302 D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CONTROL ELEMENTS CHAPTER 7: PROTECTION
Permissive Echo
Range: Enabled, Custom, Disabled
Default: Disabled
If this setting is “Enabled,” the hybrid POTT scheme sends a permissive echo signal to the
remote ends using pre-programmed logic (see the logic diagram). If set to “Custom,” the
echo signal is sent if the condition selected by the
Echo Condition setting is satisfied. The
echo is sent only once and then the logic locks out for the time specified by the Echo
Lockout
setting. The duration of the echo pulse is specified by the Echo Duration setting.
Operation of the over-reaching protection elements (distance zone 2 or the
Ground
Directional OC Forward
setting) inhibits the echo.
RX Pickup Delay
Range: 0.000 to 65.535 seconds in steps of 0.001
Default: 0.000 seconds
This setting enables the relay to cope with spurious received signals. Set the delay longer
than the longest spurious TX signal that can be received simultaneously with the zone 2
pickup. The selected delay increases the response time of the scheme.
Transient Block Pickup Delay
Range: 0.000 to 65.535 seconds in steps of 0.001
Default: 0.020 seconds
This setting defines a transient blocking mechanism embedded in the hybrid POTT
scheme for coping with the exposure of both the over-reaching zone 2 and ground
directional overcurrent function to current reversal conditions.
The transient blocking logic applies to both operate (trip) and send (transmit) paths.
Identifying the fault as a reverse fault prevents the scheme from both operating and
keying the channel. If the reverse fault condition prevails for the time specified by this
setting, the blocking operation is extended by the transient blocking timer for the time
specified by the
Transient Block Reset Delay setting. This allows riding through current
reversal conditions.
However, if distance zone 1 picks up during the transient blocking condition, the blocking
action is removed. This allows the IED to cope with evolving faults when an external fault
is followed by an internal fault. Without zone 1 feedback, the trip is delayed
unnecessarily.
When determining this delay value, make it not greater than the fastest possible trip
time for faults on an adjacent line so that extended blocking action can be established.
Take into account the pickup time of the reverse-looking elements of the scheme. Also,
the delay specified by this setting cannot be too short in order to avoid locking up a
spurious reverse fault indication that can occur during internal fault conditions.
Transient Block Reset Delay
Range: 0.000 to 65.535 seconds in steps of 0.001
Default: 0.090 seconds
This setting specifies a transient blocking mechanism embedded in the hybrid POTT
scheme for coping with the exposure of the overreaching protection functions to current
reversal conditions (see also the
Transient Block Pickup Delay setting).
Make this delay long enough to cope with transient conditions, including current
reversals as well as spurious negative and zero-sequence currents occurring during
breaker operations (in the case when neutral directional or negative-sequence
directional overcurrent functions are used). The breaker failure time of the surrounding
protection systems within the reach of the ground directional function used by the
hybrid POTT scheme need to be considered to make sure that the ground directional
function is not jeopardized during delayed breaker operations.
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