Time Synchronization Using A Built-In Gps Receiver - Hitachi 670 Series Application Manual

Section 3
Common time synchronization
Fast clock synchronization mode
At start-up and after interruptions in the GPS, PTP or IRIG B time signals, the deviation between the
external time system and the internal differential time system can be substantial. A new start-up is
also required, for example, after maintenance of the auxiliary voltage system.
When the time difference is > 16 μs, the line differential protection function is blocked and the time
regulator for the hardware clock automatically uses a fast mode to synchronize the clock systems.
Time adjustment is made with an exponential function, that is, with big time adjustment steps in the
beginning, and then smaller steps until a time deviation between the GPS time and the differential
time system of < 16 μs has been reached. The protection function is then enabled and the
synchronization remains in fast mode or switches to slow mode depending on the setting.
Slow clock synchronization mode
When Echo mode is used as GPS-error method, (GPSerr setting), a slow synchronization mode
might be preferred.
This prevents the hardware clock to make too big time steps and allows the ECHO method to "keep
up" so that no block will occur while regaining GPS synchronization.
At start-up a fast adjustment will be used in order to get synchronized as fast as possible.
When GPS synchronization is reached, and the protection is unblocked, slow clock synchronization mode is activated
(if selected).
3.2

Time synchronization using a built-in GPS receiver

Telecommunication networks with unspecified route switching require that the line differential
protection IED's built-in GPS receiver is used. This way the protection function can operate correctly
independent of asymmetric delays in communication channels.
The maximum allowed time deviation in a telecommunication network is 40 ms representing the sum
of deviations in both directions. It is also possible to make an asymmetric split, for example 10 ms in
one direction and 30 ms in the other direction, because the deviation can be accurately measured
when GPS timing is available. With longer deviations, the protection function gets blocked.
IEC07000160 V2 EN-US
Figure 9:
18
Three-end application using IEDs' built-in GPS receivers
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Communication set-up, 670/650 series
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