ABB RED615 Applications Manual

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ABB RED615 Applications Manual Manual pdf 81 page image

The factor 0.7 takes into account the protection IED inaccuracy, currenttransformer errors, and imperfections of the short circuit calculations.The adequate performance of the CT should be checked when the setting of thehigh set stage overcurrent protection is defined. The operate time delay caused bythe CT saturation is typically small enough when the overcurrent setting isnoticeably lower than Fa.When defining the setting values for the low set stages, the saturation of the CTdoes not need to be taken into account and the start current setting is simplyaccording to the formula.Delay in operation caused by saturation of current transformersThe saturation of CT may cause a delayed IED operation. To ensure the timeselectivity, the delay must be taken into account when setting the operate times ofsuccessive IEDs.With definite time mode of operation, the saturation of CT may cause a delay thatis as long as the time the constant of the DC component of the fault current, whenthe current is only slightly higher than the starting current. This depends on theaccuracy limit factor of the CT, on the remanence flux of the core of the CT, andon the operate time setting.With inverse time mode of operation, the delay should always be considered asbeing as 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 IED.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.1MRS756498 D Section 4Requirements for measurement transformersRED615 75Application Manual