It is difficult to give general recommendations for additional margins forremanence to avoid the minor risk of an additional time delay. They depend on theperformance and economy requirements. When current transformers of lowremanence type (for example, TPY, PR) are used, normally no additional margin isneeded. For current transformers of high remanence type (for example, P, PX,TPX) the small probability of fully asymmetrical faults, together with highremanence in the same direction as the flux generated by the fault, has to be kept inmind at the decision of an additional margin. Fully asymmetrical fault current willbe achieved when the fault occurs at approximately zero voltage (0°).Investigations have shown that 95% of the faults in the network will occur whenthe voltage is between 40° and 90°. In addition fully asymmetrical fault currentwill not exist in all phases at the same time.23.1.3 Fault current M11613-3 v1M11613-4 v3The current transformer requirements are based on the maximum fault current forfaults in different positions. Maximum fault current will occur for three-phasefaults or single phase-to-earth faults. The current for a single phase-to-earth faultwill exceed the current for a three-phase fault when the zero sequence impedancein the total fault loop 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.23.1.4 Secondary wire resistance and additional load M11614-3 v1M11614-4 v4The voltage at the current transformer secondary terminals directly affects thecurrent transformer saturation. This voltage is developed in a loop containing thesecondary wires and the burden of all relays in the circuit. For earth faults the loopincludes the phase and neutral wire, normally twice the resistance of the singlesecondary wire. For three-phase faults the neutral current is zero and it is justnecessary to consider the resistance up to the point where the phase wires areconnected to the common neutral wire. The most common practice is to use fourwires secondary cables so it normally is sufficient to consider just a singlesecondary wire for the three-phase case.The conclusion is that the loop resistance, twice the resistance of the singlesecondary wire, must be used in the calculation for phase-to-earth faults and thephase resistance, the resistance of a single secondary wire, may normally be usedin the calculation for three-phase faults.As the burden can be considerable different for three-phase faults and phase-to-earth faults it is important to consider both cases. Even in a case where the phase-to-earth fault current is smaller than the three-phase fault current the phase-to-earthfault can be dimensioning for the CT depending on the higher burden.Section 23 1MRK 511 407-UEN CRequirements358 Phasor measurement unit RES670 2.2 IECApplication manual
