What is differential protection in transformers?

Short Answer:

Differential protection is a protective scheme used to detect faults inside a transformer by comparing the current entering and leaving the transformer. If the difference between these currents exceeds a set limit, it indicates an internal fault, and the protection system trips to isolate the transformer.

This method quickly identifies short circuits or winding faults within the transformer, providing fast and reliable protection to prevent damage and maintain system safety.

Detailed Explanation:

Differential protection in transformers

Introduction

Transformers are critical equipment in power systems, and internal faults like winding shorts can cause severe damage and outages. Differential protection is a widely used technique to protect transformers by monitoring the difference between incoming and outgoing currents.

It is highly sensitive and selective, ensuring faults inside the transformer are detected quickly while avoiding unnecessary trips from external faults.

Working Principle

  • Current transformers (CTs) are installed at the transformer’s primary and secondary sides to measure currents.
  • Under normal conditions or external faults, the current entering and leaving the transformer is nearly equal after accounting for the turns ratio.
  • The differential relay compares these currents after scaling.
  • If there is an internal fault, such as a short circuit between windings or turn-to-turn fault, the difference between the currents increases sharply.
  • When this difference exceeds a preset threshold, the relay operates and trips the circuit breaker to isolate the transformer.

Key Features

  • High sensitivity to internal faults, even minor winding faults.
  • Selectivity to avoid tripping for external faults or normal load changes.
  • Uses percentage differential characteristic to accommodate CT errors and inrush currents.
  • Protects against phase-to-phase, phase-to-ground, and inter-turn faults inside the transformer.

Advantages

  • Fast fault detection and isolation
  • Minimizes damage and repair costs
  • Improves reliability and safety of the power system
  • Reduces downtime by preventing extensive equipment failure

Components

  • Current transformers at primary and secondary sides
  • Differential relay that compares CT inputs
  • Circuit breaker for isolation

Applications

  • Protection of power transformers in substations and power plants
  • Used in generator step-up transformers and distribution transformers
  • Also applicable to large motors and busbars for differential protection
Conclusion

Differential protection in transformers is a crucial scheme that compares incoming and outgoing currents to detect internal faults. By providing fast and selective fault detection, it safeguards transformers from serious damage and ensures continuous, safe operation of power systems.