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 highset stage O/C protection is defined. The operate time delay caused by the CTsaturation is typically small enough when the relay setting is noticeably lower thanFa.When defining the setting values for the low set stages, the saturation of the CT doesnot need to be taken into account and the start current setting is simply according tothe formula.Delay in operation caused by saturation of current transformersThe saturation of CT may cause a delayed relay operation. To ensure the timeselectivity, the delay must be taken into account when setting the operate times ofsuccessive relays.With definite time mode of operation, the saturation of CT may cause a delay that isas long as the time the constant of the DC component of the fault current, when thecurrent is only slightly higher than the starting current. This depends on the accuracylimit factor of the CT, on the remanence flux of the core of the CT, and on the operatetime setting.With inverse time mode of operation, the delay should always be considered as beingas long as the time constant of the DC component.With inverse time mode of operation and when the high-set stages are not used, theAC component of the fault current should not saturate the CT less than 20 times thestarting current. Otherwise, the inverse operation time can be further prolonged.Therefore, the accuracy limit factor Fa should be chosen using the formula:Fa > 20*Current start value / I1nThe Current start value is the primary pickup current setting of the relay.11.1.1.3 Example for non-directional overcurrent protectionThe following figure describes a typical medium voltage feeder. The protection isimplemented as three-stage definite time non-directional overcurrent protection1MRS756378 D Section 11Requirements for measurement transformersREF615 227Application Manual
