In isolated or high impedance earthed systems the phase-to-earth fault is not thedimensioning case. Therefore, the resistance of the single secondary wire canalways be used in the calculation for this kind of power systems.25.1.5 General current transformer requirements M11615-3 v3The current transformer ratio is mainly selected based on power system data forexample, maximum load and/or maximum fault current. It should be verified thatthe current to the protection is higher than the minimum operating value for allfaults that are to be detected with the selected CT ratio. It should also be verifiedthat the maximum possible fault current is within the limits of the IED.The current error of the current transformer can limit the possibility to use a verysensitive setting of a sensitive residual overcurrent protection. If a very sensitivesetting of this function will be used it is recommended that the current transformershould have an accuracy class which have an current error at rated primary currentthat is less than ±1% (for example, 5P). If current transformers with less accuracyare used it is advisable to check the actual unwanted residual current during thecommissioning.25.1.6 Rated equivalent secondary e.m.f. requirements SEMOD53723-1 v1M11616-3 v2With regard to saturation of the current transformer all current transformers of highremanence and low remanence type that fulfill the requirements on the ratedequivalent limiting secondary e.m.f. Eal below can be used. The characteristic ofthe non remanence type CT (TPZ) is not well defined as far as the phase angleerror is concerned. If no explicit recommendation is given for a specific functionwe therefore recommend contacting ABB to confirm that the non remanence typecan be used.The CT requirements for the different functions below are specified as a ratedequivalent limiting secondary e.m.f. Eal according to the IEC 61869-2 standard.Requirements for CTs specified according to other classes and standards are givenat the end of this section.25.1.6.1 Busbar protection SEMOD129059-4 v6The CT can be of high remanence or low remanence type and they can be usedtogether within the same zone of protection. Each of them must have a ratedequivalent limiting secondary e.m.f. Eal that is larger than or equal to the requiredrated equivalent limiting secondary e.m.f. Ealreq below:The high remanence type CT must fulfillsr Ral alreq f max ct L 2pr rI SE E 0.5 I R RI Iæ ö³ = × × × + +ç ÷è øEQUATION1534 V2 EN-US (Equation 536)1MRK 506 369-UEN B Section 25RequirementsLine distance protection REL670 2.2 IEC 881Application manual
