GE 845 Instruction Manual page 421

CHAPTER 4: SETPOINTS
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
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
FUNCTION
Range: Disabled, Alarm, Latched Alarm, Configurable
Default: Disabled
OUTPUT RELAY 3 (X)
Range: Do Not Operate, Operate
Default: Do Not Operate
EVENTS
Range: Enabled, Disabled
Default: Enabled
TARGETS
Range: Disabled, Self-reset, Latched
Default: Self-reset
CONTROL
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