GE Multilin L90 Line Current Differential System 5-1735 SETTINGS 5.6 GROUPED ELEMENTS5• 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 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 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 will be delayed by 1.5 of a power system cycle. The element is designed to emu-late an electromechanical directional device. Larger operating and polarizing signals will result in faster directional discrimi-nation bringing more security to the element operation.Table 5–20: 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 |I 1 | ≤ 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 5–21: 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= = =
