What diagnostic tests are performed on power transformers?

Diagnostic tests assess transformer condition: dissolved gas analysis (DGA - detects incipient faults from gas types), insulation resistance and polarization index (moisture and contamination), power factor/dissipation factor (insulation quality), winding resistance (tap changer contacts, winding integrity), turns ratio (tap positions), frequency response analysis (FRA - detects winding deformation), partial discharge (PD - detects insulation defects), oil quality (dielectric strength, moisture, acidity), and thermal imaging (hotspots). Results are trended over time and compared against industry standards.