CHAPTER 4: SETPOINTS CONTROL
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL 4–401
VT Fuse Failure (VTFF)
The 850 relay provides one VT Fuse Failure. The VT Fuse Failure detector can be used to
raise an alarm and/or block elements that may operate incorrectly for a full or partial loss
of 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, power
functions. 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 phases
2. Class B: loss of all three phases.
Different means of detection are required for each class. An indication of a Class A failure is
a significant level of negative sequence voltage, whereas an indication of a Class B failure
is the presence of positive sequence current and an insignificant amount of positive
sequence voltage. These noted indications of fuse failure could also be present when faults
are present on the system, so a means of detecting faults and inhibiting fuse failure
declarations 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.
The settings of this function are applied to three-phase voltage input (supervised with
positive, negative and zero sequence current components) to produce an Operate flag.
Path: Setpoints > Control > VT Fuse Failure 1 (2)
FUNCTION
Range: Disabled, Alarm, Latched Alarm, Configurable
Default: Disabled
OUTPUT RELAY X
For details see Common Setpoints
.
EVENTS
Range: Enabled, Disabled
Default: Enabled
TARGETS
Range: Disabled, Self-reset, Latched
Default: Self-reset
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