For this example with a fault between T and B, the measured impedance from the Tpoint to the fault will be increased by a factor defined as the sum of the currentsfrom T point to the fault divided by the IED current. For the IED at C, theimpedance on the high voltage side U1 has to be transferred to the measuringvoltage level by the transformer ratio.Another complication that might occur depending on the topology is that thecurrent from one end can have a reverse direction for fault on the protected line.For example, for faults at T the current from B might go in reverse direction fromB to C depending on the system parameters (see the dotted line in figure 128),given that the distance protection in B to T will measure wrong direction.In three-end application, depending on the source impedance behind the IEDs, theimpedances of the protected object and the fault location, it might be necessary toaccept zone 2 trip in one end or sequential trip in one end.Generally for this type of application it is difficult to select settings of zone 1 thatboth gives overlapping of the zones with enough sensitivity without interferencewith other zone 1 settings, that is, without selectivity conflicts. Careful faultcalculations are necessary to determine suitable settings and selection of properscheme communication.8.4.3 Setting guidelines SEMOD154496-1 v28.4.3.1 General SEMOD154469-4 v7The settings for Full-scheme distance protection, mho characteristic function(ZMHPDIS) are done in primary values. The instrument transformer ratio that hasbeen set for the analog input card is used to automatically convert the measuredsecondary input signals to primary values used in ZMHPDIS.The following basics should be considered, depending on application, when doingthe setting calculations:• Errors introduced by current and voltage instrument transformers, particularlyunder transient conditions.• Inaccuracies in the line zero-sequence impedance data, and their effect on thecalculated value of the earth-return compensation factor.• The effect of infeed between the IED and the fault location, including theinfluence of different Z0/Z1 ratios of the various sources.• The phase impedance of non transposed lines is not identical for all faultloops. The difference between the impedances for different phase-to-earthloops can be as large as 5-10% of the total line impedance.• The effect of a load transfer between the terminals of the protected line, thefault resistance is considerable and the effect must be recognized.• Zero-sequence mutual coupling from parallel lines.1MRK 506 369-UEN B Section 8Impedance protectionLine distance protection REL670 2.2 IEC 249Application manual
