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Two step definite and inverse time-
delayed residual
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overcurrent protection (TEF1 and TEF2)
Upol = -U
ϕ
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65°
IN Operation
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The change in operate value is small when the phase angle deviates moderately from
65°. A deviation of 20° increases the operate value by only 6.5%.
The polarizing voltage can have a high content of harmonics relative to the fundamental
frequency when the output voltage is low, particularly when capacitive VTs are used.
To secure a correct measurement, the directional function must have an effective band-
pass filtering of the voltage. In the module, the filtering secures a correct function for
fundamental frequency polarizing voltages down to 1% of the rated voltage.
The directional function has two comparators, one operates in the forward direction
(STFW) and one operates in the reverse direction (STRV). In case of an external fault,
the capacitive current generated on the line decreases the current to the earth-fault relay
situated at the line end towards the fault. So the reverse direction comparator must have
an increased sensitivity to secure reliable blocking in case of external faults when a di-
rectional comparison or a blocking communication scheme is used. The operate current
of the reverse direction measuring element in the module is, as a fixed ratio, set at 0.6
IN> Dir.
109
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