Because the large-scale transformer low-voltage side winding "â–³"-shaped connection, parallel winding mutual inductance, long charging time, test data instability, not only the lack of credibility of the test data, but also increased the difficulty of judging the condition of the equipment.
There are many ways to test DC resistance in the field. The mainstream methods include DC voltage drop method and bridge method. The principle of the DC voltage drop method is to pass a direct current in the measured winding to generate a voltage drop across the resistance of the winding. The current through the winding and the voltage drop across the winding are measured. According to Ohm's law, the DC resistance of the winding can be calculated. . The bridge principle measures the DC resistance through the principle of bridge balance. There are two kinds of dual-arm bridges and one-arm bridges. 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.
Domestic manufacturers produce various types of fast DC resistance testers, each with their own advantages and disadvantages. Some problems have also arisen in use. In Guangdong, there have been many cases in which transformers under normal conditions have been subjected to unnecessary checks due to the test that the unbalanced DC resistance of the low-voltage side has exceeded the specified range.
In order to ensure that the measurement data is accurate and reliable, if any imbalance is found during the measurement of the DC resistance, the following steps can be used to eliminate the interference first, so as to avoid unnecessary losses caused by misjudgment.
1) First check the test lead clips, the elasticity and the contact surface are all good, the test lead is not broken, exclude test leads, test instruments.
2) Fully stand and discharge, excluding the influence of residual charge or induced potential.
3) When the tap changer is switched multiple times, if the unbalance ratio is not significantly reduced, and the DC resistance unbalance ratio of each gear is similar, the influence of bad contact of the tap changer can be basically eliminated.
4) Perform temperature conversion to compare with the previous test.
5) According to the data of the factory and handover report, it is determined whether the unbalanced DC resistance exceeds the standard due to the difference in lead resistance during manufacture.
After completing these steps, if the DC resistance still exceeds the limit, the transformer may have the following defects: 1 winding is not tight or welding quality is poor; 2 tap switch internal fault; 3 winding or lead wire breaks; 4 winding layer, winding Short circuit.
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