Telecommunication Networks And Line Differential Protection; Overview; Telecommunication Network Types - Hitachi 670 Series Application Manual

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1MRK505382-UEN Rev. K

Section 2

2.1

Overview

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)

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.

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' communication modems. This clock can

also be a GPS clock but the telecommunication network synchronization is totally separate from the

line differential protection IED's internal clock synchronization.

2.2

Telecommunication network types

There are two main types of telecommunication networks used by electric power utilities:

Plesiochronous Digital Hierarchy (PDH) networks and Synchronous Digital Hierarchy (SDH)

networks (see Figure 2). These networks are mostly owned by the utilities, but it is also possible to

lease communication links from external companies.

PDH networks are used with 64 kbit/s communication. Proper synchronization of PDH networks must

be available so that hey can be used with protection applications. SDH networks are used with 2

Mbit/s communication.

Communication set-up, 670/650 series

Application Guide

Telecommunication networks and

line differential protection

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,

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.