WAZIPOINT Engineering Science & Technology: What is the Distance Relay and How does It Work?

## Wednesday, July 12, 2023

### What is the Distance Relay and How does It Work?

A distance relay, also known as a distance protection relay or impedance relay, is a protective device used in power systems to detect and isolate faults on transmission lines or other electrical equipment. Its primary function is to measure the impedance (or distance) between the relay location and the faulted section of the power system.

## Distance Relay and Its Working Principle

The basic principle behind the operation of a distance relay is that the impedance between the relay and the fault location varies depending on the distance to the fault. By comparing the measured impedance with pre-set impedance characteristics, the relay can determine the approximate location of the fault.

Here's a general overview of how a distance relay works:

Voltage and Current Measurement: The distance relay measures the voltage and current phasors at its location using current transformers and voltage transformers connected to the power system. These measurements provide information about the system conditions and fault currents.

Impedance Calculation: The relay calculates the impedance of the faulted section by dividing the measured voltage by the measured current. This impedance is expressed as a complex quantity, taking into account both magnitude and phase angle.

Impedance Comparison: The calculated impedance is compared to pre-determined impedance characteristics or a relay setting. These characteristics are typically represented on a relay's impedance reach diagram, which is a graph showing the impedance values for different fault locations.

Operation and Fault Detection: If the calculated impedance falls within the predefined impedance characteristic region, the relay determines that a fault has occurred within the specified distance. It then initiates a trip signal to disconnect the faulted section from the power system. The relay may also generate alarms or additional signals to indicate the fault condition.

It's important to note that distance relays are often equipped with various features and settings to improve their performance, such as coordination with other protective devices, fault type discrimination, adaptive settings, and communication capabilities for remote monitoring and control.

Overall, distance relays provide reliable and fast protection for power systems, enabling the rapid identification and isolation of faults to prevent further damage and ensure the safe and stable operation of the electrical network.

### Types of Distance Relay Used in Substation

Distance relays are an essential part of protective relaying systems used in substations. They are responsible for detecting faults and accurately determining the distance to the fault location. There are several types of distance relays used in substations, including:

Impedance Relay: Impedance relays are widely used in transmission and distribution systems. They measure the impedance of the faulted section by calculating the ratio of voltage to current at the relay location. Impedance relays are simple and provide reliable distance protection.

Reactance Relay: Reactance relays primarily measure the reactance of the faulted section to determine the fault location. These relays are commonly used in systems with high reactance or long transmission lines.

Mho Relay: Mho relays use a combination of impedance and phase angle measurements to calculate the distance to the fault. They are widely used in transmission lines and provide excellent high-speed fault detection and accurate distance measurement.

Quadrilateral Relay: Quadrilateral relays are an advanced type of distance relays that employ the concept of a quadrilateral characteristic. They measure voltage and current phasors at the relay location and compare them to predefined characteristic curves. Quadrilateral relays offer enhanced performance and are suitable for complex network configurations.

Permissive Overreach Transfer Trip (POTT) Relay: POTT relays are used in interconnected power systems to provide backup protection and coordination between different substations. These relays receive signals from remote substations and, if necessary, initiate a transfer trip command to disconnect the faulty section.

Communication-Assisted Distance Relay: With the advancement of communication technologies, distance relays can now exchange information with relays at remote locations. Communication-assisted distance relays utilize synchronized communication channels to exchange data and enhance the accuracy and coordination of distance protection schemes.

It's important to note that different manufacturers may have their own variations and naming conventions for distance relays. The specific relay types used in a substation depend on the system requirements, network configuration, and relay coordination considerations. Different Types of Relays and Their working Procedure are discussed in detail in another episode.