320 D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CONTROL ELEMENTS CHAPTER 7: PROTECTION
Directional comparison blocking application guidelines
The directional comparison blocking scheme typically compares the direction to a fault at
both ends of the line. Unlike the permissive schemes, the absence of a blocking signal
permits operation of the scheme. Consequently, the scheme is biased toward
dependability and requires an “on/off” type of signaling.
By default, the directional comparison blocking scheme uses only a forward-looking
overreaching zone 2 distance element to identify forward faults. Ground directional
overcurrent functions available in the D90
Plus
can be used in conjunction with the zone 2
distance element to increase the coverage for high-resistance faults.
By default, the directional comparison blocking scheme also uses only a reverse-looking
zone 4 distance element to identify reverse faults. Ground directional overcurrent functions
available in the D90
Plus
can be used in conjunction with the zone 4 distance element for
better time and sensitivity coordination.
For proper operation of the directional comparison blocking scheme, the zones 2 and 4
phase and ground distance elements must be enabled and configured per the standard
rules of distance relaying. If used by this scheme, the selected ground directional
overcurrent functions must be enabled and configured accordingly.
The directional comparison blocking scheme generates two output operands (DIR BLOCK TX
INIT and DIR BLOCK TX STOP) that are used control the transmission of signals to the remote
end. Choices of communications channel include remote inputs and outputs and
telecommunications interfaces. When used with telecommunications facilities, assign the
output operand to operate an output contact connected to key the transmitter at the
interface.
The directional comparison blocking scheme can be preferred over a hybrid POTT scheme
because of shorter reach settings for the zone 2 elements. This follows from the
fundamental difference that all zone 2 elements are required to see an internal fault for the
POTT approach, under all system conditions, which in turn means that the reversed zone 4
block initiate elements must also have an increased reach. On the other hand, a directional
comparison blocking scheme can have much shorter zone 2 reach settings if sequential
clearance can be accepted. The simple rule to ensure that all faults can be cleared is for
each terminal to have a reach setting equal to the distance to the tap plus twice the
distance from the tap to the remote terminal.
The zone 2 element must have a coordinating timer specified by the
RX Coordination
Pickup Delay setting to ensure that the blocking signal is received for all external faults that
are within the set reach of the local overreaching zone 2.
Transient blocking logic is implemented via the timer specified by the transient blocking
timer. This timer continues to send a blocking signal for the time specified by the
Transient
Block Reset Delay
setting if it was being sent for at least 30 ms during the initial reverse
fault specified via the
Transient Block Pickup Delay setting.
The output operand from the directional comparison blocking scheme (DIR BLOCK OP) must
be configured to interface with other D90
Plus
functions (output contacts in particular) to
make the scheme fully operational. Typically, you program the output operand to initiate a
trip, breaker failure, and autoreclose, as well as to drive a user-programmable LED as per
user requirements.
Directional comparison unblocking (DCUB) application guidelines
The directional comparison unblocking (DCUB) scheme is used with a frequency shift
keying (FSK) PLC that produces a loss-of-guard output during an actual loss-of-guard
signal condition and during reception of the permissive keyed frequency when the
received carrier signal changed from guard to permissive frequency. The loss-of-guard
signal is preferably a normally-closed contact, allowing the scheme to get this signal if the
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