12.5.6
Operation principle
The four step directional residual overcurrent protection EF4PTOC (51N/67N) is
configured to give input information, that is directional fault detection signals, to the
ECPSCH (85) logic:
•
Input signal PLTR_CRD is used for tripping of the communication scheme, normally
the pickup signal of a forward overreaching step of PUFW.
•
Input signal CS_STOP is used for sending block signal in the blocking
communication scheme, normally thepickup signal of a reverse overreaching step of
PUREV.
•
Input signal CSUR is used for sending permissive signal in the underreaching
permissive communication scheme, normally the pickup signal of a forward
underreaching step of STINn, where n corresponds to the underreaching step.
•
Input signal CSOR is used for sending permissive signal in the overreaching
permissive communication scheme, normally the pickup signal of a forward
overreaching step of STINn, where n corresponds to the overreaching step.
12.5.6.1
Blocking scheme
In the blocking scheme a signal is sent to the other line end if the directional element
detects a ground fault in the reverse direction. When the forward directional element
operates, it trips after a short time delay if no blocking signal is received from the opposite
line end. The time delay
tCoord
, normally 30–40 ms, depends on the remote reverse unit
operating and communication transmission times and a chosen safety margin.
One advantage of the blocking scheme is that only one channel (carrier frequency) is
needed if the ratio of source impedances at both end is approximately equal for zero and
positive sequence source impedances, the channel can be shared with the impedance
measuring system, if that system also works in the blocking mode. The communication
signal is transmitted on a healthy line and no signal attenuation will occur due to the fault.
Blocking schemes are particular favorable for three-terminal applications if there is no
zero-sequence outfeed from the tapping. The blocking scheme is immune to current
reversals because the received signal is maintained long enough to avoid unwanted
operation due to current reversal. There is never any need for weak-end infeed logic,
because the strong end trips for an internal fault when no blocking signal is received from
the weak end. The fault clearing time is however generally longer for a blocking scheme
than for a permissive scheme.
If the fault is on the line, the forward direction measuring element operates. If no blocking
signal comes from the other line end via the CR binary input (received signal) the TRIP
output is activated after the
tCoord
set time delay.
1MRK 506 335-UUS A
Section 12
Scheme communication
519
Technical manual
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