4–370 889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUALCONTROL CHAPTER 4: SETPOINTSVT Fuse Failure (VTFF)The 889 relay provides one VT Fuse Failure. The VT Fuse Failure detector can be used toraise an alarm and/or block elements that may operate incorrectly for a full or partial lossof AC potential caused by one or more blown fuses. Some elements that might be blocked(via the BLOCK input) are voltage restrained overcurrent, directional current, powerfunctions. This loss can be caused by a blown primary voltage transformer fuse (or fuses),or by voltage transformer secondary circuit protection fuse failure.There are two classes of fuse failure that may occur:1. Class A: loss of one or two phases2. Class B: loss of all three phases.Different means of detection are required for each class. An indication of a Class A failure isa significant level of negative sequence voltage, whereas an indication of a Class B failureis the presence of positive sequence current and an insignificant amount of positivesequence voltage. These noted indications of fuse failure could also be present when faultsare 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 itdisappears. An additional condition is introduced to inhibit a fuse failure declaration whenthe monitored circuit is de-energized: positive sequence voltage and current are bothbelow threshold levels.The settings of this function are applied to three-phase voltage input (supervised withpositive, negative and zero sequence current components) to produce an Operate flag.Path: Setpoints > Control > VT Fuse Failure 1 (2)FUNCTIONRange: Disabled, Alarm, Latched Alarm, ConfigurableDefault: DisabledOUTPUT RELAY XFor details see Common Setpoints.EVENTSRange: Enabled, DisabledDefault: EnabledTARGETSRange: Disabled, Self-reset, LatchedDefault: Self-reset
