Due to changes in the power system, such as generator and line outages
and changes in load and generation, the performance of distance relay
s can vary. In the case of a distance relay protecting a phase of a do
uble-circuit line, especially the state of the parallel circuit is of
major importance. Simulations show that, depending on the power system
state, a distance relay can cover from less than 50% up to far more t
han 100% of the total line length. This is demonstrated with a double-
ircuit line under SLG fault condition, since this is the most ocuuring
type of fault. In this paper the distance protection of a double-circ
uit line under SLG fault condition is formulated. To achieve correct o
peration, the relay does not use the measured quantities of the circui
t-to-be-protected only, but also the zero sequence current of the para
llel circuit. Such a relay requires extra measuring equipment, and, mo
reover, the zero sequence current of the parallel circuit cannot alway
s be measured. Therefore, another approach is chosen. A correction fac
tor is introduced, set adaptively according to the actual power system
state. In this way, the appropriate setting of the relay is provided,
in relation with the actual power system state. A side-effect of the
adaptive setting of the relay is that the safety margin in the relay s
ettings is decreased, due to the uncertainty in the power system state
. By adapting the relay to the actual power system state, maximum sele
ctivity is achieved, and the protection system as such will be more re
liable.