1.428707 LRarc I×=EQUATION1456 V1 EN (Equation 48)where:L represents the length of the arc (in meters). This equation applies for the distance protection zone 1.Consider approximately three times arc foot spacing for the zone 2 and wind speed of approximately30 m/hI is the actual fault current in A.In practice, the setting of fault resistance for both phase to ground RFPGx and phase tophase RFPPx (where x is 1-5 depending on selected zone) should be as high as possiblewithout interfering with the load impedance in order to obtain reliable fault detection.In combined quadrilateral and mho characteristic, these settings are used specifically byquadrilateral mode of operation. Dynamic mho characteristic self-expansion feature takescare of the fault resistance.6.1.2.7 Load encroachmentFive zone distance protection, quadrilateral and mho characteristic (ZQMPDIS, 21)exhibits two important characteristics of the distance protection. These characteristics canbe separately configured by the setting CharPGZx (where x = 1-5) for phase to groundloops and by CharPPZx (where x = 1-5) for phase to phase loops. This selectable featuregives an additional flexibility for line protection.In some cases the load impedance might enter the zone characteristic without any fault onthe protected line. The phenomenon is called load encroachment and it might occur whenan external fault is cleared and high emergency load is transferred on the protected line.The effect of load encroachment is illustrated to the left in figure 61 and figure 62. Theentrance of the load impedance inside the characteristic is of course not allowed and theway to handle this with conventional distance protection is to consider this with thesettings, that is, to have a security margin between the distance zone and the minimumload impedance. This has the drawback that it will reduce the sensitivity of the protection,that is, the ability to detect resistive faults.The IED has a built in function which shapes the characteristic according to the left offigure 61 and figure 62.1MRK 506 334-UUS A Section 6Impedance protection127Application manual
