By hvhipot 
The DC resistance tester under HV Hipot Electric Co., Ltd. power can help many power workers conduct various power tests more conveniently.
Purpose of Transformer DC Resistance Test:
1. Check the welding quality of the winding joints and whether there is a turn to turn short circuit in the winding;
2. Whether the contacts at each position of the on load tap changer are in good condition, and whether the actual position of the tap changer is consistent with the indicated position;
3. Whether the outgoing line is disconnected;
4. For three-phase power transformers, calculate the unbalance rate of their three-phase DC resistance based on the test results to determine whether it meets the design or relevant standards
Measurement of transformer DC resistance using a DC resistance tester
Measuring DC resistance is an important item in transformer testing. By measuring the DC resistance, the conductive circuit of the equipment can be checked for defects such as poor contact, poor welding, coil faults, and wiring errors. In the actual measurement of small and medium-sized transformers, the DC bridge method is often used. When the resistance value of the tested coil is greater than 1 Ω, a single arm bridge is generally used for measurement. When the resistance value is less than 1 Ω, a double arm bridge is used for measurement. When using a double arm bridge connection, the potential pile head of the bridge should be close to the measured resistance, and the current pile head should be connected to the potential pile head.
Before measurement, the resistance value of the tested coil should be estimated. Place the bridge ratio knob of the DC resistance tester in the appropriate position, short-circuit the untested coil to ground, and then turn on the power switch. charge. After sufficient power supply, press the galvanometer switch and quickly adjust the measuring arm to move the pointer of the current detector towards the middle zero line of the galvanometer scale. Fine tune until the pointer stabilizes and record the value when the resistance stops at zero. At this point, the resistance value of the tested coil is equal to the multiplier multiplied by the resistance value of the measuring arm. After the measurement is completed, first turn on the mirror button, and then release the power switch.
During the measurement process, in addition to strictly following electrical safety regulations and equipment testing procedures, the following points should also be noted:
1) When the coil temperature is stable, the temperature difference between the upper and lower parts of the transformer oil tank should not exceed 3 ℃;
2) Due to the inductance of the transformer coil, the charging current is unstable during measurement. It is necessary to count after the current stabilizes and shorten the charging time if necessary;
3) The contact resistance of conductors in the test circuit should be minimized as much as possible. The tap joints of transformers in operation are often affected by impurities such as oil film, resulting in poor contact. Generally, it is necessary to switch multiple times before measuring to avoid misjudgment;
4) When measuring the low voltage side of a transformer, all personnel should maintain a safe distance from the high voltage side;
5) During the testing process, the test cable clamp must be securely fixed. One of the large transformers has a high current during testing, and if the wiring is not reliable enough, it is easy to generate arcs; Secondly, if the fixture falls off during the testing process, it is easy to damage the instrument (high voltage).
6) The transformer DC resistance tester should avoid strong vibration, direct sunlight, and magnetic fields during transportation, storage, and operation. When storing and preserving instruments, attention should be paid to the ambient temperature and humidity. It should be placed in a dry, ventilated, and non corrosive gas environment. Moisture, rain, and sunlight are not allowed to affect. Handle with care when using.