CHAPTER 4: SETPOINTS PROTECTION889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–1233890 Amps *10% = 389 Amps = > 389/3000*M(w2) = 389/3000*0.4521 =0.0585 xCT (w1)In the worst case, the errors from the two CTs as expressed in xCT(w1) as a reference canbe summed:Minimum Pickup = 0.0585 + 0.0585 = 0.117 (0.2xCT setting for Pickup is sufficient)Slope 1: This setting defines the desired differential/restraint ratio for detecting internalfaults. The percent Slope 1 setting must be above the differential/restraint ratio duringexternal faults with non-significant fault current magnitude, but with long lasting DCcomponent, which is able to cause CT saturation.The Slope 1 setting can be calculated based on the restraint current during emergencyloading of the transformer:Restraint current Winding 1 = 117 Amps/200 = 0.585 xCTRestraint current Winding 2(scaled) = 3890/3000*0.4521 = 0.585 xCTDifferential current = CT error = 0.117 xCTSlope 1 = (0.117/0.585)*100 = 20%Adding 5% margin, produces a Slope 1 setting of 25%Slope 2: The Slope 2 setting ensures stability during heavy through fault conditions, whereCT saturation results in high differential current. Slope 2 must be set high enough to copewith the worst case when the CTs from one winding saturate but the CTs from the otherwinding do not saturate during external fault. In such case the ratio of the differential torestraint current can be as high as 95 to 98%.Break 1: Break 1 must be set to reflect the through current (restraint current) expressed intimes winding 1 CT (w1) below which CT saturation due to DC components and/or residualmagnetism is not expected. The latter may be as high as 80% of the nominal flux,effectively reducing the CT capabilities by factor of 5.Therefore, this setting must correspond to the end of the linear operation of the CT with theworst characteristic, counting up to 80% remnant flux in the CT core. To calculate thissetting, CTs knee-point voltage and CT secondary burden are needed.where Rb = 2 • Rlead + RCTsec + Rrelay is the CT burden resistance which includes CTsecondary resistance RCTsec , 2 times conductor resistance Rlead and the relay inputresistance Rrelay.ICTsec is the CT secondary nominal current. Vsat is the CT knee-point voltage. The Break 1 iscalculated per the formula above for each winding CT, and then the smallest per value isentered as a setting.Break 2: Break 2 setting defines the beginning of Slope 2 used to provide stability duringheavy through fault conditions, where CT saturation results in high differential current.Break 2 must be set to the through-fault current which causes the worst CT to saturate,with saturation free time of at least half power cycle.To define the degree of CT saturation, the IEEE CT saturation tool can be used:For example, let the CT 3000:5 on winding 2 be the worst one which would saturate firstduring external faults, and let the maximum external fault current be 25kA, with an X/Rratio of 15. The CT voltage kneepoint from the saturation characteristic is 800 V, withwinding resistance of 0.5 Ohms, Burden resistance of 2.19 Ohms, and Burden reactance of0.01 Ohms.
