Communications Channels For Pilot-Aided Schemes - GE D90 Plus Instruction Manual

CHAPTER 15: THEORY OF OPERATION

Communications channels for pilot-aided schemes

PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
When the trip command is issued by the trip output logic
operands asserted) and during open pole conditions
phase selector resets all of its output operands and ignores any subsequent operations of
the disturbance detector.
Plus
In the D90 , pilot-aided schemes transmit a code representing the type of fault
determined by the local phase selector according to the scheme logic. At a receiving
terminal, the local and remote data are combined to determine the action to be
performed. Schemes can be used with channels that can carry one, two, or four bits. Using
a one-bit channel, the schemes at all terminals of the line use their local phase selectors to
identify the fault type and initiate appropriate tripping actions. In 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 messages, 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-country fault condition. Using four-bit channels, the
relays share enough information about fault types seen from all the line terminals 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 operands (defined
by RX1, RX2, RX3, and RX4 settings for each scheme) and output operands (defined by TX1,
TX2, TX3, and TX4 settings for communications and trip A, trip B, trip C, and trip 3P settings
for actions, for each scheme) 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 produce an output.
(OPEN POLE OP
Figure 563: Phase selector logic
SINGLE-POLE TRIPPING
(TRIP 1-POLE or
TRIP 3-POLE
operand asserted), the
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