GE LPS-D, Instruction Manual, Page: 51 / 436

GE Power Management LPS-D Line Protection System 1-
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1 PRODUCT DESCRIPTION 1.8 PROTECTION SCHEME DESCRIPTIONS
1
1.8.6 BLOCKING SCHEME
Figure 1–14: BLOCKING SCHEME LOGIC DIAGRAM on page 1–34 is the logic diagram for the Blocking
scheme. Since a reverse-looking blocking function is required in this scheme, the Zone 4 distance functions
are automatically set for reversed reach when the Blocking scheme is selected. As far as channel operation is
concerned, a blocking scheme has opposite sense from a PUTT, POTT or Hybrid scheme.
For an external fault, the blocking functions at the line end nearest the fault key the transmitter, and the receipt
of this blocking signal at the other end of the line prevents a trip output. For an internal fault, the transmitters
are not keyed or, if keyed on initially at fault inception, they are quickly turned off by operation of the overreach-
ing trip functions. Therefore, receiver output is not required for tripping at either end.
The channel equipment generally used is an ON-OFF carrier set. The sequence of operations is similar for an
internal ground fault. Ground-distance, ground-directional-overcurrent, or both functions acting in parallel may
be selected for ground-fault protection. Ground-distance and ground-directional-overcurrent each have sepa-
rate trip and block functions as well as separate transient blocking circuits.
The LPS-D Blocking scheme has provisions for different logics to start the carrier for ground faults. The carrier
starting option is determined by Protection Settings 1204: CARRSTART and 505: IPBKFACTOR.
CARRSTART may be set to DIR, IPB, or FD. The DIR option used the IPB function ANDed with the NB func-
tion to start the carrier in the GDOC and MHOGDOC schemes. The IPB option uses the IPB overcurrent detec-
tor without NB supervision as a non-directional carrier start function. In the DIR and IPB options, the IPB
function may be selected to operate on the zero-sequence current without positive sequence current restraint
by setting IPBKFACTOR = 0.0, or to operate on zero-sequence current with positive sequence current
restraint by setting IPBKFACTOR = 0.066.
For any multiphase fault on the protected line, one or more of the overreaching zone variable-mho functions
operates at each end of the line and applies one of the inputs to the comparator AND407 via OR2. An output
from OR110 inhibits the blocking functions at AND503 via OR103 and NOT2, and any carrier that may have
been started is stopped via OR2, AND209, and OR213. Consequently, the carrier is stopped or is not started at
any terminal of the line; there is no receiver output and no blocking input applied to comparator AND407 via
CC1 and NOT3.
Assuming that the out-of-step blocking function has not operated, AND407 produces an output to initiate trip-
ping following the coordination time-delay pickup set on timer TL1. The coordinating time is required to allow
time for receiving a blocking signal from the remote terminal(s) to prevent misoperation on external faults. The
required setting is described in Chapter 2: CALCULATION OF SETTINGS Note that tripping, as in all the
schemes, is supervised by the Fault Detector at AND35, thus confirming that a trip only occurs when a distur-
bance is present on the power system.
For remote external faults within the reach of the local overreaching zone tripping functions, one of the remote
blocking functions operates to key the transmitter ON, sending a blocking signal to the local end. The receiver
output is supplied to the LPS-D via one of the contact converter inputs. The default, or as shipped contact con-
verter assigned to the receiver is CC1. An input from the receiver blocks tripping at the local end by removing
the upper input to AND407 via CC1 and NOT3. At the remote end, the output of the blocking functions is
applied to the lower NOT input of AND407 to block tripping there. This lower NOT input to AND407 forms part
of the transient blocking logic that blocks tripping when clearing external faults or during current reversals that
occur when clearing faults on a parallel line.