Based on the calculated values of the winding resistance in the product technical data, select a DC resistance tester or a dedicated bridge (with a precision no less than 0.2 grade) appropriately; the requirements are as follows:
a. Check the instrument and select the test current for the DC resistance tester based on the instrument’s user manual and the size of the winding resistance.
b. All the terminals of each winding must be in an open state; if there is a winding connected in a delta configuration, the open ends of this winding must not be connected.
c) When measuring the resistance of the transformer winding with taps, for on-load voltage-regulating transformers, there is no need to disconnect the measurement circuit. All that is required is to restart the instrument.
d. When changing the tap position of an unexcited voltage regulating transformer, the power supply must be cut off; for a D/D connected transformer, the tap position can be changed phase by phase without cutting off the power supply.
e. When the test sample is a large-capacity five-column transformer and the low voltage is connected in D configuration, the DC resistance measurement should adopt the excitation method to shorten the measurement time; after the test, it is necessary to pay attention to demagnetization.
f. When conducting the measurement, it is essential to wait until the influence of the winding self-inductance effect is minimized before reading the data. Otherwise, significant errors will be caused.
g. After each measurement is completed, the measurement circuit must be completely discharged and confirmed before proceeding to the next step.
Cautionary Notes for DC Resistance Measurement
The current used for testing the DC resistance should be 2% to 10% of the rated current. It should not exceed 20% of the rated current to avoid errors caused by the temperature rise of the windings due to the current.
When there is current I in the direct current circuit, there is energy LI2/2 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 DC resistance tester. Therefore, a discharge circuit is needed to gradually reduce the current from I through the loss on the resistor. When the current is very small, the circuit can be disconnected.
In the complete set of testing instruments, there are discharge circuits, and corresponding discharge indicators as well. When the current drops very low, the circuit is disconnected based on the discharge indicator.
Since there is still current I in the circuit when the power is disconnected, it gradually decays over time. The larger the resistance R in the discharge circuit, the greater the power consumed on R, the shorter the discharge time, and the higher the voltage IR across the discharge circuit terminals. Therefore, the selection of resistance R in the discharge circuit should not only ensure that the discharge voltage is safe, but also minimize the discharge time as much as possible.
When measuring the resistance of a 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.
It is strictly prohibited to use the tap changer to change the tap position of the transformer when there is no oil at the installation site. Because after the transformer oil in the transformer tank is discharged, there will be transformer oil gas around the transformer body, and its flash point is relatively low. When the tap position is changed under direct current, the generated electric arc or spark may ignite the transformer oil gas, causing a fire and damaging the transformer.
Post time: Jul-14-2026