ABB RELION 630 Series Applications Manual

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ABB RELION 630 Series Applications Manual Manual pdf 59 page image

The adequate performance of the CT should be checked when the setting of the highset stage overcurrent protection is defined. The operate time delay caused by the CTsaturation is typically small enough when the overcurrent setting is noticeably lowerthan Fa.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 to theformula.Delay in operation caused by saturation of current transformersThe saturation of CT may cause a delayed protection relay operation. To ensure thetime selectivity, the delay must be taken into account when setting the operate timesof successive protection relays.With definite time mode of operation, the saturation of CT may cause a delay that isas long as the time constant of the DC component of the fault current, when the currentis only slightly higher than the starting current. This depends on the accuracy limitfactor of the CT, on the remanence flux of the core of the CT, and on the operate timesetting.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, the ACcomponent 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 start current setting of the protection relay.4.1.1.3 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 protection.1MRS757582 C Section 4Requirements for measurement transformersREG630 53Application Manual