Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: What is a Protective Relay? Principle, Advantages, Applications
Share
Notification Show More
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > Electrical Basics > What is a Protective Relay? Principle, Advantages, Applications

What is a Protective Relay? Principle, Advantages, Applications

A protective relay is an electrical component that is designed to trip a circuit breaker when a fault is encountered or identified.

Last updated: October 17, 2023 2:09 pm
R Jagan Mohan Rao
Electrical Basics
No Comments
Share
5 Min Read
SHARE

A protective relay is a device that monitors system conditions like amps, volts, etc., using CTs and PTs and reacts to the detection of abnormal conditions.

Contents
Protective Relay PrincipleTypes of Protective RelaysReliabilitySelectivitySpeedSensitivityAdvantages of Protective RelaysApplications of Protective Relays

Protective Relay Principle

A protective relay is an electrical component that is designed to trip a circuit breaker when a fault is encountered or identified.

The protective relay compares the actual real-time values against preset threshold values and sends electrical control signals to trip circuit breakers to clear an abnormal condition on the equipment it is protecting, alarm indications are sent to system control and sometimes other protection operations are initiated.

Types of Protective Relays

Characteristics of Protective Relay

A protective relay is required to satisfy four basic applicable functional characteristics:

  1. Reliability
  2. Selectivity
  3. Speed
  4. Sensitivity

Reliability

The relay should be reliable as a primary requirement. It must operate when it is required. There are various components that go into operation before a relay operates.

Hence, every component and circuit which is involved in the operation of the relay plays an important role; for example, the absence of suitable current and voltage transformers may result in unreliable operation.

Since the protective relays remain idle most of the time on the power system, proper maintenance will play an important role in improving the reliable operation of the relay.

In-separable reliability is a matter of design based on long experience. This can be achieved partly by:

  1. Simplicity and robustness in construction.
  2. High contact pressure.
  3. Dust-free enclosures.
  4. Good contact material.
  5. Good workmanship. 
  6. Careful maintenance.

Selectivity

It is the basic requirement of the relay in which it should be possible to select which part of the system is defective and which is not.

It should isolate the faulty part of the system from the healthy one.

Selectivity is achieved in two ways:

  1. Unit system protection
  2. Non-unit system of protection

Unit system of protection

In this type of system, the protection responds only to faults within its own area and does not make note of the conditions somewhere else.

Faults occurring elsewhere in the power system have no influence on the unit system and they are neglected.

Unit-type systems protect a specific area of the system.

Example:

The differential protection of transformers, transmission lines, bus bars, and generators.

Here the protection scheme will work only if the fault is in the transformer, bus bar, or the generator, transmission line relay separately.

Non-unit system of protection

In this system, the selectivity is obtained by the current settings of the relays at different locations, have no boundaries.

Protect their own designated areas and as well as zones overlapped also require protection.

All of which may respond to a generated fault.

Speed

A protective relay must operate at the speed it is required at. It should neither be too slow which can lead to damage to the equipment nor should it be too fast which can end in undesired operation during transient faults.

The shorter the time that a fault is allowed to persist on the system, the more load are often transferred between given points on the facility power system without loss of synchronization.

Sensitivity

A relay should be enough sensitive so that it operates reliably. It is expressed in terms of minimum volt-amps required for the relay operation.

Sensitivity depends on the settings we do for the protective relay to act.

Advantages of Protective Relays

  • Protective relay continuously monitors power system condition.
  • Improves system performance, system stability, system reliability.
  • Disconnects the faulty parts as quickly as possible, so as to minimize the damage to the fault parts themselves.
  • Detect system failures when they occur and isolate fault areas from the rest of the system.
  • Minimize risk of fire
  • Provide safety and protect people working on the system.

Applications of Protective Relays

Overcurrent Protection

It is used for the protection of distribution lines, large motors, equipment, etc.

Distance Protection

It is used for protection of Transmission or sub-transmission lines.

Carrier-current Protection

It is used for the protection of the Extra High Voltage (EHV) and Ultra High Voltage (UHV) line.

Differential Protection

It is used for the protection of transformers, generators, motors of very large size, bus zones, etc.

Reference: Power Systems Protection and Switchgear by Badri Ram, D.N Vishwa Karma 2001.

Read Next:

  • Circuit Breaker Quiz
  • Circuit Breaker Animation
  • Oil Circuit Breakers
  • How to Select a Fuse?
  • What is Auto Reclosing?
Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

What is a Substation? Purpose of an Electrical Substation
Rotary Encoder Interview Questions and Answers
Electrical Cabinet Air Conditioner – Enclosure Cooling, Maintenance
What is SMPS? – Types of SMPS
What are Inverse Time and Instantaneous Trip Circuit Breakers?
Difference between Stepper Motor and Servo Motor
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
Leave a Comment

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

Totally Enclosed Fan Cooled (TEFC) Motors Rampant Winding Burnouts
How to Locate Faults in Cables? – Fault Analysis & Location
Things to Know When Measuring Current
Electrical Switches Principle & Types
Types of Electrical Wiring
Top 100 Electrical Machines Projects for Electrical Engineers
What is Switchgear and Electrical Substation?
What is Electrical Slip Ring? Principle, Types, Advantages, Applications

Keep Learning

Current Transformer Operation

Current Transformer Selection, Operation & Maintenance

Types of Batteries

Cells and Batteries – Types, Applications, Explanation

Lithium-ion Battery Charging System

Lithium-ion Battery Charging Systems

Switched Mode Power Supply (SMPS)

How does SMPS Works? – Switched Mode Power Supply

Difference between KW and KVA

Difference between KW and KVA

VFD Commissioning and Testing Procedure

VFD Commissioning and Testing Procedure (Variable Frequency Drive)

Difference between Servo Motor and DC Motor

Difference between Servo Motor and DC Motor

What is Electrical Busbar

What is Electrical Busbar? Types, Advantages, Disadvantages

Learn More

SWAS Analyzer Questions

SWAS Analyzers Questions and Answers

Instrumentation Diagrams Multiple Choice Questions

Instrumentation Diagrams Multiple Choice Questions

Technical Evaluation for DCS and ESD - Distributed Control System

Technical Evaluation for DCS and ESD – Distributed Control System

Free Mitsubishi PLC Online Training Courses

Free Mitsubishi PLC Online Training Course and Tutorials

PRESSURE VARIATION CONTROL VALVe - 2

Constant Flow Valve Principle

What is Oscilloscope

What is Cathode Ray oscilloscope ?

Single Beam and Double Beam Instruments

Single Beam and Double Beam Instruments

Testing of Pneumatic Systems

Panel and Field Pneumatic Controller’ Bumpless Transfer Issues

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?