5-152 G30 Generator Protection System GE Multilin5.6 GROUPED ELEMENTS 5 SETTINGS5• Transformation errors of current transformers (CTs) during double-line and three-phase faults.• Switch-off transients during double-line and three-phase faults.The positive-sequence restraint must be considered when testing for pickup accuracy and response time (multiple ofpickup). The operating quantity depends on the way the test currents are injected into the relay (single-phase injection:Iop = (1 – K) × I injected ; three-phase pure zero-sequence injection: I op = 3 × I injected).The positive-sequence restraint is removed for low currents. If the positive-sequence current is below 0.8 pu, the restraint isremoved by changing the constant K to zero. This facilitates better response to high-resistance faults when the unbalanceis very small and there is no danger of excessive CT errors as the current is low.The directional unit uses the zero-sequence current (I_0) or ground current (IG) for fault direction discrimination and maybe programmed to use either zero-sequence voltage (“Calculated V0” or “Measured VX”), ground current (IG), or both forpolarizing. The zero-sequence current (I_0) must be greater than the PRODUCT SETUP DISPLAY PROPERTIES CUR-RENT CUT-OFF LEVEL setting value and IG must be greater than 0.5 pu to be validated as the operating quantity for direc-tional current. The following tables define the neutral directional overcurrent element.where: ,,ECA = element characteristic angle and IG = ground currentWhen 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.Table 5–22: QUANTITIES FOR "CALCULATED 3I0" CONFIGURATIONDIRECTIONAL UNIT OVERCURRENT UNITPOLARIZING MODE DIRECTION COMPARED PHASORSVoltage Forward –V_0 + Z_offset × I_0 I_0 × 1∠ECAI op = 3 × (|I_0| – K × |I_1|) if |I1 | > 0.8 puI op = 3 × (|I_0|) if |I1 | ≤ 0.8 puReverse –V_0 + Z_offset × I_0 –I_0 × 1∠ECACurrent Forward IG I_0Reverse IG –I_0DualForward–V_0 + Z_offset × I_0 I_0 × 1∠ECAorIG I_0Reverse–V_0 + Z_offset × I_0 –I_0 × 1∠ECAorIG –I_0Table 5–23: 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= = =
