For single-phase transformers, the no-load test can be conducted using the wiring shown in Figure 1. For three-phase transformers, the two-wattmeter method shown in Figures 2 and 3 can be used for no-load tests. Figure 2 is the direct measurement method, suitable for transformers with relatively small rated voltage and current, where voltage and current meters can be used for direct measurement. When the rated voltage and current of the transformer are large, indirect measurement must be carried out with the help of voltage transformers and current transformers, in which case the wiring in Figure 3 is adopted.



During the no-load test, the rated voltage is applied to one side of the transformer (depending on the test conditions), and all other windings are left open. Since the single-phase test circuit (Figure 1) allows high-order harmonic currents to pass, the third harmonic component of the no-load current of a single-phase transformer is relatively large. After single-phase transformers are formed into a three-phase transformer bank, the single-phase no-load currents of the three-phase transformer are basically the same. Regardless of whether the windings are connected in Y or D, the third harmonic current cannot pass through the circuit, so the no-load current during operation of the transformer differs from the no-load current during single-phase no-load tests at the transformer test station.



The no-load current of a three-phase transformer is asymmetric due to the asymmetry of the transformer core. The cores of medium and small capacity three-phase transformers are usually three-limb cores, and the three-phase magnetic circuits are asymmetric, with the no-load currents of phases A and C being slightly larger than that of phase B.



For large-capacity transformers with three-phase five-limb cores, the core has side yokes, and part of the magnetic flux of the side phases can pass through the side yokes, so the difference in three-phase excitation currents is smaller than that of three-limb cores. However, the excitation currents of the two side phases are still basically larger than that of the middle phase. In addition, in five-limb cores, the third harmonic can form a circuit.



The degree of distortion of the test voltage waveform has an impact on the measurement of no-load current and no-load loss. When the waveform is not sinusoidal, the results of no-load loss measurement need to be corrected for the waveform. Errors in correction during the calculation process are unavoidable.