GE D90 Plus Instruction Manual page 229

CHAPTER 7: PROTECTION
Stepped distance scheme application guidelines
Phase current supervision and fuse failure for the stepped distance scheme
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Reset Delay
Range: 0.000 to 65.535 seconds in steps of 0.001
Default: 0.000 seconds
This setting specifies a delay for the reset of the load encroachment element between
the operate output state and the return to logic 0 after the input passes outside the
defined pickup range. It is normally set to zero.
Block
Range: any FlexLogic™ operand
Default: Off
Assertion of the FlexLogic™ operand assigned to this setting will block operation of the
load encroachment element.
Events
Range: Enabled, Disabled
Default: Enabled
This setting enables and disables the logging of load encroachment events in the
sequence of events recorder.
The logic for the load encroachment scheme is shown below.
Figure 194: Load encroachment scheme logic
This section provides general application guidelines for implementing stepped distance
Plus
schemes with the D90 .
The phase-to-phase (delta) current is used to supervise the phase distance elements,
primarily to ensure that in a de-energized state the distance elements will not be picked up
due to noise or induced voltages on the line.
However, this supervision feature may also be employed to prevent operation under fuse
failure conditions. This obviously requires that the setting must be above maximum load
current and less than the minimum fault conditions for which operation is expected. This
potential problem may be avoided by the use of a separate fuse fail function, which means
that the phase current supervision can be set much lower, typically two times the
capacitance charging current of the line.
The usage of the fuse fail function is also important during double-contingency events
such as an external fault during fuse fail conditions. The current supervision alone would
not prevent maloperation in such circumstances.
It must be kept in mind that the fuse failure element provided on the D90
detect fuse fail conditions. This may create a race between the instantaneous zone 1 and
the fuse failure element. Therefore, for maximum security, it is recommended to set the
GROUPED PROTECTION ELEMENTS
Plus
needs time to
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