GE D90 Plus Instruction Manual page 344

CONTROL ELEMENTS
334
The VT fuse failure detector can be used to raise an alarm or block elements that can
operate incorrectly for a full or partial loss of AC potential caused by one or more blown
fuses. Some elements that can be blocked (via the Block setting) include voltage restrained
overcurrent and directional current.
There are two classes of fuse failure that can occur, as follows:
• Class A — Loss of one or two phases
• Class B — Loss of all three phases
Different 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 an insignificant amount of positive-
sequence voltage. These noted indications of fuse failure can 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.
Select the Settings > Protection > Control > VT Fuse Failure menu to access the VT fuse
failure settings.
Figure 273: VT fuse failure configuration settings
The following setting is available for each VT fuse failure element.
Function
Range: Enabled, Disabled
Default: Disabled
This setting enables and disables the fuse failure feature for each source.
The figure shows the VT fuse failure logic.
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION