Three-phase transformers such as transformer 33kv to 400v, 400 33kv transformer, 400v transformer, etc., here to the common y, D11 wiring as an example. Under this wiring, the current introduced into the differential protection of each phase is the difference between the currents of the two transformer windings, and the excitation inrush current is naturally the difference between the excitation inrush currents of the two windings.
We know that there is a 120° phase difference between the three-phase voltages, which leads to the three-phase inrush current is not the same. In any case, as long as there is no-load input, there will be at least two phases with different degrees of inrush current. With this wiring, there will be one phase where the inrush current is no longer biased to only one side of the time axis, but becomes symmetrical. From the time axis, the other two phases still have asymmetrical inrush currents. In general, the symmetrical inflow has relatively small values. The asymmetric surge contains a large number of non-periodic components, whereas there are no non-periodic components in the symmetric surge. In a three-phase inrush, there will be a single phase or two phases with a relatively small second harmonic content, but at least one phase with a large second harmonic content. The waveform of the excitation inrush is still discontinuous, but the degree of discontinuity is significantly smaller, with the symmetrical inrush having the smallest discontinuity angle. The phase difference between the forward maximum and the reverse maximum of the symmetrical inrush current is 120°, while the steady state current has a wave width of 180°.
When a single-phase transformer is closed at no load, whether or not an inrush current is generated and the magnitude of the inrush current are very much related to the closing angle. When the closing angle α is equal to 0 or π, the excitation inrush current is the largest. The waveform of single-phase transformer excitation inrush current will be completely biased to one side of the time axis and will appear intermittent. Generally speaking, the larger the excitation inrush current is, the smaller the discontinuity angle is. And single-phase transformer excitation current contains a large number of non-periodic components, the smaller the discontinuity angle, the larger the non-periodic component. It also contains many high harmonic components, of which the second harmonic is dominated. And the smaller the interruption angle, the smaller the second harmonic.
Now you have a certain understanding of the characteristics of three-phase transformer and single-phase transformer excitation inrush.