Section 2 Telecommunication networks and line
differential protection
2.1 Overview
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Telecommunication networks have two main application areas for multi-terminal
line differential protection IEDs with up to five line ends:
• Networks with symmetric or fixed routes using echo timing (back-to-back
systems included)
• Networks with unspecified route switching using accurate global time derived
from the Global Positioning System (GPS)
Echo timing can be used as a fall-back system in the latter
application area if the GPS reference is lost in one or more IEDs,
.
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With networks using symmetric or fixed routes and echo timing, the internal clock
in each local line differential protection IED operates as a master and the internal
clock in each remote line differential protection IED operates as a slave. Time
deviations between the internal clocks are monitored continuously, and
compensated for with echo messages between all ends at 40 ms intervals over the
telecommunication network. After start-up, it takes approximately 15 seconds to
reach full synchronization of the internal clocks. Time deviation between the
internal clocks is compensated for to be within 1 μs of relative time.
The internal clock also functions as a real time clock for other
protection and monitoring functions such as event timing, but this is
totally separate from the line differential protection function.
With networks using unspecified route switching, reference for the internal clock
comes from global time provided by, for example, a built-in GPS receiver. The
internal clock in each line differential protection IED is thus set according to the
GPS systems' global time. Inaccuracy depends on the connection time to the GPS
system. After start-up (cold start), a software calibration procedure is carried out.
After less than one hour, all internal clocks' real time deviation from the global
time has decreased to 1 μs.
There can be only one master clock in the telecommunication network for the
synchronization of multiplexers, transceivers and differential protection relays'
1MRK 505 382-UEN B Section 2
Telecommunication networks and line differential protection
Communication set-up 670/650 series 2.2 13
Application Guide
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