5-186 F60 Feeder Protection System GE Multilin5.6 GROUPED ELEMENTS 5 SETTINGS5The neutral directional overcurrent element provides both forward and reverse fault direction indications the NEUTRAL DIROC1 FWD and NEUTRAL DIR OC1 REV operands, respectively. The output operand is asserted if the magnitude of the oper-ating current is above a pickup level (overcurrent unit) and the fault direction is seen as forward or reverse, respectively(directional unit).The overcurrent unit responds to the magnitude of a fundamental frequency phasor of the either the neutral current calcu-lated from the phase currents or the ground current. There are separate pickup settings for the forward-looking andreverse-looking functions. If set to use the calculated 3I_0, the element applies a positive-sequence restraint for better per-formance: a small user-programmable portion of the positive-sequence current magnitude is subtracted from the zero-sequence current magnitude when forming the operating quantity.(EQ 5.17)The positive-sequence restraint allows for more sensitive settings by counterbalancing spurious zero-sequence currentsresulting from:• System unbalances under heavy load conditions.• 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 0.05 pu to be validated as the operating quantity for direc-tional current. The following tables define the neutral directional overcurrent element.MESSAGE NEUTRAL DIR OC1 BLK:OffRange: FlexLogic operandMESSAGE NEUTRAL DIR OC1TARGET: Self-resetRange: Self-reset, Latched, DisabledMESSAGE NEUTRAL DIR OC1EVENTS: DisabledRange: Disabled, EnabledTable 5–30: 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_0Dual, Dual-V, Dual-IForward–V_0 + Z_offset × I_0 I_0 × 1∠ECAorIG I_0Reverse–V_0 + Z_offset × I_0 –I_0 × 1∠ECAorIG –I_0I op 3 I_0 K I_1×–( )×=
