Reference Clock Deviation From The Set Maximum Time Deviation - Hitachi 670 Series Application Manual
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Section 2
Telecommunication networks and line differential protection
The setting range for MaxtDiffLevel is ±200-2000 μs, and it must be coordinated with reference to the
sensitivity of the protection function. This setting is only relevant for echo timing.
MaxtDiffLevel is defined and measured at a sudden change in time difference between the line ends
induced by route switching. If the set value for MaxtDiffLevel is exceeded, the protection function
gets blocked. To avoid unwanted trips produced by a number of small changes below the set value,
these small changes are summated and checked to be below MaxtDiffLevel.
The line differential protection IED has no supervising function that could detect the difference
between asymmetric delay, buffer memory delay, telecommunication network jitter and wander, and
internal clock drift. The sum of these factors is supervised by observing the deviation between the
internal clocks in all IEDs.
GPSSyncErr denotes GPS synchronization error, and it is activated when GPS global time and
internal clock time deviate by more than 16 μs. If activated, the differential protection function can be
set to be blocked or the echo mode can be set to be activated.
In an SDH network (≥ 2 Mbit/s) with an acceptably small asymmetric delay, for example G.703 E1,
maximum time deviation of 2 x ±300 μs can be used. In a PDH network (n x 64 kbit/s) with a buffer
memory of for example 100 μs, typical maximum time deviation is 2 x ±400 μs. The actual setting
value is to be calculated accordingly.
2.3.1.2
Reference clock deviation from the set maximum time deviation
Reference clock deviation lower than the set maximum time deviation
Route switching that causes the virtual difference between the internal clocks of respective line
differential protection IEDs to be lower than the set maximum time deviation due to asymmetry in the
communication channel delay, jitter and wander, and buffer memory, does not cause a
communication failure alarm or blocking of the protection function. The deviation is considered to be
within the accuracy requirements, and will be compensated for by the normal synchronization
mechanisms of the internal clocks.
If route switching takes longer than 2 s, master and slave IEDs start to re-synchronize after a 4 s
delay incorporating the new channel asymmetry. Synchronization adjusts the internal clock
differences of differential protection IEDs at different line ends to be within ±1 μs. Time
synchronization messages are evaluated every 5 ms. If maximum time deviation is set to ±200 μs, it
takes approximately 10 seconds to reach new synchronization. Synchronization reaches the ±1 μs
accuracy after an additional 10-15 seconds.
Reference clock deviation higher than the set maximum time deviation
Route switching that causes the virtual difference between the internal clocks of respective line
differential protection IEDs to be higher that the set maximum time deviation (±200-2000 μs) due to
asymmetry in the communication channel delay, jitter and wander, and buffer memory, causes the
blocking of the internal clock synchronization. If this deviation lasts under 2 s, the protection function
still remains in operation. The total stability time of 2 seconds stems from the route switching being
supervised during 50 internal clock synchronization messages that are sent every 5 ms but evaluated
every 40 ms.
If deviation between the internal clocks is higher than the set limit after 2 s, a communication failure
alarm is issued and the protection function gets blocked. After communication channels are restored,
a re-synchronization with initial 20 μs clock adjustment steps takes place for each clock
synchronization message. The clock adjustment steps gradually decrease as the internal clock
differences are reduced.
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1MRK505382-UEN Rev. K
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Communication set-up, 670/650 series
Application Guide
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