Short Answer:

A transformer differential protection scheme works by comparing the current entering and leaving the transformer. Under normal conditions or external faults, these currents are nearly equal. If a fault occurs inside the transformer, such as winding short or internal insulation failure, the difference between the two currents increases beyond a set limit.

This difference, called differential current, is detected by a differential relay. When it exceeds the preset threshold, the relay operates and sends a trip signal to the circuit breaker, isolating the faulty transformer quickly and safely.

Detailed Explanation:

Transformer differential protection scheme

Transformers are key components in power systems and must be protected from internal faults like short circuits, inter-turn faults, or winding-to-ground faults. The most reliable and sensitive method to detect such faults is the differential protection scheme, which monitors the current at both ends of the transformer and detects any imbalance.

This method is widely used in power and distribution transformers, providing fast and selective protection. The principle is based on Kirchhoff’s Current Law, which states that the sum of currents entering and leaving a closed system should be zero.

Working principle

1. CT placement:
   • Current transformers (CTs) are installed at both the primary and secondary sides of the transformer.
2. Current comparison:
   • Under normal conditions, load currents on both sides are balanced (after adjusting for turns ratio).
   • The differential relay calculates the difference (I_diff) between primary and secondary currents.
3. Fault detection:
   • If an internal fault occurs, the current difference increases.
   • This unbalance is detected as differential current.
4. Relay operation:
   • If the differential current exceeds the set threshold, the relay operates.
   • It sends a trip signal to circuit breakers on both sides of the transformer.

Features and components

• Percentage Differential Relay:
  • Adds stability during external faults or CT saturation.
  • Uses a restraining current to avoid false tripping.
• Stabilizing Resistor or Bias:
  • Improves relay performance during through-faults or inrush conditions.
• Harmonic Restraint:
  • Prevents tripping during magnetizing inrush by detecting harmonic content in the current waveform.

Advantages

• Selective protection: Acts only during internal faults, not on external disturbances.
• Fast response: Minimizes damage by disconnecting the transformer quickly.
• Reliable detection: Accurately identifies even minor internal faults.
• Flexible for any size: Suitable for small distribution transformers to large power transformers.

Special considerations

• CT accuracy and matching: Correct CT selection and polarity are crucial.
• Inrush current handling: Use harmonic restraint to avoid false trips during energization.
• Tap changing and vector group adjustments: Compensation for turns ratio and phase shift is necessary in the relay logic.

Conclusion:

A transformer differential protection scheme works by comparing the currents at the primary and secondary of a transformer. Any difference beyond a set limit indicates an internal fault, prompting the relay to trip the breakers and isolate the transformer. This method is fast, accurate, and essential for ensuring the safety and longevity of transformers in power systems.