GE L30 Instruction Manual page 200

5.4 SYSTEM SETUP
• Change in the round trip travel time. This value is measured by the L30 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
with other factors to decide, upon GPS loss, if the relays continue to compensate using the memorized correction.
Typically, one may decide to disable compensation if the round trip time changes.
• BLOCK GPS TIME REF: This setting signals to the L30 that the time reference is not valid. The time reference may be
not accurate due to problems with the GPS receiver. The user must to be aware of the case when a GPS satellite
receiver loses its satellite signal and reverts to its own calibrated crystal oscillator. In this case, accuracy degrades in
time and may eventually cause relay misoperation. Verification from the manufacturer of receiver accuracy not worse
than 250 μs and the presence of an alarm contact indicating loss of the satellite signal is strongly recommended. If the
time reference accuracy cannot be guaranteed, it should be relayed to the L30 via contact inputs and GPS compensa-
tion effectively blocked using the contact position in conjunction with the
typically a signal from the GPS receiver signaling problems or time inaccuracy.
Some GPS receivers can supply erroneous IRIG-B signals during power-up and before locking to satellites. If the
receiver's failsafe contact opens during power-up (allowing for an erroneous IRIG-B signal), then set a dropout delay
up to 15 minutes (depending on GPS receiver specifications) to the failsafe contact via FlexLogic™ to prevent incor-
rect relay response.
• MAX CHNL ASYMMETRY: This setting detects excessive channel asymmetry. The same threshold is applied to both
the channels, while the following per-channel FlexLogic™ operands are generated:
. These operands can be used to alarm on problems with communication equipment and/or to decide
2 MAX ASYM
whether channel asymmetry compensation remains in operation should the GPS-based time reference be lost. Chan-
nel asymmetry is measured if both terminals of a given channel have valid time reference.
If the memorized asymmetry value is much greater than expected (indicating a significant problem with IRIG-B timing),
then this operand can be also used to block GPS compensation, forcing the relay to use the memorized asymmetry
5
value.
• ROUND TRIP TIME CHANGE: This setting detects changes in round trip time. This threshold is applied to both chan-
nels, while the
87L DIFF 1 TIME CHNG
ated. These operands can be used to alarm on problems with communication equipment and/or to decide whether
channel asymmetry compensation remains in operation should the GPS-based time reference be lost.
5-64
and ). These operands can be latched in FlexLogic™ and combined
87L DIFF 2 TIME CHNG
and
87L DIFF 2 TIME CHNG ASYM
L30 Line Current Differential System
setting. This setting is
BLOCK GPS TIME REF
87L DIFF 1 MAX ASYM
per-channel FlexLogic™ operands are gener-
5 SETTINGS
and
87L DIFF
GE Multilin