10-4 D30 Line Distance Protection System GE Multilin10.2 DISTANCE ELEMENTS (STEPPED DISTANCE SCHEME) 10 APPLICATION OF SETTINGS10Figure 10–1: UNDERSTANDING SYSTEM HOMOGENEITYIf , the zero-sequence network is more homogenous (the zero-sequence current better approximatesthe fault current), and zero-sequence polarizing should be selected. Otherwise, negative-sequence is a better polarizingsignal.The calculated angles reflect errors between the ideal and applied polarizing angles. Therefore, after selecting the polar-izing current, the corresponding angle should be chosen as the non-homogeneity correction angle. For example, calcula-tions yielding 0 = 5° and 2 = –1° should result in selecting the negative-sequence for polarization, and the value of –1°for non-homogeneity correction.Quite often the non-homogeneity correction angle is used to apply extra security rather than correct the angle between therelay and the fault currents. For extra security the angle should be set to negative values.Ability to select an optimum polarizing signal and to correct for non-homogeneity should be approached with care. The bestpolarizing signal and the correcting angle both depend on system parameters and intended reach of the zone. If the systemconfiguration is static, the selection is straightforward.However, if the system configuration may change significantly, all the relevant system topologies shall be considered forcalculations. Either a compromise choice is made, or different settings are applied via switchable setting groups dependingon system conditions. In highly non-homogenous and dynamic systems where the correction for non-homogeneity is notfeasible, conservative reach (reduced) and/or correction angle (negative) settings should be considered.c) GROUND DISTANCE ZONE 1The zone 1 reach must be set so that nominally instantaneous operation does not extend beyond the end of the protectedline. However this may be somewhat more complicated than for the phase elements, because of zero sequence mutualinduction with an adjacent parallel line, possibly carried on the same tower, which can be out of service and grounded atmultiple points. A fault beyond 100% of the protected line may cause overreach unless the reach is reduced significantly,sometimes as low as 65% of the line length. If the line being protected does not have a significant interaction with an adja-cent circuit, then the typical 80% setting may be used. If there is significant mutual coupling between the parallel lines, thenthe mutual compensation feature of the ground distance elements can be used instead of a drastic reduction in the reach.However, even in this case, there is more uncertainty as compared with the phase distance elements because the zero-sequence impedance of the line and thus the zero-sequence-compensating factors may vary significantly due to weatherand other conditions.842742A1.CDRdZdZ(1 – d)Z(1 – d)ZZZ0A1A0L1L0L1LZZ0B1BIIII I0220FIIFFDEPENDING ON SYSTEMPARAMETERS, I0 OR I2 MAYBE ABETTER APPROXIMATION OF THEFAULT CURRENT ANGLEabs 0 abs 2
