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Functions
2.2 Protection Data Interfaces and Protection Data Topology
Monitoring the communication
The communication is permanently monitored by the devices.
Single faulty data telegrams are not a direct risk if they occur only occasionally. They are recognized and
counted in the device which detects the disturbance and can be read out per unit of time as statistical informa-
tion (Annunciation → Statistic).
You can define a limit for the permissible rate of faulty data telegrams. When, during operation, this limit is ex-
ceeded, an alarm is given (e.g. „PI1 Error", No. 3258 if protection data interface 1 is affected). You may
use this alarm to block the differential protection, either via binary output and input, or via logical combination
by means of the integrated user-definable logic (CFC).
If several faulty telegrams or no data telegrams at all are received, this is considered a Disturbance of the
communication as soon as a disturbance time of 100 ms (default setting, changeable) is exceeded. A corre-
sponding indication is output („PI1 Data fault", No 3229 for protection data interface 1). If the system
offers no alternative way of communication (as ring topologies would do), the differential protection will stop
operating. All devices are affected by the disturbance, since the formation of differential currents and restraint
currents is no longer possible at any of the ends. The distance protection as the second main protection func-
tion assumes the complete protection over all zones, provided that it is configured as emergency function just
like the overcurrent protection. As soon as data transmission has returned to normal, the devices switch auto-
matically back to differential protection mode or differential and distance protection mode, depending on how
they are configured.
If the communication is permanently interrupted (i.e. longer than a settable time), this is considered as a Failure
of the communication. A corresponding alarm is output (e.g. „PI1 Datafailure", No 3230 for protection
data interface 1). Otherwise the same reactions apply as for the disturbance.
Transmission time jumps that, for example, can occur in case of switchover in the communication network are
recognized (e.g. „PI1 jump", No. 3254 for interface 1) and corrected by the devices. The differential protec-
tion system continues to operate without loss of sensitivity. The transmission times are measured again and
updated within less than 2 seconds. If GPS synchronization (with satellite receiver) is used, asymmetric trans-
mission times are recognized and corrected immediately.
The maximum permissible unbalance of the operating times can be set. This has a direct influence on the sen-
sitivity of the differential protection. The automatic self-restraint of the protection adapts the restraint quantities
to this tolerance so that a spurious pickup of the differential protection by these influences is excluded. Thus,
higher tolerance values reduce the sensitivity of the protection, which may be noticeable in case of very low-
current faults. With GPS synchronization transmission time differences do not affect the sensitivity of the dif-
ferential protection as long as GPS synchronization is intact. When the GPS synchronization detects that the
permissible transmission time difference is exceeded during operation, the indication „PI 1 PD unsym."
(No. 3250 for protection data interface 1) will be issued.
When a transmission time jump exceeds the maximum permissible transmission delay time, this is annunciat-
ed. If transmission time jumps occur frequently the regular operation of the differential protection is no longer
ensured. Protection communication via this communication link can be blocked via a setting parameter (e.g.
4515 PI1 BLOCK UNSYM). If a chain topology was configured, failure of one transmission link blocks the dif-
ferential protection. If a ring topology is configured, the system switches to chain topology. A corresponding
alarm is output („PI1 unsym.", No. 3256 for interface 1). This blocking of the link can only be reset via a
binary input („>SYNC PI1 RESET", No. 3252 for interface 1).
78
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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