8.3.2 ApplicationSub-transmission networks are being extended and often become more and morecomplex, consisting of a high number of multi-circuit and/or multi terminal lines of verydifferent lengths. These changes in the network will normally impose more stringentdemands on the fault clearing equipment in order to maintain an unchanged or increasedsecurity level of the power system.The distance protection function in the IED is designed to meet basic requirements forapplication on transmission and sub-transmission lines (solid grounded systems) althoughit also can be used on distribution levels.8.3.2.1 System groundingThe type of system grounding plays an important role when designing the protectionsystem. Some hints with respect to distance protection are highlighted below.Solidly grounded networksIn solidly grounded systems, the transformer neutrals are connected directly to groundwithout any impedance between the transformer neutral and ground.ANSI05000215 V2 ENFigure 109: Solidly grounded networkThe ground-fault current is as high or even higher than the short-circuit current. The seriesimpedances determine the magnitude of the fault current. The shunt admittance has verylimited influence on the ground-fault current. The shunt admittance may, however, havesome marginal influence on the ground-fault current in networks with long transmissionlines.The ground-fault current at single phase-to-ground in phase A can be calculated asequation 104:A A01 2 0 f 1 N f3 V3I Z Z Z 3Z Z Z ZV×= =+ + + + +EQUATION1710 V2 EN (Equation 104)Section 8 1MRK 506 369-UUS -Impedance protection224 Line distance protection REL670 2.2 ANSIApplication manual
