8-4 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALCURRENT DIFFERENTIAL (87L) SETTINGS CHAPTER 8: APPLICATION OF SETTINGS8This software program is also useful for establishing test parameters. It is strongly recommended this program bedownloaded. Look for the "L90 Test Tool" on the GE Grid Solutions website, for example in the L90 support documents athttp://www.gegridsolutions.com/support/l90.htmThe differential characteristic is defined by four settings: CURRENT DIFF PICKUP, CURRENT DIFF RESTRAINT 1, CURRENT DIFFRESTRAINT 2, and CURRENT DIFF BREAK PT (breakpoint). As is typical for current-based differential elements, the settings area trade-off between operation on internal faults against restraint during external faults.8.2.2 Current differential pickupThis setting establishes the sensitivity of the element to high impedance faults, and it is therefore good to choose a lowlevel, but this can cause maloperation for an external fault causing CT saturation. The selection of this setting is influencedby the decision to use charging current compensation. If charging current compensation is Enabled, set pickup to aminimum of 150% of the steady-state line charging current, to a lower limit of 10% of CT rating. If charging currentcompensation is Disabled, set pickup to a minimum of 250% of the steady-state line charging current to a lower limit of10% of CT rating.If the CT at one terminal can saturate while the CTs at other terminals do not, increase this setting by approximately 20 to50% (depending on how heavily saturated the one CT is while the other CTs are not saturated) of CT rating to preventoperation on a close-in external fault.8.2.3 Current differential restraint 1The CURRENT DIFF RESTRAINT 1 setting controls the element characteristic when current is below the breakpoint, where CTerrors and saturation effects are not expected to be significant. The setting is used to provide sensitivity to highimpedance internal faults, or when system configuration limits the fault current to low values. A setting of 10 to 20% isappropriate in most cases, but increase this to 30% if the CTs can perform quite differently during faults.8.2.4 Current differential restraint 2The CURRENT DIFF RESTRAINT 2 setting controls the element characteristic when current is above the breakpoint, where CTerrors and saturation effects are expected to be significant. The setting is used to provide security against high currentexternal faults. A setting of 30 to 40% is appropriate in most cases, but increase this to 70% if the CTs can perform quitedifferently during faults.8.2.5 Current differential break pointThe CURRENT DIFF BREAK PT setting controls the threshold where the relay changes from using the restraint 1 to therestraint 2 characteristics. Two approaches can be considered, as follows:• Program the setting to 150 to 200% of the maximum emergency load current on the line, on the assumption that amaintained current above this level is a fault• Program the setting below the current level where CT saturation and spurious transient differential currents can beexpectedThe first approach gives comparatively more security and less sensitivity; the second approach provides less security formore sensitivity.8.2.6 CT tapIf the CT ratios at the line terminals are different, the CURRENT DIFF CT TAP 1(2) setting must be used to correct the ratios toa common base. In this case, modify the CURRENT DIFF BREAK PT and CURRENT DIFF PICKUP settings because the localcurrent phasor is used as a reference to determine which differential equation is used, based on the value of local andAssigning the same value to the CURRENT DIFF RESTRAINT 1(2) settings reverts dual slope bias characteristics intosingle slope bias characteristics.
