60cm×60cm×3m of

galvanized iron of dimensions 60 cm× 60 cm×6 mm is buried into the ground with its face
vertical at a depth of not less than 3 meters from ground level.

The earth plate is inserted into

auxiliary layers of coke and salt for a minimum thickness of 15 cm. The earth wire (GI or
copper wire) is tightly bolted to an earth plate with the help of nut or bolt. The copper plate
and copper wire are usually not employed for grounding purposes because of their higher
cost.

Earthing Through Water Mains

In this type of earthing the GI or copper wire are connected to the water mains with the help
of the steel binding wire which is fixed on copper lead as shown below.
 The water pipe is made up of metal, and
it is placed below the surface of the ground, i.e. directly connected to earth. The fault current
flow through the GI or copper wire is directly get earthed through the water pipe.

Overhead Ground Wire or Earth Wire

Definition: The overhead earth wire or ground wire is the form of lightning protection using
a conductor or conductors. It is attached from support to support above the transmission line
and well grounded at regular interval. The earth wire intercepts the direct lightning strikes,
which would strike the phase conductors. The ground wire has no effect on switching surges.

When the lightning strikes an earth wire at mid-span, waves are produced which travel in
opposite directions along the line. The waves reach the adjoining tower, which passes them to
earth safely.The earth wire is effective only when the resistance between the tower foot and
earth is sufficiently low.
If the resistance between them is not low and the earth wire or tower will be struck by the
lighting, then the lighting will be raised to the very high potential, which will cause a flash
over from the tower to one or more phase conductors. Such a flashover is known as back
flashover.

The back flash over only occurs when the product of the tower conductor and tower
impedance exceeds the insulation levels of the line. It can be minimised by reducing tower
footing resistance using driven rods and counterpoises where soil resistivity is high.

The counterpoise is the conductor buried in the ground. The wire is usually made up of
galvanised steel. The counterpoise for an overhead terminal consists of a special ground
terminal that reduces the surge impedance of the ground connection and increases the
coupling between the ground wire and the conductor.

Two types of counterpoise are used in the transmission line, i.e., the parallel counterpoise and
the radial counterpoise.

Parallel Counterpoise – The parallel counterpoise is made up of one or more counterpoise

buried under the transmission line through its length. The counterpoise line connected
through the over earthed wire at all the towers and poles.

Radial Counterpoise – The radial type counterpoise is made up of many wires extending
radially from the tower legs.The number and length of wires are determined by the tower
location and soil conditions.

Shielding or Protective Angle

The shielding or protective angle is the angle between the vertical earth wire and the phase
conductor which is to be protected. Usually, the angle between the vertical through the earth
wire and the line joining the earth wire through the outermost phase conductor is taken as a
shielding angle.
For effective shielding, the protective angle should be kept as small as possible. The angle
between 20° and 30° is quite safe, and it should not be kept above 40°.

Two wires are used in modern high voltage system with wider spacing between the
conductor. The protection afforded by the two wire earth wire is much better than the single
wire. Also, the surge impedance for two earth wires is low and the coupling effect of the
wire increases.

Earth Resistance
Definition: The resistance offered by the earth electrode to the flow of current into the
ground is known as the earth resistance or resistance to earth. The earth resistance mainly
implies the resistance between the electrode and the point of zero potential. Numerically, it is
equal to the ratio of the potential of the earth electrode to the current dissipated by it. The
resistance between the earthing plate and the ground is measured by the potential fall method.

The resistance of the earthing electrode is not concentrated at one point, but it is distributed
over the soil around the electrode. Mathematically, the earth resistance is given as the ratio of
the voltage and the current shown below.

Where V is a measured voltage

between the voltage spike and I is the injected current during the earth resistance
measurement through the electrode.

The value of the earth resistance for different power stations is shown below
Large Power Station – 0.5 ohms
Major Power Station – 1.0 ohms
Small Substation – 2.0 ohms
In all other cases – 8.0 ohms

The region around the earth in which the electrode is driven is known as the resistance area or
potential area of the ground. The fault current which is injected from the earth electrode is
passing away from the electrode in all directions shown below in the figure. The flow of
current into the grounds depends on the resistivity of the soil in which the earth electrode is
placed. The resistivity of the soil may vary from 1 to 1000 ohm-m depends on the nature of
the soil.

The resistivity of the earth depends on its temperature. When the temperature is greater than
0ºC, then its effect on ground resistivity is negligible, But at 0ºC the water in the soil starts
freezing which increase their resistivity. The resistivity of the earth is also affected by the
composition of some soluble salts as shown in the figure below.

The resistance of the earth varies from layer to layer. The lower layer of soil has more
moisture and lower resistivity. If the lower layer contains hard and rocky soil, then their
resistivity increases with depth.
Soil Resistivity
Definition: The measure of the resistance offered by the soil in the flow of electricity, is
called the soil resistivity. The resistivity of the soil depends on the various factors likes soil
composition, moisture, temperature, etc. Generally, the soil is not homogenous, and their
resistivity varies with the depth. The soil having a low resistivity is good for designing the
grounding system. The resistivity of the soil is measured in ohmmeter or ohm-centimeters.

The resistivity of the soil mainly depends on its temperature. When the temperature of the
soil is more than 0º, then its effect on soil resistivity is negligible. At 0º the water starts
freezing and resistivity increases. The magnitude of the current also affects the resistivity of
the soil. If the magnitude of current dissipated in the soil is high, it may cause significant
drying of soil and increase its resistivity.

The resistivity of the soil varies with the depth. The lower layers of the soil have greater
moisture content and lower resistivity. If the lower layer contains hard and rocky layers, then
their resistivity may increase with the depth.

Measurement of Soil Resistivity

The resistivity of the soil is usually measured by the four spike methods. In this method the
four spikes arranged in the straight line are driven into the soil at equal distance. A known
current is passed between electrode C1 and C2 and potential drop V is measured across P1 and
P2. The current I developed an electric field which is proportional to current density and soil
resistivity. The voltage V is proportional to this field.

The soil resistivity is proportional to the ratio of the voltage V and current I and is given as
Where ρ is the resistivity of the soil and their unit is ohmmeters. S is the horizontal space
between the spikes in m and b is the depth of burial in metre.

If the measurement is to be carried out using the main supply, an isolating transformer should
be connected between the main supply and the test setup. So that the result may not be
affected by it.

Resistance & Reactance Grounding

Resistance grounding

In this type of neutral grounding, the neutral of the system is connected to ground through
one or more resistance. Resistance grounding limits the fault currents. It protects the system
from transient overvoltages. Resistance grounding decreases the arcing grounding risk and
permits ground-fault protection.

The value of resistance used in the neutral grounding system should neither be very high nor
be very low shown in the figure below.

A very low resistance makes the system to the solidity

grounded, whereas a very high resistance makes the system ungrounded. The value of
resistance is chosen such that the ground-fault current is limited, but still sufficient ground
current flows permit the operation of ground faults protections. In general, the ground fault
may be limited up to 5% to 20% of that which occur with a three-phase line.

Reactance Grounding

In reactance grounded system, a reactance is inserted between the neutral and ground to limit
the fault current as shown in the figure below.
 To minimize transient overvoltages, the ground fault
current in a reactance grounded system should not be less than 25% of the three phase fault
current. This is considerably more than the minimum current desirable in resistance grounded
systems.

I ask you the quotation for a reactance of setting of neutral to the ground according to
the following characteristics:
Neutral grounding inductance (reactance) 34.5 kV Quantity 1
Installation outdoor
Ambient temperature ° C 45
Cooling type AN
Line frequency Hz 50
Rated voltage kV 34.5
Reactance Ω 6 and 7 or 30

Neutral Grounding
In neutral grounding system, the neutral of the system or rotating system or transformer is
connected to the ground. The neutral grounding is an important aspect of power system
design because the performance of the system regarding short circuits, stability, protection,
etc., is greatly affected by the condition of the neutral. A three phase system can be operated
in two possible ways

1. With ungrounded neutral

2. With a grounded neutral
Ungrounded Neutral System

In an ungrounded neutral system, the neutral is not connected to the grounded. In other
words, the neutral is isolated from the ground. Therefore, this system is also known the
isolated neutral system or free neutral system shown in the figure below.

Grounded System

In neutral grounding system, the neutral of the system is connected to the ground. Because of
the problems associated with ungrounded neutral systems, the neutrals are grounded in most
of the high-voltage systems.

Some of the advantages of neutral grounding are as follows

1. Voltages of phases are limited to the line-to-ground voltages.

2. Surge voltage due to arcing grounds is eliminated.
3. The overvoltages due to lightning discharged to ground.
4. It provides greater safety to personnel and equipment.
5. It provides improved service reliability.

Method Of Neutral Grounding

The methods commonly used for grounding the system neutral are

1. Solid grounding (or effective grounding)

2. Resistance Grounding
3. Reactance Grounding
4. Peterson-coil grounding (or resonant groundings)
The selection of the type of grounding depends on the size of the unit, system voltage and
protection scheme to be used.

Balanced Earth Fault Protection

The balanced earth fault protection scheme is mainly used for protection of small generator
where differential and self-balanced protection systems are not applicable. In a small
generator, the neutral end of the three phase windings is connected internally to a single
terminal. So the neutral end is not available, and protection against earth fault is provided by
using the balanced earth protection scheme. Such scheme does not provide protection against
phase-to-phase fault until and unless they develop into earth faults.

Connection of Balanced Earth Fault Protection Scheme

In this scheme, the current transformers are mounted on each phase. Their secondary is
connected in parallel with that of CT mounted on a conductor joining the star point of the
generator to earth. A relay is connected across the secondaries of the CTs.

The balanced protection schemes

provide protection against earth fault in the limited region between the neutral and line CTs
(current transformers). It provides protection against the stator winding of the earth fault in
the stator and does not operate in case of an external earth fault. This scheme is also called
restricted earth fault protection scheme. Such type of protection is provided in the large
generator as an additional protection scheme.

Working of Balanced Earth Fault Protection Scheme

When the generator is in a normal operating condition the sum of the currents flow in the
secondary of the current transformers is zero and the current flow into secondary to neutral is
also zero. Thus the relay remains de-energized. When the fault occurs in the protected zone
(left of the line) the fault current flow through the primary of current transformers and the
corresponding secondary current flow through the relay which trips the circuit breaker.
When the fault develops external of the protective zone (right of the current transformer) the
sum of the currents at the terminal of the generator is exactly equal to the current in the
neutral connection. Hence, no current flows through the relay operating coil.

Drawback of Balanced Earth Protection Scheme

If the fault occurs near the neutral terminal or when grounding of the neutral is connected
through a resistance or a distributing transformer then the magnitude of the fault current flow
through the secondary of current transformer becomes small. This current is less than the
pick-up current of the relay. Thus, the relay remains inoperative, and the fault current
continues to persist in the generator winding which is highly undesirable.

Bulk Oil and Minimum Oil Circuit Breaker

Bulk Oil Circuit Breaker: A breaker which uses a large quantity of oil for arc extinction is
called a bulk oil circuit breaker. Such type of circuit breaker is also known as dead tank-type
circuit breaker because their tank is held at ground potential. The quantity of oil requires in
bulk oil circuit breaker depends on the system voltage. If the output rating of the voltage is
110 KV, then it requires 8 to 10 thousand kg of oil, and if their output rating is 220 KV, then
breakers need 50 thousand Kg of oil.

In bulk oil circuit breaker, oil performs mainly two functions. Firstly, it acts as an arc
extinguishing medium and secondly, it insulates the live parts of the breaker from earth. The
quantity of oil requires for arc extinction is only about one-tenth of the total and the rest
being used for the insulation.

These large quantities of oil are subject to the

carbonisation, sludging, etc., which occurs due to arc interruption and other causes reducing
the insulating properties and requires regular maintenance.

Bulk oil circuit breaker needs a large tank which increases expenses and also increases the
weight of the circuit breaker. Because of the following disadvantage the low oil circuit
breaker is developed which use minimum oil for arc extinction.

Minimum Oil Circuit Breaker

In this type of circuit breaker minimum oil is used as an arc quenching medium and it is
mounted on a porcelain insulator to insulate it from the earth. The arc chamber of such type
of circuit breaker is enclosed in a bakelised paper. The lower portion of this breaker is
supported by the porcelain and the upper porcelain enclosed the contacts.

This circuit breaker is of the single breaker type in which a moving contact tube moves in a
vertical line to make or break contact with the upper fixed contacts mounted within the arc
control devices.

A lower ring of fixed contacts is in permanent contact with the moving arm to provide the
other terminal of the phase unit. Within the moving contact, the tube is a fixed piston. When
the moving contact moves downwards, it forces the insulating oil to enter into the arc control
devices . Thus, the arc gets extinguished.

Minimum oil circuit breaker requires less space as

compared to bulk oil circuit breaker which is an important feature in large installations. But it
is less suitable in places where the frequent operation is required because the degree of
carbonisation produced in the small volume of oil is far more dangerous than in the
conventional bulk oil circuit breakers and this also decreases the dielectric strength of the
material.

The low oil circuit breakers have the advantages of a requirement of the lesser quantity of oil,
smaller space requirement, smaller tank size, smaller weight, low cost, reduced risk of fire
and reduced maintenance problems. Minimum oil circuit breaker suffers from the following
drawbacks when compared with the bulk oil circuit breakers

 Increased degree of carbonisation due to a smaller quantity of oil.

 The dielectric strength of oil decreases due to a high degree of carbonisation.
 Difficulty in removal of gases from the contact space-time