CHAPTER 5: SETTINGS CONTROL ELEMENTSN60 NETWORK STABILITY AND SYNCHROPHASOR MEASUREMENT SYSTEM – INSTRUCTION MANUAL 5-2095The 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 each source.NEUTRAL WIRE OPEN 1 DETECTION — Enables and disables the VT neutral wire open detection function. When the VT isconnected in Delta, do not enable this function because there is no neutral wire for Delta connected VT.NEUTRAL WIRE OPEN 1 3 HRAM PKP — Specifies the pickup level of 3rd harmonic of 3V0 signal for the NEUTRAL WIRE OPENDETECTION logic to pick up.Base voltage for this element is PHASE VT SECONDARY setting in the case of WYE VTs and (PHASE VT SECONDARY)/ in caseof DELTA VTs. The setting is found under SETTINGS SYSTEM SETUP AC INPUTS VOLTAGE BANK PHASE VT SECONDARY.3
