GE D90 Plus Instruction Manual page 328

CONTROL ELEMENTS
318
The scheme generates an output operand
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 timer setting can be used to ride through spurious PLC receive signals.
Pickup Delay
The scheme output operand
Plus
D90 functions (output contacts in particular) to make the scheme fully operational.
Typically, program the output operand to initiate a trip, breaker fail, and autoreclose, and
to drive a user-programmable LED as per user requirements.
Permissive overreaching transfer trip (POTT) application guidelines
The permissive overreaching transfer trip (POTT) scheme is for two-terminal line
applications. It uses an overreaching zone 2 distance element to compare the direction to
a fault at both the ends of the line.
Ground directional overcurrent functions available in the D90
conjunction with the zone 2 distance element to key the POTT scheme and initiate its
operation. This provides increased coverage for high-resistance faults.
Good directional integrity is the key requirement for an overreaching 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 apply extra conditions through FlexLogic equations—this extra signal is
primarily meant to be the output operand from either the negative-sequence directional
overcurrent element or neutral directional overcurrent element. Both of these elements
have separate forward (FWD) and reverse (REV) output operands. Use the forward
indication (NEG SEQ DIR OC1 FWD
An important consideration occurs when one of the line terminals is open. In this case, it is
necessary to identify the line terminal open 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 that is producing the fault infeed. However, with any echo
scheme, a means must be provided to avoid a permanent lock up of the transmit/receive
loop. The echo duration timer (via the
the setting) perform this function by ensuring that the permissive signal is
Echo Lockout
echoed once for a guaranteed duration of time before going to a lockout for a user-
specified length of time.
Note that in ring bus or breaker-and-a-half situations, it can be the line disconnect or a
combination of the disconnect status, the breaker status, or both, that is the indication that
the terminal is open.
The timer specified by the
RX Pickup Delay
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. However, a
situation can occur where the ground distance elements have an extended reach. This
situation is encountered when you want to account for the zero-sequence inter-circuit
mutual coupling. This is not a problem for the ground distance elements in the D90
have 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 a well-defined reach. As such, he current reversal logic is incorporated for the
extra signal supplementing zone 2 in the scheme. The transient blocking approach for this
(PUTT TX) that is used to transmit the signal to
(PUTT OP) must be configured to interface with other
or
NEUTRAL DIR OC1 FWD
setting) and echo lockout timer (via
Echo Duration
setting is included in the permissive receive path
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
Plus
can be used in
operand).
Plus
that