Siemens siprotec 7SD5, Руководство пользователя, Страница: 90 / 698

2 Functions
90
7SD5 Manual
C53000-G1176-C169-1
Figure 2-15 Basic principle of differential protection for four ends (single-line illustration)
Transmission
Measured Value If the entire protected object is located in one place — as is the case with generators,
transformers, busbars —, the measured quantities can be processed immediately.
This is different for lines where the protected zone spans a certain distance from one
substation to the other. To be able to process the measured quantities of all line ends
at each line end, these have to be transmitted in a suitable form. In this way, the trip-
ping condition at each line end can be checked and the respective local circuit breaker
can be operated if necessary.
7SD5 transmits the measured quantities as digital telegrams via communication chan-
nels. For this, each device is equipped with at least one protection data interface.
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 the
protection data interface. As a result, the currents can be added up a processed in
each device.
Figure 2-16 Differential protection for a line with two ends
In case of more than two ends, a communication chain is built up by which each device
is informed about the total sum of the currents flowing into the protected object. Figure
2-17 shows an example for three ends. Ends
1 and 2 are derived from the arrange-
ments of the current transformers shown on the left. Although this is actually only one
line end, it should be treated in terms of differential protection as two ends because
the current is measured in two places. Line end
3 is situated on the opposite side.
Each device receives its local currents from the current transformers. Device 1 mea-
sures the current i
1
and transmits its data as complex phasor I
1
to device 2 . Device 2
adds the share I
2
to its own measured current i
2
and sends this partial sum to device
3 . The partial sum I
1
+ I
2
finally reaches device 3 which then adds its share I
3
. Vice
versa, a corresponding chain leads from device 3 via device 2 to device 1 . In this way,
the total sum of the three currents measured at the measuring points is available to all
three devices.
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