By hvhipot 
The purpose of the DC resistance tester to measure the DC winding resistance of the transformer winding is to check the welding quality of the winding joints and whether there is a short circuit between the windings. Whether the contact position of the voltage tap switch is good, whether the actual position of the tap matches; whether the wire is broken. Stranded wires and windings are broken. When the transformer is overhauled, or after the tap position is changed, or after the outlet fault is short-circuited, it is necessary to measure the DC winding resistance of the winding and the bushing.
The measuring method of the transformer DC resistance tester is as follows:
(1) The current, voltmeter method, also known as the voltage drop method, the principle is to measure the voltage drop on the resistance through direct current in the measured 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, carefully consider how they are connected to the measurement circuit.
(2) Balanced bridge method. This is a method that uses the principle of bridge balance to measure the resistance of the DC winding. Commonly used balanced bridges include single-arm bridges and double-arm bridges. When measuring the DC winding resistance of the transformer, the transformer DC resistance tester should be performed after the transformer is powered off and the high-voltage wire is removed. For large and large-capacity power transformers, the charging time constant τ of the rl series circuit is very large, so that each measurement takes a long time to wait for the current and voltmeter to indicate stable, so the work efficiency is very low, and special instruments (such as constant Current). Replacing the power supply in the test with a power supply can greatly reduce the test time.