Differential Relay: Working, Types, Advantage & Its Applications

Differential relays are basic components in power system protection, which play a vital protective role in transformers, generators, and other important apparatus against their internal faults. Their main purpose is to monitor differences in the incoming and outgoing currents and give a fast response to situations that may result in equipment failure. After learning what a differential relay is, continue reading to learn more about these components. 

Differential Relay Working Principle

The differential relay's main task is to measure the current through a section of the power system and compare this current with that going out. This monitoring is done through the CTs employed on both sides of the equipment that is being protected. The category of CTs is used because they cause the currents, which can be high in the system, to be reduced to lower related currents to operate the relay. 

However, in normal circulation, the currents that flow into the equipment are almost equal to those that discharge currents. However, if an internal fault happens, such as a short circuit or winding fault, then the balance is disturbed, and there is a difference in the currents. The differential relay identifies this condition from the CT currents and compares them. If the difference between the values exceeds a specific limit, meaning the presence of a defect, the relay operates a trip. This signal triggers the circuit breaker to open the circuit, therefore removing the faulty section and avoiding further circulation to the equipment. 

The relay compares the incoming and outgoing currents, preventing it from being triggered in all cases of faults. This method helps the power system avoid unwanted shutdowns while isolating the affected part, thus reducing the overall stability of the power system. 

Types of Differential Relay

Below are the primary types of differential relay. 

Percentage Differential Relay

This relay works on the basis of the quantitative difference that occurs in the outgoing and incoming current levels, surpassing a certain percentage of the total current. It is highly sensitive and very stable, and its function is to protect transformers and generators. The percentage differential relay is popular for its tendency to cope with the inrush currents and provide stability in any given circumstance. 

High Impedance Differential Relay

This type of relay is normally applied in busbar protection schemes, and it employs the use of a high impedance connected to the relay’s operating coil. This application can be attributed to the fact that its design is uncomplicated and relatively cheap for any researcher to obtain. The high impedance design means that the relay will only react to a large difference and prevent it from tripping off at every small difference, thus making it more reliable. 

Low Impedance Differential Relay

While in operation, the low-impedance differential relay functions differently from the high-impedance relay in the sense that it measures the net current into the relay and the outgoing current. The over-current relay is normally used in transformer protection mainly because it offers better response time and sensitivity. The primary protection offered by this relay type is fast detection and diagnosis of faulty sections so that they may be isolated. 

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Applications of Differential Relay

Differential relays are used in the following schemes to protect a power system that is unique to the given equipment and operation. Some key applications of differential relay are as follows.  

• Transformer Protection: Differential relays are widely employed to protect transformers from IST, including winding short circuits and grounded faults. Due to the ability to identify intolerable imbalances of current, these relays contribute to avoiding major failures and increasing the utility of transformers. Proper protection of transformers is necessary because these devices are severely involved in distribution and transmission processes. 
• Generator Protection: Differential relays remain critical to generators since they help detect stator winding faults and minimise damage. These relays help ensure that generator units get protection in the quickest way possible; this is important for enhancing performance as well as minimising unnecessary repairs. Due to the application of the differential relays, there is a very short response time, and therefore, generator damage is avoided with the satisfaction of power production being achieved. 
• Motor Protection: Differential relays also protect motors from inside faults to ensure their correct functioning and safety. These relays help to isolate faults quickly so that time is not lost due to the failure of motors and the maintenance costs incurred in this process are minimised. Motor protection is a vital component in industries where the motor controls essential mechanisms and appliances. 
• Busbar Protection: These relays give a very fast fault clearance of the busbar, which limits the spread of power outages to other equipment. A Power Bus bar protection is very important when it comes to the safety of the systems that deal with the distribution of electrical power. 

Advantages of Differential Relay

• High Sensitivity: Differential relays are able to sense small amounts of current differences between the receiving and the transmitting ends. Its high sensitivity thus enables rapid detection of faults, thus ensuring that equipment is protected before it is badly damaged.
• Fast Operation: Differential relays’ fast response helps to immediately disconnect a defective section. These quick actions reduce the general effect on the power system, hence reducing the chances of system failures. 
• Reliability: Differential relays are very fast in sensing internal faults, and the chances of random tripping are minimal. This reliability is important in any industrial power system since it helps to eliminate as many unnecessary power blackouts and troubles with electric equipment as possible. 
• Flexibility: Since there are numerous kinds of differential relays, they can be used to meet the requirements of the particular power system element. Hence, for a variety of applications, including transformer, generator, motor, or busbar protection, it is possible to obtain the best of both worlds regarding relay performance and protection. 
• Reduced Maintenance Costs: This relaying reduces the amount of maintenance because protection is prompt, and a lot of equipment is not damaged. The extent to which they help to determine where problems are located ensures that much repair work is not required and sustains the functionality of power systems.

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Conclusion

Differential relays are indispensable components in modern power system protection, offering essential safeguarding for critical equipment such as transformers, generators, motors, and busbars. Their ability to detect and isolate internal faults swiftly and accurately is crucial for maintaining the stability and reliability of power systems. This blog has introduced you to all the advantages of differential relay. If you wish to relish these advantages, shop for the best quality from the Schneider Electric eShop. 

Frequently Asked Questions 

Q1. What is a current balance differential relay?

Ans: A current balance differential relay is a type of differential relay used in power system protection to detect and isolate faults by comparing the currents entering and leaving a protected zone. It is designed to operate on the principle of current balance.

Q2. What is a voltage balance differential relay?

Ans: A voltage balance differential relay is a type of differential relay used in electrical power systems to detect faults by comparing the voltage levels at different points within a protected zone.