Mid-term inspection of transformer relay protection ensures relays and associated devices operate reliably, maintaining sensitivity, selectivity, and stability in transformer protection schemes.

Purpose of Mid-Term Inspection

Mid-term inspections are conducted between major overhauls or annual maintenance to verify that protective relays continue to function correctly and that the transformer protection system remains reliable. This helps prevent nuisance tripping, undetected faults, and potential transformer damage due to insulation failure, overheating, or internal faults (IEEE Std C37.91-2021) .

Key Components to Inspect

1. Protective Relays: Includes electromechanical, solid-state, and microprocessor-based relays. Inspect for:
   • Proper operation of trip circuits
   • Correct settings and calibration
   • Signs of deterioration or mechanical wear in electromechanical relays
   • Self-test results in microprocessor relays (Vertiv)
2. Current Transformers (CTs) and Voltage Transformers (VTs):
   • Check for correct polarity and ratio matching
   • Inspect for insulation integrity and physical damage
   • Ensure CTs are not saturated during fault conditions, which could affect differential protection
3. Buchholz and Gas-Actuated Relays:
   • Inspect for gas accumulation or oil leaks
   • Perform gas analysis to distinguish between internal faults and external leaks
   • Verify alarm and trip functions
4. Thermal and Over-Fluxing Protection:
   • Check temperature sensors and fiber-optic devices
   • Verify hot-spot calculations and cooling system triggers

Recommended Tests

1. Primary Current Injection Test:
   • Injects current directly into the primary circuit of CTs and relays
   • Verifies the entire protection system, including CTs, wiring, and relay operation
   • Essential for detecting incorrect CT polarity or relay sequence errors
2. Secondary Current Injection Test:
   • Injects current into the relay secondary circuit
   • Confirms relay operation without stressing the primary transformer circuit
   • Suitable for routine mid-term inspections
3. Functional and Trip Tests:
   • Simulate fault conditions to ensure relays trip correctly
   • Verify coordination with upstream and downstream protection devices
   • Check timing and characteristic curves for differential and overcurrent relays
4. Diagnostic Checks:
   • Review relay event logs and self-test reports
   • Analyze trends in relay operation, such as repeated alarms or trips
   • Inspect auxiliary circuits, trip coils, and control wiring

Documentation and Compliance

• Record all test results, adjustments, and observations
• Ensure compliance with NETA, NERC, and NFPA standards for relay maintenance
• Maintain a systematic schedule for mid-term inspections to reduce risk of transformer failure and downtime

Best Practices

• Engage qualified personnel familiar with transformer protection schemes
• Use calibrated test equipment such as Omicron, Doble, or PowerDB systems
• Coordinate relay settings with system protection philosophy to avoid nuisance trips
• Include both electrical and mechanical protection devices in the inspection scope Mid-term inspection is a critical preventive measure that ensures transformer relays continue to provide reliable protection, detect internal faults promptly, and maintain the overall health of the power system.