What is the Differential Relay and How Does It Work?

A differential relay is a protective relay that is commonly used in electrical power systems to detect and protect against faults or abnormalities in electrical circuits. It operates based on the principle of current balance or current differential.

Differential Relay and Its Working Principle

The main purpose of a differential relay is to compare the current entering a system (inflow current) with the current leaving the system (outflow current). Under normal operating conditions, these currents should be equal, indicating that there are no faults in the system. However, in the presence of a fault, such as a short circuit or ground fault, the current flowing into the system will differ from the current flowing out.

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

Current Measurement: The differential relay measures the current at both the input and output terminals of the system being protected. This is typically achieved by current transformers (CTs) or similar devices that convert high currents into proportional low currents suitable for measurement.

Current Comparison: The relay compares the magnitudes of the currents at the input and output terminals. This can be done using various methods, such as a balance beam or electronic circuits.

Current Differential Calculation: The relay calculates the difference between the input and output currents. If there is no fault, the differential current will be negligible or zero.

Setting and Thresholds: The relay is typically set with specific thresholds or settings to account for normal operating conditions and acceptable tolerances. These settings determine when the relay will detect a fault condition and initiate a trip signal.

Fault Detection and Operation: If the differential current exceeds the set threshold, indicating a fault, the relay operates to initiate a trip signal. This trip signal is sent to circuit breakers or other protective devices to isolate the faulty section of the electrical system from the rest of the network.

Differential relays are commonly used to protect large electrical machines, such as transformers, generators, and motors, where detecting faults at an early stage is crucial to prevent damage and ensure the stability of the power system. They provide a reliable and sensitive means of fault detection by comparing currents rather than relying solely on magnitude or voltage thresholds.

Types Differential Relay Used in Substation

In a substation, various types of differential relays are used to protect electrical equipment, such as transformers, generators, and busbars, from faults and abnormal operating conditions. Here are some commonly used types of differential relays in substations:

Current Differential Relay: This type of relay compares the incoming and outgoing currents of a protected zone or equipment. If there is a significant difference between the currents, it indicates a fault or abnormal condition, and the relay initiates a trip signal to isolate the faulty section.

Percentage Differential Relay: Similar to a current differential relay, a percentage differential relay compares the percentage difference between the incoming and outgoing currents. This type of relay is typically used for large power transformers, where slight imbalances due to magnetizing currents and tap changer operations need to be accounted for.

Pilot-Wire Differential Relay: In this configuration, a separate communication channel, such as a pilot wire or fiber optic link, is used to transmit the current signals from the protected zone to the relay. The relay then compares these signals and operates based on the detected differential current.

High Impedance Differential Relay: High impedance differential relays utilize current transformers with high impedance characteristics to detect internal faults in the protected equipment. These relays are commonly used for busbar protection and can quickly detect faults even at low fault currents.

Restricted Earth Fault (REF) Relay: A restricted earth fault relay is used to detect earth faults within a specified zone. It measures the difference between the current flowing into the zone and the current returning from the zone via the earth's path. This relay is commonly employed for transformer winding protection.

Transformer Differential Relay: Specifically designed for transformer protection, this relay compares the currents entering and leaving the transformer windings. It detects internal faults such as short circuits and can initiate tripping to prevent further damage to the transformer.

It's important to note that the choice of the relay type depends on the specific application, equipment, and protection requirements of the substation. Different relays may be used in combination to provide comprehensive protection for the substation's electrical system. Different Types of Relays and Their working Procedure are discussed in detail in another episode.