Longitudinal Differential Protection of Transformer

   The longitudinal differential protection of the transformer is one of the main protections used to safeguard against internal faults of the transformer and faults in the bushings within the power system. Its core principle is to determine faults by comparing the difference in currents on both sides of the transformer. The following are its detailed principles, composition, and key application points:

 I. Core Principles

1. **Kirchhoff's Current Law**

   - During normal operation or external faults, the currents on both sides of the transformer (I₁ and I₂) are equal in magnitude and opposite in phase, and the differential current (I_d = I₁ - I₂) is zero.

   - In case of an internal fault, an additional current is generated at the fault point, causing a significant increase in I_d and triggering the protection action.

2. **Phase Compensation**

   - The Y/Δ connection of the transformer will result in a phase difference of 30° between the currents on both sides. It is necessary to compensate for the phase difference through the wiring of the secondary windings of the current transformers (CTs) (such as Y/Δ or Δ/Y) or software algorithms.

 II. Composition and Wiring

1. **Key Components**

   - **Current Transformer (CT)**: Installed at the bushings on both sides of the transformer to collect the primary side current.

   - **Differential Relay**: Compares the secondary currents of the CTs on both sides and calculates the differential current and the braking current.

   - **Connecting Cable**: Ensures the impedance matching of the secondary circuits of the CTs on both sides to reduce the unbalanced current.

2. **Wiring Method**

   - The polarities of the secondary windings of the CTs on both sides need to be consistent to avoid misoperation of the protection due to incorrect wiring.

   - The differential relay usually adopts the "circulating current method" for wiring, connecting the secondary windings of the CTs on both sides in series and then connecting them to the relay.

 III. Key Points of Setting Calculation

1. **Setting Value of Differential Current**

   - Minimum Operating Current: Set to avoid the maximum unbalanced current under the rated current of the transformer (usually 0.3 to 0.5 times the rated current).

   - Braking Characteristics: Introduce the braking current (such as the average value of the currents on both sides) to prevent misoperation during external faults.

2. **Inrush Current Suppression**

   - **Second Harmonic Braking**: When the content of the second harmonic in the inrush current exceeds the setting value (usually 15% to 20%), the differential protection is blocked.

   - **Waveform Symmetry Principle**: Identify the waveform differences between the inrush current and the fault current to avoid misjudgment.

3. **CT Error Compensation**

   - Select high-precision CTs (such as Class 0.5) and calculate the load of the secondary circuit to ensure that the error is within the allowable range.

 IV. Common Problems and Solutions

1. **Misoperation Caused by Inrush Current**

   - **Cause**: Inrush current is generated due to core saturation during no-load closing, which contains a large amount of non-periodic components and higher harmonics.

   - **Countermeasure**: Adopt second harmonic braking, waveform discrimination, or time delay to avoid the inrush current.

2. **Misoperation during External Faults**

   - **Cause**: The inconsistent transient characteristics of CTs lead to an increase in the unbalanced current.

   - **Countermeasure**: Select TPY-class CTs (transient type) and optimize the braking characteristic curve.

3. **Faults in the CT Secondary Circuit**

   - **Manifestation**: Abnormal differential current is caused by CT disconnection, short circuit, or poor contact.

   - **Countermeasure**: Configure the CT disconnection detection function and regularly check the impedance of the secondary circuit.

 V. Application Scenarios

- **Applicable Transformers**: Step-down transformers, step-up transformers with a capacity of ≥ 10MVA, and important distribution transformers.

- **Coordination with Other Protections**

   - Coordinate with the gas protection. The gas protection is responsible for the faults inside the oil tank, and the longitudinal differential protection covers the faults of the bushings and outgoing lines.

   - Backup protections (such as overcurrent protection) serve as supplements.

 VI. Advantages and Disadvantages

- **Advantages**

   - High sensitivity, capable of quickly removing internal faults.

   - Not affected by the winding structure of the transformer (such as autotransformers).

- **Disadvantages**

   - Requires CTs on both sides and complex wiring, resulting in a relatively high cost.

   - Has strict requirements for the accuracy of CTs and the secondary circuit.

 VII. Debugging and Maintenance

1. **Phase Testing**: Use a phase volt-ampere meter to verify whether the phases of the secondary currents of the CTs on both sides are correct.

2. **Differential Protection Actuation Test**: Simulate internal faults to test the action time of the protection and the accuracy of the setting value.

3. **Regular Calibration**: Check the characteristics of CTs, the insulation of the secondary circuit, and the parameters of the relay every 1 to 2 years.

Through reasonable design and setting, the longitudinal differential protection can effectively ensure the safe operation of the transformer and reduce the losses caused by faults. In practical applications, it is necessary to comprehensively optimize the configuration by combining the parameters of the transformer, the short-circuit capacity of the system, and the characteristics of the protection device.