GE UR Series L90, Instruction Manual

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10-14 L90 Line Current Differential System GE Multilin
10.4 DISTANCE BACKUP/SUPERVISION 10 APPLICATION OF SETTINGS
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10.4.2 PHASE DISTANCE
a) 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-ener-
gized 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 opera-
tion 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 L90 needs some time to 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 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 ele-
ment 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 may be set
below the maximum load current for the time delayed zones.
Blocking distance elements during fuse fail conditions may not be acceptable in some applications and/or under some pro-
tection philosophies. Applied solutions may vary from not using the fuse failure element for blocking at all; through 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; to using it and accepting
the fact that the distance protection will not respond to subsequent internal faults until the problem is addressed.
To be fully operational, the Fuse Failure element must be enabled, and its output FlexLogic operand must be indi-
cated as the blocking signal for the selected protection elements.
For convenience, the current supervision threshold incorporates the factor.
b) 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 may minimize this latter concern.
The total relay inaccuracy including both steady state and transient overreach even when supplied from CVTs under the
source impedance ratios of up to 30, is below 5%.
c) PHASE DISTANCE ZONE 2
Zone 2 is an overreaching element, which essentially covers the final 10 to 20% whole of the line length with a time delay.
The additional function for the zone 2 is as a timed backup for faults on the remote bus. Typically the reach is set to 125%
of the positive-sequence impedance of the line, to ensure operation, with an adequate margin, for a fault at 100% of the line
length. The necessary time delay must ensure that coordination is achieved with the clearance of a close-in fault on the
next line section, including the breaker operating time.
The zone 2 time delay is typically set from 0.2 to 0.6 seconds, although this may have to be reviewed more carefully if a
short line terminates on the remote bus, since the two zone 2 elements may overlap and therefore not coordinate in a sat-
isfactory manner.
d) PHASE DISTANCE ZONE 3
If a remote backup philosophy is followed, then the reach of this element must be set to account for any infeed at the
remote bus, plus the impedance of the longest line which terminates on this remote bus. The time delay must coordinate
with other time-delayed protections on any remote line. Circuit loading limitations created by a long zone reach may be
overcome by using lens or quadrilateral characteristics and/or a load encroachment supervising characteristic. Consider-
NOTE
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