How to measure the DC resistance of a transformer coil

　　The purpose of measuring the DC resistance of transformer windings is to check the welding quality of winding joints and whether there is short circuit between turns of windings; whether the contact of each position of voltage tap changer is good and whether the actual position of tap is consistent; whether the lead wire is broken, whether the multi-strand wire winding is broken, etc. When the transformer is overhauled or the tap position is changed, or after the outlet fault short circuit, it is necessary to measure the DC resistance of the winding together with the bushing. The measurement method is as follows.

 　(1) Amperemeter and voltmeter method. Also known as the voltage drop method, its principle is to pass a direct current through the resistor to be measured, measure the voltage drop across the resistor, and calculate the 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 a transformer, the transformer should be powered off and the high-voltage lead removed. For large and large-capacity power transformers, since the charging time constant τ of the RL series circuit is very large, each measurement requires a long time to wait for the current and voltage meter indications to stabilize, so the work efficiency is very low. Special instruments (such as constant current power supply) are often used to replace the power supply under test, which can greatly shorten the test time. The standard for measuring the DC resistance of a transformer coil is: for transformers above 1600kVA, the difference between the resistances of each phase winding should not be greater than 2% of the three-phase average value. For windings without neutral point lead wires, the difference between the lines should not be greater than 1% of the three-phase average value. For transformers of 1600kVA and below, the difference between the phases is generally not greater than 4% of the three-phase average value, and the difference between the lines is generally not greater than 2% of the three-phase average value. Compared with the value measured at the same location before, the change should not be greater than 2%