GE L30 Instruction Manual page 358

SYSTEM SETUP
• If the
CHNL ASYM COMP
ping-pong technique.
• If the
CHNL ASYM COMP
compensation is applied and the L30 effectively uses GPS time referencing tracking channel asymmetry if the latter
fluctuates.
• If
CHNL ASYM COMP
system was not compensated prior to the problem), or the memorized (last valid) compensation is used if
compensation was in effect prior to the problem.
A terminal is considered to have a valid time reference while the
the
LOCAL GPS TROUBLE
via PP, PTP, or DC Shift IRIG-B. It is also On if locked via Amplitude Modulated IRIG-B, and if locked via PP or PTP and
indicated clock accuracy is less than 250 µs.
The
CHNL ASYM COMP
several factors typically is used. The L30 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 programmed by
selecting the
CHNL ASYM COMP
• Fail-safe output of the GPS receiver — Some receivers can be equipped with the fail-safe output relay. The L30
system requires a maximum error of 250 µs. The fail-safe output of the GPS receiver can be connected to the local L30
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 L30 and a user-
programmable threshold is applied to it. The corresponding FlexLogic operands are produced if the asymmetry is
above the threshold
5
combined with other factors to decide, upon GPS loss, if the relays continue to compensate using the memorized
correction. Typically, one decides to keep compensating if the pre-existing asymmetry is low.
• 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
and
TIME CHNG
decide, upon GPS loss, if the relays continue to compensate using the memorized correction. Typically, one decides to
disable compensation if the round trip time changes.
BLOCK GPS TIME REF
the GPS receiver. 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 can eventually cause relay misoperation.
Verification from the receiver manufacturer of 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, relay
this to the L30 via contact inputs and block GPS compensation using the contact position in conjunction with the
setting. This setting is typically a signal from the GPS receiver signaling problems or time inaccuracy.
GPS TIME REF
Some GPS receivers 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 incorrect relay response.
MAX CHNL ASYMMETRY
channels, while the per-channel
operands can be used to alarm on problems with communication equipment and/or to decide whether channel
asymmetry compensation remains in operation if the GPS-based time reference is lost. Channel 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 GPS clock timing),
then this operand also can be used to block GPS compensation, forcing the relay to use the memorized asymmetry value.
ROUND TRIP TIME CHANGE
while the
87L DIFF 1 TIME CHNG
operands can be used to alarm on problems with communication equipment and/or to decide whether channel
asymmetry compensation remains in operation if the GPS-based time reference is lost.
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selected FlexLogic operand is "Off," compensation is not applied and the L30 uses only the
selected FlexLogic operand is "On" and all terminals have a valid time reference, then
is "On" and not all terminals have a valid time reference, then compensation is not applied (if the
signal is Off. The
LOCAL GPS TROUBLE
setting dynamically turns the GPS compensation on and off. A FlexLogic operand that combines
setting. Factors to consider are:
(87L DIFF 1 MAX ASYM and
CHNG). These operands can be latched in FlexLogic and combined with other factors to
87L DIFF 2 TIME
— This setting signals to the L30 that the time reference is not valid, for example due to problems with
— This setting detects excessive channel asymmetry. The same threshold is applied to both
87L DIFF 1 MAX ASYM
— This setting detects changes in round-trip time. This threshold is applied to both channels,
and
87L DIFF 2 TIME CHNG ASYM
BLOCK GPS TIME REF
signal is On if the real time clock is not locked to global time
setting.
BLOCK GPS TIME REF
ASYM). These operands can be latched in FlexLogic and
87L DIFF 2 MAX
and FlexLogic operands are generated. These
87L DIFF 2 MAX ASYM
per-channel FlexLogic operands are generated. These
L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
selected FlexLogic operand is Off and
(87L DIFF 1
BLOCK