GE L90, Инструкция по эксплуатации

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8-14 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
DISTANCE BACKUP/SUPERVISION CHAPTER 8: APPLICATION OF SETTINGS
8
Three terminal line applications generally result in larger reach settings for the distance backup and require a calculation
of the apparent impedance for a remote fault. Carry this out for each of the three terminals, as the calculated apparent
impedance differs at each terminal.
Distance Supervision essentially offers a solution for the LV fault condition, but the differential setting must still be
increased to avoid operation for an external L-g or L-L-g fault external ground fault. In addition, the distance element
reach setting must still see all faults within the protected line and be less than the impedance for a LV bus fault.
The effective source impedance ratio (SIR) for the LV fault generally is not high, so that CVT transients do not contribute to
measuring errors.
If the distance supervision can be set to avoid operation for a transformer LV fault, then generally the filtering associated
with the distance measuring algorithm ensures no operation under magnetizing inrush conditions. The distance element
can be safely set up to 2.5
×
V
nom
/ I
peak
, where V
nom
is the system nominal voltage and I
peak
is the peak value of the
magnetizing inrush current.
For applications where the tapped station is close to one terminal, then it can be difficult to set the distance supervision to
reach the end of the line, and at the same time avoid operation for an LV fault. For this system configuration, use a three-
terminal L90; the third terminal is then fed from CT on the high side of the tapped transformer.
8.4.2 Phase distance
8.4.2.1 Phase current supervision and the fuse failure element
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 are not picked up due to noise or induced voltages on the line.
This supervision feature can also be employed to prevent operation under fuse failure conditions. This requires that the
setting be above maximum load current and less than the minimum fault conditions for which operation is expected. This
potential problem can 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 does not prevent maloperation in such circumstances.
Keep in mind that the fuse failure element provided on the L90 needs some time to detect fuse fail conditions. This can
create a race between the instantaneous zone 1 and the fuse failure element. Therefore, for maximum security, it is
recommended to both set the current supervision above the maximum load current and use the fuse failure function. The
current supervision prevents maloperation immediately after the fuse fail condition giving some time for the fuse failure
element to take over and block the distance elements permanently. This is of a secondary importance for time-delayed
zones 2 and up as the fuse failure element has some extra time for guaranteed operation. The current supervision can be
set below the maximum load current for the time delayed zones.
Blocking distance elements during fuse fail conditions are not be acceptable in some applications and/or under some
protection philosophies. Applied solutions vary from not using the fuse failure element for blocking at all, to using it and
modifying—through FlexLogic and multiple setting groups mechanisms—other protection functions or other relays to
provide some protection after detecting fuse fail conditions and blocking the distance elements, and to using it and
accepting the fact that the distance protection does not respond to subsequent internal faults until the problem is
addressed.
8.4.2.2 Phase distance zone 1
As typically used for direct tripping, the zone 1 reach must be chosen so that it does not extend beyond the far end(s) of the
protected line. Zone 1 provides nominally instantaneous protection for any phase fault within a pre-determined distance
from the relay location. To ensure that no overreach occurs typically requires a setting of 80 to 90% of the line length,
which covers CT and VT errors, relay inaccuracy and transient overreach, as well as uncertainty in the line impedance for
each phase, although transposition can minimize this latter concern.
To be fully operational, the Fuse Failure element must be enabled, and its output FlexLogic operand must be
indicated as the blocking signal for the selected protection elements.
For convenience, the current supervision threshold incorporates the
factor.