The purpose of measuring the DC resistance of the transformer windings is: the high-current generator checks the welding quality of the winding joints and whether the windings are short-circuited; whether the contacts of the voltage tap switch are good and the actual sign of the tap; whether the lead wire is broken , Whether the strands of the multi-strand wires and windings are broken. When the transformer is overhauled or after changing the position of the tap, or after an outlet fault is short-circuited, it is necessary to measure the winding and sleeve resistance. 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. The internal resistance of the pressure gauge will affect the measurement results, so 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. Measure the DC resistance of the transformer when it is switched on and remove the high-voltage lead. For large and large-capacity power transformers, due to the large charging time constant τ of the RL series circuit, it takes a long time for each measurement to wait for the current and voltmeters to indicate that the working efficiency is very low. Special instruments (such as constant current power supplies) are often used. Instead of the power supply in the test, this can greatly shorten the test time. The standard for measuring the DC resistance of the transformer coil is: For 160, the difference between the winding resistances of the phases should not be greater than 2% of the three-phase average value, and for the windings without a neutral point lead wire, the difference between the lines should not be greater than the three-phase average. 1% of the value. For 1600kVA and below, the difference is generally not more than 4% of the three-phase average value, and the difference between lines is generally not more than 2% of the three-phase average value. Compared with the previous measured value at the same position, the change should not be more than 2 %.