margin. Fully asymmetrical fault current will be achieved when the fault occurs atapproximately zero voltage (0°). Investigations have shown that 95% of the faults inthe network will occur when the voltage is between 40° and 90°. In addition fullyasymmetrical fault current will not exist in all phases at the same time.24.1.3 Fault currentThe current transformer requirements are based on the maximum fault current forfaults in different positions. Maximum fault current will occur for three-phase faultsor single phase-to-earth faults. The current for a single phase-to-earth fault will exceedthe current for a three-phase fault when the zero sequence impedance in the total faultloop is less than the positive sequence impedance.When calculating the current transformer requirements, maximum fault current forthe relevant fault position should be used and therefore both fault types have to beconsidered.24.1.4 Secondary wire resistance and additional loadThe voltage at the current transformer secondary terminals directly affects the currenttransformer saturation. This voltage is developed in a loop containing the secondarywires and the burden of all relays in the circuit. For earth faults the loop includes thephase and neutral wire, normally twice the resistance of the single secondary wire. Forthree-phase faults the neutral current is zero and it is just necessary to consider theresistance up to the point where the phase wires are connected to the common neutralwire. The most common practice is to use four wires secondary cables so it normallyis sufficient to consider just a single secondary wire for the three-phase case.The conclusion is that the loop resistance, twice the resistance of the single secondarywire, must be used in the calculation for phase-to-earth faults and the phase resistance,the resistance of a single secondary wire, may normally be used in the calculation forthree-phase faults.As the burden can be considerable different for three-phase faults and phase-to-earthfaults it is important to consider both cases. Even in a case where the phase-to-earthfault current is smaller than the three-phase fault current the phase-to-earth fault canbe dimensioning for the CT depending on the higher burden.In isolated or high impedance earthed systems the phase-to-earth fault is not thedimensioning case. Therefore, the resistance of the single secondary wire can alwaysbe used in the calculation for this kind of power systems.24.1.5 General current transformer requirementsThe current transformer ratio is mainly selected based on power system data forexample, maximum load and/or maximum fault current. It should be verified that thecurrent to the protection is higher than the minimum operating value for all faults thatSection 24 1MRK 502 071-UEN -Requirements746 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062Application manual
