As the burden can be considerable different for three-phase faults and phase-to-earth faults itis important to consider both cases. Even in a case where the phase-to-earth fault current issmaller than the three-phase fault current the phase-to-earth fault can be dimensioning for theCT 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 always be usedin the calculation for this kind of power systems.18.1.5 General current transformer requirements M11615-3 v3The current transformer ratio is mainly selected based on power system data for example,maximum load and/or maximum fault current. It should be verified that the current to theprotection is higher than the minimum operating value for all faults that are to be detectedwith the selected CT ratio. It should also be verified that the maximum possible fault current iswithin the limits of the IED.The current error of the current transformer can limit the possibility to use a very sensitivesetting of a sensitive residual overcurrent protection. If a very sensitive setting of this functionwill be used it is recommended that the current transformer should have an accuracy classwhich have an current error at rated primary current that is less than ±1% (for example, 5P). Ifcurrent transformers with less accuracy are used it is advisable to check the actual unwantedresidual current during the commissioning.18.1.6 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 rated equivalentlimiting secondary e.m.f. Eal below can be used. The characteristic of the non remanence typeCT (TPZ) is not well defined as far as the phase angle error is concerned. If no explicitrecommendation is given for a specific function we therefore recommend contacting ABB toconfirm that the non remanence type can be used.The CT requirements for the different functions below are specified as a rated equivalentlimiting secondary e.m.f. Eal according to the IEC 61869-2 standard. Requirements for CTsspecified according to other classes and standards are given at the end of this section.18.1.6.1 Transformer differential protection SEMOD54689-4 v4The current transformers must have a rated equivalent limiting secondary e.m.f. Eal that islarger than the maximum of the required rated equivalent limiting secondary e.m.f. Ealreqbelow:sr Ral alreq rt ct L 2pr rI SE E 30 I R RI Iæ ö³ = × × × + +ç ÷è øEQUATION1412 V2 EN-US (Equation 92)sr Ral alreq tf ct L 2pr rI SE E 2 I R RI Iæ ö³ = × × × + +ç ÷è øEQUATION1413 V2 EN-US (Equation 93)1MRK 504 158-UEN A Section 18Requirements265Application manual
