The Breakdown Voltage (BDV) of transformer oil is a critical parameter that indicates its dielectric strength and ability to withstand electrical stress. The acceptable and good BDV value depends on the voltage class of the transformer and industry standards.
Standard BDV Values for Transformer Oil:
New Transformer Oil (Unused):
Minimum BDV: 30 kV (as per IEC 60156 and ASTM D1816)
Good Quality Oil: Above 60 kV (indicating very high purity)
In-Service Transformer Oil (Used):
Acceptable BDV: Above 30 kV (for continued operation)
Marginal/Needs Attention: 20–30 kV (may require filtration or reconditioning)
Unacceptable/Failure Risk: Below 20 kV (immediate action needed-filtration or replacement)
Factors Affecting BDV:
Moisture content (major factor-higher moisture reduces BDV)
Contaminants (dust, fibers, particles)
Aging & Oxidation (forms sludge, reducing dielectric strength)
Testing Conditions (gap distance, electrode shape-standard is 2.5 mm gap in most tests)
Industry Standards:
IEC 60156 / ASTM D1816: Standard test method for BDV at 2.5 mm gap.
IEEE C57.106 / IS 335: Recommends minimum 30 kV for in-service oil.
