7.1.6.2 Impedance characteristicThe Mho distance function ZMHPDIS is present with four instances so that fourseparate zones could be designed. Each instance can be selected to be either forwardor reverse with positive sequence polarized mho characteristic; alternatively selfpolarized offset mho characteristics is also available. One example of the operatingcharacteristic is shown in Figure 65 A) where zone 5 is selected offset mho.The directional mho characteristic of Figure 65 has a dynamic expansion due to thesource impedance. Instead of crossing the origin as for the mho to the left of Figure65, which is only valid where the source impedance is zero, the crossing point ismoved to the coordinates of the negative source impedance giving an expansion of thecircle of Figure 65 D) .RjxMho, zone4Mho, zone3Mho, zone2Mho, zone1XZs=0RZs=Z1Zs=2Z1IEC11000223-2-en.vsdC DIEC11000223 V2 ENFigure 65: Mho characteristic and the source impedance influence on the mho characteristicThe polarization quantities used for the mho circle are 100% memorized positivesequence voltages. This will give a somewhat less dynamic expansion of the mhocircle during faults than a plain cross polarized characteristic. However, if the sourceimpedance is high, the dynamic expansion of the mho circle might lower the securityof the function too much with high loading and mild power swing conditions.The mho distance element has a load encroachment function which cuts off a sectionof the characteristic when enabled. The function is enabled by setting the settingparameter LoadEnchMode to On. Enabling of the load encroachment functionincreases the possibility to detect high resistive faults without interfering with the loadimpedance. The algorithm for the load encroachment is located in the Faulty phase1MRK502052-UEN B Section 7Impedance protection181Technical manual
