9. Enable one overcurrent stage (for example, OC1)10. By parameterCurveType_OC1 select appropriate TOC/IDMT or definite time delayed curvein accordance with your network protection philosophy11. SetStartCurr_OC1 to value between 3-10% (typical values)12. SettDef_OC1 or parameter “k” when TOC/IDMT curves are used to insure proper timecoordination with other earth-fault protections installed in the vicinity of this power line13. SetDirMode_OC1 toForward14. SetDirPrinc_OC1 toIcosPhi&U15. SetActLowVolt1_VM toBlock• In order to insure proper restraining of this element for CT saturations during three-phase faults it is possible to use current restraint feature and enable this element tooperate only whenNegSeq current is bigger than a certain percentage (10% istypical value) of measuredPosSeq current in the power line. To do this the followingsettings within the same function shall be done:16. SetEnRestrainCurr toOn17. SetRestrCurrInput toPosSeq18. SetRestrCurrCoeff to value 0.10If required, this CVGAPC function can be used in directional comparison protection scheme forthe power line protection if communication channels to the remote end of this power line areavailable. In that case typically twoNegSeq overcurrent steps are required. One for forwardand one for reverse direction. As explained before the OC1 stage can be used to detect faultsin forward direction. The built-in OC2 stage can be used to detect faults in reverse direction.However the following shall be noted for such application:• the set values forRCADir andROADir settings will be as well applicable for OC2 stage• settingDirMode_OC2 shall be set toReverse• setting parameterStartCurr_OC2 shall be made more sensitive than pickup value offorward OC1 element (that is, typically 60% of OC1 set pickup level) in order to insureproper operation of the directional comparison scheme during current reversal situations• start signals from OC1 and OC2 elements shall be used to send forward and reversesignals to the remote end of the power line• the available scheme communications function block within IED shall be used betweenmultipurpose protection function and the communication equipment in order to insureproper conditioning of the above two start signalsFurthermore the other built-in UC, OV and UV protection elements can be used for otherprotection and alarming purposes.8.1.3.2 Negative sequence overcurrent protection M13088-5 v4Example will be given how to use one CVGAPC function to provide negative sequence inversetime overcurrent protection for a generator with capability constant of 20s, and maximumcontinuous negative sequence rating of 7% of the generator rated current.The capability curve for a generator negative sequence overcurrent protection, often usedworld-wide, is defined by the ANSI standard in accordance with the following formula:1MRK 505 355-UEN A Section 8Multipurpose protection135Application manual
