Relay protection for 35kV power transformers

Relay protection for 35kV transformers relies on layered primary, backup, and auxiliary protections, with differential protection as the first line and coordinated settings to ensure fast, reliable fault clearance.

Primary Protection

Differential Protection is the most critical first-line protection for 35kV transformers. It detects internal faults such as inter-turn short circuits and winding-to-ground faults with high sensitivity and fast response. Key principles include:

• Harmonic restraint to prevent false trips during transformer energization.
• CT matching: Differential CTs must have identical ratios, characteristics, and correct polarity to avoid maloperation.
• Pickup settings: Typically set at 0.2–0.5× rated current, with restraint ratios of 0.3–0.6 to balance sensitivity and stability .

Backup Protection

Overcurrent Protection serves as secondary protection when differential protection fails or for external faults. Principles include:

• Multi-stage settings (I>, II>, III>) to coordinate with upstream line protection.
• Directional or voltage-restrained overcurrent for selective fault isolation.
• Zero-sequence protection: Selected based on system grounding (direct grounding uses overcurrent; isolated or compensated grounding may use overvoltage plus overcurrent), .

Auxiliary Protection

Temperature Protection is essential for dry-type transformers, which are sensitive to overheating:

• PT100 or NTC sensors monitor winding temperature.
• Alarm thresholds (e.g., 130°C) and trip thresholds (e.g., 155°C) are set to prevent insulation degradation.
• Automatic fan control ensures proper cooling . Grounding Protection detects low-current ground faults to prevent insulation deterioration, using zero-sequence overcurrent or overvoltage schemes depending on the grounding method. Gas or Partial Discharge Monitoring is optional for dry-type transformers, replacing Buchholz relays used in oil-filled transformers. This is particularly useful in data centers, tunnel substations, or wind turbine transformers .

Coordination and Setting Principles

• Graded Protection: Primary protection acts first, backup protection provides redundancy, and auxiliary protection monitors environmental and operational conditions.
• Time Grading: Backup delays must exceed primary protection plus breaker operation time (typically 0.3–0.5s margin) to avoid miscoordination.
• Customized Settings: Protection settings should match transformer capacity, load characteristics, operational environment, and system importance. Critical-load transformers may require redundant dual protections .
• Intelligent Protection: Modern relays may include real-time monitoring, network communication, and data analytics for early fault detection and remote supervision.

CT/PT Requirements

• Differential CTs: Identical ratios and dedicated cores to prevent imbalance.
• Zero-sequence CTs: Polarized toward the transformer neutral.
• Polarity and wiring verification: Essential to ensure accurate fault detection and prevent maloperation .

Preventive Testing and Maintenance

• Routine tests: Winding DC resistance, insulation resistance, and AC withstand voltage.
• Relay testing: Verify pickup, trip times, and coordination.
• Temperature system calibration: Ensure PT100 sensors and cooling fans operate correctly.
• Environmental control: Maintain ambient temperature below 40°C and prevent dust, moisture, or corrosive gases from affecting insulation .

Summary

A reliable 35kV transformer protection system depends on layered protection, accurate relay settings, CT/PT coordination, and regular preventive testing. Differential protection provides fast internal fault clearance, backup overcurrent and zero-sequence protections ensure redundancy, and auxiliary monitoring safeguards against thermal and environmental risks. Proper configuration and verification are essential for long-term operational reliability and safety.