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 ofvery different lengths. These changes in the network will normally impose morestringent demands on the fault clearing equipment in order to maintain an unchangedor increased security level of the power system.The high speed distance protection function (ZMFCPDIS) in the IED is designed toprovide sub-cycle, down to half-cycle operate time for basic faults. At the same time,it is specifically designed for extra care during difficult conditions in high voltagetransmission networks, like faults on long heavily loaded lines and faults generatingheavily distorted signals. These faults are handled with outmost security anddependability, although sometimes with reduced operating speed.8.3.2.1 System earthingThe type of system earthing plays an important role when designing the protectionsystem. Some hints with respect to distance protection are highlighted below.Solidly earthed networksIn solidly earthed systems, the transformer neutrals are connected directly to earthwithout any impedance between the transformer neutral and earth.IEC05000215 V2 ENFigure 95: Solidly earthed networkThe earth-fault current is as high or even higher than the short-circuit current. Theseries impedances determine the magnitude of the fault current. The shunt admittancehas very limited influence on the earth-fault current. The shunt admittance may,however, have some marginal influence on the earth-fault current in networks withlong transmission lines.The earth-fault current at single phase-to-earth in phase L1 can be calculated asequation 33:L1 L101 2 0 f 1 N f3 U U3I Z Z Z 3Z Z ZZ×= =+ + + + +EQUATION1267 V3 EN (Equation 81)Section 8 1MRK 502 071-UEN -Impedance protection214 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062Application manual
