The DC resistance tester under UHV power can help many power workers conduct various power tests more conveniently.

DC resistance measurement is used to check the quality of winding welding and inter turn short circuits; Whether the position of the tap changer is good and whether the actual position is consistent with the indication; Whether the lead wire is broken or loose; The winding of the parallel winding has no broken strands or other phenomena. The measurement of DC resistance is not only an essential inspection item after transformer overhaul, pre testing, and changes in tap changer position, but also an important inspection item after faults.

Generally speaking, measuring the DC resistance of transformer windings and high-power inductive devices using traditional methods such as bridge method and voltage drop method is a time-consuming and laborious task. In order to change this situation, shorten measurement time, and reduce the workload of testers, ultra-high voltage power has developed a fast DC resistance 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 measurement and testing of DC resistance must be handled with care to minimize measurement errors as much as possible. According to regulations, for transformers above 160kVA, the phase resistance difference is generally less than 2% of the average three-phase value, and the line to line resistance difference is generally less than 1% of the average three-phase value. For transformers of 160kVA and below, the phase resistance difference is generally less than 4% of the three-phase average value, and the line to line resistance difference is generally less than 2% of the three-phase average value. The measured phase difference should not exceed 2% compared to the measured phase difference of the corresponding part.

When the measured resistance of the DC resistor exceeds the limit value:

(1) Firstly, consider whether there are measurement errors (such as whether there are external leads, whether the probes are too long or too thin, whether the contact is good, and whether the battery voltage inside the bridge is insufficient).

(2) The resistance value of DC resistors is greatly affected by temperature, so it must be converted to the same temperature (usually 20 ℃, R20=(T+20)/(T+T), T copper=235) for comparison, usually based on the above layer oil temperature.

(3) The currently used three-phase distribution transformers adopt a Y-shaped connection for the high-voltage winding. When the resistance exceeds the limit, the following equations [RA=(RAB+rac rbc)/2, RB=(RAB+rac rac)/2, RC (RBC+rac rab)/2] can also be used to identify the defective phase.

(4) Poor contact of tap changers is common, resulting in high resistance values, such as switch cleaning, plating detachment, insufficient spring pressure, uneven stress, and carbon deposition on overvoltage contacts, all of which can lead to high resistance values. At this point, the tap changer cover should be opened and rotated a few times to generally eliminate it.

After the above inspection and processing, it still exceeds the limit and there is an internal fault, which is likely due to virtual welding, virtual welding, wire breakage, or layer short circuit of the winding and lead, or winding burnout.