Winding resistance measurement applies direct current to the winding and reads the voltage drop by the four-wire method, which removes the resistance of the test leads. Values range from a few milliohms in low-voltage windings to tens of ohms in high-voltage ones.
It reveals shorted turns, cold solder joints, loose connections and worn tap changer contacts. Comparison is made between phases, where the imbalance should not exceed 2 %, and against the equipment history after correcting to a reference temperature, since copper changes by 0.4 % per degree.
The practical difficulty is winding inductance: the current has to settle before reading, which on a large power transformer can take several minutes, and the core must be demagnetized afterwards so that subsequent tests are not skewed.