5-176 C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUALGROUPED ELEMENTS CHAPTER 5: SETTINGS5The 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) × Iinjected ; 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 canbe 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 CURRENTCUT-OFF LEVEL setting value and IG must be greater than 0.05 pu to be validated as the operating quantity for directionalcurrent. The following tables define the neutral directional overcurrent element. V_0 is the zero-sequence voltage, I_0 isthe zero-sequence current, ECA is the element characteristic angle, and IG is the ground current.Table 5-30: Quantities for "calculated 3I0" configurationTable 5-31: Quantities for "measured IG" configurationwhere:ECA = element characteristic angleIG = 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 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)Take the bias into account when using the neutral directional overcurrent element to directionalize other protectionelements.Directional unit Overcurrent unitPolarizing mode Direction Compared phasorsVoltage Forward –V_0 + Z_offset × I_0 I_0 × 1∠ECA Iop = 3 × (|I_0| – K × |I_1|) if |I 1 | > 0.8 puIop = 3 × (|I_0|) if |I1 | ≤ 0.8 puReverse –V_0 + Z_offset × I_0 –I_0 × 1∠ECACurrent Forward IG I_0Reverse IG –I_0Dual, Dual-V, Dual-I Forward –V_0 + Z_offset × I_0 I_0 × 1∠ECAorIG I_0Reverse –V_0 + Z_offset × I_0 –I_0 × 1∠ECAorIG –I_0Directional unit Overcurrent unitPolarizing mode Direction Compared phasorsVoltage Forward –V_0 + Z_offset × IG/3 IG × 1∠ECA Iop = |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= = =
