Section 11 1MAC309294-MB FRequirements for measurement transformers550 RER620Technical ManualIn practise, the actual accuracy limit factor (Fa ) differs from the rated accuracy limit factor(F n) and is proportional to the ratio of the rated CT burden and the actual CT burden.The actual accuracy limit factor is calculated using the formula:Fn the accuracy limit factor with the nominal external burden SnSin the internal secondary burden of the CTS the actual external burden11.1.1.2 Non-directional overcurrent protectionThe current transformer selectionNon-directional overcurrent protection does not set high requirements on the accuracyclass or on the actual accuracy limit factor (F a ) of the CTs. It is, however, recommendedto select a CT with Fa of at least 20.The nominal primary current I1n should be chosen in such a way that the thermal anddynamic strength of the current measuring input of the relay is not exceeded. This isalways fulfilled whenI 1n > Ikmax / 100,I kmax is the highest fault current.The saturation of the CT protects the measuring circuit and the current input of the relay.For that reason, in practice, even a few times smaller nominal primary current can be usedthan given by the formula.Recommended pickup current settingsIf I kmin is the lowest primary current at which the highest set overcurrent stage is to trip,the pickup current should be set using the formula:Current pickup value < 0.7 x (Ikmin / I1n)I 1n is the nominal primary current of the CT.The factor 0.7 takes into account the protection relay inaccuracy, current transformererrors, and imperfections of the short circuit calculations.The adequate performance of the CT should be checked when the setting of the high setstage overcurrent protection is defined. The trip time delay caused by the CT saturation istypically small enough when the overcurrent setting is noticeably lower than F a .When defining the setting values for the low set stages, the saturation of the CT does notneed to be taken into account and the pickup current setting is simply according to theformula.F F S SS Sa nin nin≈ × ++
