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covers the network. It should be noted that high voltage side 1-pole faults cause im-
pedance measurement errors due to the star-delta connection of the unit transformer
on the lower voltage side. An unwanted operation of the stage can be excluded since
the fault impedances of power system faults are modeled too high.
Faults outside this range are switched off by the T END final time stage.
Depending on the switching status of the system, it may be useful to extend the ZONE
Z1, T-Z1 undelayed tripping zone. If, for example, the high-voltage side circuit
breaker is open, the pickup can only be caused by a fault in the power unit. If consid-
eration of the circuit breaker auxiliary contact is possible, a so-called overreach zone
ZONE Z1B can be made effective (see also Section 2.14.2, Grading of the Machine
Impedance Protection figure).
Figure 2-28
Tripping Logic
The T END time delay is started subsequent to the protection pickup, establishing the
the fault loop. The loop impedance components are compared with the limit values of
the zones previously set. The tripping is executed if the impedance is within its zone
during the the course of the corresponding time stage.
For the first Z1 zone and also for the Z1B overreach zone, the time delay will in most
cases be zero or at least very short. i.e. tripping occurs as soon as it is established that
the fault is within this zone.
The Z1B overreach stage can be enabled from outside, via a binary input.
For the Z2 zone which may extend into the network, a time delay is selected over-
reaching the first stage of the power system protection.
7UM61 Manual
C53000-G1176-C127-3
Tripping Characteristics of the Impedance Protection
2.14 Impedance Protection (ANSI 21)
95
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