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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 defini-
tion. 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
exist in the protected area before a fault occurs, e.g. if a line with the voltage trans-
formers 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 deter-
mined. 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 local
currents (differential currents) are calculated (see „Transmission of measured values",
above), each device calculates the total sum of the restraining quantities.
It is due to the self-restraint that the differential protection always operates with the
maximum possible sensitivity 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 synchronisation, the
self-restraint when using communication networks is once more minimised since dif-
ferences in the transmission times are compensated automatically. A maximum sen-
sitivity 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 ex-
pected 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 harmonic 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 exceed-
ed, it will not be effective any more, since there must be an internal current-intensive
short-circuit.
Figure 2-18 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 con-
dition is also effective at the other device.
7SD610 Manual
C53000-G1176-C145-4
2.3 Differential Protection
65
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