ABB REO 517 Applications Manual page 72

62
3 zone impedance protection (Z(n)RW)
R
U
)LJXUH  $SSDUHQW LPSHGDQFH ZLWK UHVLVWDQFH DQG UHDFWDQFH FRQQHFWHG LQ VHULHV
The measuring elements receive information about currents and voltages from the A/D
converter. The checksums are calculated and compared, after which the information is
saved in memory. Squared RMS amplitudes of voltage and current are calculated and
compared according to the formula:
2
U
2
< ⋅
--------- - Z I
set
Z
set
and results in one inner and one outer impedance circle. The outer circle, circumscribed,
indicates a start zone, which gives an efficient delimitation outwards towards the tran-
sient fault during normal load conditions. The inner circle, inscribed, delimits the near-
field, where the rapid function for short circuit faults can be obtained without jeopar-
dizing the selectivity.
X
I
en00000379
&KDSWHU 
/LQH LPSHGDQFH
(Equation 19)