Introduction
1.2 Application Scope
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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1.2
Application Scope
The SIPROTEC 4 7SD5 line protection is a protection relay that combines differential and distance protection.
A multi-end fault locator allows to precisely locate faults in two-end lines, even in case of unfavourable operat-
ing or fault conditions.
The combined line protection is a selective short-circuit protection for overhead lines and cables with single-
and multi-ended infeeds in radial, ring or any type of meshed systems of any voltage level. Measuring data are
compared separately for each phase. The network neutral can be earthed, compensated or isolated.
The device incorporates the functions which are normally required for the protection of an overhead line feeder
and is therefore capable of universal application. It may also be applied as time graded back-up protection to
all types of comparison protection schemes used on lines, transformers, generators, motors and busbars of all
voltage levels.
The inrush current restraint also allows the application of the 7SD5 even if a power transformer is situated
within the protected zone (ordering option) whose starpoint(s) might also be isolated, earthed or provided with
a Petersen coil.
A major advantage of the differential protection principle is the instantaneous tripping in the event of a short-
circuit at any point within the entire protected zone. The current transformers limit the protected zone at the
ends towards the remaining system. This rigid limit is the reason why the differential protection scheme shows
such an ideal selectivity.
The line protection system requires a 7SD5 device as well as a set of current transformers at either end of the
protected zone.
Voltage transformers are required if protection functions requiring a voltage measurement (e.g. distance pro-
tection, fault locator) are used in addition to the differential protection. They are also needed for the acquisition
and display of measured values (voltages, power, power factor).
The devices located at the ends of the protected zone exchange measuring information via protection data in-
terfaces using dedicated communication links (usually fibre optic cables) or a communication network, provided
that they operate with differential protection. The distance protection can exchange measuring information via
teleprotection functions with conventional connections (contacts), or transmit it through fast command channels
on the protection data interfaces (can be configured with DIGSI). Two type 7SD5 devices can be used for a
protected object with two ends: Cables, overhead line or both, even with transformer in the protected zone
(order variant). With type 7SD5 protected objects having 3 (three-terminal lines) or more ends can be protected
in addition to two-end lines, also with or without unit-connected transformer(s) (order variant). A maximum of 6
ends can be covered, which means that smaller busbar arrangements can also be protected. For each end a
7SD5*3 is used. If you set up a communication chain between more than two devices, 7SD5*2 can also be
used at the ends of the chain. For more information please refer to Section 2.2.1.
The protection data communication can be set up as a ring. This enables redundant operation in the event that
one communication line fails; the devices will automatically find the remaining healthy communication lines. But
even with two ends, the communication can be doubled to create redundancies.
Since fault-free data transmission is the prerequisite for the proper operation of the differential protection, it is
continuously monitored internally.
In the event of a communication failure, if there is no backup channel available, the devices can automatically
be switched to the second main protection function, i.e. distance protection, or to emergency operation using
an integrated time overcurrent protection, until communication is restored.
The communication can be used for transmitting further information. Apart from measured values, the trans-
mission of binary commands or other information is also possible.
Alternatively the distance protection can be used as backup protection, just as the time overcurrent protection
can be used as backup time overcurrent protection, i.e. both operate independently and in parallel to the differ-
ential protection at each end.
