GE Multilin D30 Line Distance Relay 8-178 THEORY OF OPERATION 8.4 SERIES COMPENSATED LINES8Figure 8–7: DYNAMIC REACH CONTROLSection (a) of the figure below shows the effect of adaptive reach control for low-current external fault. The reach is reducedsufficiently to cope with both transient and steady-state overreach. Section (b) shows a high-current external fault. The airgaps or MOVs conduct majority of the fault current and neither steady-state nor transient overreach takes place. The relaydoes not reduce its reach as it is not necessary. Section (c) shows a high-current internal fault. Because of the large cur-rent, the reach is not reduced and the element responds to this internal fault. Traditional approach would leave this fault outof the relay reach.The neutral and negative-sequence directional protection functions of the relay cope with the voltage and/or current inver-sions by adding appropriate offset to their polarizing signals as explained in the Ground Directional Overcurrent section.The offset impedance can always be successfully selected to guarantee correct fault direction discrimination regardless ofthe degree of compensation and location of the series capacitors and the potential source.Refer to Chapter 9: Application of Settings for detailed recommendations on settings for series compensation applications.837729A1.CDRTHE REACH ISDYNAMICALLYREDUCED BYV L/abs(I)SET REACH (Z R )ACTUAL REACH ISA FUNCTION OFCURRENTMAGNITUDEACTUAL REACHFOR VERY HIGHCURRENTSACTUAL REACHFOR VERY SMALLCURRENTS RXFAR-ENDBUSBAR
