The purpose, method, and precautions of transformer DC resistance testing

Purpose of testing transformer DC resistance:

1. Check the welding quality of the winding joints and whether there is a turn to turn short circuit in the winding;

2. Check whether the contact between each position of the on-load tap changer is good, 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 imbalance rate of their three-phase DC resistance based on the test results to determine whether it meets the design or relevant standards

Measurement of DC resistance of transformers 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 measured coil is greater than 1 Ω, it is generally measured using a single arm bridge. When the resistance value is less than 1 Ω, it is measured using a double arm bridge. When using a double arm bridge for wiring, 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 coil to be tested should be estimated, the bridge ratio knob of the DC resistance tester should be placed in an appropriate position, the untested coil should be short circuited to ground, and then the power switch should be turned on. After turning on the charging and sufficient power supply, press the galvanometer switch to quickly adjust the measuring arm, so that the pointer of the current detector moves towards the zero line in the middle of the galvanometer scale, and fine tune it until the pointer stabilizes and records the value of the resistance when it stops at zero. At this point, the resistance value of the measured coil=multiplier × Measure the resistance value of the arm. After the measurement is completed, first turn on the galvanometer button and then release the power switch.

During the measurement process, in addition to strictly adhering to electrical safety regulations and equipment testing regulations, the following points should also be noted:

1. When the coil temperature stabilizes, the temperature difference between the upper and lower parts of the transformer oil tank shall not exceed 3 ℃;

2. Due to the inductance of the transformer coil, the charging current during measurement is unstable. It is necessary to count after the current stabilizes, and if necessary, shorten the charging time;

3. The contact resistance of conductors in the test circuit should be minimized as much as possible. The tap joints of transformers during operation are often affected by dirt such as oil film, resulting in poor contact. Generally, it is necessary to switch several times before measuring to avoid misjudgment;

4. When measuring the low voltage side of the transformer, all personnel should maintain a safe distance from the high voltage side;

5. During the testing process, the test wire clamp must be firmly fixed. One of the large transformers applied a high current during the test, and if the wiring is not reliable enough, it is easy to generate an arc; Secondly, if the clamp falls off during the testing process, it is easy to damage the instrument (high voltage).

6. During transportation, storage, and operation, the transformer DC resistance tester should avoid strong vibrations, direct sunlight, and magnetic fields. When storing and keeping instruments, attention should be paid to environmental temperature and humidity. It should be placed in a dry, ventilated, and non corrosive gas free place. Do not allow exposure to moisture, rain, or sunlight. Handle with care when using.

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Kvtester Electronics Technology Co.,Ltd. is a high-tech enterprise specializing in power testing, testing, research and development, production, and sales of testing equipment. It has been engaged in the electrical testing industry for many years, and its products are of high quality. We welcome customers to come and purchase. Service hotline: 0086-27-81778799, to learn more, visit the official website: www.kvtester.com