How to Read a Transformer Test Report
A test report is only useful if you can tell what was actually tested, what was skipped, and whether the numbers agree with each other. This guide walks through a typical report in the order a reviewer checks it, and flags the gaps that matter most before you accept, energise, or buy a transformer.
Start with the nameplate and the test conditions
Check that the report names the same unit you are looking at: rating, voltage ratio, vector group, cooling type, oil type and serial number. Then check the conditions under which tests were done — oil temperature for resistance measurements, test voltages for insulation tests, and the date. A resistance value without a temperature, or an insulation value without a test voltage, cannot be compared with anything, and a report that omits the unit identity entirely cannot be trusted for that unit at all.
Insulation, ratio and winding resistance: do they agree?
Insulation resistance and polarisation index speak about insulation condition; turns ratio speaks about winding and tap-changer integrity; winding resistance speaks about connections and conductor condition. Read them together: a healthy ratio with a bad resistance on one phase points at a connection, not a winding; a falling insulation trend across all phases points at moisture or ageing, not a single fault. Any single alarming number deserves a repeat test under stated conditions before a verdict, because test leads, temperature and surface leakage all move results.
Oil tests and DGA: trend, not one sample
Oil breakdown voltage, moisture, acidity and dissolved gas analysis are most useful as trends against the same unit's history and against sister units. One DGA sample can suggest a question; it rarely proves a fault on its own. Look for which gases are rising, how fast, and whether the pattern matches a known fault type — then confirm with electrical tests and inspection. A report with no oil data at all is not a clean bill of health; it is an untested area.
The most important line: what was NOT tested
A report that lists only passing tests may still hide the tests that were never run: no short-circuit impedance, no frequency response, no bushing tests, no load or temperature-rise data. Before you rely on a report, list the missing tests explicitly and decide whether each missing test matters for your decision. That missing-test list is often more valuable than the passing numbers, and it is exactly what a pre-purchase review should produce.
If you want a second pair of eyes on your own report or nameplate before you decide, see the remote transformer services — documents only, fixed price, fixed turnaround, no site visit. For the wider testing picture, start with the transformer testing and commissioning guide.