Due to the triangular connection of the low-voltage side windings of the large transformers, the transformer resistance meter manufacturers have mutual inductance in the parallel windings and long charging time. Not only the test data lacks credibility, but also the judgment time of the equipment status and unstable test data are increased.

There are many ways to test DC resistance in the field. The main methods include DC voltage drop method and bridge method. The principle of the DC voltage drop method is to pass a DC current through the winding to be tested and produce a voltage drop on the winding resistance. The current through the winding and the voltage drop across the winding can be measured. According to Ohm's law, the DC resistance of the winding can be calculated. The principle of the electric bridge is to use the principle of bridge balance to measure DC resistance, using double-arm bridge and double-arm bridge, in addition, there are high-voltage charging low-voltage measurement method, magnetic flux pump method, second-order oscillation method, dynamic measurement method, short circuit demagnetization method and Constant current source method.

In order to ensure the accuracy and reliability of the measurement data, the transformer direct resistance meter can eliminate interference according to the following steps and avoid unnecessary losses caused by misjudgment when imbalance occurs in the DC resistance measurement process.

(1) First check the test lead clamp, the elasticity and contact surface are good, the test lead is not broken, and the influence of the test lead and the test instrument is excluded.

(2) There is enough lifting capacity to eliminate the influence of residual charge or induced potential.

(3) If the unbalance rate of the multi-switch tap changer is not significantly reduced, and the unbalance rate of the DC resistance of each gear is almost the same, the impact of poor contact of the tap changer can be basically eliminated.

(4) Perform temperature conversion and compare with the previous test.

(5) According to the factory data and the handover report, determine whether the DC resistance unbalance rate exceeds the standard due to the difference in lead resistance during the manufacturing process.

After these steps are completed, if the DC resistance still does not meet the specifications, the transformer has the following defects: poor connection of the secondary winding or poor welding quality; internal failure of the secondary tap switch; broken primary winding or lead wire; primary winding layer and turns Short circuit.