1. When measuring the DC resistance of the transformer, due to the large inductance, it must be charged in place to minimize the self-inductance effect. Read the resistance value after the pointer of the DC resistance tester is basically stable to improve the correctness and accuracy.
2. The result of the DC resistance tester should be compared horizontally and vertically, and the influencing factors such as temperature, phase difference, wiring method, tap switch, poor contact, disconnection, etc. should be fully analyzed.
a, temperature influence
The DC resistance is proportional to the temperature. The DC resistance at different oil temperatures is different. The DC resistance measured at different temperatures should be converted to the same temperature for easy comparison. The DC resistance temperature conversion formula is:
(Where R1 and R2 are the resistance values at temperatures t1 and t2 respectively, T is the resistance temperature constant, and the copper wire is 235)
b, the influence of phase difference
The calculation of the percentage difference between the DC resistance lines or the difference between phases can be carried out as follows:
(△Rx is the percentage of DC resistance line difference or phase difference, Rmax is the maximum value of three-wire or three-phase DC resistance measured value, Rmin is the minimum value of three-wire or three-phase DC resistance measured value, Rp is three-wire or three-phase (Average value of measured value of DC resistance)
c. The influence of the wiring method
The three-phase winding Y of the transformer is connected or connected. If the unbalanced value of the three-phase line resistance exceeds the standard, the line resistance must be converted into phase resistance, so that the defective phase can be easily found.
d. Influence of tap changer
In the process of measuring the on-load tapping transformer, the measurement result is incorrect. First, check the DC resistance of all tap positions to see if it is caused by poor contact of the polarity switch or a loose lead.
e. Poor contact
If the conductive rod and the internal lead in the transformer bushing are in poor contact, causing the joint to heat up, infrared imaging can be used to analyze the heat-generating part.
f. The influence of disconnection
If one of the phases of the transformer windings with delta connection is disconnected, the resistance value of the two-phase terminal without disconnection is 1.5 times the normal value, and the resistance value of the disconnected phase terminal is 3 times the normal value.
3. To judge the result of the DC resistance tester, relevant factors and criteria should be comprehensively considered, and the development and change process of the measurement data of the equipment should be analyzed in detail.
4. The judgment of the result of the DC resistance tester attaches importance to the analysis, judgment and verification of the comprehensive method.