Transformer Insulation Integrity and Protection Testing: Ensuring Reliability and Performance

Verification of Insulation Integrity

When a new or overhauled transformer is energized under open-circuit (no-load) conditions, switching surges—caused by operations such as opening or closing the no-load transformer circuit—can generate overvoltages. These reach 4.0–4.5 times the phase voltage if the neutral point is isolated or earthed through a Petersen coil, and up to 3.0 times the phase voltage when the neutral is solidly grounded. The full-voltage, no-load impact test deliberately subjects insulation to these switching overvoltages before service, exposing any weak spots in the transformer windings or auxiliary circuits.

Assessment of Differential Protection Performance

Energizing a de-energized, unloaded transformer produces inrush (magnetizing) currents reaching 6–8 times the rated current. Though this inrush decays relatively rapidly—typically to 0.25–0.5 times rated current within 0.5–1 second—total decay may take several seconds in small-to-medium units and 10–20 seconds in large transformers. Early-stage inrush can falsely trigger differential protection, preventing closure. Repeated no-load closing operations allow protection engineers to observe actual inrush waveforms, verify relay wiring, characteristic curves, and settings, and confirm proper differential protection operation under real inrush conditions.

Evaluation of Mechanical Strength

Substantial electromagnetic forces generated during inrush transients subject the transformer's core, windings, and structural components to mechanical stress. Repeated no-load closing tests verify that all internal and support structures can withstand these forces without deformation or damage.

Test Procedure Requirements

• New Units: Five consecutive full-voltage no-load closing operations.

• Overhauled Units: Three consecutive operations.

• Test Interval: At least 5 minutes between operations.

• On-Site Monitoring: Qualified technicians should observe the transformer throughout testing, checking for abnormalities (unusual sounds, vibrations, or thermal signs) and halting immediately if defects are detected.

These multiple impact tests ensure the transformer's insulation reliability, protection coordination, and mechanical robustness before continuous service.