2 Functions
94
7UM61 Manual
C53000-G1176-C127-3
Figure 2-27 Logic Diagram of the Pickup Stage of the Impedance Protection
Tripping Characteristic
The tripping characteristic of the impedance protection is a polygon (see also Figure
2-28). It is a symmetrical characteristic, even though a fault in reverse direction (neg-
ative R and/or X values) is 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 criterion is 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 trip characteristic, the protection
issues a trip command which may be delayed according to the relevant 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)
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