The apparent impedance is considered as an impedance loop with resistance R andreactance X.The formula given in equation 24 is only valid for radial feeder application without load.When load is considered in the case of single phase-to-ground fault, conventional distanceprotection might overreach at exporting end and underreach at importing end. The IEDhas an adaptive load compensation which increases the security in such applications.The directional evaluations are performed simultaneously in both forward and reversedirections, and in all six fault loops. Positive sequence voltage and a phase locked positivesequence memory voltage are used as a reference. This ensures unlimited directionalsensitivity for faults close to the IED point.6.1.6.7 CVT filterIn HHV networks, use of CVT is very common due to their reduced cost and size. But dueto the capacitance effect the voltage will not change instantaneously and this causes theCVT transients into post fault voltage signals. Many times, this causes the relays to overreach. ZQMPDIS (21) handles the CVT transients internally and if setting CVTFltr isEnabled then input voltage profile is corrected accordingly.CVT filter is designed to reduce the CVT transients. Use CvtFltr setting to enable ordisable the CVT filter. The filter detects the fault and switches the filter coefficient to givecorrect voltage values to the measurement loops. When CvtFltr is Enabled all the loopsand all the zones will have the corrected voltage value. In applications where the line hasa high SIR and the voltage signals are obtained by CVT, it is highly recommended to usethe CVT Filter.6.1.6.8 Simplified logic diagramsDistance protection zonesThe design of the distance protection zones are presented for all measuring loops: phase-to-ground as well as phase-to-phase.Phase-to-ground related signals are designated by AG, BG and CG. The phase-to-phasesignals are designated by AB, BC and CA.Fulfillment of two different measuring conditions is necessary to obtain the one logicalsignal for each separate measuring loop:• Zone measuring condition, which follows the operating equations described above.• Group functional input signal (PHSEL), as presented in figure 49.Section 6 1MRK 506 335-UUS AImpedance protection122Technical manual
