GE L90 Instruction Manual page 132

5.3 SYSTEM SETUP
• ZERO-SEQUENCE CURRENT REMOVAL: This setting facilitates application of the L90 to transmission lines with
tapped transformer(s) without current measurement at the tap(s). If the tapped transformer is connected in a grounded
wye on the line side, it becomes a source of the zero-sequence current for external ground faults. As the transformer
current is not measured by the L90 protection system, the zero-sequence current would create a spurious differential
signal and may cause a false trip. If enabled, this setting forces the L90 to remove zero-sequence current from the
phase currents prior to forming their differential signals, ensuring protection stability on external ground faults. How-
ever, zero-sequence current removal may cause all three phases to trip for internal ground faults. Consequently, a
phase selective operation of the L90 is not retained if the setting is enabled. This does not impose any limitation, as
single-pole tripping is not recommended for lines with tapped transformers. Refer to Chapter 9 for guidelines.
• LOCAL (TERMINAL 1 and TERMINAL 2) ID NUMBER: In installations using multiplexers or modems for communica-
tion, it is desirable to ensure the data used by the relays protecting a given line comes from the correct relays. The L90
performs this check by reading the ID number contained in the messages sent by transmitting relays and comparing
this ID to the programmed correct ID numbers by the receiving relays. This check is used to block the differential ele-
ment of a relay, if the channel is inadvertently set to Loopback mode, by recognizing its own ID on a received channel.
If an incorrect ID is found on a either channel during normal operation, the FlexLogic™ operand 87 CH1(2) ID FAIL is
set, driving the event with the same name. The result of channel identification is also available in
!"
STATUS CHANNEL TESTS
"0" at local relay ID setting indicates that the channel ID number is not to be checked. Refer to the Current Differential
section in this chapter for additional information.
• CHNL ASYM COMP: This setting enables/disables channel asymmetry compensation. The compensation is based on
absolute time referencing provided by GPS-based clocks via the L90 IRIG-B inputs. This feature should be used on
multiplexed channels where channel asymmetry can be expected and would otherwise cause errors in current differ-
ential calculations. The feature takes effect if all terminals are provided with reliable IRIG-B signals. If the IRIG-B signal
is lost at any terminal of the L90 protection system, or the Real Time Clock not configured, then the compensation is
not calculated. If the compensation is in place prior to losing the GPS time reference, the last (memorized) correction is
5
applied as long as the value of
The GPS-based compensation for channel asymmetry can take three different effects:
• If
CHNL ASYM COMP
• If
CHNL ASYM COMP
then compensation is applied and the L90 effectively uses GPS time referencing tracking channel asymmetry if
the latter fluctuates.
• If
CHNL ASYM COMP
set or IRIG-B FAILURE operand is not asserted), then compensation is not applied (if the system was not com-
pensated prior to the problem), or the memorized (last valid) compensation is used if compensation was in effect
prior to the problem.
The
CHNL ASYM COMP
bines several factors is typically used. The L90 protection system does not incorporate any pre-defined way of treating
certain conditions, such as failure of the GPS receiver, loss of satellite signal, channel asymmetry prior to the loss of
reference time, or change of the round trip time prior to loss of the time reference. Virtually any philosophy can be pro-
grammed by selecting the
• Fail-safe output of the GPS receiver. Some receivers may be equipped with the fail-safe output relay. The L90 sys-
tem requires a maximum error of 250 µs. The fail-safe output of the GPS receiver may be connected to the local
L90 via an input contact. In the case of GPS receiver fail, the channel compensation function can be effectively
disabled by using the input contact in conjunction with the
• Channel asymmetry prior to losing the GPS time reference. This value is measured by the L90 and a user-pro-
grammable threshold is applied to it. The corresponding FlexLogic™ operands are produced if the asymmetry is
above the threshold (87L DIFF MAX 1 ASYM and 87L DIFF 2 MAX ASYM). These operands can be latched in
FlexLogic™ and combined with other factors to decide, upon GPS loss, if the relays continue to compensate using
the memorized correction. Typically, one may decide to keep compensating if the pre-existing asymmetry was low.
• Change in the round trip travel time. This value is measured by the L90 and a user-programmable threshold
applied to it. The corresponding FlexLogic™ operands are produced if the delta change is above the threshold
(87L DIFF 1 TIME CHNG and 87L DIFF 2 TIME CHNG). These operands can be latched in FlexLogic™ and com-
bined with other factors to decide, upon GPS loss, if the relays continue to compensate using the memorized cor-
rection. Typically, one may decide to disable compensation if the round trip time changes.
5-42
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VALIDITY OF CHANNEL CONFIGURATION
is "On". See Chapter 9 for additional information.
CHNL ASYM COMP
(GPS) is "Off", compensation is not applied and the L90 uses only the ping-pong technique.
(GPS) is "On" and all L90 terminals have a valid time reference (
(GPS) is "On" and not all L90 terminals have a valid time reference (
setting dynamically turns the GPS compensation on and off. A FlexLogic™ operand that com-
setting. Factors to consider are:
CHNL ASYM COMP
L90 Line Differential Relay
for commissioning purposes. The default value
BLOCK GPS TIME REF
(GPS) setting.
BLOCK GPS TIME REF
5 SETTINGS
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ACTUAL VALUES
not set),
not
BLOCK GPS TIME REF
GE Multilin