IEC TR 60909-4-2021 Short-circuit currents in three-phase AC systems – Part 4: Examples for the calculation of short-circuit currents.
If the magnetic coupling between the three coils without or with iron core is small, the zerosequence impedance Z (0) is approximately equal to the positive-sequence impedance Z (1 ) .
When calculating short-circuit currents in high-voltage systems, it is generally sufficient to use the reactance only.
4.3 Transformers
4.3.1 General
Unit transformers of power station units are also dealt with in 4.4.
Network transformers have two, or three or even more three-phase windings. Figure 3 gives an example for the positive-sequence [Figure 3 b)] and the zero-sequence system impedances [Figure 3 c)] of a two-winding transformer with the vector group YNd5 [Figure 3 a)].
In the case of three-winding transformers (examples are given in Table 3 of IEC TR 60909-2:2008), it is necessary to measure three different impedances and then to calculate the three impedances of the equivalent circuit in the positive-sequence or the zero-sequence system of the transformer (see 6.3.2 of IEC 60909-0:201 6 and the example in 4.3.2 of this document).
Table 1 gives examples for the equivalent circuits in the positive-sequence and the zerosequence system of two- and three-winding transformers with different earthing conditions on the HV- and the LV-side. The impedances of Table 1 are related to side A, which may be the HV-side or the LV-side of the transformer.
As shown in Table 2, transformers with the vector group Yy should not be used in low-voltage systems with low-impedance earthing on the LV-side (TN-network), because Z (0) may be very high, so that short-circuit protection may fail. For feeding TN-networks, transformers of no. 2 or 3 in Table 1 should be used. Transformers with the vector group YNyn,d are typical in high-voltage networks, with neutral point earthing normally only on one side (A or B). The examples no. 4b and 6 of Table 1 show that the zero-sequence system of both networks are coupled, if both the neutral points A and B are earthed (earthing switch ES in case no. 4b closed). In these cases, additional considerations are necessary, especially if the transformation ratio is high, to find out if this coupling is admissible. Case no. 5 of Table 1 gives an example how to avoid this coupling in the zero- sequence system. Case no. 9 of Table 1 gives a further example to avoid the coupling in the zero-sequence system if two parallel transformers at the same place or at different places are present.IEC TR 60909-4 pdf download.