5-172 G30 Generator 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 above bias should be taken into account when using the neutral directional overcurrent element to directionalize otherprotection elements.Figure 5–65: NEUTRAL DIRECTIONAL VOLTAGE-POLARIZED CHARACTERISTICS• NEUTRAL DIR OC1 POLARIZING: This setting selects the polarizing mode for the directional unit.– If “Voltage” polarizing is selected, the element uses the zero-sequence voltage angle for polarization. The usercan use either the zero-sequence voltage V_0 calculated from the phase voltages, or the zero-sequence voltagesupplied externally as the auxiliary voltage V_X, both from the NEUTRAL DIR OC1 SOURCE.The calculated V_0 can be used as polarizing voltage only if the voltage transformers are connected in Wye. Theauxiliary voltage can be used as the polarizing voltage provided SYSTEM SETUP AC INPUTS VOLTAGE BANK AUXILIARY VT CONNECTION is set to “Vn” and the auxiliary voltage is connected to a zero-sequence voltagesource (such as broken delta connected secondary of VTs).V_0 13--- VAG VBG VCG+ +( ) zero sequence voltage= =I_0 13---IN 13--- IA IB IC+ +( ) zero sequence current= = =827805A1.CDRVAG(reference)VBGVCG–3I_0 line3I_0 lineECA line–ECA lineLALALALAECAFWD LAlineFWD OperatingRegionREV OperatingRegionFWD LAlineREV LAlineREV LAline–3V_0 line3V_0 line
