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Functions
2.19 Impedance Protection (ANSI 21)
Figure 2-61
Tripping Characteristic
The tripping characteristic of the impedance protection is a polygon (see also Figure 2-62). It is symmetrical
even though a fault in reverse direction (negative R and/or X values) is physically impossible provided the usual
connection to the current transformers at the star-point side of the generator is used. The polygon is fully iden-
tified by one parameter (impedance Z).
As long as the pickup criteria are met, impedance calculation is done continuously using the current and voltage
vectors derived from the loop selection measured values. If the calculated impedance is within the tripping char-
acteristic, the protection sends a tripping command according to the specified delay time.
Since the impedance protection is multi-stage, the protected zones can be chosen such that the first stage
(ZONE Z1, T-Z1) covers faults in the generator and the lower voltage side of the unit transformer, whereas the
second stage (ZONE Z2, ZONE2 T2) covers the entire power station block. It should be noted, however, that
high voltage side 1-pole earth faults cause impedance 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 station block. If consideration of the circuit breaker auxiliary contact is possible, a so-called
overreach zone ZONE Z1B can be made effective (see also Section 2.19.3, Figure „Grading of the machine
impedance protection").
154
Logic Diagram of the Pickup Stage of the Impedance Protection
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
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