Telecommunication Networks With Symmetric Or Fixed Routes - Hitachi 670 Series Application Manual

Section 2
Telecommunication networks and line differential protection
IEC07000240 V2 EN-US
Figure 2:
2.3 Telecommunication networks with symmetric or fixed
routes
It is possible to use networks with fixed routes where symmetric time delay is applied, or networks
with fixed route switching where both directions have a symmetric time delay even after route
switching has been performed. In these types of networks, different channel delay times are
automatically compensated for, and echo timing can be used. If a fixed route has specified
asymmetry, this can be compensated for by using the AsymDelay parameter.
The maximum interruption time for route switching and echo timing (for example when a
communication channel is lost) without affecting the synchronization of internal clocks is 2 s. The
protection function gets blocked for the duration of the interruption. The 2 s time limit is derived from
the stability of the internal clocks. If route switching takes longer than that, the protection function
gets blocked until the internal clocks are re-synchronized. Proper re-synchronization of the internal
clocks takes an additional 15 seconds after the channel is restored.
From protection point-of-view, route switching interruptions should be < 50 ms (normally route
switching takes < 0.1 ms). The maximum time delay in a telecommunication network can be set to 40
ms (default delay being 20 ms). In echo timing mode, this time represents round-trip delay divided by
two. If the channel delay is longer than the set value, the protection function gets blocked.
The protection function also gets blocked if the maximum time deviation (MaxtDiffLevel) between the
line differential protection IEDs' internal clocks is higher than ±200-2000 μs (see Figure 3). Blocking
of the function lasts until the internal clocks' deviation reaches the set value range. Time required for
new synchronization depends on the interruption time.
12
Telecommunication networks for electric power utilities
The recommended value for maximum time delay when using echo timing is 20 ms.
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Communication set-up, 670/650 series
Application Guide