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
130 has to be interpreted as the relation
between series capacitor reactance X
C
and the total positive sequence reactance X1 to
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 earth 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-earth fault loops on the
other side. Different settings of the reach for the ph-ph faults and ph-E loops makes it
possible to minimise the necessary decrease of the reach for different types of faults.
1MRK 502 071-UEN - Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 251
Application manual
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