A SUMMER TRAINING REPORT

on
220/132/33 kV SALAKATI GRID SUB-STATION,
KOKRAJHAR

Duration: - 24/07/2023 to 11/08/2023

BACHELOR OF TECHNOLOGY
IN
ELECTRICAL ENGINEERING

Submitted By
Ambedkar Basumatary 220910003004
Deep Jyoti Bhuyan 230950003007
Satyajee Swargiary 230950003016
Thulunga Gayary 220910003048
Uddhav Krishna Konwar 220910003049

Department Of Electrical Engineering

Bineswar Brahma Engineering College,
Kokrajhar, Assam- 783370
 DECLARATION

We hereby declare that the internship report entitled “A Training Report on 220/132/33
kV Salakati Grid Sub-Station, Kokrajhar, Assam” submitted by Ambedkar Basumatary
(220910003004), Deep Jyoti Bhuyan (230950003007), Satyajee Swargiary
(230950003016), Thulunga Gayary (220910003048), and Uddhav Krishna Konwar
(220910003049) to Bineswar Brahma Engineering College has been prepared under the
guidance of Dr. Dayal Ch. Shill, Head of the Department of Electrical Engineering,
Bineswar Brahma Engineering College, Kokrajhar, Assam – 783370.
We further declare that the information presented in this report is true, original, and
based on our internship experience and has not been submitted to any other university
or institute for the award of any degree.
 ACKNOWLEDGEMENT

Training plays a vital role in bridging academic knowledge with real-life industrial
practices. Undertaking our training at Assam Electricity Grid Corporation Limited
(AEGCL) provided us with valuable exposure to professional work environments and
practical insights into the functioning and protection of high-voltage power systems. This
experience has enhanced our technical understanding and prepared us for future
professional challenges.
We are deeply thankful to Mr. Uttam Debnarh, Assistant General Manager, 220/132/33
kV Salakati Grid Sub-Station, Kokrajhar, Assam, for his constant encouragement, valuable
assistance, and for acquainting us with the operational facilities and safety practices during
the course of our training. His guidance helped us gain a clear understanding of substation
operation and maintenance.
We extend our heartfelt thanks to Dr. Medalson Ronghang, Training and Placement
Officer, Bineswar Brahma Engineering College, for granting us the opportunity to undergo
this internship. We are equally grateful to our Head of Department, Dr. Dayal Ch. Shill, and
the faculty members of the Department of Electrical Engineering, Bineswar Brahma
Engineering College, for their continuous encouragement and support, which contributed
greatly to the success of our training.

I
 ABSTRACT

We successfully completed our summer industrial internship at the 220/132/33 kV Salakati

Grid Sub-Station (GSS) under the Assam Electricity Grid Corporation Limited
(AEGCL) with the guidance and support of the station members. The internship was
carried out from 1st July to 31st July, during which we were assigned to study and
understand the functioning of the substation and its equipment.

Throughout the training, we gained practical knowledge about the arrangement of the
substation, its major equipment, transmission lines, control panels, and power
transformers. This internship enhanced our understanding of the role of substations in
the power system, the layout and arrangement of equipment, and the operation and
protection mechanisms involved in substation activities. Additionally, our
communication and fieldwork skills were significantly improved through interaction
with engineers and staff during practical sessions.

II
 CONTENTS
DECLARATION
ACKNOWLEDGEMENT I
ABSTRACT II
LIST OF FIGURES III
LIST OF TABLES IV
CHAPTER 1. INTRODUCTION 1
CHAPTER 2. POWER SYSTEM 2
2.1. STRUCTURE OF POWER SYSTEM 3

CHAPTER 3. SUB-STATION 4
3.1. DIFFERENT TYPES OF SUB-STATION 5
3.2. OVERVIEW OF SALAKATI GRID SUB-STATION 6
CHAPTER 4. EQUIPMENTS IN SUBSTATIONS 8
4.1. LIGHTNING ARRESTOR 8
4.1.1. Expulsion-type Lightning arrester 9
4.1.2. Meatal-Oxide Lightning Arrester 9
4.1.3. Horn-Gap Arrester 10
4.2. CAPACITIVE VOLTAGE TRANSFORMER 10
4.3. WAVE TRAP 11
4.4. CURRENT TRANSFORMER 12
4.5. ISOLATOR 14
4.6. CIRCUIT BREAKER 15
4.6.1. Air Circuit Breaker 16
4.6.2. Oil Circuit Breaker 17
4.6.3. SF₆ Circuit Breaker 17
4.6.4. Vacuum Circuit Breaker 18
4.6.5. Air Blast Circuit Breaker 18
4.7. BUSBAR 19
4.7.1. Single Busbar 20
4.7.2. Double Busbar 20
4.8. BUS COUPLER 20
4.9. POTENTIAL TRANSFORMER 21
4.10. POWER TRANSFORMERS 22
 4.10.1. Laminated core 23
4.10.2. Windings 23
4.10.3. Insulating material 24
4.10.4. Main Tank 25
4.10.5. Terminals and bushings 25
4.10.6. Transformer oil 26
4.10.7. Tap changer 27
4.10.8. Buchholz relay 28
4.10.9. Oil conservator 28
4.10.10. Breather 29
4.10.11. Explosion vent 29
4.10.12. Radiator and fans 30
4.11. BATTERY BANK 31
4.12. TRANSMISSION TOWER 32
4.13. CONDUCTOR USED IN THE SYSTEM 33
CHAPTER 5. PROTECTION 34
5.1. RELAY BASED PROTECTION SYSTEM 34
5.3.1. Features Of Numerical Relays 36
5.3.2. Relay Coordination and Selectivity 37
5.2. EARTHING SYSTEM FOR PROTECTION 37
5.3. PROTECTION OF MAIN TRANSFORMER 38
5.3.1. Electrical Protection Schemes 38
5.3.2. Mechanical and Thermal Protection 39
5.3.3. Nitrogen Injection Fire Protection System (NIFPS) 40
CHAPTER 6. SHUTDOWN 41
6.1. REASONS FOR SHUTDOWN 41
6.2. PROCESS OF SHUTDOWN 41
CHAPTER 7. CONCLUSION 43
PHOTO GALLERY 44
 LIST OF FIGURES
Figure Name Page No.

Fig 2.1: Structure of power system 2

Fig 3.1: 220/132/33 kV Salakati Grid Sub-Station 4
Fig 3.2: Single Line Diagram of 220/132/33kv Salakati Grid Sub-Station 6
Fig 3.3: Block Diagram of Connected GSS with Salakati GSS 7
Fig 4.1: Lightning Arrestor 8
Fig 4.2: Capacitor Voltage Transformer 11
Fig 4.3: Wave Trap 12
Fig 4.4: Current transformers 13
Fig 4.5: Isolator 14
Fig 4.5: SF6 Circuit breaker 15
Fig 4.6: Air Circuit Breaker 16
Fig 4.7: Oil Circuit Breaker 17
Fig 4.8: SF6 Circuit Breaker 17
Fig 4.9: Vacuum Circuit Breaker 18
Fig 4.10: Air Blast Circuit Breaker 18
Fig 4.11: Busbar 19
Fig 4.12: SLD of a 33kV side Single Busbar in Salakati GSS 20
Fig 4.13: Bus Coupler 20
Fig 4.14: Potential Transformer 21
Fig 4.15: 160 MVA, 220/132 kV auto-transformers (ATR-I and ATR-II 22
Fig 4.16: Parts of a transformer 23
Fig 4.17: Primary and secondary windings inside a transformer 24
Fig 4.18: Bushings of transformer 26
Fig 4.19: Tap changer of transformer 27
Fig 4.20: Buchholz Relay 28
Fig 4.21: Oil Conservator Tank 28
Fig 4.22: Explosion Vent of transformer 29
Fig 4.23: Radiator and fans of a transformer 30
Fig 4.24: Battery Bank 31
Fig 4.25: Transmission Tower 32

III
 LIST OF TABLES
Table Name Page No.

Table 4.1: Circuit breaker remarks 15

Table 4.2: Different types of ACSR based on its current carrying capacity 34

IV
CHAPTER 1. INTRODUCTION

We know electricity plays a crucial role in various aspects of modern life, from lighting and
electronics to powering homes, industries, and transportation. Its discovery and
understanding revolutionized the world, making it one of the most transformative and
essential discoveries in human history. The study of electricity is essential in understanding
how electric charges, and the principles behind electrical engineering and technology. Life
wouldn’t be what it is today, without the discovery of electricity. There are several ways to
produce electricity. Hydroelectricity production from water, windmills generate electricity
from wind, burning of coal produce electricity and solar panels are used to make energy
through the sun.

Overview:

The Salakati Grid Sub-Station receives power from National Thermal Power Plant (NTPC)
and the Power Grid Corporation of India Limited (PGCIL) and also includes a Line-In Line-
Out (LILO) connection to the Rangia Substation. While it operates primarily as a step-down
substation, reducing voltage from 220 kV to 132 kV and 33 kV, it is classified as a
transmission-type substation due to its role in high-voltage power transfer between major
substations. The Salakati Grid Sub-Station consists of two 220/132 kV auto-transformers,
each rated at 160 MVA, one 132/33 kV power transformer of 16 MVA, and two 33/0.44 kV
station transformers rated at 250 kVA each for auxiliary supply.
CHAPTER 2. POWER SYSTEM

A power system is a network that supplies and distributes electrical energy from power
plants to end-users. It consists of various components such as generators, transmission lines,
substations, transformers, distribution lines, and consumers. The main objective of a power
system is to ensure the reliable delivery of electricity to meet the demand of consumers.
Power plants, which can be based on various sources such as fossil fuels, nuclear energy,
or renewable sources, generate electrical energy. This energy is then transmitted over long
distances through high-voltage transmission lines to substations. At substations,
transformers are used to step down the voltage of the electricity to a suitable level for
distribution. The electricity is then distributed through a network of lower voltage
distribution lines to residential, commercial, and industrial consumers. Power systems also
include control and protection systems to monitor and regulate the flow of electricity,
maintain stability, and ensure safety. These systems enable utilities to manage power
generation, transmission, and distribution efficiently and effectively. Overall, power
systems play a crucial role in providing reliable and affordable electricity to power homes,
businesses, and industries, supporting economic growth and the well-being of societies.

Fig 2.1: Structure of power system

2
2.1. Structure of Power System:

The main components of a power system include:

• Generation: This level consists of power plants that produce electrical energy.
Power plants can be based on various sources such as fossil fuels (coal, oil, natural
gas), nuclear energy, or renewable sources (solar, wind, hydroelectric, geothermal).
• Transmission: Once electricity is generated, it is transmitted over long distances
through high voltage transmission lines. The transmission network connects power
plants to substations and is responsible for transporting large amounts of electricity
at high voltages to reduce losses during transmission.
• Substations: Substations act as intermediaries between the transmission and
distribution systems. They are equipped with transformers to step down the high
voltage electricity from transmission lines to lower voltage levels suitable for
distribution.
• Distribution: At the distribution level, electricity is distributed to consumers
through a network of lower voltage distribution lines. This level includes
transformers that further step down the voltage to levels suitable for various end-
users, such as residential, commercial, and industrial consumers.
• Consumers: This is the final level of the power system, where the electrical energy
is finally consumed. Consumers can be households, businesses, industries, or any
entity that requires electricity for its operations.
• Control and Protection Systems: Power systems also include control and
protection systems that monitor and regulate the flow of electricity, maintain system
stability, and ensure the safety of the grid. These systems include protective relays,
monitoring devices, automatic control systems, and SCADA (Supervisory Control
and Data Acquisition) systems.

3
CHAPTER 3. SUB-STATION

A substation is a part of an electrical generation transmission and distribution system.

Substations transform voltage from high to low, or the reverse, or perform any of several
other important functions. Between the generating station and consumer, electric power
may flow through several substations at different voltage levels. A substation may include
transformers to change voltage levels between high transmission voltages and lower
distribution voltages, or at the interconnection of two different transmission voltages. A
substation that has a step-up transformer increases the voltage while decreasing the current,
while a step-down transformer decreases the voltage while increasing the current for
domestic and commercial distribution.

Fig 3.1: 220/132/33 kV Salakati Grid Sub-Station

3.1. DIFFERENT TYPES OF SUB-STATIONS

1. Based on Function
• Transmission Substation: A transmission substation transfers bulk power from
generating stations to distribution networks using high-voltage transmission lines.
It steps up or steps down voltage levels (e.g., 132 kV, 220 kV, 400 kV) for efficient
long-distance transmission and includes equipment like transformers and circuit
breakers.

4
 • Distribution Sub-station: A distribution sub-station receives high-voltage power
from the transmission system and steps it down to levels like 33 kV or 11 kV for
supply to homes, businesses, and industries. It acts as the main point of connection
between the transmission and consumer systems.
• Switching Sub-station: Switching sub-stations do not contain transformers and are
mainly used for connecting or disconnecting transmission lines. They help in
isolating faults, rerouting power, and performing maintenance without affecting the
entire system.

2. Based on Voltage Level

• Low Voltage Substation: Low voltage substations operate below 33 kV, typically
at 11 kV or 440 V. These substations deliver power directly to end-users and are the
final step in the distribution process.
• High Voltage (HV) Substation: A heigh voltage substation operates within the
voltage range of 33kV to 220kV.
• Extra High Voltage (EHV) Substation: EHV substations operate above 220 kV
(e.g., 400 kV or 765 kV) and are used for bulk power transmission over long
distances. They are critical for maintaining the stability of national or inter-state
grids.

3. Based on Construction Type

• Outdoor Substation: Outdoor substations have equipment installed in open air and
are common in rural or semi-urban areas. They are easier to maintain and are more
economical compared to indoor substations, especially where space is not an issue.
• Indoor Substation: Indoor substations are housed within buildings and protect
equipment from weather and external damage. These are commonly used in urban
or industrial areas where space is tight and safety, noise reduction, and aesthetics
are important.

5
3.2. OVERVIEW OF SALAKATI GRID SUB-STATION

Fig 3.2: Single Line Diagram of 220/132/33kv Salakati Grid Sub-Station

The Salakati Grid Substation, located under Assam Electricity Grid Corporation Limited
(AEGCL), is a 220/132/33 kV Extra High Voltage (EHV) transmission substation. It plays
a vital role in receiving high-voltage power from generating stations and transmitting it to
nearby regions after appropriate voltage transformation.
The sub-station receives 220 kV incoming supply from National Thermal Power
Corporation (NTPC-I and NTPC-II) and Power Grid Corporation of India Limited (PGCIL-
I and PGCIL-II)

The 220 kV system is equipped with standard protective and switching equipment such
as lightning arresters, capacitive voltage transformers, wave traps, isolators, current
transformers and SF6 circuit breakers. These lines connect to two main buses: Main Bus-I
and Main Bus-II, forming a double bus arrangement that ensures reliability and flexibility.
A Transfer Bus acts as a secondary backup bus for switching operations during maintenance
or faults. Bus couplers and transfer bus couplers facilitate the shifting of loads between
buses without interruption.

The sub-station includes two 160 MVA, 220/132 kV auto-transformers (ATR-I and ATR-
II), which step down the voltage from 220 kV to 132 kV. These transformers are equipped
with On-Load Tap Changers (OLTCs) for regulating output voltage.

6
 The 132 kV system feeds several important outgoing lines including Dhaligaon
(DHALIGAON-I and DHALIGAON-II) and Kokrajhar (KOKRAJHAR-I and
KOKRAJHAR-II). Additionally, a 220 kV double circuit outgoing line is connected to
RANGIA-I and RANGIA-II, allowing high-voltage transmission directly from the
substation to another grid node.

Fig 3.3: Block Diagram of Connected GSS with Salakati GSS

The 132 kV voltage is further stepped down to 33 kV using 132/33 kV, 16 MVA power
transformers, supplying power to local distribution feeders.

All transformer and feeder bays are equipped with appropriate protection and metering
devices, including CTs, PTs, CVTs, and circuit breakers. Communication is supported by
wave traps that block high-frequency signals from entering unwanted parts of the network.

Overall, the Salakati GSS is a transmission-type substation, serving as a major voltage

step-down and power distribution hub in the lower part of Assam.

7
CHAPTER 4. EQUIPMENTS IN SUBSTATIONS
Substations are equipped with lightning arrestor, circuit breakers and protective devices to
control the flow of electrical power, isolate faculty sections of the system and protect the
equipment from damage. These devices ensure the safe and reliable operation of the
electrical grid.

4.1. LIGHTNING ARRESTOR

Fig 4.1: Lightning Arrestor

A Lightning Arrester (LA) is an essential protective device used in electrical substations to
protect equipment from high-voltage surges caused by lightning strikes or switching
operations. These sudden voltage spikes can damage critical equipment such as
transformers, circuit breakers, insulators, and other components. The lightning arrester
functions by safely diverting the excessive voltage to the ground, thus ensuring the
protection and stability of the substation system.
The modern lightning arrester consists of zinc oxide (ZnO) blocks, which are enclosed
in a weather-resistant porcelain or polymer insulator housing. These ZnO blocks act as
highly non-linear resistors, also known as metal oxide varistors (MOVs). They do not
require a spark gap and respond quickly to voltage surges. The ZnO varistors provide very
high resistance under normal voltage conditions and very low resistance during surge
conditions.

8
 The working principle of a lightning arrester is based on its non-linear voltage-current
characteristic. Under normal operating voltage, the arrester remains non-conductive.
However, when a surge occurs, the arrester becomes conductive almost instantly, allowing
the surge current to flow directly to the ground. Once the surge is discharged, it quickly
returns to its insulating state without interrupting the power supply.
In a substation, lightning arresters are strategically placed at critical locations to ensure
maximum protection. Common placement points include:
• At the incoming and outgoing transmission line gantries.
• Near power transformers on both high-voltage and low-voltage sides.
• Along busbars and breaker bays.
• At the terminals of capacitor banks or reactors.
• On surge-prone equipment such as CTs, PTs, and isolators.

Types of Lightning Arrester

There are many types of lightning arresters. Some of them are:

• Expulsion-type Lightning Arrester
• Metal-Oxide Lightning Arrester
• Horn-Gap Arrester

4.1.1. Expulsion-type Lightning arrester

These LAs are used in electrical systems operating at a voltage of 33kV or above. It is a
better technique for arresting surge than the rod-gap arrester. It is similar to the rod-gap
arrester except for the fact that these arresters have an additional gap in series with the rod
gap. This gap is enclosed within a fibre tube which is formed by two electrodes. The upper
electrode is connected to the rod gap while the lower one is connected to the ground.

4.1.2. Meatal-Oxide Lightning Arrester

These Las contain zinc oxide (ZnO) as the metal-oxide resistor. Hence, it is also known as
a Zinc Oxide diverter. ZnO is an N-type semiconductor material that is doped with some
fine powder of insulating oxides. These powders are then compressed into a disc shape and
enclosed in a porcelain casing which is filled with Nitrogen or (SF6).

9
 The boundaries of each ZnO disc act as a barrier that controls the flow of current during
normal operating conditions. But during lightning or a high-voltage surge, these barriers
break down and the material behaves as a conductor rather than an insulator. This allows
the surge current to pass to the ground without affecting the other systems.

4.1.3. Horn-Gap Arrester

These LAs have limited use due to their unreliability. It is used as backup or secondary
protection with the main arrester. Two metallic plates are bent to form a horn shape. One of
these plates is connected to the system to be protected while the other one is connected to
the earth. There is a measured air gap between the two plates which acts as the insulating
media. During the time of fault or lightning, the air gap insulation breaks down and allows
an arc to jump.
Thus, as the warm air around the arc pushes it up, the wider metal gap at the top causes
the arc length to increase thus increasing its resistance and eventually the arc extinguishes.
However, this method is unreliable as primary protection as it takes a longer time (2-3
seconds) to work appropriately.

4.2. CAPACITIVE VOLTAGE TRANSFORMER

A Capacitive Voltage Transformer (CVT), also known as a Capacitor Voltage Divider, is a
device used in high-voltage substations to step down high voltages to measurable and safer
levels for metering, protection, and control purposes. In addition to voltage measurement,
CVTs are widely used for Power Line Carrier Communication (PLCC) due to their
capability to couple high-frequency signals with the transmission line. CVTs are typically
used for voltage levels 132 kV and above, where electromagnetic voltage transformers
become bulky and expensive.

The construction of a CVT consists mainly of two parts: a capacitor voltage divider and
an electromagnetic unit (or intermediate transformer). The capacitor voltage divider is
made up of a series of high-voltage capacitors connected in a stack, which divides the
incoming high voltage into a lower voltage. This stepped-down voltage is then fed into the
electromagnetic unit, which further reduces the voltage to standard levels (110 V or 100 V)
for secondary usage. The electromagnetic unit also includes ferro resonant circuits to
provide voltage stabilization and a tuning unit for PLCC signal coupling.

10
 Fig 4.2: Capacitor Voltage Transformer

The working principle of a CVT is based on the capacitive voltage division. When a
high voltage is applied across the series-connected capacitors, a proportional fraction of the
voltage is developed across the lower capacitor section. This low voltage is then
transformed and isolated by the intermediate transformer and supplied to protective relays,
meters, and communication equipment. The CVT also includes a drain coil or wave trapper
connection point for blocking PLCC signals from entering the substation.

In a substation, CVTs are installed between the line and ground, typically connected to
the busbars or transmission lines, and are often located adjacent to circuit breakers and
lightning arresters. They are visually identified by their tall, stacked cylindrical capacitor
units mounted vertically on insulators.

4.3. WAVE TRAP

A Wave Trapper, also known as a Line Trap, is a device used in electrical substations and
high-voltage transmission lines as a part of the Power Line Carrier Communication (PLCC)
system. Its main function is to block high-frequency communication signals (typically in
the range of 30 kHz to 500 kHz) from entering into unwanted parts of the power system
while allowing the power frequency (50 Hz or 60 Hz) current to pass without obstruction.
It is essential component for enabling communication, telemetry, protection, and remote
control through the existing transmission lines without the need for separate
communication lines.

11
 Fig 4.3: Wave Trap
The construction of a wave trap typically consists of a high-reactance coil (inductor)
wound on a non-magnetic core, sometimes in combination with capacitors, depending on
the required tuning. It is designed to offer high impedance to high-frequency signals and
low impedance to power frequency. The coil is enclosed in a weatherproof housing suitable
for outdoor installation. It is often mounted on insulators and placed in series with the
transmission line conductor.

The working principle of the wave trap is based on frequency selectivity. When high-
frequency carrier signals are injected into the transmission line for communication, the
wave trap prevents these signals from entering the substation equipment such as
transformers or circuit breakers, where they might be attenuated or cause interference. It
essentially acts as a band-stop filter for high frequencies, directing the carrier signals
towards the communication equipment (coupling capacitor and line tuner) while allowing
the 50 Hz power frequency to pass freely.

In substations, wave trappers are placed in series with the transmission line conductors,
usually near the CVTs (Capacitive Voltage Transformers) and line gantries. They are most
commonly seen on high-voltage (132 kV and above) transmission lines where PLCC is
used.

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4.4. CURRENT TRANSFORMER
A Current Transformer (CT) is an instrument transformer used in electrical substations to
step down high current levels to a lower, measurable value, typically 1 A or 5 A, suitable
for protection, control, and metering equipment. It provides electrical isolation between the
high-voltage power circuit and low-voltage measuring instruments, ensuring both safety
and accurate current measurement. CTs are essential for operating protective relays, energy
meters, fault recorders, and monitoring systems.

Fig 4.4: Current transformers

The construction of a CT typically consists of a primary winding, a magnetic core, and

a secondary winding. The primary winding is usually just one or a few turns (often the main
power conductor itself), and it carries the actual line current. The secondary winding is
wound with many turns of fine wire and is connected to measuring or protection devices.
The entire assembly is enclosed in a porcelain or polymer housing filled with oil or
insulation gas for high-voltage applications. In outdoor substations, CTs are mounted on
insulated bushings and are designed to withstand environmental and electrical stress.
The working principle of a CT is based on electromagnetic induction. The high current
flowing through the primary winding induces a proportional current in the secondary
winding. The ratio of transformation is fixed and known (e.g., 1000/5 A), allowing accurate
current measurement and protection. CTs must never be operated with the secondary
winding open, as it can cause dangerously high voltages and damage to the winding or
connected devices.

13
4.5. ISOLATOR

Fig 4.5: Isolator

An Isolator, also known as a Disconnect Switch, is a mechanical device used in substations

to ensure complete disconnection of a part of the electrical system for maintenance,
inspection, or repair. It is designed to operate only when the circuit is completely de-
energized, i.e., after the associated circuit breaker has interrupted the load current. The
isolator provides a visible open point in the circuit to confirm safe isolation and to prevent
accidental energization of the equipment under maintenance.
It typically includes fixed and moving contacts mounted on porcelain or polymer
insulators that support and insulate live parts from the grounded structure. A key safety
feature in many isolators is the presence of a ground terminal or associated earthing switch.
When the isolator is opened, the grounding switch is then closed, which connects the
isolated section of the system to earth potential. This discharges any trapped, static, or
induced voltages, ensuring the isolated part is completely safe for human interaction. Some
isolators have a mechanically interlocked earthing blade that can only be operated after
isolation is confirmed.
After the load current is interrupted by the circuit breaker, the isolator is opened to create
a physical break in the line. If required, the earthing switch is operated next to ground the
circuit, eliminating any residual voltages. This sequence ensures maximum safety during
substation maintenance or repair.

14
4.6. CIRCUIT BREAKER

A Circuit Breaker (CB) is a crucial switching and protection device used in electrical
substations to automatically interrupt the flow of current under normal as well as abnormal
conditions such as short circuits, overloads, or equipment faults. It is capable of breaking
the circuit under load and safely extinguishing the arc formed during contact separation.
However, a circuit breaker does not detect faults on its own—it operates in coordination
with a protective relay, which senses the fault and sends a trip signal to the breaker. Among
various types, the SF₆ (Sulphur hexafluoride) circuit breaker is the most commonly used
type in high-voltage substations in India, due to its excellent arc-quenching ability, compact
size, and high reliability.

Fig 4.5: SF6 Circuit breaker

S. No. Description Status

1 Type 200-SFM-40B
2 Rated Voltage 245 kV
3 Rated Normal Current 3150 A
4 Rated Frequency 50 Hz
5 Rated line charging Breaking current 125 A
6 Rated Short-Circuit Breaking current 40 kA
7 Rated Opening/Closing Voltage 220 V (DC)
8 Rated SF6 Gas Pressure 6 kg/cm2-g (at 200C)
9 Total Weight with Gas 2400 kg

Table 4.3: Circuit breaker remarks

15
 The circuit breaker includes fixed and moving contacts housed within an arc-quenching
chamber. This chamber is filled with an appropriate insulating medium- such as air, oil, SF₆
(Sulphur hexafluoride gas), or vacuum- to extinguish the arc during interruption. The
operating mechanism, which may be spring, pneumatic, or hydraulic, controls the
movement of the contacts. The breaker is triggered by trip coils, which receive signals from
protective relays when a fault is detected.

The working principle involves the detection of abnormal current by relays, which then
send a signal to energize the trip coil. This causes the contacts to separate rapidly,
interrupting the current. The arc that forms between the separating contacts is quickly
cooled and extinguished by the insulating medium inside the chamber, restoring insulation
and stopping current flow.

CBs are classified by their arc quenching medium, which includes:

4.6.1. Air Circuit Breaker (ACB)

Air Circuit Breakers use compressed air or atmospheric air as the arc-extinguishing
medium. When the breaker opens, the arc is formed between contacts and is cooled by
blowing air across it. In modern ACBs, arc chutes are used to lengthen and cool the arc.
Voltage Range: Typically used in low-voltage systems up to 11 kV.
Applications:
• Indoor installations
• Low-voltage switchboards
• Industrial facilities
Advantages:
• Simple design
• Safe and quick interruption
• Easy to maintain Fig 4.6: Air Circuit Breaker
Disadvantages:
• Not suitable for high-voltage applications
• Large size compared to other types

16
4.6.2. Oil Circuit Breaker (OCB)
Oil Circuit Breakers use insulating oil (usually mineral oil) as both an arc-quenching
medium and an insulator. When contacts separate, the arc vaporizes the oil around it,
creating hydrogen gas that cools and extinguishes the arc. The oil also insulates live parts.
Voltage Range: Suitable for medium to high-voltage systems, up to 220 kV.
Applications:
• Outdoor substations (older installations)
• Rural grid systems
Advantages:
• Good insulation strength
• Effective arc quenching
Disadvantages:
• Fire hazard due to flammable oil
• Requires frequent maintenance and oil testing Fig 4.7: Oil Circuit Breaker

4.6.3. SF₆ Circuit Breaker (Sulphur Hexafluoride)

SF₆ Circuit Breakers use sulphur hexafluoride gas, a highly electronegative gas, as the arc-
quenching and insulating medium. When contacts open, the arc is formed in SF₆ gas, which
quickly absorbs free electrons and extinguishes the arc. It provides superior dielectric
strength and insulation.
Voltage Range: Suitable for high-voltage and extra-high-voltage systems (66 kV to 765
kV).
Applications:
• High-voltage transmission substations
• Switchyards and grid substations
• Power generation stations
Advantages:
• Excellent arc-quenching properties
• Compact and space-efficient
• Low maintenance
Disadvantages: Fig 4.8: SF₆ Circuit Breaker
• SF₆ is a potent greenhouse gas (needs proper handling)
• Expensive compared to air or oil types

17
4.6.4. Vacuum Circuit Breaker (VCB)
VCBs use a high vacuum as the arc-extinguishing medium. When the contacts separate, the
arc forms in the vacuum but is extinguished quickly because vacuum does not support arc
formation for long. The contact gap is small, and the mechanism is sealed in a vacuum
interrupter.
Voltage Range: Ideal for medium-voltage systems (3.3 kV to 33 kV).
Applications:
• Indoor substations
• Industrial plants
• Transformer protection
Advantages:
• Long life
• Maintenance-free operation
• No fire or gas hazard
Fig 4.9: Vacuum Circuit Breaker
• Compact and environmentally safe
Disadvantages:
• Not suitable for very high voltages
• Vacuum bottles must be sealed properly

4.6.5. Air Blast Circuit Breaker (ABCB)

Air Blast CBs use high-pressure air to blow out the arc. The arc is extinguished as
compressed air is released through a nozzle when the contacts separate. There are different
types like axial blast and cross-blast.
Voltage Range: Used in high-voltage applications (132 kV and above) in the past.
Applications:
• Old transmission substations (now mostly replaced)
• Advantages:
• Fast operation
• No oil, so less fire risk
Disadvantages:
• Noisy and requires air compressor system
• High maintenance
• Obsolete technology Fig 4.10: Air Blast Circuit

18 Breaker
4.7. BUSBAR

Fig 4.11: Busbar

A busbar is a metallic bar in a switchgear panel used to carry electric power from incoming
feeders and distributes to the outgoing feeders. In simple terms, bus bar is an electrical
junction where incoming and outgoing currents exchange. Electrical busbar consists the
number of lines electrically, which are operating at the same voltage and frequencies.
Generally, copper or aluminium conducting material is used in the construction of bus bars.
The first requirement of any substation design is to avoid a total shutdown of the
substation for the purpose of maintenance, or due to fault somewhere out on the line. A total
shutdown of the substation means complete shutdown of all the lines connected to the
substation. A substation bus scheme is the arrangement of overhead bus bar and associated
switching equipment (circuit breakers and isolators) in a substation. The operational
flexibility and reliability of the substation greatly depends upon the bus scheme.
Busbars are installed at the centre of the substation layout, typically connecting
transformers, feeders, circuit breakers, and isolators. Proper spacing, clearance, and support
are maintained to avoid phase-to-phase and phase-to-ground faults. In AIS systems, they
are mounted on post insulators, while in GIS systems, they are enclosed inside SF₆-filled
metal tubes.

19
Busbars are classified into several types based on configuration:

4.7.1. Single Busbar

As the name implies, the single bus substation configuration consists of all circuits
connected to a main bus. A fault on the bus or between the bus and circuit breaker will
result in an outage of the entire bus or substation. Failure of a single circuit breaker will
also result in an outage of the entire bus

Fig 4.12: SLD of a 33kV side Single Busbar in Salakati GSS

4.7.1. Double Busbar

A Double Busbar system is a bus configuration in which the substation is equipped with
two parallel busbars. This arrangement allows any circuit (incomer, feeder, or transformer)
to be connected to either of the two busbars through isolators, enabling flexible load
management and uninterrupted operation. It provides greater system reliability, as power
can be transferred from one bus to the other during maintenance or fault conditions without
interrupting the power supply. The system enhances operational flexibility, ensures
redundancy, and is widely used in high-voltage and critical substations.

4.8. BUS COUPLER

Fig 4.13: Bus Coupler

20
A bus coupler is a device used to connect two separate busbars using a circuit breaker along
with isolators, current transformers and protection relays. It allows for the transfer of power
from one bus to another without interrupting the power supply, thereby increasing the
operational flexibility, reliability, and maintainability of the substation. In normal operating
conditions, the bus coupler remains open. However, during maintenance, switching
operations, or faults on one busbar, it is closed to redirect the power flow through the
healthy bus. The bus coupler typically includes a circuit breaker along with isolators on
either side and current transformers for protection and metering. Protection relays are
integrated to monitor faults and ensure the breaker operates only under fault conditions,
thus preventing unnecessary tripping.

4.9. POTENTIAL TRANSFORMER

It is an instrument transformer which is used for the protection and measurement purpose
in the power systems. A potential transformer is mainly used to measure high alternating
voltage in a power system. Potential transformers are step-down transformers, i.e., they
have many turns in the primary winding while the secondary has few turns. The stepped
down voltage by the Potential transformer can be measure using a low range AC voltmeter.
The potential transformer has shell type construction of its magnetic core for better
accuracy. One end of the secondary winding of the potential transformer is grounded to
provide the proper protection to the operator.

Fig 4.14: Potential Transformer

21
4.10. POWER TRANSFORMER
A Power Transformer is a key electrical device used in substations to transfer electrical
energy between two or more voltage levels through the principle of electromagnetic
induction. It operates on AC and is designed for high efficiency when transferring large
amounts of power over long distances. In grid and transmission substations, it is typically
used as a step-down transformer, reducing high transmission voltages to lower levels
suitable for regional or local distribution.

Fig 4.15: 160 MVA, 220/132 kV auto-transformer

It consists of a core made of laminated silicon steel sheets to reduce eddy current loss,
and windings made of copper or aluminium. It has primary and secondary windings, wound
around the limbs of the core. The entire assembly is placed in a steel tank filled with
insulating oil, which serves both as an insulator and a cooling medium. Radiators, fans,
conservators, bushings, and breather systems are attached externally to support cooling and
pressure regulation.
The transformer is based on Faraday's law of electromagnetic induction. When AC
voltage is applied to the primary winding, it creates a changing magnetic flux in the core,
which induces a voltage in the secondary winding. The voltage level is either stepped up or
stepped down based on the turn’s ratio between the windings.
In a substation, power transformers are installed on separate transformer bays, connected
through isolators, lightning arresters, current transformers and circuit breakers for
protection and switching. Bushings on the transformer allow safe external connections to
incoming and outgoing lines. Buchholz relay, oil level indicators, and temperature sensors
are installed for protection and monitoring.

22
Parts of a Transformer:
1. Laminated core 7. Tap changer
2. Winding 8. Buchholz relay
3. Insulating material 9. Oil conservator tank
4. Tank 10. Explosion vent
5. Terminals and Bushings 11. Breather
6. Transformer oil 12. Radiator and Fans

Fig 4.16: Parts of a transformer

4.10.1. Laminated Core

Laminated cores are commonly found in power transformers used in electrical grids,
distribution transformers, and various types of electrical equipment where efficient energy
transfer is essential. The laminated core in a transformer is designed to minimize eddy
current losses by using insulated layers of magnetic material.
CRGO (Cold Rolled Grain Oriented) Steel material is used to minimize hysteresis
losses. The composition of core material depends on the voltage, current, and frequency of
supply to the transformer.

4.10.2. Windings
In a transformer always two sets of windings are placed on laminated core and these are
insulated from each other. Winding consists of several no of turns of copper conductors that
is bundled together and connected in series. The main function of windings is to carry
current and produce working magnetic flux and induce mutual EMF for transformer action.
Classification of windings:
Based on the input and output of supply
• Primary winding: The winding at which the input supply is connected is known
as the primary winding.
• Secondary winding: - The winding from which output is taken to the load is
known as the secondary winding.

23
Based on the voltage level of supply
• High voltage (HV) winding: The winding that is connected with higher voltage is
known as high voltage winding. It is made up of a thin copper conductor with a
large no of turns. It can be either primary or secondary winding of the transformer.
• Low voltage (LV) winding: The winding that is connected with lower voltage is
known as low voltage winding. It is made up of a thick copper conductor with few
no. of turns. It can also be either primary or secondary winding of the transformer.

Fig 4.17: Primary and secondary windings inside a transformer

4.10.7. Insulating material

Insulating materials in transformers serve to prevent electrical leakage, ensure safe
operation, and maintain efficient energy transfer. Common insulating materials include

• Transformer Oil: Mineral oil or synthetic oil is used as a coolant and insulating
material in oil-filled transformers. It dissipates heat and enhances insulation
between windings and the core.
• Solid Insulation: Materials like paper, pressboard, and epoxy are used to insulate
windings and other internal components, preventing electrical contact and
supporting structural integrity.
• Insulating Tape and Varnish: Layering windings with insulating tape and
applying varnish helps in preventing short circuits and enhancing resistance to high
voltages.
• Mylar and Kapton: These are thin yet robust materials used for insulation between
layers of windings, ensuring electrical separation and heat resistance.
• Ceramic Insulators: Used in bushings and other high-voltage areas, ceramics
provide strong insulation in harsh conditions.
24
 • Insulating Gases: In gas-insulated transformers, sulphur hexafluoride (SF6) is
employed as an insulating gas that prevents arcing and enhances insulation.
• Composite Insulation: Modern transformers often use composite materials
combining fibres and resins for high strength and enhanced insulation.
These insulating materials collectively enable transformers to handle high voltages,
maintain isolation between different components, and ensure safe and efficient operation.

4.10.7. Main Tank

Main tank is the robust part of transformer that serves mainly two purposes:
• It protects core and windings from the external environment and provide housing
for them.
• It is used as a container for transformer oil and provides support for all other
external accessories of the transformer.
Tanks are made up of fabricated rolled steel plates. They are provided with lifting hooks
and inbuilt cooling tubes. In order to minimize the weight and stray losses, aluminium
sheets are also being used instead of Steel plates. However, due to its lightweight property,
nowadays aluminium tank is more familiar and costly than a steel tank.

4.10.5. Terminals and Bushings

Terminals and bushings play specific roles in the functioning and safety of transformers.

• Terminals: Terminals in a transformer refer to the points or connections where

external electrical circuits are connected to the internal winding of the transformer.
Transformers have primary and secondary windings, and each winding has
terminals associated with it. The primary terminals are connected to the input power
source, while the secondary terminals are connected to the load or the output circuit.
These terminals facilitate the flow of electrical current into and out of the
transformer, enabling energy transfer.
• Bushings: Bushings are insulating devices that provide a safe and secure way to
connect external electrical equipment to the internal components of a transformer.
They are used to pass conductors or cables through the walls of the transformer's
tank or casing without allowing any electrical contact with the grounded metal
parts. Bushings are made of insulating materials, such as porcelain or composite
materials, to prevent electrical leakage and ensure the safety of both the transformer
and personnel.

25
 Fig 4.18: Bushings of transformer

As windings are of two types and so bushings are also of two types as named below:
• High-voltage bushing
• Low-voltage bushing
Oil impregnated paper (OIP) is a term used for bushings that utilize plain craft paper,
with the condenser core saturated with transformer-grade mineral oil. OIP bushings usually
have upper air-side and lower oil-side porcelain insulators.
Resin impregnated paper (RIP) is a term used for dry bushings in which the main
insulation consists of a core wound from crepe paper, which is then impregnated with a
curable resin. The outer insulation is a composite insulator with silicone sheds or a porcelain
insulator.

4.10.7. Transformer Oil

The function of transformer oil is to provide insulation between windings as well as cooling
due to its chemical properties and very good dielectric strength. It dissipates the heat
generated by the core and windings of a transformer to the external environment. When the
windings of transformer get heated due to flow of current and losses, the oil cools down
the windings by circulating inside the transformer and transfer heat to the external
environment through its cooling tubes.
Hydro-carbon mineral oil is used as transformer oil and acts as coolant. It is composed
of aromatics, paraffin, naphthene, and olefins.

26
4.10.7. Tap Changer
A tap changer is a crucial component in transformers that allows for the adjustment of the
transformer's turns ratio, and consequently, its output voltage. The tap changer mechanism
enables this adjustment by changing the point at which the electrical connection is made to
the winding. This can be achieved through different types of tap changers:

• On-Load Tap Changer (OLTC): It allows for voltage adjustments while the
transformer is under load, meaning it's still connected to the power supply. This type
of tap changer is commonly used in applications where continuous power supply is
crucial, such as in power distribution networks. OLTCs can be operated remotely
and are designed to change the tap position without interrupting the power flow.
• Off-Circuit Tap Changer (OCTC): It is designed to change the tap position when
the transformer is disconnected from the power supply. This type of tap changer is
typically used in situations where interrupting the power flow for adjustment is
acceptable, such as in industrial settings.

Fig 4.19: Tap changer of transformer

4.10.10. Buchholz Relay

Buchholz relay is the most important part of a power transformer rated more than 500kVA.
It is a gas-actuated relay mounted on the pipe connecting the main tank and conservator
tank.

27
 Fig 4.20: Buchholz Relay

The function of the Buchholz relay is to protect the transformer from all internal faults
such as short circuit fault, inter-turn fault, etc. When short circuit occurred in winding then
it generates enough heat to decompose transformer oil into gases (hydrogen, carbon
monoxide, methane, etc). These gases move towards the conservator tank through a
connecting pipe, then due to these gases, Buchholz relay gets activated. It sends signal to
trip and alarm circuits and activate it. Then circuit breaker disconnects the transformer from
the supply.

4.10.10. Oil Conservator

The function of the oil conservator tank is to provide adequate space for expansion and
contraction of transformer oil according to the variation in the ambient temperature of
transformer oil inside the main tank. It is a cylindrical drum-type structure installed on the
top of the main tank of the transformer. It is connected to the main tank through a pipe and
a Buchholz relay mounted on the pipe. A level indicator is also installed on the oil
conservator to indicate the quantity of oil inside the conservator tank. It is normally half-
filled with transformer oil.

Fig 4.21: Oil Conservator Tank

28
4.10.10. Breather
Breather is a cylindrical container filled with silica gel and directly connected with the
conservator tank of the transformer.

The transformer breather serves as a protective barrier against moisture and airborne
contaminants, preventing them from entering the transformer's main tank. This is crucial to
maintain the insulation properties of the oil and extend the transformer's operational life.

As the transformer operates and the oil temperature fluctuates, the breather allows
controlled air exchange between the transformer's interior and the external environment.
This helps prevent over-pressurization or vacuum conditions within the tank and ensures
safe operation.

4.9.12. Explosion Vent

Explosion vent is a metallic pipe with a diaphragm at one end and installed on the main
tank slightly above than conservator tank. It is available only in high rated power
transformer. The main function of the explosion vent is to protect power transformer against
explosion during excessive pressure build up in the main tank due to severe internal faults.
It acts as an emergency exit for oil and hot air gases inside the main tank of the transformer.

The explosion vent works on the same principle as the safety valve works in the pressure
cooker. Hence In other words we can also called the explosion vent as safety valve of the
transformer.

Fig 4.22: Explosion Vent of transformer

29
4.9.12. Radiator and Fans
Radiator and fans are components commonly used in transformers and other electrical
equipment to manage heat generated during operation.

• Radiator: A radiator is a heat exchanger designed to dissipate excess heat from a

transformer's insulating oil. The radiator is attached to the transformer's main tank.
It typically consists of a series of fins or tubes that increase the surface area exposed
to the air. The hot oil from the transformer flows through the radiator, and the large
surface area of the fins or tubes facilitates the transfer of heat to the surrounding air
by convection. The air movement around the radiator helps in dissipating heat and
keeping the transformer's temperature within safe limits.
• Fans: Fans are used in conjunction with radiators to enhance heat dissipation by
increasing air circulation. Depending on the transformer's size, design, and cooling
requirements, fans can be attached to the radiator to improve the efficiency of heat
transfer. Fans draw ambient air through the radiator, increasing the rate of heat
exchange and preventing the transformer from overheating. Fans can be controlled
manually or automatically based on temperature sensors.

Fig 4.23: Radiator and fans of a transformer

30
4.11. BATTERY BANK

A substation battery charger ensures all the essential electrical systems in a substation
continue to operate in the event of a power outage. An absence of an electrical supply could
result in damage to equipment and personnel. The DC system is the most important
component of a high voltage industrial/utility substation. It supplies the energy needed to
manage the protective devices and high voltage components and allows electrical faults to
be safely isolated.
Most high voltage substations house either a sealed or flooded cell battery bank. In a
normal functioning system, the batteries provide very little current. A continuous load
current maintains a constant charge on the battery. The battery charger provides a current if
the charge exceeds the output capability. A failing substation battery charger or if the
charger trips is a good indication of whether the system is working effectively.

Fig 4.24: Battery Bank

31
4.12. TRANSMISSION TOWERS
A transmission tower (also known as a power transmission tower, power tower, or
electricity pylon) is a tall structure (usually a steel lattice tower) used to support an overhead
power line. In electrical grids, they are used to carry high voltage transmission lines that
transport bulk electric power from generating stations to electrical substations.

Fig 4.25: Transmission Tower

Transmission Tower Parts:

• Peak: The portion above the top cross arm is called peak of transmission tower.
Generally, earth shield wire connected to the tip of this peak.
• Cross arm: Cross arms of transmission tower hold the transmission conductor. The
dimension of cross arm depends on the level of transmission voltage, configuration
and minimum forming angle for stress distribution
• Cage: The portion between tower body and peak is known as cage of transmission
tower. This portion of the tower holds the cross arms.
• Body: The portion from the bottom cross arms up to the ground level is called the
transmission tower body. This portion of the tower plays a vital role in maintaining
the required ground clearance of the bottom conductor of the transmission line.

32
4.13. CONDUCTORS USED IN THE SYATEM
The cost and life of Power system primarily depend on the material used for making
conductor for generation, transmission and distribution line.
The much suitable material for conductor is copper as it having-
• High conductivity
• High tensile strength.
• Good ductility.
The only limitation is its cost.
Overcoming the cost problem, most extensively used material in transmission line is now
Aluminium.
• Aluminium is having sufficient conductivity.
• light in weight.
• low conductor weight
• less sag.
• The only limitation is its low tensile strength.
To overcome this limitation steel core is used for increasing the tensile strength of
aluminium conductor such as in:
• AAC: All Aluminium Conductor
• AAAC: All Aluminium Alloy Conductor
• ACSR: Aluminium Conductor, Steel Reinforced
• ACAR: Aluminium Conductor, Alloy Reinforced

Table 4.6: Different types of ACSR based on its current carrying capacity

Conductor Code Current carrying capacity at Current carrying capacity at

Name 400C temp. (approx.) 450C temp. (approx.)
Dog 324 A 300 A
Wolf 430 A 398 A
Panther 520 A 482 A
Deer 806 A 747 A
Zebra 795 A 736 A
Moose 900 A 835 A

33
CHAPTER 5. PROTECTION IN SUBSTATION
The protection system in a substation plays an essential role in maintaining system stability,
equipment safety, and uninterrupted power supply. It ensures that electrical faults such as
short circuits, earth faults, and overloads are detected rapidly and cleared selectively. The
key elements of substation protection include protective relays, grounding systems, and
specific protection schemes for vital components like power transformers.

5.1. RELAY BASED PROTECTION SYSTEM

Relays are the core elements of the protection system in a substation. Their function is to
constantly monitor electrical parameters such as current, voltage, frequency, and phase
angle. When these parameters deviate from their normal ranges, the relay detects the
abnormality and immediately sends a trip signal to the corresponding circuit breaker. This
action isolates the faulted section of the system, preventing damage to equipment and
avoiding large-scale outages.
In older sub-station, electromechanical and static relay were used, but modern
substations primarily use numerical (digital) relays. Numerical relays use microprocessors
to provide multifunctional protection, event recording, disturbance analysis, remote setting
configuration, and integration with SCADA (Supervisory Control and Data Acquisition)
systems. Types of Protective Relays:

1. Overcurrent Relay (OCR):

These relays operate when the load current exceeds a predefined setting. Overcurrent
protection is fundamental and is widely used as primary protection for outgoing feeders and
backup protection for transformers and transmission lines. The relay can be configured for:
• Instantaneous operation, for detecting high fault currents near the substation.
• Time-delayed operation, for coordination with downstream relays (graded
protection).

2. Earth Fault Relay (EFR):

Earth fault relays are used to detect current leakage to the ground. In a star-connected
system, any imbalance in the three-phase currents indicates an earth fault. These relays are
highly sensitive and commonly used in:
• Transformer neutral protection
• Feeder protection
• Busbar protection (in conjunction with differential schemes)

34
3. Differential Relay:
This is a unit protection scheme where the current entering and leaving a component (like
a transformer or busbar) is compared. If the difference exceeds a certain threshold, the relay
operates. It is the most sensitive and fastest protection method for internal faults and is used
for:
• Power transformer protection (e.g., 160 MVA unit)
• Generator protection
• Busbar zones

4. Restricted Earth Fault (REF) Relay:

REF protection is a type of differential protection, but it is confined to the zone between
the winding and the neutral grounding point of a transformer. It detects low-magnitude
earth faults, which might not be sensed by the main differential relay due to CT mismatch
or low current levels. It provides added sensitivity, especially in high-value transformer
protection.

5. Buchholz Relay:
Though mechanical, this relay is crucial for transformer protection. It is installed in the oil
circuit between the main tank and conservator and operates on the principle of gas
accumulation or oil movement. It works in two stages:
• The first stage (alarm) triggers if gas slowly accumulates, indicating insulation
failure or overheating.
• The second stage (trip) activates during rapid oil movement caused by internal short
circuits.

6. Distance Relay
Used for transmission line protection, especially over long distances where fault current
magnitude may not be reliable for detection. It measures the impedance between the relay
location and the fault point. The lower the impedance, the closer the fault. These relays
divide the line into zones:
• Zone 1: Instantaneous protection for nearby faults

• Zone 2 & 3: Delayed protection for remote sections (backup)

35
7. Over/Under Voltage Relay:
These relays operate when system voltage goes beyond or falls below safe limits. They are
used in:
• Busbar protection
• Load shedding schemes
• Transformer secondary protection

9. Frequency Relay (Over/Under Frequency):

Frequency relays monitor the deviation of system frequency from the nominal value (50
Hz in India). Frequency variation indicates imbalance between generation and load. These
relays are part of:
• Grid islanding systems
• Generator protection
• Load rejection schemes

9. Thermal Overload Relay:

This relay protects equipment against overheating due to continuous overload. Numerical
versions model the thermal profile of the transformer or motor and initiate an alarm or trip
before the equipment reaches critical temperature.

5.1.1. Features of Numerical Relays

Modern digital relays (numerical relays) combine several protection functions into a single
unit and offer advanced features such as:
• Multiple Protection Zones: Overcurrent, earth fault, differential, voltage, and
frequency in one relay.
• Communication Protocols: IEC 61850, MODBUS, etc., for integration with
SCADA and automation systems.
• Disturbance Recording: Automatically stores waveform and event data during
faults.
• Programmable Logic: Users can define custom tripping conditions using logic
gates (AND, OR, NOT).
• Self-Monitoring: Relays continuously monitor their own health and
communication status.

36
5.1.2. Relay Coordination and Selectivity

Relay coordination ensures that only the faulty section is isolated while keeping the rest of
the system operational. This is achieved by setting:
• Pick-up values (minimum current/voltage for operation)
• Time delays (so the farthest relay operates first)
• Zone grading (especially in distance and overcurrent protection)
For example, in a transformer-feeder system:
• The feeder relay will trip first for a downstream fault.
• If the feeder relay fails, the transformer relay will act after a delay.
• This prevents unnecessary disruption and maintains supply reliability.

5.2. EARTHING SYSTEM FOR PROTECTION

An effective earthing system is fundamental to protection in any substation. It provides a

low-resistance path for fault current to flow safely into the ground and ensures the correct
functioning of earth fault relays. Earthing also protects both equipment and personnel
during normal and fault conditions.

There are primarily three types of earthing used in substations:

1. Neutral Earthing:
In star-connected systems like power transformers, the neutral point is grounded either
directly or through a Neutral Earthing Resistor (NER). This limits earth fault current to safe
levels and enables sensitive detection by earth fault relays. In 160 MVA transformers, high-
resistance grounding is often used on the low-voltage (132 kV) side.

2. Equipment Earthing (Body Earthing):

All metallic non-current-carrying parts such as enclosures of circuit breakers, panels, and
transformer tanks are connected to the earth grid. This ensures that no dangerous potential
is developed during insulation failure.

37
3. Substation Earth Grid (Earth Mat):

A buried conductor mesh (usually copper or galvanized iron) spreads across the substation
yard, forming a grid that keeps touch and step voltages within safe limits. It also acts as the
reference ground for all protective devices.

4. Lightning Protection Earthing:

Lightning arresters and surge protectors are directly grounded to safely discharge high-
voltage surges from the system.

5.3. PROTECTION OF MAIN TRANSFORMER

The power transformer is the heart of the substation and represents one of its most
valuable assets. A fault in a transformer can cause catastrophic damage, long downtime,
and safety hazards. Therefore, multiple layers of protection are employed to detect and
isolate faults early, while also incorporating mechanical, thermal, and fire protection
systems to prevent escalation.

5.3.1. Electrical Protection Schemes

1. Differential Protection (Main Internal Fault Protection):

This relay compares the current entering and leaving the transformer. If the difference
exceeds a preset value, it indicates an internal fault (e.g., winding-to-winding short circuit).
This scheme offers fast, zone-specific protection.

2. Restricted Earth Fault (REF) Protection:

REF protection is more sensitive than differential protection and detects low-magnitude
earth faults within a defined zone near the transformer neutral. It ensures reliable tripping
for faults that may not cause a significant current imbalance.

38
3. Overcurrent and Earth Fault Protection (Backup):
These relays provide backup for faults not detected by the main protection or if the primary
protection fails. They are time-delayed to allow coordinated operation.

4. Buchholz Relay:
This is a gas and oil movement detector installed between the main tank and conservator.
It operates in two stages:
1. Alarm stage: Triggers when gas accumulates slowly, indicating minor faults.
2. Trip stage: Activates during rapid oil flow, usually caused by a major internal fault like
arcing.

5. Sudden Pressure Relay:

This detects a rapid rise in pressure within the transformer tank due to high-energy faults.
It is faster than the Buchholz relay in some cases and immediately initiates a trip.

6. Surge Protection:
Lightning Arresters (LAs) are installed at transformer terminals to absorb surges due to
lightning or switching operations. CVTs may also assist in voltage monitoring and
insulation coordination.

7. Thermal Overload Protection:

Modern numerical relays use real-time current and ambient temperature inputs to calculate
hot-spot winding temperature and initiate trip or alarm signals if the transformer is running
under prolonged overload conditions.

5.3.2. Mechanical and Thermal Protection

1. Temperature Monitoring System:
• Winding and oil temperature indicators (WTI and OTI) monitor the transformer's
internal temperatures.
• Thermistors or RTDs are embedded in windings and oil.
• If the temperature crosses alarm or trip thresholds, cooling fans are turned on, and
trip signals can be issued in case of high risk.

39
2. Cooling System Protection
• A forced air–oil cooling system (OFAF) is generally used for 160 MVA
transformers. Any failure in the cooling fans or oil pumps is detected by:
• Auxiliary protection relays
• Control panel alarms and interlocks
• These systems help prevent thermal runaway, which can damage insulation and
degrade oil.

5.3.3. Nitrogen Injection Fire Protection System (NIFPS)

To protect the transformer against fire hazards, especially during oil-based internal faults,
substations install a Nitrogen Injection Fire Protection System. This system is automatic
and provides an additional layer of safety.

Working of NIFPS:

• The transformer oil catches fire due to an internal arc or tank rupture.
• Temperature or flame sensors detect the fire.
• The NIFPS control panel activates and releases compressed nitrogen gas stored in
a cylinder.
• Nitrogen is injected into the transformer tank, which increases internal pressure,
displacing oxygen and suppressing combustion. Activates a shut-off valve to stop
oil flow from the conservator. Initiates trip signals to isolate the transformer from
the system.

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CHAPTER 6. SHUTDOWN

In the context of an electrical substation, a "shutdown" refers to the controlled de-

energization or disconnection of equipment or an entire substation from the power grid.
Substation shutdowns are essential for various purposes, such as maintenance, repair,
testing, and ensuring the safety of personnel and equipment.

6.1. REASONS FOR SHUTDOWN:

Regular maintenance and inspection of substation equipment are crucial to ensure its
reliable and efficient operation. During maintenance activities, specific equipment may
need to be de energized or taken out of service to perform tasks like cleaning, lubrication,
testing, and replacement of faulty components.
• Repairs: If any equipment within the substation becomes faulty or malfunctions, it
may require a shutdown to allow for repairs or replacement. Temporary shutdowns
are done to address the issues before bringing the equipment back into service.
• Testing and Commissioning: Before new equipment or modifications are put into
operation, they need to undergo testing and commissioning to ensure their
functionality and compliance with safety standards. During this process, selective
shutdowns may be necessary to test specific components or circuits.
• Emergency Situations: In some cases, substation shutdowns may be necessary to
respond to emergencies, such as a fault or short circuit, to prevent further damage
or hazards.
• Load Shedding: During periods of high demand and low supply, utility companies
may implement controlled load shedding, which involves planned power outages in
specific areas or substations to balance the power grid and avoid widespread
blackouts.

6.2. PROCESS OF SHUTDOWN:

• Preparation: Before initiating the shutdown, the operating personnel should review
the operating procedures, safety guidelines, and any specific shutdown
requirements for that particular substation. They should also notify the appropriate
authorities and personnel about the planned shutdown to ensure everyone is aware
of the situation.

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• Load Reduction: If the substation is carrying a load, the first step is to reduce the
load gradually. This is to avoid sudden power fluctuations that could impact the
stability of the grid or create voltage surges.
• Isolation of Equipment: After reducing the load, the equipment within the
substation needs to be isolated. This involves opening the circuit breakers and
switches to disconnect the transformers, lines, and other electrical components from
the power sources and transmission lines. This step ensures that no electricity flows
through the equipment during the shutdown process. • Safety Checks: Once the
equipment is isolated, the personnel must conduct safety checks to ensure that all
circuits are de-energized and there are no residual electrical charges. They may use
appropriate testing equipment like voltage detectors or grounding devices to verify
the absence of voltage.
• Lockout and Tagout: To prevent accidental re-energization of equipment while
maintenance or repair work is being carried out, the equipment should be locked
out and tagged with appropriate warning signs. This step is essential for the safety
of maintenance personnel.
• Draining Oil and Coolants: If the substation contains oil-filled transformers or
other equipment with coolants, these fluids should be drained or safely disposed of
as per regulations. This step is necessary to prevent environmental hazards and
equipment damage during maintenance.
• Ventilation and Dehumidification: Depending on the substation's design and
environmental conditions, it may be necessary to ventilate or dehumidify the
substation to remove any accumulated gases or moisture that could be potentially
hazardous.
• Documentation: Throughout the shutdown process, detailed documentation should
be maintained, recording each step taken, safety checks performed, and any issues
or observations noted. This documentation is vital for future reference and
regulatory compliance.
• Shutdown Completion: Once all the necessary steps have been completed, and
safety checks are carried out, the shutdown is considered complete. The personnel
should ensure that all the lockout/tagout procedures are in place and that the
substation is secure.

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CHAPTER 7. CONCLUSION

The period of our training has been an instructive and enriching experience. The
internship at the 220/132/33 kV Salakati Grid Sub-Station (AEGCL) provided us with
an opportunity to work under highly experienced engineers and gain first-hand
exposure to the operation and maintenance of high-voltage substations.
During the training, we acquired practical knowledge about substation switchgears,
major equipment, protection systems, and power transmission processes. Many of the
activities and concepts we encountered were closely related to the theoretical topics we
had studied in our academic curriculum, allowing us to strengthen our understanding of
real-world applications.
We gained insight into the construction, operation, and maintenance of transformers,
circuit breakers, isolators, busbars, and other essential components of the substation.
Despite the high demand for power and the heavy workload, the substation’s smooth
functioning is ensured through proper maintenance and skilled handling by the staff.
The engineers guided us patiently, explained new concepts, and answered our queries,
which greatly enhanced our learning experience.
This report presents an overview of the substation and its role in the transmission
system. We conclude that this internship has significantly improved our technical
knowledge and practical understanding of the power sector, which will be invaluable
for our future professional careers in the field of electrical engineering. We are sincerely
thankful to AEGCL and the staff of Salakati Grid Sub-Station for providing us this
valuable learning opportunity.

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PHOTO GALLERY

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