Differential Protection of Transformer: Working, Settings & Calculation

Understanding Differential Protection of Transformer

Differential protection of transformer is a crucial method used to detect internal faults in power transformers. It plays a vital role in the overall safety and reliability of a power system. Transformers are expensive and critical equipment. Therefore, their protection is a top priority in any electrical installation.

This protection scheme operates on the principle of comparing the current entering and leaving the transformer windings. Under normal conditions, these two currents are nearly equal. But during internal faults, such as phase-to-phase or phase-to-earth faults, the difference between these currents becomes significant. This difference is what the differential protection detects and responds to.

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Working Principle of Differential Protection of Transformer

The basic working of the differential protection of transformer relies on Kirchhoff’s current law. According to this principle, the incoming and outgoing currents in a closed loop must be equal.

In practical terms, a transformer differential relay continuously monitors the current flowing into the primary winding and the current flowing out of the secondary winding.

If the system is healthy, these currents are the same when referred to a common base. However, in case of an internal fault, the sum of these currents will not be zero. The differential relay senses this mismatch and quickly sends a trip signal to isolate the transformer.

The key components in this protection scheme include:

• Current Transformers (CTs) on both sides of the transformer
• Differential Relay to detect current differences
• Stabilizing Resistors or restraining elements to avoid mal-operation during external faults or inrush currents

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Why Differential Protection is Essential for Transformers

Transformers are vulnerable to several types of internal faults such as turn-to-turn short circuits, interwinding faults, and insulation failures. External protection like overcurrent or earth fault relays may not react fast enough or may fail to detect such faults accurately.

Differential protection of transformer provides fast and selective operation. It ensures that the transformer is disconnected quickly, limiting damage and minimizing system disturbance. It is the most sensitive and reliable form of protection for internal transformer faults.

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CT Connection and Polarity in Differential Protection of Transformer

Correct CT connection and polarity are crucial for the successful operation of the protection system. The CTs on both sides of the transformer must be connected in such a way that during normal conditions, their secondary currents cancel each other.

This is achieved by matching their polarity and considering the transformer vector group.

For example, if a transformer has a Delta-Star configuration (Dyn11), the CTs on the delta side are connected in star and those on the star side are connected in delta. Also, a phase shift compensation is made in the relay settings to account for the 30-degree phase shift introduced by the transformer.

Incorrect CT polarity or connection leads to unwanted relay operations or failure to trip during faults.

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Types of Faults Detected by Differential Protection of Transformer

Differential protection of transformer can detect the following internal faults:

• Phase-to-phase short circuits
• Phase-to-earth faults
• Turn-to-turn faults
• Winding insulation failure
• Interwinding faults

It is important to note that this protection does not detect external faults, overloading, or through faults unless they lead to an internal fault.

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Differential Protection Settings for Transformer

Setting the differential relay correctly is essential for avoiding nuisance tripping and ensuring security during external faults. The typical settings include:

Pickup Current (Id): This is the minimum differential current required to operate the relay. It is usually set to 20-30% of the rated full load current.

Restraint or Bias Current (Ib): It is a stabilizing element that prevents operation during external faults or magnetizing inrush. It is calculated as: Ib = (|I1| + |I2|) / 2
Where I1 and I2 are the primary and secondary currents referred to the same side.

Slope Settings: Some relays use a two-slope or dual-slope characteristic to improve stability during external faults with CT saturation.

Slope 1 (low bias region): 20-30%

Slope 2 (high bias region): 40-60%

Harmonic Restraint: To block operation during transformer energization, a second harmonic restraint is used. The relay blocks tripping if the second harmonic content in the differential current exceeds a set value, usually 15%.

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Inrush Current and its Impact on Differential Protection of Transformer

Inrush current occurs when a transformer is energized. It is a high-magnitude, non-fault current that can cause false operation of the differential relay. This is mitigated by the harmonic restraint technique.

Inrush current contains a significant amount of second harmonic component. During such conditions, the relay uses second harmonic blocking to differentiate inrush from actual faults.

Without this restraint, the relay could trip unnecessarily, causing outages and damaging transformer life.

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Calculation Example: Differential Protection of Transformer

Let’s understand the calculation through a simplified example:

Given:

• Transformer rating: 10 MVA, 11kV / 0.415kV
• CT ratio on 11kV side: 500/1
• CT ratio on 0.415kV side: 12000/1
• Relay pickup setting: 30% of rated current

Step 1: Calculate Full Load Current

Primary side current (I1):

= (10 × 10⁶) / (√3 × 11 × 10³) = 525 A

Secondary side current (I2):

= (10 × 10⁶) / (√3 × 0.415 × 10³) = 13,904 A

Step 2: CT Secondary Current

CT Secondary Current Primary Side:

= 525 A / 500 = 1.05 A

CT Secondary Current Secondary Side:

= 13,904 A / 12,000 = 1.1587 A

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Step 3: Differential Current (Id)

Id = |I1 – I2|

= |1.05 – 1.1587| = 0.1087 A

Pickup Setting = 30% of rated CT secondary current

= 0.3 × 1 A = 0.3 A

Since 0.1087 A < 0.3 A, relay will not trip under normal load.

But during fault:

Let’s assume current on one side becomes 2.0 A and other side 0.8 A

Id = |2.0 – 0.8| = 1.2 A

This exceeds pickup setting → Relay will trip.

This example demonstrates how the relay detects imbalance and operates only during internal faults.

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Advantages of Differential Protection of Transformer

• High sensitivity to internal faults
• Fast tripping time (within milliseconds)
• Selective operation with no impact on other equipment
• Security against through faults and inrush currents
• Stable under CT saturation and external disturbances

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Challenges and Limitations

• Requires accurate CT matching and polarity
• Complex settings if multiple winding transformer is used
• Cannot detect external faults or overloading conditions
• Needs proper inrush and overexcitation blocking to avoid nuisance tripping

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Comparison Table: Differential vs Overcurrent Protection

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Conclusion

Differential protection of transformer is a dependable and efficient technique for safeguarding transformers against internal electrical faults. By monitoring the current entering and exiting the transformer, it ensures that only genuine internal issues result in isolation. Proper configuration of CTs, phase compensation, harmonic restraints, and relay settings is critical for its effective performance.

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