Time allowed for a signal to be received
Where directional functions are configured in both line terminals, each can send a signal from
the MeasBwd output to the Receive input of the function at the opposite end of the line (for
example, via a PLC channel) when it is measuring a fault in the reverse direction. This signal
prevents the respective directional overcurrent function from tripping, because the fault
cannot be in the zone between them. The functions therefore have to allow time, that is, the
wait time, for the signal from the opposite line terminal to be received. If none is received
within t
Wait
, the circuit-breakers are tripped at both ends.
The time set for
Delay acts in this type of scheme as a backup, which does not rely on the
communication channel. Thus, when the Receive input is being used, the setting for
Delay
must be longer than the setting for t
Wait
:
Delay > t
Wait
Response after decay of the memorized voltage
The voltage measured by the protection can quickly decay to almost zero for a close fault and
make determining direction unreliable. For this reason, the function includes a voltage
memory feature which for the first 200 milliseconds after the start of an overcurrent
memorizes the voltage measured immediately before it and this is used as reference to
determine fault direction. After this time, the last valid direction is used for an adjustable
period.
MemDirMode provides facility for setting how the protection must respond after this time or
in the event that the circuit-breaker is closed onto a fault and no voltage could be memorized
beforehand. The two possible settings are the protection can trip or it can block.
Time during which the memorized direction is valid
The
MemDuration setting determines how long the last valid direction measurement shall be
used. The setting should be as short as possible (200 ms) when the function is being used as
backup for a distance function in an HV power system, because an actually measured voltage
is only available during this time and therefore it is only possible to detect a reversal of
direction during this time. For longer settings, the last valid power direction is used instead of
the actually memorized voltage.
5.5
Directional overcurrent inverse time protection 67
(DIROCINV)
5.5.1
Mode of operation
GUID-F34E6B2E-4EEB-4BDC-BB79-7CDA8BE8C034 v1
Directional inverse time overcurrent function for:
•
detecting phase faults on ring lines
•
detecting phase faults on double-circuit lines with an infeed at one end
•
backup protection for a distance protection scheme
5.5.2
Features
GUID-DF5218ED-8A78-4AB6-BB78-13B1B75FFC11 v1
•
Directionally sensitive three-phase phase fault protection
•
Operating characteristics according to British Standard BS 142:
Section 5
1MRK 505 406-UEN B
Bay protection functions
104
Bay protection functions REB500
Technical manual
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