5-302 F60 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUALMONITORING ELEMENTS CHAPTER 5: SETTINGS5Figure 5-165: Incipient cable fault detector logic5.9.0.8 VT fuse failureSETTINGS CONTROL ELEMENTS MONITORING ELEMENTS VT FUSE FAILURE 1(4)Every signal source includes a fuse failure scheme.The VT fuse failure detector is used to raise an alarm and/or block elements that operate incorrectly for a full or partial lossof AC potential caused by one or more blown fuses. Some elements that can be blocked (via the BLOCK input) are distance,voltage restrained overcurrent, and directional current.There are two classes of fuse failure that occur:• Class A — Loss of one or two phases• Class B — Loss of all three phasesDifferent means of detection are required for each class. An indication of class A failures is a significant level of negative-sequence voltage, whereas an indication of class B failures is when positive sequence current is present and there is aninsignificant amount of positive sequence voltage. Also, a rapid decrease in the phase voltages magnitude from a healthyvoltage level without disturbance in current can indicate VT fuse fail conditions. These noted indications of fuse failure canalso be present when faults are present on the system, so a means of detecting faults and inhibiting fuse failuredeclarations during these events is provided.Once the fuse failure condition is declared, it is sealed-in until the cause that generated it disappears.An additional condition is introduced to inhibit a fuse failure declaration when the monitored circuit is de-energized;positive-sequence voltage and current are both below threshold levels.VT FUSE FAILURE 1 FUNCTION — Enables and disables the fuse failure feature for Source 1 VT Fuse Fail. VT FUSE FAILURE 1 VT FUSE FAILURE 1FUNCTION: DisabledRange: Disabled, Enabled VT FUSE FAILURE 1ALARM DELAY: 1.000 sRange: 0.000 to 65.535 s in steps of 0.001 NEUTRAL WIRE OPEN 1DETECTION: DisabledRange: Disabled, Enabled NEUTRAL WIRE OPEN 13 HARM PKP: 0.100 puRange: 0.004 to 3.000 pu in steps of 0.001
