1. 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 up to 4.0–4.5 times phase voltage if the neutral point is isolated or earthed through a Petersen coil, and up to 3.0 times phase voltage when the neutral is solidly grounded. The full‑voltage, no‑load impact test exposes any weak spots in the transformer windings or auxiliary circuits by deliberately subjecting the insulation to these switching overvoltages before the unit enters service.
2. Assessment of Differential Protection Performance
Energizing a de‑energized, unloaded transformer produces inrush (magnetizing) currents that reach 6–8 times the rated current. Although this inrush current decays relatively rapidly—typically down to 0.25–0.5 times rated current within 0.5–1 second—the total decay may take several seconds in small‑ to medium‑sized units and 10–20 seconds in large transformers. Early‐stage inrush can falsely trigger the transformer's differential protection, preventing closure. By performing repeated no‑load closing operations, protection engineers can observe the actual inrush waveform, verify relay wiring, characteristic curves, and setting values, and confirm whether the differential protection will operate correctly under real inrush conditions.
3. Evaluation of Mechanical Strength
The 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.
Interval Between Tests: Each operation must be spaced by at least 5 minutes.
On‑Site Monitoring: A qualified technician should observe the transformer throughout the tests, looking for abnormalities (e.g., unusual sounds, vibrations, or thermal signs) and halting the procedure immediately if a defect is detected.
By executing these multiple impact tests, one ensures the transformer's insulation reliability, protection coordination, and mechanical robustness before it is placed into continuous service.