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Two step definite and inverse time-
delayed residual
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overcurrent protection (TEF1 and TEF2)
on the other lines gives a time difference of 0.3-0.4 seconds. A logarithmic characteris-
tic is generally the most suitable for this purpose, because the time difference is constant
for a given ratio between the currents.
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In the directional comparison scheme, information of the fault current direction must be
transmitted to the other line end. A short operate time enables auto-reclosing after the
fault.
A communication logic block for residual overcurrent protection can be included in the
REx 5xx terminal to provide this feature. The function contains circuits for blocking
overreach and permissive overreach schemes. See the section &RPPXQLFDWLRQ ORJLF
IRU UHVLGXDO RYHUFXUUHQW SURWHFWLRQ .
In the permissive directional comparison system, the forward direction measuring ele-
ment will send a permissive signal to the other line end if a fault is seen in the forward
direction. The direction element at the other line end must wait for a permissive signal
before it can trip.
The total operate time for the system will be the sum of the pick-up time of the measur-
ing element and the transmission time of the permissive signal. An operate time of 50-
60 ms, including a channel transmission time of 20 ms, can be achieved. This short op-
erate time allows auto-reclosing after the fault.
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The functions (TEF1 andTEF2) measures the residual current I
and the residual volt-
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age U
Figure 47 shows the current measuring, time delay and logic circuits of this
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protection function. The two functions are identical except that TEF1 uses the current
I4 and TEF2 the current I5. References will therefore only be made to TEF1 in the fol-
lowing description, but the also apply for TEF2.
105
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