IEC TR 60909-4:2021 pdf download.Short-circuit currents in three-phase AC systems
Formulae for the calculation of the positive-sequence and the zero-sequence system impedances of overhead lines with one or two parallel circuits (double circuit line) and without or with one or two earth wires are given in IEC TR 60909-2. The negative-sequence impedance is equal to the positive-sequence impedance assuming transposed lines and cross-bonded cables, respectively. The measurements to find the positive-sequence and the zero-sequence impedances of cables with sheath, shielding and armouring are similar to those given in Figure 1. Examples are given in IEC TR 60909-2. In the case of the zero-sequence impedance, the earthing of the sheath or the shielding or the armouring is important as well as the number of parallel cables. In the case of low-voltage four-core cables, the cross-section of the earthed core has an influence on the zero-sequence impedance .
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 60909-0:2016 and the example in 4.3.2 of this document).
Table 1 gives examples for the equivalent circuits in the positive-sequence and the zero-sequence 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.
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.