Hybrid Pott Scheme (Hyb-Pott) - GE L90 Instruction Manual

PROTECTION SIGNALING SCHEMES CHAPTER 8: APPLICATION OF SETTINGS
a permanent lock up of the transmit/receive loop. The echo co-ordination (ECHO DURATION) and lock-out (ECHO LOCKOUT)
timers perform this function by ensuring that the permissive signal is echoed once for a guaranteed duration of time
before going to a lockout for a settable period of time.
Recognize that in ring bus or breaker and a half situations it can be the line disconnect or a combination of the disconnect
and/or the breaker(s) status that is the indication that the terminal is open.
The timer is included in the permissive receive path to ride through spurious receive outputs that can
POTT RX PICKUP DELAY
be produced during external faults, when power line carrier is utilized as the communications medium.
No current reversal logic is included for the overreaching phase and ground distance elements, because long reaches are
not usually required for two terminal lines. A situation can occur however, where the ground distance element has an
extended reach. This situation is encountered when it is desired to account for the zero sequence inter-circuit mutual
coupling. This is not a problem for the ground distance elements in the L90 that do have a current reversal logic built into
their design as part of the technique used to improve ground fault directionality.
Unlike the distance protection elements, the ground directional overcurrent functions do not have their reach well defined,
therefore the current reversal logic is incorporated for the extra signal supplementing Zone 2 in the scheme. The transient
blocking approach for this POTT scheme is to recognize that a permissive signal has been received and then allow a
settable time for the local forward looking directional element to pick up.
TRANS BLOCK PICKUP DELAY
The scheme generates an output operand (POTT TX) that is used to transmit the signal to the remote end. Choices of
communications channel include Remote Inputs/Outputs and telecommunications interfaces. When used with
telecommunications facilities the output operand is assigned to operate an output contact connected to key the
transmitter at the interface. Power Line Carrier (PLC) channels are not recommended for this scheme since the PLC signal
can be interrupted by a fault.
For proper operation of the scheme the Zone 2 phase and ground distance elements must be enabled and configured per
rules of distance relaying. The Line Pickup element needs to be enabled and configured properly to detect line-end-open/
weak-infeed conditions.
If used by this scheme, the selected ground directional overcurrent function(s) must be enabled and configured
accordingly The output operand from the scheme (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.

8.5.5 Hybrid POTT scheme (HYB-POTT)

This scheme uses an over-reaching Zone 2 distance element to compare the direction to a fault at both ends of the line.
Ground directional overcurrent functions available in the relay 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 three-terminal applications and for weak-infeed conditions. As a long reach of the
overreaching distance element can be required for three-terminal applications, transient blocking logic is provided for
both distance and ground directional overcurrent elements. In order to cope with weak-infeed conditions, an echo feature
is made available.
By default, the scheme uses the reverse-looking Zone 4 distance element to identify reverse faults. Additionally, reverse-
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looking ground directional overcurrent functions can be used in conjunction with Zone 4.
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. The Line Pickup element needs to be enabled and configured properly to detect
line-end-open/weak-infeed and undervoltage conditions.
If used by this scheme, the selected ground directional overcurrent function(s) must be enabled and configured
accordingly.
The scheme generates an output operand (HYBRID POTT TX) that is used to transmit the signal 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.
For more application recommendation, see the POTT scheme.
8-18 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL