2 Functions
92
7SD5 Manual
C53000-G1176-C169-1
The frequency of the measured quantities, which is decisive for the comparison of
complex phasors, is also continuously measured and with the calculation, if neces-
sary, corrected to achieve a synchronous comparison of the phasors. If the device is
connected to voltage transformers and at least one voltage of a sufficient level is avail-
able, the frequency is derived from this voltage. If not, the measured currents are used
for the determination of the frequency. The measured frequencies are interchanged
between the devices via the communication link. Under these conditions all devices
work with the currently valid frequency.
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 transformers and
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 in-
fluences.
Charging Current
Compensation
Charging current compensation is an ancillary function of the differential protection. It
allows to achieve a higher sensitivity by partially compensating the charging currents
caused by the capacitances of the overhead line or cable. Charging currents flow
through the capacitance of the line.
Due to the phase-to-earth and phase-to-phase capacitances, charging currents are
flowing even in healthy operation and cause a difference of currents at the ends of the
protected zone. Especially when cables and long lines have to be protected, the ca-
pacitive 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 compensation 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 sim-
plicity, Figure 2-18 shows a single-phase system.
Figure 2-18
Charging current compensation for a line with two ends (single-phase system)
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