A transformer tank is a pressure-tight steel enclosure that has to hold oil for decades, withstand vacuum during filling and survive transport and short-circuit forces. Most problems found at site, from weeping welds to distorted covers, can be prevented by testing the tank properly before it leaves the factory. This guide sets out the tests buyers usually specify.
Agree an inspection and test plan first
An inspection and test plan (ITP) lists every check, the acceptance criteria, the record produced and the hold or witness points for the buyer. Agreeing it before fabrication starts avoids disputes later and lets your inspector plan visits.
Leak testing
The most common method is an air pressure test with soap solution brushed on every weld, looking for bubbles. Long seams are often also checked with kerosene and chalk, where kerosene applied to one side shows through as a stain on the chalked side if there is a leak path. Dye penetrant testing is used on critical welds.
Vacuum withstand
Large power transformers are usually filled with oil under vacuum. The tank, cover and stiffeners must withstand that vacuum without permanent distortion. The test holds the specified vacuum for an agreed time while deflection is measured at defined points on the walls and cover, and permanent set is recorded after release.
Pressure withstand
An over-pressure test, at the level set by the transformer designer, checks strength and gasketed joints. Deflection readings are again recorded at defined points.
Insulation resistance
Where core clamps, magnetic shunts or frames must be insulated from the tank, insulation resistance is measured with a megohmmeter to confirm there is no unintended earth path.
Dimensional and coating checks
Overall dimensions, bushing turret positions, flange flatness and fitting locations are checked against the approved drawing. Surface preparation (for example Sa 2½ to ISO 8501-1) and dry film thickness of the paint system are recorded.
Documents to ask for
- Approved drawings and the agreed ITP
- Mill test certificates traced to heat numbers
- Welding procedures and welder qualifications
- Leak, vacuum and pressure test records with deflection readings
- Insulation resistance and coating thickness reports
Rating bands and construction styles are covered on our power transformer tanks page.