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

SINGLE-POLE TRIPPING

Communications channels for pilot-aided schemes

568
The pre-fault quantities are captured and the calculations start when the disturbance
detector (ANSI 50DD) operates.
When the trip command is issued by the trip output logic
operands asserted) and during open pole conditions
phase selector resets all 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 is 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 the RX1, RX2, RX3, and RX4 settings for each scheme) and output operands (defined by
the 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 477: Phase selector logic
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 15: THEORY OF OPERATION
(TRIP 1-POLE or
TRIP 3-POLE
operand asserted), the