Longer Route Switching Interruptions; Detecting Accumulated Time Delay Changes With Echo Timing - Hitachi 670 Series Application Manual

1MRK505382-UEN Rev. K
2.3.2

Longer route switching interruptions

When echo timing is used and route switching takes longer than 2 s, internal clocks are synchronized
with asymmetric delay included. Influence from asymmetric delay is 2 s/40 ms x 10 μs (average 20
ms and 1 μs) = 0.5 ms, which can cause an unwanted trip depending on the sensitivity set for the
protection function.
When global time (GPS) is used, communication failure in the range of 10–30 seconds can cause a
synchronization loss of the internal clocks. Communication failure longer than 30 seconds always
requires a re-synchronization of the internal clocks. If synchronization is lost, 5–15 seconds of
healthy communication is required to get the internal clocks synchronized again and to get them
working with normal tripping times.
2.3.3

Detecting accumulated time delay changes with echo timing

Echo timing has an additional feature that detects accumulated time delay changes below the
MaxtDiffLevel of ±200-2000 μs. This is done to avoid small changes from creating an accumulated
asymmetry which can cause an unwanted trip. Due to asymmetry in communication channel delay,
jitter and wander, and buffer memory, this feature is restricted to four time delay changes.
The parameter used for detecting the accumulated time delay changes (sensitivity) is DeadbandtDiff,
and its setting must be coordinated with that of MaxtDiffLevel. The influence of MaxtDiffLevel on
sensitivity is shown in Figure 5.
A A
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4500 4500
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IEC07000225 V1 EN-US
Figure 5:
The DeadbandtDiff setting range is ±200-1000 μs. If the accumulated time delay of up to four
changes is greater then the set value for DeadbandtDiff but below the set value for MaxtDiffLevel, no
action is taken. When the fifth time delay change occurs, the protection function is blocked even if the
maximum deviation is not reached. This restriction is included because of measuring inaccuracy with
too many consecutive, accumulated time delay changes. After blocking, the communication set-up
must be checked before restarting.
Communication set-up, 670/650 series
Application Guide
0 0 5 5 10 10
Fault current at external faults
Virtual error in amperes at different asymmetric delay times in a
telecommunication network
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Telecommunication networks and line differential protection
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Section 2
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1 ms
0.8 ms 0.8 ms
0.6 ms 0.6 ms
0.4 ms
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0.2 ms
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