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If an influencing parameter cannot be determined — e.g. the frequency if no sufficient measured quantities are
available — the device will assume nominal values by definition. 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 frequency 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 decreased correspondingly. This is important during operation if no measured quantities exist
in the protected area before a fault occurs, e.g. if a line with the voltage transformers on the line side is switched
onto a fault. Since the frequency is not yet known at this time, an increased restraint will be active until the
actual frequency is determined. This may delay the tripping, but only close to the pickup threshold, i.e. in case
of very low-current faults.
The self-restraining quantities are calculated in each device from the total sum of the possible deviations and
transmitted to the other devices. In the same way as the total currents (differential currents) are calculated (see
„Transmission of measured values" above), each device thus calculates the total sum of the restraining quan-
tities.
It is due to the self-restraint that the differential protection always operates with the maximum possible sensi-
tivity since the restraining quantities automatically adapt to the maximum possible errors. In this way, also high-
resistance faults, with high load currents at the same time, can be detected effectively. Using GPS synchroni-
sation, the self-restraint when using communication networks is once more minimised since differences in the
transmission times are compensated by the precise calculation of the two-way transmission times. A maximum
sensitivity of the differential protection consists of an optical-fiber connection.
Inrush restraint
If the protected area includes a power transformer, a high inrush current can be expected when connecting the
transformer. This inrush current flows into the protected zone but does not leave it again.
The inrush current can amount to a multiple of the rated current and is characterised by a considerable 2nd
harmonic content (double rated frequency) which is practically absent during a short-circuit. If the second har-
monic content in the differential current exceeds a selectable threshold, tripping is blocked.
The inrush restraint has an upper limit: if a certain (adjustable) current value is exceeded, it will not be effective
any more, since there must be an internal current-intensive short-circuit.
Figure 2-28 shows a simplified logic diagram. The condition for the inrush restraint is examined in each device
in which this function has been activated. The blocking condition is transmitted to all devices so that it is effec-
tive at all ends of the protected object.
Figure 2-28
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
Logic diagram of the inrush restraint for one phase
Functions
2.3 Differential Protection
97
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