5-210 L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALGROUPED ELEMENTS CHAPTER 5: SETTINGS5The neutral directional overcurrent element provides both forward and reverse fault direction indications for the NEUTRALDIR OC1 FWD and NEUTRAL DIR OC1 REV operands, respectively. The output operand is asserted if the magnitude of theoperating 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 either the neutral currentcalculated 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 betterperformance: a small user-programmable portion of the positive-sequence current magnitude is subtracted from thezero-sequence current magnitude when forming the operating quantity.I op = 3 x (|I_0| - K x |I_1|) Eq. 5-24The 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 faultsThe 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. NEUTRAL DIR OC1OFFSET: 0.00 ΩRange: 0.00 to 250.00 Ω in steps of 0.01 NEUTRAL DIR OC1 FWDECA: 75° LagRange: –90 to 90° in steps of 1 NEUTRAL DIR OC1 FWDLIMIT ANGLE: 90°Range: 40 to 90° in steps of 1 NEUTRAL DIR OC1 FWDPICKUP: 0.050 puRange: 0.006 to 30.000 pu in steps of 0.001 NEUTRAL DIR OC1 REVLIMIT ANGLE: 90°Range: 40 to 90° in steps of 1 NEUTRAL DIR OC1 REVPICKUP: 0.050 puRange: 0.006 to 30.000 pu in steps of 0.001 NEUTRAL DIR OC1 BLK:OffRange: FlexLogic operand NEUTRAL DIR OC1TARGET: Self-resetRange: Self-reset, Latched, Disabled NEUTRAL DIR OC1EVENTS: DisabledRange: Disabled, Enabled
