A transformer is proven in three distinct layers. Routine tests are carried out on every single unit before it leaves the factory. Type tests are performed once on a representative unit to validate a design. Special tests are agreed between purchaser and manufacturer when the application demands them. The categories are defined in IEC 60076-1, and confusing them is the most common source of misunderstanding in a test report.
Routine tests: every unit, no exceptions
Routine tests confirm that this particular transformer was built correctly and matches its rating plate. They are quick, non-destructive and repeatable, which is why they can be applied to the whole production run.
- Winding resistance measurement — detects bad joints and wrong conductor sizes.
- Voltage ratio, and check of vector group and phase displacement.
- Impedance voltage (uk%), short-circuit impedance and load loss.
- No-load loss and no-load current at rated voltage and frequency.
- Separate-source (applied) voltage withstand and induced overvoltage withstand, per IEC 60076-3.
- Insulation resistance and, for liquid-immersed units, a leak and pressure check on the tank.
The loss figures from the routine test are the ones that matter for EcoDesign compliance and for your energy budget, because they are measured on the unit you are actually buying rather than on a sibling.
Type tests: proving the design, once
Type tests validate a design rather than a serial number. They are performed on one unit representative of a design family, and the certificate then covers other units of the same design. Because they are expensive and some are stressful to the unit, they are not repeated for every order.
The two classic type tests are the temperature-rise test to IEC 60076-2, which confirms that oil and winding temperatures stay within limits at rated load, and the lightning impulse voltage test to IEC 60076-3, which confirms the insulation withstands a simulated surge.
When a supplier offers a type-test certificate, check that it belongs to the same design family — same rating range, same core and winding construction — and not merely to the same manufacturer.
Special tests: agreed for the application
Special tests are everything else, ordered when the installation justifies the cost. Short-circuit withstand testing to IEC 60076-5 is the most demanding: the unit is subjected to a real fault current, and it is not always feasible to test large ratings, so withstand is frequently demonstrated by calculation instead.
Other common special tests include sound power level measurement, partial discharge measurement, measurement of zero-sequence impedance, harmonic content of the no-load current, and dissolved gas analysis on the oil as a delivery baseline.
Witnessing the results
A factory acceptance test (FAT) is not a fourth category of test — it is the routine test programme performed with you, or your consultant, present. Nothing extra is measured; you simply see the measurements taken and can query them on the spot.
As a rule of thumb: insist on the full routine test report for every unit, ask for type-test certificates covering the design, and specify special tests only where the application really drives them — a data centre or an offshore substation justifies partial discharge and sound measurement in a way a rural distribution station does not. Protrafo supplies the routine test report with every transformer and can arrange witnessed testing at the production partner's works.
