To avoid induced high voltage damaging instruments, selection recommendations for distribution transformers



By measuring DC resistance, the following hidden defects can be detected: Inter-turn short circuit in windings: Short circuit will cause the resistance value of that phase to be significantly lower. Poor contact of tap changer: If the resistance value of a certain tap is too high or unstable, it usually means oxidation of contacts, insufficient pressure, or burning. Poor soldering of leads: Virtual soldering between windings and bushing leads will cause an increase in resistance. Broken strands: If any strand of multi-strand parallel wound conductors is broken, the resistance will increase. Three-phase unbalance: Regulations usually require that the difference between three-phase resistances should not exceed 2% (above 1.6MVA) or 4% (below 1.6MVA). If it exceeds the standard, it is considered abnormal. 5. Operation precautions Discharge safety: Since the test object (especially transformers) has large inductance, a large amount of magnetic energy is stored after the test. It is necessary to first allow the instrument to automatically discharge or manually fully discharge, and confirm that there is no residual voltage before removing the test leads. Otherwise, high-voltage arcs may occur, endangering personal and equipment safety. Temperature conversion: Resistance values are significantly affected by temperature. During measurement, it is necessary to record the oil temperature or winding temperature, and convert the results to a standard temperature (usually 20°C or 75°C) for comparison. Formula: