For single-phase transformers, the no-load test can be carried out using the wiring shown in Figure 1. For three-phase transformers, the two-wattmeter method as shown in Figure 2 and Figure 3 can be used for the no-load test. 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 method in Figure 3 is adopted.



During the no-load test, rated voltage is applied to one side of the transformer (depending on test conditions), and all other windings are left open. Since the single-phase test circuit (Figure 1) can pass high-order harmonic currents, the third harmonic current component of the no-load current of a single-phase transformer is relatively large. After single-phase transformers form a three-phase transformer bank, the single-phase no-load currents of the composed 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 of the transformer in operation 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, so the no-load currents of phases A and C are somewhat 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, basically, the excitation currents of the two side phases are still 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. The results of no-load loss measurement under non-sinusoidal waveform conditions need to be corrected. The correction error in the calculation process is unavoidable.