The measurement of DC resistance is used to check the welding quality of the windings and whether there are inter-turn short circuits in the windings; to determine whether the position of the tap changer is correct and whether the actual position matches the indication; to check for any breakage or loosening of the leads; and to ensure that the strands of the winding are not broken. The measurement of DC resistance is an indispensable test item for transformers during major overhauls, pre-tests, and changes in tap changer positions, and is also an important inspection item after faults occur.
Under normal circumstances, using traditional methods (such as the bridge method and the voltage drop method) to measure the DC resistance of transformer windings and high-power inductive equipment is a time-consuming and labor-intensive task. To change this situation and shorten the measurement time as well as reduce the workload of the testers, Wuhan Guodian Xigao has developed a DC resistance rapid tester (hereinafter referred to as the “DC Resistance Tester”). It is an ideal device for measuring the DC resistance of transformer windings and high-power inductive equipment.
Therefore, the DC resistance measurement test must be carried out carefully, minimizing measurement errors as much as possible. The regulations stipulate that for transformers with a capacity of 160 kVA and above, the difference between phase-to-phase resistances should generally not exceed 2% of the average value of the three phases, and the difference between line-to-line resistances should generally not exceed 1% of the average value of the three phases; for transformers with a capacity of 160 kVA and below, the difference between phase-to-phase resistances should generally not exceed 4% of the average value of the three phases, and the difference between line-to-line resistances should generally not exceed 2% of the average value of the three phases; the difference between the measured phase-to-phase resistances should not change by more than 2% compared with the previous measured phase-to-phase resistances at the corresponding positions.
When the measured DC resistance exceeds the standard:
① First, consider whether there is measurement error (such as whether the external leads are connected, whether the test leads are too long or too thin, whether the contact is good, whether the battery voltage of the bridge is sufficient, etc.).
② The DC resistance value is greatly affected by temperature, so it must be converted to the same temperature (generally with 20℃ as the reference, R20=(T+20)/(T+t), T copper = 235) for comparison, and generally the upper layer oil temperature is used as the basis.
③ For three-phase distribution transformers currently in use, the high-voltage winding adopts Y connection, when the DC resistance exceeds the standard, the following formula can be used [RA=(RAB+RAC-RBC)/2, RB=(RAB+RBC-RAC)/2, RC(RBC+RAC-RAB)/2] to find the defective phase.
④ Poor contact of the tap changer can cause the resistance to be higher. Common causes include unclean tap changer, fallen electroplating, insufficient spring pressure, uneven force, and carbon deposits on the contacts during overvoltage. All of these will cause the resistance to be higher. At this time, the tap changer cover should be opened and rotated several times to eliminate it.
After the above checks and treatments, if the resistance still exceeds the standard, it indicates an internal fault. It is very likely that there is a false soldering, detachment, breakage of the winding and lead, inter-turn short circuit, or winding burnout. If it cannot be handled on site, it needs to be sent to the maintenance room for core lifting and major overhaul.
Post time: Aug-25-2026