8.13.3.6 Underreaching and overreaching schemes GUID-3090702F-508B-480C-9AC7-2D5A82B9AD1E v1It is a basic rule that the underreaching distance protection zone should under nocircumstances overreach for the fault at the remote end bus, and the overreachingzone should always, under all system conditions, cover the same fault. In order toobtain section selectivity, the first distance (underreaching) protection zone must beset to a reach less than the reactance of the compensated line in accordance withfigure 209.en06000618.vsdX 11X 12-j XCA BDA DBZone 1AZone 1BZone 2AZone 2BGIEC06000618 V1 EN-USFigure 209: Underreaching (Zone 1) and overreaching (Zone 2) on seriescompensated lineThe underreaching zone will have reduced reach in cases of bypassed seriescapacitor, as shown in the dashed line in figure 209. The overreaching zone (Zone2) can this way cover bigger portion of the protected line, but must always coverwith certain margin the remote end bus. Distance protection Zone 1 is often set to( )1 11 12= × + -Z S CX K X X XEQUATION1914 V1 EN-US (Equation 369)Here KS is a safety factor, presented graphically in figure 210, which covers forpossible overreaching due to low frequency (sub-harmonic) oscillations. Here itshould be noted separately that compensation degree KC in figure 210 relates tototal system reactance, inclusive line and source impedance reactance. The samesetting applies regardless MOV or spark gaps are used for capacitor overvoltageprotection.Equation 369 is applicable for the case when the VTs are located on the bus side ofseries capacitor. It is possible to remove XC from the equation in cases of VTsinstalled in line side, but it is still necessary to consider the safety factor KS .If the capacitor is out of service or bypassed, the reach with these settings can beless than 50% of protected line dependent on compensation degree and there willbe a section, G in figure 209, of the power line where no tripping occurs fromeither end.Section 8 1MRK 506 369-UEN BImpedance protection386 Line distance protection REL670 2.2 IECApplication manual
