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Figure 2-25
The communication chain can also be connected to a ring, as shown in dashed lines in Figure 2-25. This pro-
vides for redundancy of transmission: even if one communication link fails, the entire differential protection
system will be fully operational. The devices detect communication failures and switch automatically to another
communciation channel. It is also possible to switch off one line end, e.g. for a check or a revision, and put the
local protection out of operation. With a communication ring, the rest of the operation can proceed without dis-
turbances.
You will find detailed information on the topology of device communication in Section 2.2.1.
Restraint
The precondition for the basic principle of the differential protection is that the total sum of all currents flowing
into the protected object is zero in healthy operation. This precondition is only valid for the primary system and
even there only if shunt currents of a kind produced by line capacitances or magnetizing currents of transform-
ers and parallel reactors can be neglected.
The secondary currents which are applied to the devices via the current transformers, are subject to measuring
errors caused by the response characteristic of the current transformers and the input circuits of the devices.
Transmission errors such as signal jitters can also cause deviations of the measured quantities. As a result of
all these influences, the total sum of all currents processed in the devices in healthy operation is not exactly
zero. Therefore, the differential protection is restrained against these influences.
Charging current compensation
Charging current compensation is an additional function for the differential protection. It allows to achieve a
higher sensitivity by compensating the charging currents that flow through the capacitance of the line and that
are caused by the capacitances of the overhead line or the cable.
Due to the phase-to-earth and phase-to-phase capacitances, charging currents are flowing even in healthy op-
eration and cause a difference of currents at the ends of the protected zone. Especially when cables and long
lines have to be protected, the capacitive charging currents can reach considerable magnitude.
If the feeder-side transformer voltages are connected to the devices, the influence of the capacitive charging
currents can be compensated to a large extent arithmetically. It is possible to activate a charging current com-
pensation which determines the actual charging current. With two line ends, each device takes over half of the
charging current compensation, with M devices each device takes the Mth part. For more simplicity, Figure 2-
26 shows a single-phase system.
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
Differential protection for a line with three ends
Functions
2.3 Differential Protection
95
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