GE L90 Instruction Manual page 563

9 THEORY OF OPERATION
single pole operation applications however, a three-pole trip can be performed in the event of an in-line single-phase fault
co-incident with a fault on a different phase (cross-country fault) that is within the reach of the local phase selector, which is
considerably longer than the line. This possibility can be reduced by using a two-bit channel, and eliminated by using a
four-bit channel.
Using two-bit channels, the relays can share limited information about their local phase selection, improving considerably
the accuracy of single-pole tripping on cross-country faults. Two-bit channels however can only provide four different mes-
sages, one of which must be "no fault has been detected." With only three messages available it is not possible to transmit
sufficient information to eliminate the use of local phase selector data, so a three-pole operation can occur in a cross-coun-
try fault condition. Using four-bit channels, the relays share enough information about fault types seen from all the line ter-
minals that local fault selector data can be rejected. In addition, in multiple bit systems the relays do not respond to non-
valid bit combinations, making the protection system more immune to communication problems than in a single bit system.
Each scheme within the relay has a setting that specifies the number of bits available on the associated communications
channel. This setting defines the input (RX1, RX2, RX3, RX4) and output (TX1, TX2, TX3, TX4 for communications and
[Scheme Abbreviation] TRIP A, TRIP B, TRIP C, TRIP 3P for action) operands used by the scheme, the data codes used to
convey fault data between terminals, and the method of combining information from the local and remote terminals to pro-
duce an output.
b) SINGLE BIT CHANNELS
The TX1 and RX1 operands are used, and fault data is coded per the following tables.
Table 9–11: PERMISSIVE SCHEME TRANSMIT CODES FOR 1-BIT CHANNELS
PHASE SELECTOR DETERMINATION OF
FAULT TYPE
AG, BC, BCG, BG, CA, CAG, CG, AB, ABG, 3P
Unrecognized or AR FORCE 3P TRIP
None of the above
Table 9–12: BLOCKING SCHEME TRANSMIT CODES FOR 1-BIT CHANNELS
PHASE SELECTOR DETERMINATION OF
FAULT TYPE
AG, BC, BCG, BG, CA, CAG, CG, AB, ABG, 3P
Unrecognized or AR FORCE 3P TRIP
None of the above
Table 9–13: UNBLOCKING SCHEME TRANSMIT CODES FOR 1-BIT CHANNELS
PHASE SELECTOR DETERMINATION OF
FAULT TYPE
AG, BC, BCG, BG, CA, CAG, CG, AB, ABG, 3P
Unrecognized or AR FORCE 3P TRIP
None of the above
The action output is generated per the following tables.
Table 9–14: PERMISSIVE SCHEME TRIP TABLE FOR 1-BIT CHANNELS
REMOTE DATA
BIT PATTERN RECEIVED
RX1
1
1
1
GE Multilin
BIT PATTERN TRANSMITTED
TX1
BIT PATTERN TRANSMITTED
TX1
BIT PATTERN TRANSMITTED
TX1
REMOTE DETERMINATION
OF FAULT TYPE
Any
Any
Any
L90 Line Current Differential System
9.5 SINGLE-POLE TRIPPING
1
1
0
0
0
1
1
1
0
LOCAL DATA
LOCAL DETERMINATION OF
FAULT TYPE
AG Fault
BG Fault
CG Fault
9
TRIP OUTPUT
Trip Phase A (A+N/G)
Trip Phase B (B+N/G)
Trip Phase C (C+N/G)
9-39