Time Synchronization Using Irig-B - Hitachi 670 Series Application Manual

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1MRK505382-UEN Rev. K

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:

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 restart time after a short GPS interruption.

If time deviation after the interruption is less than the set MaxtDiffLevel, the protection function can

be enabled directly.

In the case of GPS interruption, the protection function is disabled or echo timing is enabled as a

back-up synchronization method. Enabling echo timing has a delay which depends on how long the

GPS system has been in service after start. Inaccuracy of the internal clock starts at 16 μs and goes

down to ±1μs after some minutes.

In telecommunication networks with unspecified route switching, echo timing can only be used with

unchanged time delay in both directions. As soon as the time delay is outside the set limits, the

protection function gets blocked. Some additional features are available to increase the performance

of echo timing:

•

If redundant or alternative routes are available, they are also supervised for changes in time

delay. If no time delay difference is observed in these routes, echo timing continues to use the

reserve channel.

•

In telecommunication networks with unspecified route switching, time deviations below the set

limits are accumulated to handle multiple route switching, and these accumulated deviations can

reach the set time deviation limit.

3.3