18.1.5 General current transformer requirementsThe current transformer ratio is mainly selected based on power system data for example,maximum load. However, it should be verified that the current to the protection is higherthan the minimum operating value for all faults that are to be detected with the selected CTratio. The minimum operating current is different for different functions and normallysettable so each function should be checked.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 thisfunction will be used it is recommended that the current transformer should have anaccuracy class which have an current error at rated primary current that is less than ±1%(for example, 5P). If current transformers with less accuracy are used it is advisable tocheck the actual unwanted residual current during the commissioning.18.1.6 Rated equivalent secondary e.m.f. requirementsWith 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 remanencetype CT (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 ABBto confirm 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 forCTs specified according to other classes and standards are given at the end of this section.18.1.6.1 Distance protectionThe current transformers must have a rated equivalent secondary e.m.f. Eal that is largerthan the maximum of the required secondary e.m.f. Ealreq below:s n Ra l a lre q CT L 2pn nk ma xI I SE E R RI I³ = × + +æ ö× ç ÷è øEQUATION1675 V1 EN (Equation 178)s n Ra l a lre q CT L 2pn nkzone 1I I SE E R RI I³ = × + +æ ö× ç ÷è øEQUATION1676 V1 EN (Equation 179)Section 18 1MRK 506 334-UUS ARequirements492Application manual
