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# Differential relay: How it works, circuit & types

Sep 8, 2023

I. Introduction

A. Definition of a Differential Relay

A differential relay is a type of electrical protection device mainly used to detect faults and abnormalities in power systems. It works by comparing the difference between the outgoing and incoming currents in the circuit to detect if there is an electrical fault. The differential relay consists of a current transformer, a differential relay, a computer, and an output contact. When the current between two terminals in the power system is not equal, the current transformer will detect these changes and pass the signal to the differential relay. If a fault is detected, the differential relay will immediately cut off the power supply to protect the power system.
A “Differential Relay” may also be referred to as a “Differential Protection Relay” or a “Differential Current Relay“, both of which are protection relays used to detect internal faults in equipment in a power system, such as transformers, generators or motors.

figure 1: differential relay diagram

B. Brief overview of the importance of differential relays in power systems

What are the function of differential relays? Differential relays are widely used in all aspects of power systems. In substations, differential relays are used to protect equipment such as transformers, cables, and capacitors to prevent faults in the power system. In power plants, differential relays are used to protect generators and transformers, ensuring power generation efficiency and power stability.

In addition, differential relays can also be applied to industrial automation, photovoltaic power transmission, and other fields. With the continuous upgrading of power systems and the improvement of technology, the application of differential relays is becoming more and more extensive, playing a crucial role in ensuring the safety of power equipment and personnel.

II.Differential Relay Working Principle

The principle of the differential relay is based on Kirchhoff’s current law, which states that the sum of the alternating currents in a circuit is zero. When the circuit is working normally, the sum of the currents is zero, so the differential relay does not operate. However, when a fault occurs in the circuit, causing an imbalance in the current, the differential relay will detect the differential current signal and send out a protection signal, automatically cutting off the part of the circuit affected by the fault.

The working principle of the differential relay mainly includes two parts: the current transformer and the differential device. The current transformer is used to measure and transform the current, converting the high current in the power system into low current signals needed for measurement and protection. The differential device generates a protection signal based on the differential current, achieving circuit protection.

Figure 2: Differential Current Relay

III. Circuit of a Differential Relay

A. Description of the main components
A differential relay typically consists of the following main components:

Current Transformers (CTs): These are used to measure the current in the different sections of the system being protected. The CTs are usually placed at each end of the system.

Relay Coil: This is the part of the relay that responds to the difference in current. If the current input from the CTs is not balanced (indicating a fault), the coil will energize and trigger the relay to operate.

Trip Circuit: This is the circuit that disconnects the faulty section of the system from the rest of the system when the relay operates.

Settings: These are adjustments made to the relay to determine its operating characteristics, such as the level of current difference that will cause the relay to operate.

Please note that the exact components and their configuration can vary depending on the specific type and model of the differential relay.

B. Explanation of the flow of current in the circuit
In a differential relay, the current flow is monitored by current transformers (CTs) which are placed at different points in the system. The relay is designed to operate when the difference in current between these points exceeds a certain threshold.

Here’s a simplified explanation of the current flow:

Under normal conditions, the current entering and leaving the system should be equal. The CTs at each end of the system will sense these currents and send corresponding signals to the relay. Since the currents are equal, the difference is zero, and the relay remains inactive.

If a fault (like a short circuit) occurs within the system, the current at the fault point will change. This change in current will be sensed by the CTs. The CT at the fault end will now send a different signal to the relay compared to the CT at the other end.

The relay senses this difference in current (hence the term ‘differential’) and if the difference exceeds a pre-set threshold, the relay will activate.

Once activated, the relay operates the trip circuit which disconnects the faulty section of the system, thus preventing any further damage.

IV. Types of Differential Relays
Differential relays can be categorized based on several different criteria, including their construction and the type of system they are designed to protect. Here are a few common types:

Transformer Differential Relays: These are designed specifically to protect transformers. They monitor the current entering and leaving the transformer, and trigger if a difference (indicating a potential fault) is detected.

Generator Differential Relays: These are used to protect generators. They operate on the same principle as transformer differential relays, monitoring the current at different points in the generator circuit.

Busbar Differential Relays: These protect busbars in power distribution systems. They monitor the total current entering and leaving the busbar, and trigger if a difference is detected.

High Impedance Differential Relays: These relays are used in applications where the normal operating current is low, but the fault current is very high. They have a high operating impedance, which prevents them from triggering under normal conditions but allows them to operate quickly under fault conditions.

Low Impedance Differential Relays: These relays are the opposite of high impedance differential relays. They have a low operating impedance, which means they can operate under conditions of low fault current.

Percentage Differential Relays: These relays are designed to prevent operation under conditions of heavy external fault current or transformer magnetizing inrush current. They operate when the differential current exceeds a certain percentage of the restrained current.

V. Conclusion
In summary, a differential relay is an important protective device that is widely used in fields such as power systems, industrial control, building electricity, and rail transit. Its function is to protect and control circuits by detecting differences in current within the circuit.

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