ABB REO 517, Applications Manual

Page: 125 / 440

115
Scheme communication logic for residual
overcurrent protection (EFC)
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In the permissive scheme, the forward directed measuring element sends a permissive
signal to the other line end if a fault is detected in the forward direction. The directional
element at the other line end must wait for a permissive signal before giving a trip sig-
nal. Independent channels (frequencies) must be available for the communication in
each direction.
An impedance-measuring relay which works in an underreach permissive mode with
one channel in each direction can share the channels with the earth-fault overcurrent
protection. If the impedance measuring relay works in the permissive overreach mode,
common channels can be used in single-line applications. In case of double lines con-
nected to a common bus at both ends, use common channels only if the ratio Z
1S
/ Z
0S
(positive through zero-sequence source impedance) is about equal at both line ends. If
the ratio is different, the impedance measuring and the directional earth-fault current
system of the healthy line may detect a fault in different directions, which could result
in unwanted tripping.
In case of an internal fault, the forward directed measuring element operates and sends
a permissive signal to the remote end via the EFC--CS output (carrier send). Local trip-
ping is permitted when the forward direction measuring element operates and a permis-
sive signal is received via the EFC--CR binary input (carrier receive).
The total operate-time for the system is the sum of the Pick-up time (of the measuring
element) and the Transmission time (of the permissive signal)
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t
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EFC-CSBLK
EFC-BLOCK
EFC-CACC
EFC-CR
EFC-CS
EFC-TRIP
EFC-CRL
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