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L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CONTROL ELEMENTS
CHAPTER 5: SETTINGS
5
In single-pole tripping applications, the scheme uses local fault type identification provided by the phase selector together
with information received from the remote terminal. The scheme generates output operands (
HYBRID POTT TX1
through
HYBRID POTT TX4
) that are used to transmit the signal to the remote terminal(s). Choices of communications channel include
remote inputs/outputs and telecommunications interfaces. When used with telecommunications facilities, assign the
output operand to operate an output contact connected to key the transmitter at the interface. When used with
telecommunications facilities, assign the output operands to operate output contacts connected to assert the individual
bits at the interface.
To make the scheme fully operational as a stand-alone feature, the scheme output operands must be configured to
interface with other relay functions, output contacts in particular. Typically, you program the output operands to initiate a
trip, breaker fail, and autoreclose, and drive a user-programmable LED as per user application.
When used in conjunction with the trip output element, the scheme is pre-configured to initiate trip, breaker fail, and
single-pole autoreclose actions.
HYB POTT BLOCK
— This setting allows the user to assign any FlexLogic operand to block the scheme. Contact inputs from a
pilot cut-out switch are typically used for this purpose.
HYB POTT PERMISSIVE ECHO
— If set to “Enabled”, the scheme sends a permissive echo signal to the remote end(s) using a
pre-programmed logic (see logic diagram below). If set to “Custom”, the echo signal is sent if a condition selected via the
HYB POTT ECHO COND
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 settable as
ECHO DURATION
. Operation of the overreaching
protection elements (distance zone 2 or
GND DIR O/C FWD
setting) inhibits the echo.
HYB POTT ECHO COND
— This setting specifies a user-selected echo condition and applies if the
HYB POTT PERMISSIVE ECHO
is
set to “Custom”.
HYB POTT WEAK INFEED
— If this setting is set to “Enabled”, the scheme activates both the keying and operating paths using
a pre-programmed weak infeed logic (see logic diagram below). If this setting is set to “Custom”, the weak infeed condition
is to be specified by the user via the
HYB POTT W/I COND
setting.
HYB POTT W/I COND
— This setting specifies user-selected weak infeed condition and applies if the
HYB POTT WEAK INFEED
is
set to “Custom”.
HYB POTT RX PICKUP DELAY
— 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.
TRANS BLOCK PICKUP DELAY
— 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
TRANS BLOCK
PICKUP DELAY
, the blocking operation is extended by the transient blocking timer for
TRANS BLOCK RESET DELAY
. 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 is to cope
with evolving faults when an external fault is followed by an internal fault. Without the zone 1 feedback, the trip is delayed
unnecessarily.
Do not set the
TRANS BLOCK PICKUP DELAY
longer 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.
Do not set the delay too short, in order to avoid locking up a spurious reverse fault indication that can occur during internal
fault conditions.
TRANS BLOCK RESET DELAY
— This setting defines 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
TRANS BLOCK
PICKUP DELAY
).