5-186 D30 Line Distance Protection System GE Multilin5.6 GROUPED ELEMENTS 5 SETTINGS5where: ,,ECA = element characteristic angleIG = ground currentZ_offset is the offset impedance, for which magnitude is the OFFSET setting and angle is the FWD ECAWhen NEUTRAL DIR OC1 POL VOLT is set to “Measured VX”, one-third of this voltage is used in place of V_0. The followingfigure explains the usage of the voltage polarized directional unit of the element.The figure below shows the voltage-polarized phase angle comparator characteristics for a phase A to ground fault, with:• ECA = 90° (element characteristic angle = centerline of operating characteristic)• FWD LA = 80° (forward limit angle = the ± angular limit with the ECA for operation)• REV LA = 80° (reverse limit angle = the ± angular limit with the ECA for operation)The element incorporates a current reversal logic: if the reverse direction is indicated for at least 1.25 of a power systemcycle, the prospective forward indication is delayed by 1.5 of a power system cycle. The element is designed to emulate anelectromechanical directional device. Larger operating and polarizing signals results in faster directional discriminationbringing more security to the element operation.The forward-looking function is designed to be more secure as compared to the reverse-looking function, and therefore,should be used for the tripping direction. The reverse-looking function is designed to be faster as compared to the forward-looking function and should be used for the blocking direction. This allows for better protection coordination.The above bias should be taken into account when using the neutral directional overcurrent element to directionalize otherprotection elements.Table 5–27: QUANTITIES FOR "MEASURED IG" CONFIGURATIONDIRECTIONAL UNIT OVERCURRENT UNITPOLARIZING MODE DIRECTION COMPARED PHASORSVoltage Forward –V_0 + Z_offset × IG/3 IG × 1∠ECA I op = |IG|Reverse –V_0 + Z_offset × IG/3 –IG × 1∠ECAV_0 13--- VAG VBG VCG+ +( ) zero sequence voltage= =I_0 13---IN 13--- IA IB IC+ +( ) zero sequence current= = =
