10-38
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
SINGLE-POLE TRIPPING
CHAPTER 10: THEORY OF OPERATION
10
The trip output element receives requests for single and three-pole trips and three-pole reclose initiation, which it then
processes to generate outputs that are used to
•
Determine whether a single or three-pole operation is to be performed
•
Initiate tripping of breaker poles A, B, and C, either individually or as a group
•
Initiate breaker failure protection for phases A, B, and C, either individually or as a group
•
Notify the open pole detector when a single pole operation is imminent
•
Initiate either single or three-pole reclosing
•
Notify the phase selector when a trip operation is imminent
When notified that a single pole operation has been initiated, the open-pole detector
•
Initiates blocking of protection elements that can potentially maloperate when a breaker pole is open
•
Instructs the phase selector to de-assert all outputs, as an open pole invalidates calculations
The operation of the scheme on a line in a single breaker arrangement is described as follows. The line is protected by a
L90 relay using the line pickup and zone 1 phase and ground distance elements, and a permissive overreaching transfer
trip scheme (using zone 2 phase and ground distance elements as well as negative-sequence directional overcurrent
elements,
GND DIR O/C FWD
: “
NEG SEQ OC1 FWD,
”
GND DIR O/C REV
: “
NEG SEQ OC1 REV
”). Zone 1 is configured to issue a single-
pole trip when appropriate (
TRIP 1-POLE INPUT-1
: “
GND DIST Z1 OP,
”
TRIP 1-POLE INPUT-2
: “
PHS DIST Z1 OP
”). By default, the POTT
scheme issues a single-pole trip. It is assumed that when tripping three poles, both the zone 1 and the POTT initiate three-
pole reclosing. This is achieved by setting
TRIP RECLOSE INPUT-1
: “
POTT TRIP 3P,
”
TRIP RECLOSE INPUT-2
: “
GND DIST Z1 OP,
” and
TRIP RECLOSE INPUT-3
:
“
PHS DIST Z1 OP.
”
It is assumed for this discussion that the relay features that are shown in the Single-Pole Operation figure have been
programmed for the application and are in service. The description begins with line breakers open at both the local and
remote ends, and the operation of the scheme is described in chronological order.
Because the line is de-energized, the line pickup element is armed. The recloser is presently enabled. An operator requests
that breaker control close the breaker, and it operates output relays to close breaker poles A, B, and C. This operator
manual-close request is also forwarded from breaker control to recloser, which becomes disabled, de-asserting its
“Enabled” output. This output is transferred to trip output, where it converts any input request for a single pole operation
into a three-pole operation. At the recloser, the
AR1 BLK TIME @ MAN CLOSE
timer is started.
The breaker closes and status monitoring contacts on the breaker poles change state; the new breaker pole states are
reported to breaker control, which in turn transfers these states to the recloser, trip output, breaker failure, and open pole
detector. Because a fault is not detected the
AR1 BLK TIME @ MAN CLOSE
times out and the recloser is enabled, which
asserts the “Enabled” output, informing the trip output element that single-pole trip operations are now permitted. When
normal voltage appears on the line, the line pickup element is disarmed. As the local line breaker has not tripped, the
operator closes the breaker at the remote end of the line, placing the line in service.
Several scenarios are considered as follows.
10.5.1.2 SLG fault
An AG fault occurs close to the considered relay. Immediately after the fault, the disturbance detector (50DD) picks-up and
activates the phase selector. The phase selector recognizes an AG fault by asserting its
PHASE SELECT AG
operand. Ground
distance zone 1 (AG element) responds to the fault.
At this moment, the request to trip is placed for the trip output. As the fault is recognized as an AG fault, the
TRIP PHASE A
operand is asserted by the trip output. This signal is passed to the breaker control scheme and results in tripping pole A of
the breaker.
Simultaneously with the
TRIP PHASE A
operand, the
TRIP 1-POLE
operand is asserted to activate the open pole detector. This
detector responds to the
TRIP PHASE A
signal by declaring phase A open by asserting
OPEN POLE OP FA
(even before it is
actually opened). The
TRIP PHASE A
signal resets only after the breaker actually operates as indicated by its auxiliary
contact. At this moment the open pole detector responds to the breaker position and continues to indicate phase A
opened. This indication results in establishing blocking signals for distance elements (
OPEN POLE BLK AB
,
OPEN POLE BLK CA
operands are asserted). If neutral and negative-sequence overcurrent elements are mapped into the trip output to trigger
single-pole tripping, they must be blocked with the
OPEN POLE BLK N
operand, specifically provided for this purpose. The
OPEN POLE BLK N
operand must be assigned through the block setting of the overcurrent element. The two latter operands
