8-12
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
SERIES COMPENSATED LINES
CHAPTER 8: APPLICATION OF SETTINGS
8
Good directional integrity is the key requirement for an over-reaching forward-looking protection element used to
supplement Zone 2. Even though any FlexLogic operand can be used for this purpose, allowing the user to combine
responses of various protection elements or to apply extra conditions through FlexLogic equations, this extra signal is
primarily meant to be the output operand from the Neutral Directional IOC. Both of these elements have separate forward
(FWD) and reverse (REV) output operands. Use the forward indication NEUTRAL DIR OC1 FWD).
An important consideration is when one of the line terminals is open. It is then necessary to identify this condition and
arrange for a continuous sending of the permissive signal or use a slower but more secure echo feature to send a signal to
the other terminal, which is producing the fault infeed. With any echo scheme however, a means must be provided to avoid
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
POTT RX PICKUP DELAY
timer is included in the permissive receive path to ride through spurious receive outputs that can
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 L60 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
TRANS BLOCK PICKUP DELAY
for the local forward looking directional element to pick up.
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.
8.4 Series compensated lines
8.4.1 Distance settings
Traditionally, the reach setting of an underreaching distance function is set based on the net inductive impedance
between the potential source of the relay and the far-end busbar, or location for which the zone must not overreach. Faults
behind series capacitors on the protected and adjacent lines need to be considered for this purpose. For further
illustration, a sample system shown in the figure is considered.
