Short Answer:

Differential relays protect transformers by detecting any difference in current between the input (primary side) and output (secondary side) of the transformer. Under normal operation, the current entering and leaving the transformer should be nearly equal. But if there is an internal fault, like a short circuit inside the transformer, the current becomes unequal. The relay senses this imbalance and quickly sends a signal to trip the circuit breaker.

This method is highly accurate and fast. Differential relays only respond to internal faults and ignore external disturbances or overloads. That’s why they are widely used in transformer protection systems to avoid major damage and ensure system safety.

Detailed Explanation:

Differential relays protect transformers

Transformers are critical and expensive components in any power system. Protecting them from internal faults is very important because even a small fault can lead to large failures, long power outages, and costly repairs. One of the most effective ways to protect transformers is by using differential relays.

These relays work on a very simple and powerful idea – comparing the current going into the transformer with the current coming out of it. If everything is working properly, these two currents should be equal (or nearly equal considering small losses). But when there is a fault inside the transformer, the balance is disturbed, and the differential relay detects it immediately.

Working Principle of Differential Relay

Differential relays are based on Kirchhoff’s Current Law, which says that the total current entering a closed system must be equal to the total current leaving it. In transformers, this means:

Input current (primary side) ≈ Output current (secondary side)

The relay measures these currents with the help of Current Transformers (CTs) placed on both sides of the transformer. The currents are brought to a common relay point where they are compared. The difference between the two currents is called the differential current.

If this differential current is higher than a set value (called the pickup value), it means an internal fault has occurred. The relay operates and trips the circuit breaker, disconnecting the transformer from the system.

Steps of Operation

1. Normal Condition
   In normal operation or during external faults, the currents on both sides of the transformer are balanced. The differential current is almost zero. The relay does not operate.
2. Internal Fault
   If a short circuit happens inside the transformer winding, the current balance is lost. The differential current increases beyond the safe limit. The relay detects this and operates.
3. Tripping Action
   Once the fault is detected, the relay sends a trip signal to the circuit breaker. The breaker then disconnects the transformer to prevent further damage.

Advantages of Differential Protection

• High Sensitivity: It can detect even small internal faults.
• Fast Operation: The relay acts in milliseconds, reducing damage.
• Selective: It only operates for internal faults, ignoring external issues.
• Reliable: Used in large transformers and critical systems due to its accuracy.

Types of Faults Detected

Differential relays can detect various types of internal faults in transformers such as:

• Short circuits between turns in the same winding
• Phase-to-phase faults inside the transformer
• Ground faults within the transformer windings
• Winding deformation or insulation failure

These faults can occur due to insulation breakdown, overheating, mechanical damage, or aging. If not cleared quickly, they can cause fire, explosion, or complete transformer failure.

Challenges and Solutions

Sometimes, external factors like transformer inrush current or CT mismatch may create false differential current. To avoid this:

• Harmonic Restraint: Used to block relay operation during magnetizing inrush at transformer start-up.
• CT Matching and Slope Characteristics: Helps avoid false tripping during load changes or external faults.

Modern digital differential relays are smart enough to handle these conditions and only operate when a real internal fault is present.

Conclusion

Differential relays are one of the most trusted and widely used methods for transformer protection. They work by measuring the difference in current between the input and output of the transformer. If there is a fault inside the transformer, this difference increases, and the relay immediately operates to disconnect the transformer. This protection is fast, accurate, and essential for preventing serious damage. Without differential protection, transformers would be more vulnerable to internal faults, leading to costly failures and long downtimes in power systems.