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Two step definite and inverse time-
delayed residual
overcurrent protection (TEF1 and TEF2)
where:
I
is I
/IN>Inv
N
k
is a time multiplying factor, settable in the range of 0.05 to 1.10
All inverse time characteristic settings are a compromise between short fault clearing
time and selective operation in a large current range. The main determining factors are
the maximum allowed fault-clearing time at the maximum fault resistance to be covered
and the selectivity at maximum fault current.
Set the minimum operate current (IN>Low) of the earth-fault overcurrent protection to
one to four times the set characteristic quantity (IN>Inv) of the inverse time delay. So
an inverse characteristic with a low set IN>Inv set to get a short operate time at mini-
mum fault current can be combined with a higher set IN>Low minimum operate cur-
rent, to avoid unwanted operation due to false earth-fault currents.
Set the minimum operate (tMinInv) time independent of the inverse time characteristic.
Normally, set this time longer than the time delay of distance zone 2 in REx 5xx to
avoid interference with the impedance measuring system in case of earth-faults with
moderate fault resistance within zone 2.
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The high current step has a definite time delay characteristic. It is set to give a trip with
set delay time in case of a phase to earth fault on the protected line, with large residual
current. The step might also serve as main or remote back-up protection for phase to
earth faults on the remote busbar.
The chosen setting (IN>High and tHigh) must be coordinated with other earth fault pro-
tections in the system.
When a solidly earthed, power transformer is energized, an inrush current normally
flows in the neutral-to-earth connection of the transformer. This current is divided
among other earthed transformers and lines connected to the same bus, inversely pro-
portional to their zero-sequence impedance. The amplitude and time duration of this
current can be sufficiently large to cause the unwanted operation of a sensitive earth-
fault overcurrent protection.
The earth-fault overcurrent protection has a built-in second harmonic current stabiliza-
tion, which prevents unwanted operation if the inrush current has a second harmonic
content of 20% or more. This is normally the case. On rare occasions, it may be neces-
sary to increase the setting of the operate value for the residual earth-fault overcurrent
protection to avoid unwanted operation due to transformer inrush current.
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