A Comprehensive Guide to Differential Relays!

Differential relay is one of the protective devices employed in electrical power systems where it makes a protective fence for equipment and guarantor of power supply continuity. This guide will offer insights on the differential relays, the working of this relay, the different types of this relay, the application, the advantages and disadvantages and many others.  

What is Differential Relay?

A differential relay is protective equipment which is used in power systems to identify faults by measuring the differential current through a protection zone’s input and output. In a nutshell, it measures differential current, which is the difference between two or more currents and gives an alarm or trips the circuit breaker if the difference is beyond a certain limit. It makes sure that only the faults that occur within the protected zone are detected, giving exact protection to different equipment like transformers, generators, and motors. 

Differential relay protection is very efficient for internal faults like phase to phase, phase to ground, winding fault etc. Due to these reasons different relay protection is considered very essential in electrical power systems. By constantly comparing these currents it helps to reduce the probabilities of damaging equipment and blackouts. 

Types of Differential Relay

Differential relays are of several types depending upon the usage and protection standards that are desired in the particular network. Understanding these types helps in selecting the most suitable relay for your needs:

• Current Differential Relay: This is the simplest form of differential relay, this depends on the comparison of the current entering in the protected zone with the one leaving the zone. As there is a difference in phase, the relay triggers the circuit breaker.
• Voltage Differential Relay: The current type is different from the current differential relay; this one measures voltage differences. It is mostly applied where voltage measurements are preferred during fault detections over current measurements.
• Percentage Differential Relay: The percentage differential relay type is significantly popular among all the types due to its high level of stability and reliability due to the percentage restraint. This is because the difference in current is compared to a percentage of load current hence minimizing possibilities of the relay to trip unnecessarily.
• High Impedance Differential Relay: This relay includes a high impedance element for improving its stability at the occurrences of the external faults and the CT saturation conditions. It is widely used in busbar protection and wherever highly sensitive current transformers are required. 

Differential Relay Working

A differential relay is operated with the help of the differential current measuring technique. Here’s a step-by-step overview of the differential relay working:

• Current Monitoring: This relay continuously measures the currents in the input and output points of the zone that require protection. These measurements are obtained via a current transformer which is also referred to as the current probe.
• Comparison of Currents: The relay measures the currents and then makes comparisons out of them. In steady state conditions, these currents are almost equal and hence the difference between them is small.
• Fault Detection: It is self-understood that if a fault happens inside the protected area, the currents are significantly different. The given differential relay detects this anomaly.
• Trip Signal Activation: If the difference is greater in magnitude compared to the pre-set value, then the relay broadcasts a trip signal, and the circuit breaker isolates the faulty section from the rest of the circuits.  

Advantages of Differential Relays

Differential relays offer several advantages, making them a preferred choice for protecting electrical equipment:

• High Sensitivity: These relays offer protection to equipment by disconnecting the faulty section the moment an internal fault occurs owing to the fact that they are very sensitive to them.
• Selective Fault Detection: This is made possible by the ability of the zone to detect only the faults confined within its functioning zone hence minimizing disturbances of external faults to the system.
• Fast Response Time: Differential relays have a short cycle of operation; they perceive the faults as soon as possible and do not allow several pieces of equipment to be damaged.
• Reliable Protection: This is because the flow of input and output currents are constantly being compared by the differential relay thereby ensuring that the protection is still effective regardless of the loads being carried.
• Minimal Maintenance: Differential relays do not have several mechanical parts as those found in traditional overload relays. Hence, they hardly require any maintenance throughout their useful life.  

Application of Differential Relay

Differential relays are in common use in many applications where the settings of each particular application are unique. Here are some key application of differential relay:

• Transformer Protection: Differential relays protect transformers from internal faults, winding short circuits, and phase-to-phase faults, ensuring a stable power supply.
• Generator Protection: Differential relays are commonly used in power plants to protect generators from internal faults that could cause significant damage if not addressed promptly.
• Motor Protection: In industrial settings, differential relays protect motors from issues in the windings, helping prevent costly repairs and downtime.
• Busbar Protection: Differential relays are also used in busbar protection systems, providing fast and accurate fault detection in substations.

Percentage Differential Relay

Percentage differential relays are preferred for their stability and reliability compared to simple differential relays. They feature a percentage restraint characteristic, meaning the relay only operates when the differential current exceeds a set percentage of the operating load current. This design reduces false trips due to external interference or CT errors.

Differential Relay Price

The price of differential relays depends on their type, application, and manufacturer. Slope-type current differential relays are generally more affordable, while advanced percentage differential relays with additional features tend to be more expensive. Special applications, such as high impedance differential relays, may also come at a higher cost.

Also Read: What Is Electromagnetic Relay - Types and Its Working Principle?

Conclusion

Differential relays are essential in modern electrical systems, detecting internal faults in transformers, generators, motors, and other devices. Their sensitivity, selectivity, and fast response make them ideal for protecting critical equipment. Whether you need a simple current differential relay or a more advanced percentage differential relay, investing in these protective devices ensures long-term, safe operation and protection of valuable assets.

For high-quality differential relays and expert advice, visit Lauritz Knudsen Electrical and Automation Smartshop. They offer a wide range of products to meet your electrical protection needs, helping you safeguard your investments effectively and conveniently.