4–126 889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUALPROTECTION CHAPTER 4: SETPOINTSPICKUPRange: 0.05 to 1.00 x CT in steps of 0.01Default: 0.20 x CTThis setting defines the minimum differential current required for operation. The settingis selected based on the amount of differential current that might be seen under normalconditions. This differential current is a result of the transformer magnetizing current, CTinaccuracies, and transformer ratio that would change by the transformer on-load tapchanger. When the tap changer feature is set properly, the 889 compensates thetransformation ratio which impacts the differential protection automatically.SLOPE 1Range: 1 to 100% in steps of 1Default: 25%This setting is applicable for restraint currents (through currents) of zero to kneepoint 1,and defines the ratio of differential/restraint currents, above which the element willoperate. This slope is set to ensure sensitivity on internal faults at normal transformerloading. The value of slope 1 must be above the maximum differential currents whichmay be seen for through currents with magnitudes less than break 1 setting. Thedifferential currents can be caused by winding CT inaccuracies, the constanttransformer magnetizing current, and the leakage current from in-zone groundingtransformers. Usually a margin of 2% to 5% is added to this setting.BREAK 1Range: 0.50 to 2.00 x CT in steps of 0.01 x CTDefault: 1.50 x CTThe setting for Break 1 defines the limit of linear operation of the magnitude referenceCT, accounting for up to 80% residual flux, that effectively reduces the capability of theCT by a factor of 5. Break 1 must be set below the current that can cause CT saturationdue to DC components and/or residual magnetism. Very often the Break 1 setting isbased on the transformer winding 1 nominal current (100% transformer loading). In suchcases, one must check whether or not this setting complies with the aboverecommendation regarding the CT linear performance. This breakpoint marks the end ofslope 1, where no CT saturation is expected for restraining (through) currents smallerthan that breakpoint.BREAK 2Range: 2.00 to 30.00 x CT in steps of 0.01 x CTDefault: 4.00 x CTBreak 2 must be set below the fault current that is most likely to saturate any of thetransformer CTs due to an AC component alone. The setting is expressed in times CT,where the CT rating is the magnitude reference CT either user selected, or selectedautomatically.SLOPE 2Range: 1% to 100% in steps of 1%Default: 95%The Slope 2 setting ensures stability during through fault conditions, resulting in CTsaturation and spurious high differential current. Slope 2 must be set high to cater forthe worst case scenario, where only CTs from one winding saturate, but the CTs from theother winding(s) do not saturate. In such cases the differential/restraint ratio can go ashigh as 95% to 98%.
