2.4 Distance protection Z<
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The protection strategy usually applied for double-earth faults
is aimed at isolating one of the fault locations with the
expectation that the second fault location will then extinguish
on its own, similar to a single-phase earth-fault, or will be
tripped by a hand after successful earth fault searching.
DEF –algorithm in Vamp 259 –relay is enabled together with
distance protection Z1e<. Enabling is done by selecting network
grounding as ―Comp‖ compensated. When DEF -function is
enabled earth fault loop Z1e< is blocked during faults as long
as DEF -sequence is fulfilled.
During first earth-fault the fault is recognized due to several
terms. One of the phase voltages has to drop below ―Phase
under-voltage limit‖. Two of the phase voltages need to increase
above ―Phase over-voltage limit‖. Now the relay memorises that
in which phase the first earth-fault in the network appeared. In
case impedance measurement goes inside the zone Z1e< during
voltage drop caused by the first earth-fault the trip will be
blocked.
When earth fault turns into double earth fault the fault is
recognized as follows. Second faulty phase has to decrease 10%
below the healthy phase. Healthy phase still has to stay above
the ―Phase over-voltage limit‖. Also certain amount of zero
sequence voltage (U0) is required in the final phase.
Additionally if comparison condition is selected as U0_I0 also
residual current has to exceed the set limit.
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