CHAPTER 7: PROTECTION
CONTROL ELEMENTS
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
307
Directional comparison unblocking
The directional comparison unblocking scheme is available for single-pole tripping
applications and can be used with one, two, or four bit communications channels. Choices
of communications channel include remote inputs, remote outputs, and
telecommunications interfaces.
The directional comparison unblocking scheme typically uses an over-reaching zone 2
distance element to compare the direction to a fault at all terminals of the line. Ground
directional overcurrent functions available in the device can be used in conjunction with
the zone 2 distance element to key the scheme and initiate its operation. This increases the
coverage for high resistance faults. The scheme is intended for two or three-terminal
applications. As a long reach of the overreaching distance element may be required for
three-terminal applications, transient blocking logic is provided for both distance and
ground directional overcurrent elements. An echo feature is made available to cope with
line open conditions.
By default the scheme uses the reverse-looking zone 4 distance element to identify reverse
faults. Additionally, reverse-looking ground directional overcurrent functions can be used
in conjunction with zone 4.
For proper operation, the zone 2 and 4 phase and ground distance elements must be
enabled and configured according to the principles of distance relaying. The line pickup
element should be also be enabled and configured to detect line-end-open conditions. If
used by the directional comparison unblocking scheme, the selected ground directional
overcurrent functions must be enabled and configured accordingly.
In single-pole tripping applications, the directional comparison unblocking scheme uses
local fault type identification provided by the phase selector together with information
received from the remote terminal. The scheme generates output operands (
DCUB TX1
through
DCUB TX4
) that are used to transmit the signal to the remote terminals. The
directional comparison unblocking scheme was developed for use with frequency shift
keying communications channels, though different telecommunications interfaces can be
used (for example, digital communication channels utilizing the D90
Plus
teleprotection
inputs and outputs).
To make the directional comparison unblocking scheme fully operational as a stand-alone
feature, the scheme output operands must be configured to interface with other relay
functions, in particular the output contacts. Typically, the output operands should be
programmed to initiate a trip, breaker fail, and autoreclose, and drive an annunciator
indication as per the intended application.
When used in conjunction with the trip output element, the directional comparison
unblocking scheme is pre-configured to initiate trip, breaker fail and autoreclose actions.
Select the
Settings > Protection > Control > Pilot Schemes > DCUB Scheme
menu item to
access the directional comparison unblocking scheme settings.
