Siemens SIPROTEC 7SD610 Manual page 66

Functions
2.3 Differential Protection
protection devices is called protection 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.
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 Currents
Due to the capacitances of the three phases against ground and against one another, charging currents are
flowing even in healthy operation and cause a difference of currents at the ends of the protected zone. Espe-
cially when cables are used, the capacitive charging currents can reach considerable magnitude.
Charging currents do not depend on the intensity of the measured currents. In healthy operation they can be
considered as being almost constant under steady-state conditions, since they are only determined by the
voltage and the capacitances of the lines. They can therefore be taken into account during the setting of the
sensitivity of the differential protection (see also Subsection 2.3.2 under „Pickup Value of Differential Current").
The same is true for the steady-state magnetizing currents across shunt reactances. The devices have a sep-
arate inrush restraint feature for transient inrush currents (see below under the margin heading „Inrush Re-
straint").
Current transformer errors
To consider the influences of current transformer errors, each device calculates a self-restraining quantity I
This is calculated by estimating the possible local transformer errors from the data of the local current trans-
formers and the intensity of the locally measured currents (see Figure 2-17). The current transformer data have
been parameterized in the power system data 1 (Section 2.1.2.1 under margin heading „Current Transformer
Characteristic" and apply to each individual device. Since each device transmits its estimated errors to the other
device, each device is also capable of forming the total sum of possible errors; this sum is used for restraint.
66
Differential protection for a line with two ends
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
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