CHAPTER 5: SETTINGS GROUPED ELEMENTSL60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 5-2735There are two negative-sequence directional overcurrent protection elements available. The element provides bothforward and reverse fault direction indications through its output operands NEG SEQ DIR OC1 FWD and NEG SEQ DIR OC1 REV,respectively. The output operand is asserted if the magnitude of the operating current is above a pickup level (overcurrentunit) and the fault direction is seen as forward or reverse, respectively (directional unit).The overcurrent unit of the element essentially responds to the magnitude of a fundamental frequency phasor of either thenegative-sequence or neutral current as per user selection.A positive-sequence restraint is applied for better performance: a small user-programmable portion of the positive-sequence current magnitude is subtracted from the negative or zero-sequence current magnitude, respectively, whenforming the element operating quantity.Iop = |I_2| - K x |I_1| or Iop = 3 x (|I_0| - K x |I_1|) Eq. 5-26The positive-sequence restraint allows for more sensitive settings by counterbalancing spurious negative-sequence andzero-sequence currents resulting from• System unbalances under heavy load conditions• Transformation errors of current transformers (CTs)• Fault inception and switch-off transientsThe positive-sequence restraint must be considered when testing for pickup accuracy and response time (multiple ofpickup). The positive-sequence restraint is removed for low currents. If the positive-sequence current is less than 0.8 pu,then the restraint is removed by changing the constant K to zero. This results in better response to high-resistance faultswhen the unbalance is very small and there is no danger of excessive CT errors, since the current is low.The operating quantity depends on the way the test currents are injected into the L60. For single phase injection• Iop = ⅓ × (1 – K) × Iinjected for I_2 mode• Iop = (1 – K) × Iinjected for I_0 mode if I_1 > 0.8 puThe directional unit uses the negative-sequence current (I_2) and negative-sequence voltage (V_2).The following tables define the negative-sequence directional overcurrent element.Table 5-38: Negative-sequence directional overcurrent unit NEG SEQ DIR OC1 POS-SEQ RESTRAINT: 0.063Range: 0.000 to 0.500 in steps of 0.001 NEG SEQ DIR OC1 FWDECA: 75° LagRange: 0 to 90° Lag in steps of 1 NEG SEQ DIR OC1 FWDLIMIT ANGLE: 90°Range: 40 to 90° in steps of 1 NEG SEQ DIR OC1 FWDPICKUP: 0.050 puRange: 0.015 to 30.000 pu in steps of 0.005 NEG SEQ DIR OC1 REVLIMIT ANGLE: 90°Range: 40 to 90° in steps of 1 NEG SEQ DIR OC1 REVPICKUP: 0.050 puRange: 0.015 to 30.000 pu in steps of 0.005 NEG SEQ DIR OC1 BLK:OffRange: FlexLogic operand NEG SEQ DIR OC1TARGET: Self-resetRange: Self-reset, Latched, Disabled NEG SEQ DIR OC1EVENTS: DisabledRange: Disabled, EnabledMode Operating currentNegative-sequence I op = |I_2| – K × I_1|Zero-sequence I op = 3 × (|I_0| – K × |I_1|) if |I_1| > 0.8 puI op = 3 × |I_0| if |I_1| ≤ 0.8 pu
