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Figure 2-16 shows this for a line with two ends. Each device measures the local
current and sends the information on its intensity and phase relation to the opposite
end. The interface for this communication between protection devices is called protec-
tion data interface. As a result, the currents can be added up and processed in each
device.
Figure 2-16
You will find detailed information on the topology of device communication in Section
2.2.1.
Measured Value
The devices measure the local currents in an asynchronous way. This means that
Synchronization
each device measures, digitizes and pre-processes the associated currents of the
current transformers with its own, random processor pulse. If the currents of two or
more line ends are to be compared, it is necessary, however, to process all currents
with the same time base.
Both devices exchange their time with each telegram. The device with index 1 func-
tions as a "timing master" thus determining the time base. The other device then cal-
culates the time delay from the transmission and processing times related to the
"timing master". With this "rough synchronization" the equality of the time bases with
a precision of ± 0.5 ms is provided.
To reach a sufficiently precise synchronization all current values are marked with a
„time stamp" before they are transmitted from one device to the other as digital tele-
grams. This time stamp indicates at which point in time the transmitted current data
were valid. Therefore, the receiving devices can carry out an optimized synchroniza-
tion of the current comparisons based on the received time stamp and their own time
management, i.e. they can compare the currents which were actually measured at
exactly the same time (<5 µs tolerance).
The transmission periods are permanently monitored by the devices using the time
data stamps and considered at the respective receiving end.
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.
7SD610 Manual
C53000-G1176-C145-4
Differential protection for a line with two ends
2.3 Differential Protection
63
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