Transformer Load Loss and Short-Circuit Impedance Measurement Procedures and Methods

Transformer load loss and short-circuit impedance measurement are routine tests for transformers. The purpose of the load test conducted by the manufacturer is to measure the load loss and short-circuit impedance of the transformer. It is to determine whether these two important performance parameters meet the requirements of standards and technical agreements, as well as to check for any defects in the transformer windings.
The load loss consists of the following components
The DC resistance loss R in the winding is the main part of the load loss. In addition, there is also additional loss caused by the leakage magnetic field generated by the winding current, which includes: eddy current loss in the winding conductors due to the leakage magnetic field.
The imbalance current loss in the parallel conductors of the winding due to the leakage magnetic field, the eddy current loss caused by the leakage magnetic field in the core, and the increase in loss due to the uneven distribution of magnetic flux in the core caused by the leakage magnetic field.
The leakage magnetic field losses occur in the oil tank and within the oil tank shielding. The leakage magnetic field losses also occur in the structural components such as the busbar and the pull plate.

The standard stipulates: The loss must not exceed +10%. For transformers that are about to undergo the temperature rise test, the load loss and short-circuit impedance should also be measured at the larger loss tap position under the rated capacity to provide data for the temperature rise test.
According to the requirements of the GB1094.1-1996 standard, the measurement of the load loss and short-circuit impedance of the transformer should be conducted on the main tap. For transformers with a tap range exceeding 5%, the short-circuit impedance should be measured on the main tap as well as the two extreme taps. When conducting the load test on a power transformer, the common connection method is to input at the high-voltage side and short-circuit at the low-voltage side. At the rated frequency, the winding to which the voltage is applied is measured when the current reaches the rated value.
The load test of the transformer should apply the rated load current of the transformer. When restricted by the test equipment, a current not less than 50% of the rated current can be applied. The measured load loss value should be multiplied by the square of the ratio of the test current to the rated current, and then corrected to the reference temperature.
The load test of a three-phase transformer requires measuring the voltages and currents of all three phases, and using the average value of the three phases as the reference.
When measuring load loss and short-circuit impedance, it is important to correctly select the capacity of the test power supply. During the measurement, the power factor meter used (requiring a high-precision low-power-factor meter) should be employed.
The measurement results of the transformer’s load loss need to be converted to the reference temperature of 75°C.