Siemens siprotec 7SD5 User Manual page 93

In healthy operation charging currents 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. Without charging current compensation, they must therefore
be taken into account when setting the sensitivity of the differential protection (refer
also to Section 2.3.2 under „Pickup Value of Differential Current"). With charging
current compensation, 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 separate inrush re-
straint feature for transient inrush currents (see below under the margin heading
„Inrush Restraint").
Current To consider the influences of current transformer errors, each device calculates a self-
Transformers restraining quantity I
Errors er errors from the data of the local current transformers and the intensity of the mea-
sured currents (see Figure 2-19). The current transformer data have been parameter-
ized under the power system data (cf. Section 2.1.2.1 under margin heading „Current
Transformer Characteristics" and apply to each individual device. Since each device
transmits its estimated errors to the other devices, each device is capable to form the
total sum of possible errors; this sum is used for restraint.
Figure 2-19
Further Influences Further measuring errors which may arise in the device itself by hardware tolerances,
calculation tolerances, deviations in time or due to the „quality" of the measured quan-
tities such as harmonics and deviations in frequency, are also estimated by the device
and increase the local self-restraining quantity automatically. Here, the permissible
variations in the protection data transmission and processing periods are also consid-
ered.
Deviations in time are caused by residual errors during the synchronization of mea-
sured quantities, data transmission and operating time variations, and similar events.
When GPS synchronization is used, these influences are eliminated and do not in-
crease the self-restraining quantity.
If an influencing parameter cannot be determined — e.g. the frequency, if no sufficient
measured quantities are available — the device will assume by definition nominal fre-
quency. In this example, frequency means that if the frequency cannot be determined,
because no sufficient measured quantities are available, the device will assume
nominal frequency. But since the actual frequency can deviate from the nominal fre-
quency within the permissible range (± 20% of the nominal frequency), the restraint
will be increased automatically. As soon as the frequency has been determined (max.
100 ms after reappearance of a suitable measured quantity), the restraint will be de-
creased correspondingly. This is important during operation if no measured quantities
are existing in the protected area before a fault occurs, e.g. if a line with the voltage
7SD5 Manual
C53000-G1176-C169-1
. This is calculated by estimating the possible local transform-
Error
Approximation of the current transformer errors
2.3 Differential Protection
93