When a maintenance manager asks 'what tests should I run on my transformer?', the right answer is not a generic list: it is understanding which problem each test rules out. A transformer ages on four fronts — insulation, windings, core and oil — and each test illuminates one of them. Running them without criteria wastes money; running them with diagnostic logic protects the asset.
The turns ratio test (TTR) compares the measured turns ratio against the nameplate, at every tap-changer position. It is the most direct way to detect shorted turns, open windings or a tap changer that fails to make contact. It is usually the first acceptance test after a repair.
Power factor and Tan Delta measure the dielectric losses of the paper-oil insulation — the most sensitive indicator of moisture, contamination and ageing. Insulation resistance, with its polarization index, is the fast screening that decides whether the unit is fit for the elevated-voltage tests.
Dissolved gas analysis (DGA) is the most powerful predictive test, with a big advantage: it is done in service. From the gases the oil dissolves it identifies arcing, partial discharge and overheating. Its value is in the trend: the gas generation rate says more than an isolated value.
Finally, sweep frequency response analysis (SFRA) is the only test that detects mechanical deformation of windings and core — damage a through-fault, transport or earthquake causes without changing the turns ratio or insulation. Capturing its baseline while the unit is healthy enables a confident diagnosis after a future event.
In practice, a full protocol combines several of these tests and reads them together and against the transformer's own history. That interpretation is the difference between a report and a diagnosis.