CHAPTER 7: PROTECTION
CONTROL ELEMENTS
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
299
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 (refer to the logic diagram below). 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. The delay should be
set longer than the longest spurious TX signal that can be received simultaneously with
the zone 2 pickup. The selected delay will increase 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 will be 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 would be delayed
unnecessarily.
This setting should not be greater than the fastest possible trip time for faults on an
adjacent line so that extended blocking action could be established. This should take
into account the pickup time of the reverse-looking elements of the scheme. The delay
specified by this setting should also not 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).
This delay should be 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 should be considered to make sure that the ground directional
function is not jeopardized during delayed breaker operations.
