Types of Relay Protection for 35kV Transformers

A 35kV transformer should be protected using a combination of differential, overcurrent, temperature, and grounding protection, with carefully coordinated relay settings and CT/VT configurations.

Primary Protection

Differential Protection (87T) Differential protection is the most sensitive and essential protection for internal transformer faults, such as inter-turn short circuits or winding-to-ground faults. It requires matched current transformers (CTs) on all windings, with correct ratios and polarity to avoid maloperation. The relay should be set to detect differential currents above the threshold while remaining stable during inrush conditions or external faults ( ).

Backup Protection

Overcurrent Protection (50/51) Overcurrent relays act as secondary protection if differential protection fails or for external short circuits. Multi-stage settings (I>, II>, III>) are recommended, coordinated with upstream line protection to prevent unnecessary tripping. Time-current curves should be adjusted to ensure proper grading margins, typically 0.2–0.5 seconds between relays ( ).

Temperature Protection

Thermal Monitoring For dry-type transformers, PT100 or NTC sensors monitor winding and core temperatures. Typical alarm and trip thresholds are 130°C for alarm and 155°C for over-temperature tripping. Automatic fan control should be integrated to prevent insulation damage due to overheating ( ).

Grounding Protection

Zero-Sequence Protection Grounding protection detects low-current earth faults and prevents insulation deterioration. Depending on the system grounding, either zero-sequence overcurrent or zero-sequence overvoltage protection can be applied ( ).

Additional Monitoring

Gas and Partial Discharge Monitoring For dry-type transformers, Buchholz relays are not required. Optional monitoring includes partial discharge detection and humidity monitoring, especially in sensitive environments like data centers or tunnel substations ( ).

Relay Settings and Coordination

• CT/VT Selection: Ensure CTs and VTs match system parameters and relay requirements. Correct polarity and ratio are critical for accurate fault detection ( ).
• Instantaneous vs. Time-Delayed Settings: Instantaneous tripping for severe faults near the transformer, time-delayed for coordinated upstream protection ( ).
• Zone Definition: Each relay should protect a specific zone (transformer, feeder, or line) to minimize system impact ( ).
• Verification: Regular testing of relay settings, CT circuits, and breaker trip circuits is essential to maintain reliability ( ).

Preventive Maintenance

• Routine tests: winding DC resistance, insulation resistance, and AC withstand voltage.
• Temperature sensor calibration and fan operation checks.
• Maintain proper ventilation and ambient temperature below 40°C, and prevent dust, moisture, or corrosive gases from entering the transformer room ( ).

Summary

A reliable 35kV transformer protection scheme combines differential, overcurrent, temperature, and grounding protection, with careful relay coordination, CT/VT matching, and preventive maintenance. Intelligent monitoring and real-time condition assessment can further enhance operational reliability and reduce the risk of transformer failure ( ).