CHAPTER 5: SETTINGS
CONTROL ELEMENTS
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
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This scheme is intended for two-terminal line applications only. It uses an over-reaching zone 2 distance element to
essentially compare the direction to a fault at both the ends of the line. Ground directional overcurrent functions available
in the relay can be used in conjunction with the zone 2 distance element to key the scheme and initiate its operation. This
provides increased coverage for high resistance faults.
For proper scheme operation, enable and configure the zone 2 phase and ground distance elements per the rules of
distance relaying. Enable and configure the line pickup element to detect line-end-open/weak-infeed conditions. If used by
this scheme, enable and configure the selected ground directional overcurrent functions accordingly.
POTT PERMISSIVE ECHO
— When "Enabled," this setting results in a permissive echo signal being sent to the remote end. The
permissive signal is echoed back upon receiving a reliable
POTT RX
signal from the remote end while the line-end-open
condition is identified by the line pickup logic. The permissive echo is programmed as a one-shot logic. The echo is sent
only once and then the echo logic locks out for a settable period of time (
ECHO LOCKOUT
setting). The duration of the echo
pulse does not depend on the duration or shape of the received
POTT RX
signal but is settable as
ECHO DURATION
.
POTT RX PICKUP DELAY
— This setting enables the relay to cope with spurious receive signals. Set the delay longer than the
longest spurious
TX
signal that can occur simultaneously with the zone 2 pickup. The selected delay increases the response
time of the scheme.
TRANS BLOCK PICKUP DELAY
— This setting defines a transient blocking mechanism embedded in the POTT scheme for
coping with the exposure of a ground directional overcurrent function (if used) to current reversal conditions. The transient
blocking mechanism applies to the ground overcurrent path only as the reach settings for the zone 2 distance functions is
not expected to be long for two-terminal applications, and the security of the distance functions is not endangered by the
current reversal conditions. Upon receiving the
POTT RX
signal, the transient blocking mechanism allows the
RX
signal to be
passed and aligned with the
GND DIR O/C FWD
indication only for a period of time defined as
TRANS BLOCK PICKUP DELAY
.
After that the ground directional overcurrent path is virtually disabled for a period of time specified as
TRANS BLOCK RESET
DELAY
.
Set the
TRANS BLOCK PICKUP DELAY
long enough to give the selected ground directional overcurrent function time to
operate, but not longer than the fastest possible operation time of the protection system that can create current reversal
conditions within the reach of the selected ground directional overcurrent function. For this setting, also take into account
the
POTT RX PICKUP DELAY
. The
POTT RX
signal is shaped for aligning with the ground directional indication as follows: the
original
RX
signal is delayed by the
POTT RX PICKUP DELAY
, then terminated at
TRANS BLOCK PICKUP DELAY
after the pickup of
the original
POTT TX
signal, and eventually, locked-out for
TRANS BLOCK RESET DELAY
.
TRANS BLOCK RESET DELAY
— This setting defines a transient blocking mechanism embedded in the POTT scheme for
coping with the exposure of a ground directional overcurrent function (if used) to current reversal conditions (see also the
TRANS BLOCK PICKUP DELAY
). Set this delay long enough to cope with transient conditions including not only current
reversals but also spurious negative and zero-sequence currents occurring during breaker operations. The breaker failure
time of the surrounding protection systems within the reach of the ground directional function used by the POTT scheme
can be considered to make sure that the ground directional function is not jeopardized during delayed breaker operations.
ECHO DURATION
— This setting defines the guaranteed and exact duration of the echo pulse. The duration does not depend
on the duration and shape of the received
POTT RX
signal. This setting enables the relay to avoid a permanent lock-up of the
transmit/receive loop.
ECHO LOCKOUT
— This setting defines the lockout period for the echo logic after sending the echo pulse.
LINE END OPEN PICKUP DELAY
— This setting defines the pickup setting for validation of the line end open conditions as
detected by the Line Pickup logic through the
LINE PICKUP LEO PKP
FlexLogic operand. The validated line end open condition
is a requirement for the POTT scheme to return a received echo signal (if the echo feature is enabled). Also for this setting,
take into account the principle of operation and settings of the line pickup element.
GND DIR O/C FWD:
Off
Range: FlexLogic operand
POTT RX:
Off
Range: FlexLogic operand
POTT SCHEME TARGET:
Self-Reset
Range: Self-Reset, Latched, Disabled
POTT SCHEME EVENTS:
Disabled
Range: Disabled, Enabled
