Power System Protection

UNIT-I

Dr G.Ravi kumar
Professor
Electrical and Electronics Engineering
Bapatla Engineering college
Bapatla

February 20, 2025

Page 1
Protective Relays
Introduction
basic requirement of protective relaying
zones of protection
primary and backup protection
classification of relays
attracted armature
balanced beam
induction disc
thermal relays
Buchholz‘s relay
Over current - under voltage
directional and non-directional relays
Distance relays-impedance, reactance, mho and off set mho relays.
Differential relays-circulating current and opposite voltage scheme.
Negative sequence relays.
Page 2
Introduction
Safeguard electrical equipment from damage caused by faults.
Minimize downtime by isolating faulted sections of the system.
Enhance safety for personnel working with electrical systems.
Basic Working Principle
Monitor electrical parameters like current, voltage, and impedance.
Detect faults when parameters exceed predetermined limits.
Trigger circuit breakers to isolate the faulted section.
Types of Protective Relays
Electromechanical Relays: Traditional, mechanical-based
protection.
Solid-State Relays: Use semiconductor components, more reliable
and faster.
Digital/Programmable Relays: Use microprocessors for advanced
protection and communication.
Page 3
Common Faults Detected
Overcurrent: Occurs when current exceeds safe limits.
Under-voltage: When voltage drops below a safe level.
Differential Protection: Detects current differences (e.g.,
transformers).
Distance/Impedance Protection: Measures impedance in
transmission lines.
Relay Characteristics
Time-Current Characteristics: Relationship between fault current
and clearing time.
Selectivity: Ensures only the faulty part of the system is isolated.
Sensitivity: Ability to detect faults with minimal delay.
Relay Coordination
Proper coordination of relays ensures fault isolation at the nearest
protective device.
Precise settings for time delays and fault current thresholds are
critical for system reliability.
Page 4
Applications of Protective Relays
Power Plants: Protecting generators, transformers, and transmission
lines.
Substations: Ensuring safe power distribution and equipment
protection.
Industrial Plants: Safeguarding critical electrical equipment.
Summary
Protective relays are essential for electrical system safety and
reliability.
Advanced relay technology allows for faster response times and
improved protection schemes.
Proper relay coordination ensures minimal disruption to the overall
power system.

Page 5
Basic Requirements of Protective Relays
Sensitivity: The relay must detect faults at the earliest possible
stage.
Selectivity: It should isolate only the faulted section without
affecting the healthy parts of the system.
Speed: The relay should operate quickly to minimize damage to
equipment and ensure safety.
Reliability: It must function accurately under various system
conditions, without failure.
Coordination: Relays must be properly coordinated with each other
to ensure appropriate fault isolation.
Sensitivity in Protective Relaying
The ability to detect faults at the earliest possible moment is critical.
If a relay is too insensitive, faults may not be detected, resulting in
prolonged damage.
Proper sensitivity allows for fault detection even with low fault
currents or in the presence of noise.
Page 6
Selectivity in Protective Relaying
Selectivity ensures that only the faulty section is disconnected.
Prevents unnecessary power outages in healthy sections of the system.
Proper coordination of time-delay settings and current thresholds is
essential.
Speed of Operation
Speed is critical to minimize damage to equipment and prevent injury.
The faster the relay operates, the quicker the faulty section is isolated
from the rest of the system.
Time delays must be set to balance between fault detection and
system stability.

Page 7
Reliability of Protective Relays
The relay must function accurately under a wide range of operating
conditions.
It must be able to withstand electrical and environmental stress
without failure.
Regular testing and maintenance are necessary to ensure reliability.
Coordination of Protective Relays
Coordination involves setting relay operating characteristics to ensure
that faults are cleared by the closest device.
Time-current coordination curves are used to ensure that protection
devices operate in the correct sequence.
Proper coordination minimizes system disruptions and ensures
reliability.

Page 8
Zones of Protection

In electrical power systems, protection zones define areas of the

system that are monitored and protected by relays.
Each zone is responsible for detecting and isolating faults that occur
within its defined boundaries.
The goal is to ensure that faults are cleared promptly while
minimizing the impact on the rest of the system.
Definition of Protection Zones
A protection zone is a section of the electrical system that is
monitored by one or more protective relays.
Each zone is designed to detect faults within its area and initiate
actions to isolate or protect the equipment.
Zones are typically defined by physical boundaries such as
transformers, generators, or transmission lines.

Page 9
Figure: Zones of Protection
Page 10
Types of Protection Zones
Zone 1: Immediate protection zone. This zone protects the
equipment directly affected by the fault.
Zone 2: Backup protection zone. This zone provides protection for
faults that are not cleared by Zone 1.
Zone 3: Extended protection zone. This zone is typically used for
protection in the case of more distant faults.
Each zone operates in a time-coordinated manner, with backup zones
operating only if the primary zone fails to clear the fault.
Zone 1 Protection
Zone 1 is the first line of defense and provides the fastest response
time.
It covers the equipment directly affected by the fault, such as a
transformer or generator.
The relay in Zone 1 operates immediately when a fault occurs, with
minimal time delay to disconnect the faulted section.
It is designed to clear faults within its own protection boundary
without affecting other sections of11 the system.
Page
Zone 2 Protection
Zone 2 provides backup protection if Zone 1 fails to isolate the fault.
It covers a larger portion of the system, typically extending beyond
Zone 1 but still within a reasonable distance from the fault.
Zone 2 operates with a longer time delay than Zone 1 to avoid
unnecessary disconnections.
It ensures that faults are cleared even if the primary protection (Zone
1) fails to detect them.
Zone 3 Protection
Zone 3 is typically the most distant protection zone, covering the
farthest sections of the system.
It is used to protect against faults that may occur at locations far
from the primary equipment.
Zone 3 operates with the longest time delay to avoid unnecessary
trips of remote sections.
It acts as a last-resort backup to clear faults if Zones 1 and 2 fail to
operate.
Page 12
Figure: Time Coordination of Protection Zones

Page 13
Time Coordination of Protection Zones
Proper time coordination is essential to ensure the correct operation
of protection zones.
Zone 1 operates with the shortest time delay, followed by Zone 2 and
then Zone 3.
The purpose of time coordination is to avoid unnecessary
disconnections of healthy sections while ensuring that the fault is
cleared quickly.
Time-current coordination curves are used to define the delay times
for each zone.
Importance of Protection Zones
Protection zones help ensure the safety of the system by isolating
faults before they cause widespread damage.
They allow for the selective isolation of faulted sections without
affecting the entire system.
Properly coordinated protection zones minimize downtime and
prevent unnecessary interruptions in power supply.
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Primary and Backup Protection
Protective relaying systems are designed to detect faults and isolate
the affected sections of the electrical system.
Primary Protection: The first line of defense that directly protects
the equipment or section experiencing the fault.
Backup Protection: A secondary protection mechanism that
operates if the primary protection fails or is delayed.
Proper coordination of both protection types ensures minimal
downtime and system stability.
Primary Protection
Purpose: To detect and clear faults in the earliest possible time to
prevent damage to equipment.
Characteristics:
Fast operation with minimal time delay.
Protects the equipment closest to the fault (e.g., generators,
transformers, or transmission lines).
Operates as soon as a fault is detected within its defined zone.
Page 15
Page 16
Backup Protection
Purpose: To provide protection if primary protection fails to clear a
fault or if a fault occurs outside the primary zone.
Characteristics:
Operates with a time delay to allow primary protection to clear the
fault first.
Ensures that faults are cleared even if the primary protection fails or is
delayed.
Covers a broader area than primary protection.
Examples: Backup overcurrent protection, distance protection (for
remote areas).
Coordination Between Primary and Backup Protection
Proper coordination is crucial to avoid unnecessary operation of
backup protection.
Primary protection should always operate first, with backup
protection stepping in only if primary protection fails.
Coordination is achieved through:
Time delays: Backup protection operates after a preset delay.
Current settings: Different current thresholds for primary and backup
relays.
Page 17
Example of Primary and Backup Protection System

Consider a transformer with both primary and backup protection:

Primary Protection: Differential protection relay to detect faults
inside the transformer.
Backup Protection: Overcurrent protection relay to protect the
transformer if the differential protection fails.
The differential protection relay operates first to isolate the
transformer in case of a fault.
If the differential protection fails to operate or the fault is outside the
primary protection zone, the overcurrent relay operates as backup
protection.

Page 18
Advantages of Primary and Backup Protection

Reliability: Provides an additional layer of security by ensuring that

faults are cleared even if the primary protection fails.
Fault Isolation: Quickly isolates the faulty section, preventing
damage to equipment and improving system stability.
Minimized Downtime: Proper coordination ensures that backup
protection only operates when necessary, minimizing unnecessary
system outages.
Flexibility: Backup protection provides a flexible safety net for
unanticipated fault conditions.

Page 19
Disadvantages of Primary and Backup Protection

Coordination Complexity: Achieving proper coordination between

primary and backup protection can be challenging.
Increased Cost: The need for additional protection relays and
systems can increase the cost of the system.
Delay in Fault Clearance: Backup protection introduces delays in
fault clearance compared to primary protection, which can be
problematic in certain scenarios.

Page 20
Classification of Relays

Relays are essential components of a protective relaying system.

They detect abnormal conditions in electrical systems and trigger
protective devices such as circuit breakers.
Relays are classified based on:
Operating principle.
Function.
Application.
Proper classification helps in selecting the right relay for different
protection schemes.

Page 21
Classification Based on Operating Principle

Electromagnetic Relays:
The most common type of relay.
Operates based on electromagnetic induction, using a coil to generate
a magnetic field.
Examples: Overcurrent relays, differential relays.
Static Relays:
Relays that use electronic components, such as diodes, transistors, and
capacitors.
They do not have moving parts, making them faster and more reliable.
Examples: Numerical relays.
Hybrid Relays:
A combination of electromagnetic and static relays.
Used in modern protection systems for enhanced speed and accuracy.

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Classification Based on Function
Overcurrent Relays:
Protect against excessive current flow.
Operates when current exceeds a preset value.
Overvoltage and Undervoltage Relays:
Protect against overvoltage or undervoltage conditions.
Operates when the voltage goes above or below a certain threshold.
Differential Relays:
Protect against faults within a zone, such as transformers or generators.
Operates when there is a difference between the current entering and
leaving a protection zone.
Distance Relays:
Protect against faults at a specific distance from the relay.
Operates when the impedance (measured by the relay) changes due to
a fault.
Earth Fault Relays:
Detects earth faults by measuring ground current.
Operates when the current flows into the ground.
Page 23
Classification Based on Application

Generator Protection Relays:

Protects generators from faults such as overcurrent, overvoltage, and
underfrequency.
Examples: Generator differential relay, overcurrent relay.
Transformer Protection Relays:
Protects transformers against faults like overcurrent, differential, and
earth faults.
Examples: Transformer differential relay, overcurrent relay.
Transmission Line Protection Relays:
Protects transmission lines from faults such as short circuits.
Examples: Distance relay, overcurrent relay.
Busbar Protection Relays:
Protects busbars from faults by detecting any short circuit between
busbar sections.
Examples: Busbar differential relay, overcurrent relay.

Page 24
Advantages of Different Relay Classifications

Electromagnetic Relays:
Reliable and widely used.
Simple in design and operation.
Static Relays:
Faster response time.
No moving parts, leading to greater reliability and longevity.
Hybrid Relays:
Combines the advantages of electromagnetic and static relays.
Provides enhanced accuracy and speed.

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Attracted Armature Relays

An attracted armature relay is an electromagnetic relay used in

electrical protection systems.
It operates based on the magnetic field generated by an energizing
coil.
The relay’s armature is attracted towards the electromagnet when
current flows through the coil, causing the relay to operate.
Commonly used for overcurrent protection, differential protection, and
other relay applications.

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Figure: Attracted Armature Relay

Page 27
Working Principle of Attracted Armature Relay

The relay consists of an electromagnet (coil) and an armature (a

movable part).
When the current passes through the coil, a magnetic field is created.
This magnetic field attracts the armature towards the coil, causing
the relay contacts to change state (e.g., open or close).
The movement of the armature is directly related to the magnitude of
the current flowing through the coil.
If the current exceeds a preset threshold, the armature moves enough
to activate the protective mechanism (such as closing a contact to
trip a circuit breaker).

Page 28
Construction of Attracted Armature Relay

Electromagnet: A coil wound around a soft iron core.

Armature: A movable iron piece that is attracted towards the
electromagnet when current flows through the coil.
Spring Mechanism: A spring is often used to return the armature to
its resting position once the magnetic field is removed.
Relay Contacts: The armature operates the contacts to either make
or break a circuit, providing the protective action.

Page 29
Types of Attracted Armature Relays

Single-Pole, Single-Throw (SPST) Relays:

Operate a single set of contacts.
Commonly used in simple on/off applications.
Double-Pole, Double-Throw (DPDT) Relays:
Operates two separate sets of contacts.
Used in more complex protection schemes requiring multiple operations.
Directional Relays:
The armature movement is influenced by the direction of current.
Used to detect faults in specific directions, such as reverse current flow.

Page 30
Applications of Attracted Armature Relays

Overcurrent Protection:
Protects electrical circuits from excessive current by triggering a
breaker.
Differential Protection:
Used in transformer protection to detect differences between incoming
and outgoing currents.
Motor Protection:
Detects overload conditions in motors, preventing damage due to
excessive current.
Control Circuits:
Provides on/off control in electrical systems.

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Advantages and Disadvantages
Advantages
Simple and Reliable: These relays are easy to understand and
maintain, with a simple design.
Cost-Effective: Less expensive compared to other types of relays,
making them a cost-effective choice for many applications.
Fast Operation: Relatively quick response time, which is essential
for protection applications.
Versatility: Can be used in a variety of applications such as
overcurrent protection, motor protection, and control circuits.
Disadvantages of Attracted Armature Relays
Limited Accuracy: The accuracy of the operation can be affected by
mechanical wear and external environmental factors.
Mechanical Wear: Over time, moving parts may wear out or
become less reliable.
Slower Response Time: Compared to solid-state or static relays, the
response time of attracted armature relays may be slower.
Page 32
Balanced Beam Relays
A balanced beam is a type of mechanical structure that remains in
equilibrium when the forces acting on it are balanced.
It is commonly used in electromechanical relays, including those used
for protection in electrical systems.
The balanced beam operates by maintaining a position where the
forces on both sides are equal, ensuring stability and accurate
operation.
Used in various applications like load balancing, mechanical
measurements, and protection systems.

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Working Principle of a Balanced Beam
The balanced beam has two arms, and a central pivot or fulcrum,
with forces applied at both ends.
For equilibrium to be maintained, the sum of moments on either side
of the pivot must be zero:
Moment on Left = Moment on Right
This is achieved when the torque (force multiplied by distance) on
each side of the beam is equal.
The forces acting on the beam can include weights, springs, or
electromagnetic forces, depending on the application.
Construction of a Balanced Beam
Beam: A rigid, usually straight piece of material (metal, wood, etc.)
that serves as the main body of the system.
Fulcrum: A central support or pivot that allows the beam to rotate.
Forces: Forces are applied at each end of the beam, either through
weights, springs, or electrical forces (as in relays).
Arm Lengths: The lengths of the arms on either side of the fulcrum
are crucial for balancing the moments.
Page 34
Applications of a Balanced Beam

Electromechanical Relays:
Used in protection systems to trigger a response when an electrical
fault is detected.
The balanced beam can actuate a switch or breaker when a certain
current or voltage threshold is exceeded.
Load Measurement:
Used in systems like weighing scales, where the beam remains balanced
under the applied weight.
Mechanical Balancing Systems:
In mechanical systems to balance rotating machinery or other
mechanical devices.

Page 35
Advantages and Disadvantageous of Balanced Beam
Simplicity: Simple mechanical design, making it easy to understand
and implement.
Accuracy: Provides accurate measurement and control due to its
ability to maintain equilibrium.
Reliability: Low maintenance and high reliability, especially in
electromechanical systems.
Versatility: Can be used in various applications, from load
measurement to electrical protection.
Disadvantages of Balanced Beam
Mechanical Wear: Over time, mechanical parts like pivots and
springs can wear out, affecting performance.
Limited Sensitivity: May not be as sensitive as modern digital or
solid-state systems.
Environmental Sensitivity: Can be affected by external factors such
as temperature, which may affect the materials and equilibrium.
Page 36
Balanced Beam in Protective Relays

In protective relays, the balanced beam mechanism is used to detect

abnormalities in electrical systems.
It is commonly used in electromechanical relays to trigger actions like
opening a circuit breaker during faults (e.g., overcurrent or differential
protection).
The balance of forces in the beam is disrupted when an abnormal
condition, such as a fault, causes a shift in the applied forces.
The beam’s movement or deflection activates the relay mechanism,
resulting in protection actions.

Page 37
Introduction to Induction Disc Relays

Induction disc relays are electromechanical devices used in protective

relaying systems.
They operate based on electromagnetic induction and are used
primarily for overcurrent protection and other fault detection.
The relay contains a rotating disc that moves in response to
electromagnetic forces caused by the current in the protected circuit.
Commonly used in systems for detecting overloads, underfrequency, or
abnormal electrical conditions.

Page 38
Figure: Induction Disc Relay

Page 39
Working Principle of Induction Disc Relays

The basic principle is electromagnetic induction, where a rotating disc

experiences torque due to the interaction of magnetic fields.
A current is passed through a coil, creating a magnetic field.
This magnetic field induces a torque on a disc, causing it to rotate.
The speed of rotation is proportional to the magnitude of the current
flowing through the coil.
If the current exceeds a certain threshold, the disc will rotate enough
to actuate the relay mechanism, such as closing or opening a contact.

Page 40
Construction of Induction Disc Relay

Induction Disc: A circular metal disc that rotates when influenced

by the magnetic field.
Coil: A coil wound around the disc’s axis through which the current
to be monitored flows.
Eddy Currents: Eddy currents are induced in the disc when it
rotates, which provide the braking torque to control the rotation
speed.
Spindle and Bearings: The disc is mounted on a spindle with
bearings to allow it to rotate freely.
Magnetic Field: A permanent magnet or electromagnet is placed
close to the disc to provide the necessary magnetic field for induction.

Page 41
Operation of Induction Disc Relay

When current flows through the coil, it generates a magnetic field

that induces a rotational force (torque) on the disc.
The torque causes the disc to rotate, and the speed of rotation is
directly proportional to the current flowing through the coil.
The rotation of the disc is opposed by eddy currents induced in the
disc itself, which provide a damping effect.
If the current exceeds a preset value, the disc will rotate enough to
actuate the relay mechanism, closing or opening contacts to trigger a
protective action (e.g., tripping a circuit breaker).

Page 42
Applications of Induction Disc Relays

Overcurrent Protection:
Induction disc relays are widely used in protecting electrical circuits
from excessive current by detecting overcurrent conditions.
Underfrequency Protection:
Used to monitor frequency variations and provide protection in cases of
underfrequency conditions.
Overload Detection:
These relays are employed in load protection systems, where they help
prevent overloads in motors and other equipment.
General Fault Detection:
Can be used for detecting various types of faults and abnormal
operating conditions in electrical systems.

Page 43
Advantages of Induction Disc Relays

Simple Construction: Induction disc relays have a simple

mechanical structure, making them easy to understand and maintain.
Reliability: They are known for their long-lasting reliability and
robustness in demanding environments.
Fast Response Time: The induction disc’s rotational movement
allows for quick response times to detect abnormal conditions.
Durability: They are mechanically durable and can withstand harsh
conditions without significant wear.

Page 44
Disadvantages of Induction Disc Relays

Mechanical Wear: The moving parts of the induction disc relay are
subject to mechanical wear over time, which may affect performance.
Limited Accuracy: The accuracy of the relay may not be as high as
modern solid-state relays, especially for precise fault detection.
Slower Response Time: Compared to electronic relays, induction
disc relays may have a slightly slower response time.
Sensitive to Environmental Factors: Extreme temperatures,
vibrations, or contamination can affect the performance of the relay.

Page 45
Induction Disc Relay in Protective Systems

Induction disc relays are commonly used in protection systems to

detect faults in electrical circuits, including short circuits and
overloads.
The relay operates by sensing the abnormal current and activating
protective devices like circuit breakers to prevent damage to the
system.
These relays play an essential role in maintaining the stability and
reliability of electrical networks.

Page 46
Introduction to Thermal Relays

Thermal relays are used to protect electrical equipment from damage

due to overheating caused by excessive current.
They work on the principle that an increase in current leads to an
increase in temperature.
These relays are typically used in motors, transformers, and other
equipment where overheating can result in failure or damage.
Thermal relays operate by monitoring the temperature rise of the
equipment and disconnecting the circuit if the temperature exceeds a
safe threshold.

Page 47
Working Principle of Thermal Relays

Thermal relays operate based on the principle of heat generation in a

conductor when an electric current flows through it.
As the current increases, the heat generated by the resistance of the
conductor also increases, leading to a temperature rise.
Thermal relays typically use a bimetallic strip or other
temperature-sensitive components that bend or deform when heated.
The bending of the bimetallic strip, which occurs as a result of the
temperature rise, triggers a mechanical action to open or close
contacts, thereby disconnecting the circuit.

Page 48
Construction of Thermal Relays

Bimetallic Strip: A key component in thermal relays, made of two

metal strips with different coefficients of thermal expansion. When
heated, the different expansion rates cause the strip to bend.
Heating Coil: A coil through which the current to be monitored
flows. The coil generates heat, which heats up the bimetallic strip.
Switching Mechanism: The bending of the bimetallic strip moves a
contact or mechanism that either opens or closes to interrupt the
circuit.
Thermal Sensing Element: A temperature-sensitive element that
detects the rise in temperature and activates the relay.

Page 49
Figure: Thermal Relay

Page 50
Operation of Thermal Relays

The current flows through the heating coil, causing it to heat up.
As the current increases, the temperature of the coil rises.
The bimetallic strip, which is in contact with the heating coil, starts to
bend due to the difference in thermal expansion between the metals.
Once the bimetallic strip bends to a certain point, it activates the
switching mechanism, either opening or closing the relay contacts.
This action disconnects the electrical circuit to prevent further
damage due to overheating.

Page 51
Applications of Thermal Relays

Overload Protection: Thermal relays are widely used for overload

protection in motors, preventing overheating and damage to windings.
Transformers: Protect transformers from overcurrent and
overheating that could result in insulation failure or damage.
Generators: Used to protect generators from overcurrent conditions
that could lead to catastrophic failure.
Home Appliances: Used in home appliances, such as refrigerators,
air conditioners, and washing machines, to prevent overheating.

Page 52
Advantages of Thermal Relays

Simplicity: Simple in design and operation, making them easy to

implement and maintain.
Cost-Effective: Relatively inexpensive compared to electronic
protection devices.
Effective Overload Protection: Highly effective in protecting
against long-term overloads and preventing overheating of equipment.
Reliable: Provides reliable protection without requiring external
power sources or complex electronics.

Page 53
Disadvantages of Thermal Relays

Slow Response Time: Thermal relays have a slower response time

compared to other types of relays, as they depend on temperature
rise, which takes time.
Limited Precision: They are less precise than modern electronic
relays, which can be adjusted to more specific current thresholds.
Wear and Tear: The mechanical components, such as the bimetallic
strip, can wear out over time, reducing the effectiveness of the relay.
Temperature Sensitivity: Their performance can be affected by
ambient temperature changes, which may cause incorrect tripping.

Page 54
Thermal Relay in Protective Systems

Thermal relays play an essential role in protecting electrical

equipment by preventing overheating and potential damage.
They are especially important in applications where equipment is
subject to varying loads over time, such as motors and transformers.
Thermal relays provide a cost-effective and simple solution for
protecting against prolonged overloads and thermal damage.
Despite their limitations, such as slower response times and sensitivity
to ambient temperature, thermal relays remain a widely used and
trusted protective device.

Page 55
Introduction to Buchholz Relay

The Buchholz relay is a gas-actuated protection device used to detect

internal faults in oil-immersed transformers.
It operates based on the detection of gases that are generated when
the transformer oil breaks down due to a fault.
The relay is installed in the pipeline between the conservator tank and
the main tank of a transformer.
It provides early warning of transformer faults such as insulation
failure or short circuits.

Page 56
Figure: Buchholz Relay
Page 57
Working Principle of Buchholz Relay

The Buchholz relay works by detecting the presence of gases that are
produced inside the transformer during a fault.
When a fault occurs (e.g., short circuit or insulation failure), the
transformer oil heats up, causing the oil to decompose and produce
gases.
These gases accumulate in the Buchholz relay, which is located in the
pipe connecting the transformer’s main tank and conservator.
The accumulation of gas or oil movement causes the relay’s float
mechanism to trigger, signaling a fault.
The relay also detects any oil movement due to sudden pressure
changes, providing protection against sudden internal faults.

Page 58
Construction of Buchholz Relay

Main Components:
Float Mechanism: A float inside the relay detects the level of gas or
oil movement.
Gas Chamber: A chamber within the relay where gas accumulates
during a fault condition.
Contacts: Contacts that open or close to trigger an alarm or trip
action when gas is detected or when oil movement is sensed.
Bimetallic Strip: A bimetallic strip is used for detecting slow-moving
faults.
Conservator Pipe: The relay is connected to the pipeline between the
transformer’s main tank and conservator.

Page 59
Operation of Buchholz Relay

During normal operation, the Buchholz relay remains inactive.

If a fault occurs within the transformer, gases such as hydrogen,
methane, and ethylene are generated.
These gases collect in the Buchholz relay gas chamber.
The accumulated gases lift the float, triggering the relay and
activating the contacts.
The relay may trigger an alarm for minor faults or send a trip signal
for severe faults, depending on the design of the system.
The relay can also detect oil flow caused by a sudden pressure rise in
the transformer.

Page 60
Applications of Buchholz Relay

Transformer Protection:
Used primarily in oil-immersed transformers to detect internal faults
such as winding short circuits and insulation breakdown.
Fault Detection:
Provides early warning for faults that may lead to catastrophic
transformer failure if left undetected.
Protection Against Fire:
By detecting internal faults early, Buchholz relays help prevent fires
caused by overheating or oil decomposition.
Safety in Power Stations:
Commonly used in power stations and substations where large
transformers are in operation.

Page 61
Advantages of Buchholz Relay

Early Fault Detection: Detects faults early, enabling quick action to

prevent severe transformer damage.
Cost-Effective: Provides a relatively low-cost means of protecting
expensive transformer equipment.
No Power Supply Required: The relay operates independently of an
external power supply.
Reliability: Proven reliability in detecting faults due to gas buildup
and oil movement.
Simple Construction: The Buchholz relay is simple to construct and
maintain.

Page 62
Disadvantages of Buchholz Relay

Limited Detection: The Buchholz relay can only detect faults that
generate gases or cause oil movement.
Maintenance: Periodic maintenance is required to check for gas
accumulation and the mechanical operation of the float mechanism.
Slow Response: It may not respond quickly enough for some types
of faults, particularly external faults.
Sensitive to Oil Level: The performance of the relay depends on the
oil level and proper installation, which can be affected by
environmental conditions.

Page 63
Buchholz Relay in Transformer Protection Systems

Buchholz relays provide essential protection for oil-immersed

transformers by detecting faults before they become severe.
They are often used in conjunction with other protection systems,
such as overcurrent and differential relays, to provide comprehensive
protection for transformers.
Early detection of faults through Buchholz relays can prevent damage
to transformer windings, insulation, and prevent potential fire hazards.

Page 64
Introduction to Overcurrent Relay

An overcurrent relay is a protective device used to monitor and

respond to excessive current in an electrical circuit.
It is designed to detect fault conditions such as short circuits or
overloads, which can cause equipment damage if not cleared promptly.
The relay is typically used in conjunction with circuit breakers or fuses
to isolate faulty sections of a power system.
Overcurrent relays are widely used in transformers, generators,
motors, and feeders to protect the system from damage due to
overcurrent conditions.

Page 65
Working Principle of Overcurrent Relay

The overcurrent relay continuously monitors the current flowing

through a circuit.
When the current exceeds a preset threshold (the pickup value), the
relay activates and triggers an alarm or trip action.
The overcurrent relay can be designed to operate based on either
instantaneous overcurrent (no time delay) or with a time delay
(inverse time characteristic).
The time delay allows the relay to distinguish between short-term
current surges and sustained overload conditions.

Page 66
Page 67
Construction of Overcurrent Relay

Sensing Element:
The current flowing through the circuit is sensed by a current
transformer (CT), which steps down the current to a lower, measurable
level.
Relay Coil:
The CT feeds the sensed current into a relay coil, where the magnetic
field generated by the current operates the relay.
Setting Mechanism:
The relay has a setting mechanism to adjust the current threshold or
pickup value.
Operating Mechanism:
The relay’s mechanical components trip the circuit breaker when the
relay is activated.

Page 68
Types of Overcurrent Relays
Instantaneous Overcurrent Relay:
Triggers immediately when the current exceeds the preset value.
Provides protection against short circuits and fault conditions with no
intentional delay.
Time-Delayed Overcurrent Relay:
Introduces a time delay before tripping the circuit.
Typically used for overload protection, allowing temporary current
surges to pass without causing unnecessary trips.
Inverse Time Overcurrent Relay:
The tripping time decreases as the current increases, providing more
rapid response to higher fault currents.
This characteristic is commonly used in power systems to coordinate
with other relays and prevent unnecessary trips.
Definite Time Overcurrent Relay:
The relay trips after a fixed time delay, regardless of the magnitude of
the current.
Page 69
Page 70
Applications of Overcurrent Relays

Transformer Protection:
Protects transformers from damage due to overcurrent conditions
caused by short circuits or overloads.
Feeder Protection:
Used in distribution feeders to protect against faults that may result in
excessive current flow.
Motor Protection:
Used to protect motors from damage due to sustained overloads or
fault conditions.
Generator Protection:
Ensures that generators are not subjected to damaging overcurrent
conditions.
Protection of Power Cables:
Protects cables in distribution systems by detecting short circuits or
overload conditions.

Page 71
Advantages of Overcurrent Relays

Simple Design:
Overcurrent relays have a straightforward and simple design, making
them cost-effective.
Easy Coordination:
Overcurrent relays can be coordinated with other protective devices in
the system to ensure selective tripping.
Versatility:
Suitable for a wide range of applications, including transformer, feeder,
motor, and generator protection.
Reliability:
Overcurrent relays are reliable and provide quick detection of faults,
helping to minimize equipment damage.

Page 72
Disadvantages of Overcurrent Relays

Time Delay in Overload Conditions:

Time-delay characteristics may cause a delay in tripping during
overload conditions.
Miscoordination:
If not properly coordinated with other relays, overcurrent relays can
lead to unnecessary tripping of circuit breakers.
Not Suitable for All Fault Types:
Overcurrent relays may not be suitable for detecting some types of
faults, such as earth faults in some systems.
Sensitive to Harmonics:
Overcurrent relays can sometimes be sensitive to harmonic currents,
which can cause them to trip unnecessarily.

Page 73
Overcurrent Relay in Protection Systems

Overcurrent relays are a crucial component in protection systems to

prevent damage to electrical equipment and ensure reliable operation.
They are commonly used in combination with other relays, such as
differential or distance relays, for a more comprehensive protection
scheme.
Overcurrent relays help isolate faults, minimize damage, and restore
power quickly by tripping the affected circuit.

Page 74
Under Voltage Relay (UVR)

An Under Voltage Relay (UVR) is a protective device used to detect when

the supply voltage falls below a certain threshold and disconnect the load
to prevent equipment damage or malfunction.
UVRs are commonly used in:
Industrial machines
Motors
Transformers
Power distribution systems
Purpose of Under Voltage Relay The primary purpose of a UVR is to
protect electrical equipment from damage due to:
Under voltage conditions causing malfunction.
Overheating in motors and transformers under low voltage.
Efficiency loss and potential failure.

Page 75
Key Components of Undervoltage Release Circuit
Power Supply: Provides the necessary voltage for the system.
Undervoltage Release Unit (UVL): Detects voltage drops and
disconnects the load.
Outgoing Load: Equipment or system being protected.
Additional Components: Resistors, diodes, and transistors for
controlling and regulating the circuit.
Undervoltage Release Circuit Diagram
Displays connections between power supply, UVL, and load.
Includes additional components like resistors, diodes, and transistors.
Shows the control parameters regulating the circuit’s operation.

Page 76
Figure: Under Voltage Relay Circuit Diagram

Page 77
How Under Voltage Relay Works

UVRs constantly monitor the supply voltage. If the voltage drops below a
predefined threshold:
The UVR detects the under voltage condition.
The relay disconnects the equipment from the power supply.
The relay can be reset when the voltage returns to a safe level.
The condition for triggering a UVR is mathematically expressed as:

U < Uthreshold

where:
U is the supply voltage.
Uthreshold is the minimum allowable voltage for safe operation.

Page 78
Applications of Under Voltage Relay UVRs are used in various
applications to protect critical systems:
Motors: To prevent damage from low voltage operation.
Transformers: To avoid faults or overheating caused by under
voltage.
Generators: To protect generators from voltage dips that could
cause damage.
Industrial Equipment: To ensure that equipment operates within
safe voltage levels.
Benefits of Under Voltage Relay The use of UVRs provides several
benefits:
Protection against equipment damage due to low voltage.
Prevention of overheating and failure of sensitive equipment like
motors and transformers.
Increased reliability and safety in electrical systems.
Reduced downtime by automatically disconnecting faulty equipment.
Page 79
Directional and Non-Directional Relays

Relays are protective devices used in electrical systems to detect faults and
isolate the affected part of the system. The operation of a relay depends
on the nature of the fault and the protection mechanism required.
Two major types of relays are used to protect electrical circuits:
Directional Relays and Non-Directional Relays.
The key difference between the two lies in how they operate concerning
the direction of the fault current.
Directional Relays
Directional relays are designed to detect faults based not only on the
magnitude of the current but also on the direction of the fault current.
These relays are typically used in power transmission systems and
generators where knowing the direction of fault currents is essential for
isolating faults in a network.

Page 80
Figure: Directional power Relay
Page 81
Operation Principle
Directional relays function by comparing the phase angle between the
voltage and current. When a fault occurs, the relay determines whether
the fault current is flowing in the predetermined direction (towards or away
from the relay). If the current flows in the right direction, the relay will
trigger the protective action. This method is crucial when dealing with
complex systems such as multi-directional fault currents in transmission
lines or network interconnections.
Key Features of Directional Relays
Direction Sensitivity: Operates when the fault current flows in a
predefined direction.
Complex Operation: Involves both magnitude and direction
analysis, making the relay more complex.
Applications: Used in transmission lines, generators, and feeders
where determining the direction of fault current is crucial for isolating
only the faulty part of the network.

Page 82
Advantages of Directional Relays
Provides selective protection in networks with multiple possible fault
directions.
Helps in distinguishing between faults occurring upstream and
downstream in a power system.
Prevents unnecessary disconnections by isolating only the faulty
section of the system.
Disadvantages of Directional Relays
More expensive due to their complexity.
Requires more time to operate because of the additional phase
comparison.
Requires an accurate supply of voltage and current measurements to
operate correctly.

Page 83
Non-Directional Relays

Non-directional relays are simpler in design and operate based only on the
magnitude of the fault current. These relays do not consider the direction
in which the current is flowing but merely detect whether the current
exceeds a threshold value, such as in overcurrent protection.
Operation Principle
Non-directional relays work by monitoring the magnitude of the fault
current. When the current exceeds a preset value, the relay activates,
regardless of whether the fault current is flowing in or out of the protected
section. Since they do not consider the direction of the current, these
relays are typically used in simpler networks.

Page 84
Key Features of Non-Directional Relays
No Direction Sensitivity: Operates based purely on the magnitude
of the current, regardless of its direction.
Simpler Operation: Only requires current magnitude analysis,
making the operation straightforward.
Applications: Used in local feeders, simple distribution networks, and
systems where the fault direction is irrelevant.
Advantages of Non-Directional Relays
Simpler and less expensive than directional relays.
Faster operation because it only requires current magnitude analysis.
Easier to install and maintain in simpler systems.
Disadvantages of Non-Directional Relays
Cannot differentiate the direction of fault currents, which may result
in broader disconnection areas.
Not suitable for complex protection systems where fault direction
matters.
Page 85
Comparison of Directional and Non-Directional Relays
In the following section, we compare the two types of relays based on
several key factors.
1. Operating Principle
- Directional Relays: Operate based on both the magnitude and direction
of the fault current. They require an analysis of the phase angle between
voltage and current to determine fault direction.
- Non-Directional Relays: Operate based only on the magnitude of the
fault current, without considering its direction.
2. Complexity
- Directional Relays: More complex due to the requirement of phase
comparison and additional components for direction detection.
- Non-Directional Relays: Simpler to design and operate, as they only
monitor the magnitude of the current.

Page 86
3. Applications
- Directional Relays: Used in systems with multiple fault directions, such
as transmission lines, generators, and complex feeder systems.
- Non-Directional Relays: Used in simpler systems where fault current
direction does not need to be considered, such as in local feeders or basic
overcurrent protection.
4. Cost
- Directional Relays: Typically more expensive due to the added
complexity of directional sensing and phase angle analysis.
- Non-Directional Relays: More cost-effective due to their simpler design.
5. Speed of Operation
- Directional Relays: Generally slower because they involve additional
steps for phase comparison and fault direction analysis.
- Non-Directional Relays: Faster operation since they only monitor the
magnitude of the fault current.

Page 87
6. Fault Detection Scenarios
- Directional Relays: Best suited for complex networks where faults may
occur in multiple directions, and selective fault isolation is necessary.
- Non-Directional Relays: Ideal for simpler systems where the direction
of the fault is irrelevant, and only magnitude-based protection is needed.
summary
Directional Relays: Ideal for complex systems where the fault
direction is important.
Non-Directional Relays: Best suited for simpler systems where only
the magnitude of fault current is a concern.
The choice of relay depends on the system’s complexity and
protection requirements.

Page 88
Distance Relays
Distance relays are protective devices used in power systems to detect
faults based on the distance from the relay to the fault location.
They are primarily used in high-voltage transmission lines.
The operation of the relay depends on the impedance measured
between the fault location and the relay.
Distance relays are an essential part of the protection system for the
transmission network.

Figure: Distance Relay Principle

Page 89
Working Principle of Distance Relays
Distance relays measure the impedance (Z ) between the fault and the
relay.
The impedance is a combination of the fault’s resistance and
reactance.
The relay operates when the impedance falls within a predetermined
range, indicating a fault at a specific distance from the relay.
The relay typically operates in three stages (zones):
Zone 1: Primary protection, operates for faults close to the relay.
Zone 2: Backup protection, operates for faults further away.
Zone 3: Backup for Zone 2, typically used in case of a fault in the
next section of the transmission line.
Components of Distance Relays
Current Transformer (CT): Measures the current flowing through
the transmission line.
Voltage Transformer (VT): Provides the voltage for the relay to
calculate the impedance.
Impedance Measurement Unit: Calculates the impedance between
the fault location and the relay.
Page 90
Types of Distance Relays
Impedance Relays: Operate based on the total impedance measured
between the fault and the relay.
Reactance Relays: Operate based on the reactance (imaginary part)
of the measured impedance.
Mho Relays: Use the admittance (Y = Z1 ) to determine the distance
to the fault, offering a circular characteristic on the impedance plane.
Quadrilateral Relays: Use both reactance and resistance to provide
protection for faults located at different distances.
Distance Relay Characteristics
Impedance-Characteristic Curve: Distance relays have a
characteristic impedance curve that indicates how far a fault can be
from the relay before it operates.
Operating Time: Distance relays generally have a faster operating
time compared to overcurrent relays.
Zones of Protection: Zone1:Fastest operation, closest to the relay.
Zone2:protection further away but with slightly delayed operation.
Zone3:backup protection for the longest distance with the slowest
response. Page 91
Applications of Distance Relays
Transmission Line Protection: Distance relays are mainly used to
protect high-voltage transmission lines against faults.
Power System Protection: They ensure selective protection by
isolating the faulted section and maintaining system stability.
Backup Protection: They provide backup protection in case the
primary protection system fails.
In Feeders and Distribution Systems: Distance relays can also be
used for protection in long feeders or distribution networks.
Advantages of Distance Relays
High Selectivity: Distance relays provide selective protection by
isolating only the faulted section of the system.
Faster Operation: They typically have faster operation compared to
overcurrent relays.
Improved Reliability: Distance relays improve the reliability of the
power system by providing protection against faults based on distance.
Minimal Coordination: They reduce the need for coordination with
other protective devices in the system.
Page 92
Challenges of Distance Relays

Impedance Measurement Errors: Errors in impedance

measurement due to line impedance changes can affect relay accuracy.
Relay Setting Complexity: Setting the correct zones and impedance
characteristics for the relay can be complex.
Inaccurate Operation During Faults Near Busbars: Distance
relays may have difficulty detecting faults close to the relay or at
busbars.
Communication Failures: In some systems, distance relays rely on
communication networks, and failures in communication can lead to
delayed protection operation.

Page 93
Impedance Relays
Impedance relays are protective devices used in electrical systems to
detect faults based on the measured impedance.
They are most commonly used in the protection of transmission lines.
The relay operates when the measured impedance between the relay
and the fault location falls within a predetermined range.
Impedance relays are one of the earliest and simplest types of distance
relays.

Page 94
Working Principle of Impedance Relays

The impedance relay calculates the impedance between the fault

location and the relay by measuring the voltage and current.
The relay operates when the impedance (Z) falls below a predefined
threshold value, indicating the presence of a fault.
Impedance relays typically operate based on the following equation:
V
Z=
I
where:
Z is the impedance,
V is the voltage,
I is the current.
A fault occurs when the impedance value falls below the set threshold.

Page 95
Impedance Relay Characteristics: The relay’s operating
characteristic is generally represented as a circle in the impedance
plane.The operating zone is typically inside this circle, indicating a
faulted condition when the impedance falls within it. Impedance
relays can provide protection based on both resistance and reactance,
but they are most sensitive to faults that cause a low impedance
(e.g., short circuits).
Page 96
Types of Impedance Relays
Series Impedance Relays: Measure the impedance in the series path
of the transmission line.
Shunt Impedance Relays: Measure the impedance in parallel to the
transmission line, typically at the source end.
Balanced Impedance Relays: Operate based on the impedance in
both the series and shunt paths, providing more accurate protection.
Components of Impedance Relays
Current Transformer (CT): Measures the current flowing through
the transmission line.
Voltage Transformer (VT): Measures the voltage applied to the
transmission line.
Relay Logic Unit: Processes the measured voltage and current to
calculate the impedance and determine whether a fault is present.
Impedance Characteristic Curve: Defines the operating
characteristic of the relay, indicating when it should operate.
Page 97
Advantages of Impedance Relays
Selective Protection: Impedance relays provide selective protection
by isolating the faulty section of the system.
Fast Operation: They typically have faster response times than
other types of relays, such as overcurrent relays.
Simple Design: Impedance relays have a relatively simple design
compared to more complex distance relays.
Applicable to Long Transmission Lines: They are particularly
useful for long transmission lines where the fault location can vary
significantly.
Disadvantages of Impedance Relays
Accuracy Issues: Impedance relays can experience errors due to
variations in system voltage or load conditions.
Cannot Differentiate Between Fault Types: They may not
distinguish between different types of faults (e.g., short circuits versus
open circuits).
Influence of System Conditions: Impedance relays can be affected
by system conditions like line loading, voltage fluctuations, and fault
resistance. Page 98
Applications of Impedance Relays

Transmission Line Protection: Impedance relays are commonly

used to protect high-voltage transmission lines against faults.
Backup Protection: They can be used as backup protection for
other protective devices in the system.
Feeder Protection: Impedance relays are also used in feeder
protection in distribution networks.
Shunt Capacitor Banks: They are used to protect shunt capacitors
and other components that can be affected by short circuits or
overloads.

Page 99
Reactance Relays
A reactance relay is a type of distance relay used for protection of
transmission lines.
It measures the reactance (the inductive or capacitive impedance) of
the system.
The relay operates when the measured reactance falls within a
predefined zone.
Reactance relays are particularly useful for short transmission lines
where the fault is predominantly reactive in nature.

Figure: Induction cup type reactance relay

Page 100
Figure: (a) Operating characteristic of a reactance relay(b) Reactance relay with
starting unit

Page 101
Working Principle of Reactance Relays
Reactance relays calculate the reactance X using the formula:
V
X =
I
where:
V is the voltage, I is the current,X is the reactance.
If the calculated reactance falls within a set threshold, the relay will
activate and disconnect the faulty section of the system.
Reactance Relay Characteristics
Reactance relays operate on a characteristic curve, typically shown as
a circle in the impedance plane.
The operating zone is inside the circle, and the relay will trip if the
impedance falls within this zone.
These relays are sensitive to faults that cause a low reactance,
typically inductive faults.
Reactance relays are often used for short transmission lines where
resistive components are less significant.
Page 102
Advantages of Reactance Relays
Fast Operation: Reactance relays have a fast response time due to
their simple operation.
Effective for Short Lines: They are ideal for protecting short
transmission lines where faults are predominantly inductive.
Simplicity: Reactance relays are simpler in design compared to other
distance relays.
Disadvantages of Reactance Relays
Limited Fault Detection: Reactance relays are not effective for
detecting faults with high resistive components.
Influence of System Conditions: They can be affected by system
conditions such as load fluctuations and voltage variations.

Page 103
Applications of Reactance Relays
Transmission Line Protection: Primarily used to protect short to
medium-length transmission lines.
Feeder Protection: Reactance relays can be used for feeder
protection in electrical distribution networks.
Backup Protection: Often employed as a backup protection
mechanism to other relays in the system.
Reactance Relay vs Impedance Relay
Impedance Relays measure both the resistance and reactance, while
Reactance Relays only measure the reactance.
Reactance Relays are more suitable for short lines where faults are
predominantly reactive.
Impedance Relays provide more comprehensive protection, detecting
both resistive and reactive faults.

Page 104
MHO Relay

The MHO relay is a type of distance relay used in power system

protection.
It is based on the principle of measuring impedance (or admittance)
between a fault and the source.
It is particularly used for the protection of transmission lines.

Page 105
Working Principle
The relay continuously measures the impedance between the
protected section and the source.
It operates based on admittance (inverse of impedance).
The relay’s characteristic is a circle on the impedance diagram,
passing through the origin.
It detects faults only in the forward direction.
Electromechanical MHO Relay Components
Electromagnetic Induction Mechanism: Induction cup produces the
mechanical torque.
Operating and Polarizing Circuits: Polarizing current creates a
reference phase for operating current.
Moving Coil and Armature: The moving coil and armature move to
operate the relay.
Trip Circuit: Sends a signal to trip the circuit breaker once the fault is
detected.
Page 106
Figure: MHO Relay

Page 107
MHO Relay Characteristics in Impedance Plane
The characteristic appears as a circle on the impedance diagram.
The relay operates only when the fault impedance is within the circle.
Directional Component: It only operates for faults in the forward
direction.

Figure: MHO Relay Characteristic on Impedance Diagram

Page 108
Advantages of Electromechanical MHO Relays
Directional Protection: Only detects faults in the forward direction.
Simple and Reliable: Robust and widely used for transmission line
protection.
Impedance Measurement: Continuous impedance measurement for
distance protection.
Selectivity: Ensures accurate fault detection.
Disadvantages of Electromechanical MHO Relays
Slower Response Time: Relatively slower compared to digital relays.
Mechanical Wear: Sensitive to wear and tear due to mechanical parts.
Limited Flexibility: Not as flexible as modern digital relays.

Page 109
Comparison with Static and Numerical MHO Relays
Electromechanical vs. Static: Static relays offer faster operation and
higher reliability.
Electromechanical vs. Numerical: Numerical relays offer advanced
features like self-monitoring and faster response times.
MHO Relay in Action
The relay measures the fault impedance and compares it to the
operating circle.
If the fault impedance is within the circle, the relay sends a trip signal
to the circuit breaker.
This ensures isolation of the faulted section from the rest of the
power system.

Page 110
Offset MHO Relay

Figure shows an offset MHO characteristic. A rectifier bridge type

amplitude comparator, as shown in Figure , can be used to realize the
offset MHO characteristic.
 The actuating quantities to be compared are I
V
and Zr − nI . Only a fraction of the CT output current is injected into
the restraint circuit. Thus, n is a fraction, i.e., n < 1.
The relay operates when:
V
|I | > Zr − nI
|IZr | > |V − nIZr |
V
|Zr | > I − nZr
|Zr | > |Z − nZr |

Page 111
Figure: (a)Offset MHO characteristic (b)Schematic diagram of a static MHO relay

Page 112
The offset MHO relay has more tolerance to arc resistance. It can also
detect close-up faults and faults which lie behind the busbar. Hence, it is
able to clear busbar faults. A typical value of offset is 10% of the
protected line length. It will operate for close-up faults resulting in V = 0.
When V = 0, the relay operates because the condition

|I | > |0 − nI |

is satisfied, where n is less than 1. In a distance protective scheme

employing MHO relays, the third unit may be an offset MHO, as shown in
Fig. 6.30. The III zone unit provides busbar zone back-up protection in
such a scheme. The main applications of offset MHO relays are: (i) busbar
zone back-up (ii) carrier starting unit in distance/carrier blocking schemes
(iii) power swing blocking. When a fault occurs, the voltage, current and
phase angle change instantaneously, whereas in case of power swings, they
change slowly.

Page 113
 Figure: MHO relays scheme with III unit an offset MHO

This property is utilised for the out of step blocking relay. The III zone
offset unit operates with some time-delay. When a fault occurs in the zone
of the II unit, it operates first and its tripping is not blocked. In case of
power swings, the III zone unit operates first and blocks the tripping of the
II zone unit. The offset characteristic gives a sufficient time-delay for the
III zone unit for this purpose.
Page 114
Differential Relays

Differential relays are essential for power system protection.

They compare the electrical quantities (usually current) entering and
leaving a protected zone.
Designed to detect faults by identifying differences in the current.
A differential relay is a suitably connected overcurrent relay which operates
when the phasor difference of currents at the two ends of a protected
element exceeds a predetermined value. Most of the differential relays are
of current differential type.
The following are the various types of differential relays. (i) Simple (basic)
differential relay (ii) Percentage (biased) differential relay (iii) Balanced
(opposed) voltage differential relay,

Page 115
Behaviour of Simple Differential Protection during Normal
Condition
Figure 8.1 illustrates the principle of simple differential protection
employing a simple differential relay.
The CTs are of such a ratio that their secondary currents are equal under
normal conditions or for external (through) faults.
If the protected element (equipment) is either a 1:1 ratio transformer or a
generator winding or a busbar, the two currents on the primary side will be
equal under normal conditions and external (through) faults. Hence, the
ratios of the protective CTs will also be identical.

Page 116
Working Principle

Figure: Simple differential protection scheme behaviour under normal condition

Relays measure the difference between input signals (e.g., currents at

different points).
If the difference exceeds a preset value, the relay trips.
Assumes that under normal conditions, the current entering and
leaving the zone are equal.
Page 117
Figure: Behaviour of simple differential protection scheme on external (through)
fault.

Page 118
Types of Differential Relays

1 Simple Differential Relay

2 High-Set Differential Relay
3 Low-Set Differential Relay
4 Restricted Earth Fault (REF) Protection
Working Mechanism
The relay continuously compares the currents entering and leaving the
zone.
For ideal conditions (no faults), the currents should be equal.
If the difference exceeds a preset threshold, the relay operates and
triggers a trip signal.

Page 119
Advantages of Differential Relays

High sensitivity to faults inside the protected zone.

Selective operation: isolates only the faulty section.
Fast fault detection.
Simple operating principle based on current comparison.
Disadvantages of Differential Relays
Sensitive to CT saturation during high fault currents.
Requires accurate CT ratio matching.
Higher cost compared to simpler protection schemes.
Calibration is critical to avoid misoperation.

Page 120
Applications of Differential Relays

Transformer protection.
Generator protection.
Feeder protection.
Busbar protection.
Motor protection.
Mathematical Expression
Let I1 be the current entering the zone.
Let I2 be the current leaving the zone.
The relay operates if:

|I1 − I2 | > set threshold value

Page 121
Conclusion

Differential relays are crucial for protecting vital components in power

systems.
They ensure selective tripping and fast fault detection.
Proper calibration and installation are necessary to avoid operational
issues.

Page 122
Negative Sequence relay

Definition: A relay which protects the electrical system from negative

sequence component is called a negative sequence relay or unbalance
phase relay.
The negative sequence relay protects the generator and motor from the
unbalanced load which mainly occurs because of the phase-to-phase faults.
The negative sequence relay has a filter circuit which operates only for the
negative sequence components. The relay always has a low current setting
because the small magnitude overcurrent can cause dangerous situations.
The negative sequence relay has earthing which protects them from phase
to earth fault but not from phase to phase fault. The phase to phase fault
mainly occurs because of the negative sequence components.

Page 123
The construction of the negative sequence relay is shown in the figure
below. The Z1, Z2, Z3, and Z4 are the four impedance of the circuit
which is connected in the form of the bridge. The impedance is energized
by the current transformers. The relay operating coil is connected to the
midpoint of the circuit as shown in the figure below.

Page 124
The operation for Positive Sequence Currents – The current IR and IY
flow through the primary windings of the relay. The current flows in the
opposite direction. The current I’R and I’Yare equal in magnitude. The
balanced current kept the relay inoperative.
The operation for Negative Sequence currents – The negative sequence
current I flow in the primary winding of the relay because of the fault
current.
The relay starts operating when the magnitude of the fault current is more
than that of the relay setting.
Applications of Negative Sequence Relay
Negative sequence relays are protective devices used to detect unbalanced
conditions or faults in power systems. Their key applications include:

Page 125
1 Detection of Unbalanced Loads:
Negative sequence relays identify unbalanced load conditions,
preventing prolonged operation under unbalanced currents that could
damage equipment like generators and motors.
2 Generator Protection:
Generators are protected from:
Rotor overheating caused by unbalanced currents.
Damage due to external faults or unbalanced loads.
3 Motor Protection:
Negative sequence relays protect motors from:
Overheating caused by phase loss or phase reversal.
Mechanical stress due to unbalanced currents.
4 Phase Loss or Phase Reversal Detection:
These relays detect phase loss or reversal conditions by identifying
significant increases in negative sequence components.
5 Transformer Protection:
Transformers are safeguarded against unbalanced currents caused by
asymmetrical faults, such as line-to-line or line-to-ground faults.
Page 125
6 Transmission Line Protection:
Negative sequence relays are used for detecting phase unbalance due
to:
Single line-to-ground faults.
Line-to-line faults.
These relays help isolate faulty sections to maintain system stability.
7 Fault Detection in Electrical Networks:
These relays detect asymmetrical faults, such as:
Open conductor faults.
High-impedance faults.
8 Protection Against Harmonics:
Negative sequence relays detect abnormal conditions caused by
harmonic distortion, ensuring system performance.
9 Preventive Maintenance and Monitoring:
Continuous monitoring of negative sequence components allows for
early detection of unbalance, enabling proactive maintenance and
reducing downtime.

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