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Functions
2.3 Differential Protection
Figure 2-26
In healthy operation charging currents can be considered as being almost constant under steady-state condi-
tions, since they are only determined by the voltage and the capacitances of the lines. Without charging current
compensation, they must therefore be taken into account when setting the sensitivity of the differential protec-
tion (refer also to Section 2.3.2 under „Pickup Value of Differential Current"). With charging current compensa-
tion, no charging currents need to be taken into account here. With charging current compensation, the steady-
state magnetizing currents across shunt reactances are taken into account as well. The devices have a sepa-
rate 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 2-27). 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
devices, each device is also capable of forming the total sum of possible errors; this sum is used for restraint.
Figure 2-27
Further influences
Further measuring errors which may arise in the actual device by hardware tolerances, calculation tolerances,
deviations in time or due to the „quality" of the measured quantities such as harmonics and deviations in fre-
quency are also estimated by the device and automatically increase the local self-restraining quantity. Here,
the permissible variations in the data transmission and processing periods are also considered.
Deviations in time are caused by residual errors during the synchronization of measured quantities, data trans-
mission and operating time variations, and similar events. When GPS synchronization is used, these influences
are eliminated and do not increase the self-restraining quantity.
96
Charging current compensation for a line with two ends (single-phase system)
Approximation of the current transformer errors
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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error
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