ABB RELION RET670 Applications Manual page 443

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
X is the reactance of the series capacitor
c
p is the maximum allowable reach for an under-reaching zone with respect to the sub-
harmonic swinging related to the resulting fundamental frequency reactance the zone is
not allowed to over-reach.
The degree of compensation C in figure
between series capacitor reactance X
the driving source to the fault. If only the line reactance is used the degree of
compensation will be too high and the zone 1 reach unnecessary reduced. The highest
degree of compensation will occur at three phase fault and therefore the calculation
need only to be performed for three phase faults.
The compensation degree in ground return path is different than in phases. It is for this
reason possible to calculate a compensation degree separately for the phase-to-phase
and three-phase faults on one side and for the single phase-to-ground fault loops on the
other side. Different settings of the reach for the ph-ph faults and ph-G loops makes it
possible to minimise the necessary decrease of the reach for different types of faults.
Impedance protection
223 has to be interpreted as the relation
and the total positive sequence reactance X1 to
C
Section 8
437