5-274 L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALCONTROL ELEMENTS CHAPTER 5: SETTINGS5Before a permanent cable fault occurs, there are usually signs of degrading insulation manifesting itself as a short, mostlyhalf-cycle spikes asserting at the phase voltage peak. Due to shortness of such spikes, they are not usually detected by theinstantaneous protection of the feeder, which operates on the RMS or fundamental component of the phase current with arelatively high pickup.The number of detected incipient faults in each phase is counted and available in the ACTUAL VALUES STATUS INCIPIENT FAULT menus. The counters can be reset with the COMMANDS CLEAR RECORDS CLEAR INCIPENT FAULTCOUNTERS command.The figure illustrates a recorded field case of an incipient phase B fault. The top portion of the figure shows the raw A, B,and C currents. The bottom portion shows the neutral current (blue) and reveals the fault period from under the load andthe superimposed phase B current (red). The superimposed current shows two fault current blips as the data slides throughthe two-cycle memory window. During the actual fault, the neutral current and the superimposed phase B currents closelycorrespond, confirming the incipient fault hypothesis and identifying the affected phase.Figure 5-145: Illustration of the incipient fault detector algorithmThe following settings are available for each incipient cable fault detector.INCIPIENT FAULT 1 FUNCTION — This setting enables and disables operation of the incipient fault detection element.INCIPNT FLT 1 BLOCK — Blocks operation of the incipient cable fault detector element. Assertion of the FlexLogic operandassigned to this setting blocks operation.INCIPIENT FAULT 1 SOURCE — Selects a current source for the incipient cable fault detector element. This source must beassigned a valid CT bank.INCIPIENT FAULT 1 PICKUP — Specifies the pickup level of the overcurrent detector in per-unit values of the CT nominalcurrent.Changes to any of the incipient cable fault detector settings resets of the number of the incipient faults detected tozero.To provide a clear timing indication when the incipient fault occurred, the incipient fault event is time-stamped withthe time the fault actually occurred. However, the FlexLogic operand is asserted four cycles later, when the incipientfault pattern is confirmed and therefore detected.
