Directional Comparison Blocking; Directional Comparison Unblocking - GE L90 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS PROTECTION SIGNALING SCHEMES
The output operand from the scheme (HYBRID POTT OP) must be configured to interface with other relay functions, output
contacts in particular, in order to make the scheme fully operational. Typically, the output operand is programmed to
initiate a trip, breaker fail, and auto-reclose, and to drive a user-programmable LED as per user application.

8.5.6 Directional comparison blocking

The scheme 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, this scheme uses only a forward-looking over-reaching Zone 2 distance element to identify forward faults.
Ground directional overcurrent functions available in the relay can be used in conjunction with the Zone 2 distance
element to increase the coverage for high-resistance faults.
By default, the scheme uses only a reverse-looking Zone 4 distance element to identify reverse faults. Ground directional
overcurrent functions available in the relay can be used in conjunction with the Zone 4 distance element for better time
and sensitivity coordination.
For proper operation of the scheme, the Zone 2 and 4 phase and ground distance elements must be enabled and
configured per rules of distance relaying.
If used by this scheme, the selected ground directional overcurrent function(s) must be enabled and configured
accordingly.
The scheme generates output operands (BLOCKING SCHEME TX INIT and BLOCKING SCHEME TX STOP) that are used
control the transmission of signals to the remote end. Choices of communications channel include Remote Inputs/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.
A 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. A blocking scheme on the other hand, 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 , to ensure that the blocking signal is
BLOCK RX CO-ORD PKP DELAY
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, which continues to send a blocking signal for a settable
TRANS BLOCK
time , if it was being sent for at least 30 ms during the initial reverse fault set via
TRANS BLOCK RESET DELAY TRANS BLOCK
.
PICKUP DELAY
The output operand from the scheme (BLOCKING SCHEME OP) must be configured to interface with other relay functions,
output contacts in particular, in order to make the scheme fully operational. Typically, the output operand is programmed
to initiate a trip, breaker fail, and auto-reclose, and to drive a user-programmable LED as per user application.

8.5.7 Directional comparison unblocking

8
8.5.7.1 Overview
The directional comparison unblocking 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 communications equipment failed. This loss-of-
guard output is connected to a contact input of the L90. The power line carrier also provides an output contact when the
permissive frequency is received. This output is wired to any other contact input of the L90.
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 8-19