What is the standard breakdown voltage of transformer oil and why is it critical?

There is no single global number, because BDV depends on the test standard, electrode gap, and whether oil is new or in service. Many utilities expect new or well treated oil to achieve relatively high BDV values under IEC 60156 conditions, while in service oils can have lower acceptable thresholds depending on risk policy. BDV is critical because low dielectric strength increases the chance of internal discharge and insulation failure, especially during transients, high load temperatures, or moisture ingress. Trend and context matter.