By hvhipot 
The purpose of measuring the DC resistance of transformer windings is to check the welding quality of winding joints and whether there is a short circuit between turns in the winding; Whether the contact of each position of the voltage tap changer is good and whether the actual position of the tap matches; Is there any breakage in the lead wire, and is there any breakage in the winding of multiple wires. During major repairs or after changing the position of the tap changer, or after an outlet fault short circuit, it is necessary to measure the DC resistance of the winding and bushing together. The measurement method is as follows.
(1) Current and voltage meter method. Also known as the voltage drop method, its principle is to apply a direct current to the measured resistor, measure the voltage drop across the resistor, and calculate the measured resistance value based on Ohm’s law. Due to the impact of the internal resistance of ammeters and voltmeters on measurement results, the way they are connected to the measurement circuit should be carefully considered.
(2) Balanced bridge method. It is a method of measuring DC resistance using the principle of bridge balance, and commonly used balance bridges include single arm and double arm bridges. When measuring the DC resistance of a transformer, it should be done after the transformer is powered off and the high-voltage leads are removed. For large capacity power transformers, due to the large charging time constant τ of the RL series circuit, each measurement requires a long time to wait for the current and voltage meter to indicate stability, resulting in low work efficiency. Special instruments (such as constant current power supplies) are often used to replace the power supply in the test, which can greatly shorten the testing time. The standard for measuring the DC resistance of transformer coils is: for transformers above 1600kVA, the difference in resistance between each phase winding should not exceed 2% of the average value of the three phases. For windings without neutral point leads, the difference between lines should not exceed 1% of the average value of the three phases. For transformers below 1600kVA, the difference between phases should generally not exceed 4% of the average value of the three phases, and the difference between lines should generally not exceed 2% of the average value of the three phases. Compared with the values measured in the same part before, the change should not exceed 2%.