Maximum transmission time
T
d
= 40 ms (forward + reverse)
IEC07000160-1-en.vsdx
Unspecified
route switching
A
B
C
GPS
clock
GPS
clock
GPS
clock
IED
IED
IED
IEC07000160 V1 EN-US
Figure 9: Three-end application using IEDs' built-in GPS receivers
With good GPS antenna visibility, it takes up to 15 minutes for the GPS system to
find the satellites (connection to a minimum of four satellites required), and start
synchronizing the IED's internal clock with global time from the GPS system.
IEC10000203 V1 EN-US
Figure 10: Setting the GPS clock
At synchronization start, the internal clock is adjusted to have the correct global
time in seconds with only integers set. The remaining fractional parts, milliseconds
(ms) and microseconds (μs), to reach global time, are used to slowly synchronize
the internal clock to global time.
Synchronizing the internal clock to global time is done at a rate of 1 ms/s. When
the internal clock is within 16 μs from global time, the protection function is
enabled. If time deviation between the internal clock time and global time is more
than 16 μs, GPS timing is deactivated and the differential protection is either
blocked or changed to Echo mode depending on the setting for LDCM (GPSerr).
After the protection function is enabled, soft calibration is performed to have the
internal clock remain within 1 μs from global time for < 1 hour. This reduces the
1MRK 505 382-UEN B Section 3
Common time synchronization
Communication set-up 670/650 series 2.2 23
Application Guide
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