Measurement of the DC resistance of transformers is carried out to shorten the charging time and obtain accurate resistance values. The charging current is generally set at 2-10% of the rated current. For large-capacity transformers, a smaller value is chosen, while for small-capacity transformers, a larger value is selected.
The magnetization method involves connecting the high-voltage and low-voltage windings in series, with the high-voltage winding providing the magnetization. This method enables rapid measurement of the resistance of the low-voltage winding. It is typically used in the measurement of the DC resistance of large-capacity transformers with a three-phase five-column core and a d-connected low-voltage winding.
1) During measurement, it should be noted that large-capacity transformers take a longer charging time, so sufficient charging time should be ensured. The difficulty of measuring the DC resistance of the windings increases as the single-unit capacity of the transformer increases. Especially for transformers with a five-column core and a triangularly connected low-voltage winding for extremely large capacity, the time for the current to stabilize becomes very long when measuring the DC resistance. If the reading is taken before the current reaches stability, the true value of the current cannot be measured.
2) Check if the connection clips are properly contacting; eliminate errors caused by the connections;
3) Temperature deviation impact: When the temperature deviation of the three-phase coils is 1℃, the error of the coils at normal temperature will increase by approximately 0.4%.
4) The no-excited tap changer should ensure that the positioning device reaches the designated position, while the on-load tap changer should be operated electrically.
5) During the test of the DC resistance tester, it is necessary to record the ambient temperature and the temperature of the transformer oil.
6) When there is an electric current I in the DC circuit, there is energy in the magnetic field of the transformer core. When the circuit is disconnected, a high voltage will be generated, which may endanger personal safety and damage the instruments. Therefore, a discharge circuit is needed to gradually reduce the current through the resistance, and the circuit should be disconnected when the current is very small.
7) When measuring the resistance of the transformer, the no-excitation tap changer must not be switched to change the tap position. When the no-excitation tap changer is changed, an arc will occur between the contacts, causing the decomposition of the oil and the formation of combustible gases and carbon, which will deteriorate the quality of the transformer oil and also damage the bridge.
Post time: Jul-28-2026