The purpose of measuring the DC resistance of the transformer windings is to check the welding quality of the winding joints and whether the windings are short-circuited; whether the contacts of the voltage tap switch are in good contact and whether the actual positions of the taps are consistent; whether the lead wires are broken, and more Whether the stranded wires and windings have broken strands, etc. When the transformer is overhauled or after the tap position is changed, or after the outlet is faulty and short-circuited, it is necessary to measure the DC resistance of the winding together with the bushing. The measurement method is as follows.

(1) Current and voltmeter method. Also known as the voltage drop method, its principle is to pass a direct current through the measured resistance, measure the voltage drop on the resistance, and calculate the measured resistance value according to Ohm's law. Since the internal resistance of the ammeter and voltmeter will affect the measurement results, the way they are connected to the measurement circuit should be carefully considered.

(2) Balanced bridge method. It is a method that uses the principle of bridge balance to measure DC resistance. Commonly used balanced bridges include single-arm and double-arm bridges. When measuring the DC resistance of the transformer, it should be performed after the transformer is powered off and the high-voltage lead is removed. For large and large-capacity power transformers, because the charging time constant τ of the RL series circuit is very large, it takes a long time for each measurement to wait for the current and voltmeter to indicate stable, so the work efficiency is very low, and special instruments (such as constant current Power supply) to replace the power supply in the test, which can greatly shorten the test time. The standard for measuring the DC resistance of transformer coils is: For transformers above 1600kVA, the difference between the winding resistances of the phases should not be greater than 2% of the three-phase average value, and for windings without a neutral point lead wire, the difference between the lines should not be greater than three-phase. 1% of the average value. For transformers of 1600kVA and below, the phase-to-phase difference is generally not more than 4% of the three-phase average value, and the line-to-line difference is generally not more than 2% of the three-phase average value. The change should not be greater than 2%.