Blocking Scheme - GE ALPS Instruction Manual

1.8 PROTECTION SCHEMES 1 PRODUCT DESCRIPTION

1.8.6 BLOCKING SCHEME

1
The logic for the Blocking scheme is shown in the following diagram. Since a reverse-looking blocking function is required
in these schemes, the Zone 4 distance functions are automatically set for reversed reach when the Blocking scheme is
selected. As for channel operation, a blocking scheme has the 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 block-
ing 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 overreaching 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 separate trip and block functions
as well as separate transient blocking circuits.
The Blocking scheme has provision 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 uses the IPB function ANDed with the NB function to start carrier in the GDOC or MGDOC schemes.
The IPB option uses the IPB overcurrent detector 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 IPBKFACTOR = 0.0, or with positive sequence current restraint, IPBKFACTOR = 0.066.
The IPB option uses the IPB overcurrent detector without NB supervision as a non-directional carrier start function. The FD
option uses the fault detector (FD) without NB supervision as a type, rather than an FSK type. Note that both Carrier Start
and Carrier Stop contact outputs are provided to control the transmitter in the GE CS28A ON-OFF carrier set.
For any multiphase fault on the protected line, one or more of the overreaching zone variable-Mho functions operates at
each end and apply one of the inputs to the comparator AND407 via OR2. An output from OR110 inhibits the blocking func-
tions 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.
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 separate trip and block functions as well as separate transient blocking circuits.
The FD option uses the fault detector (FD) without NB supervision as a non-directional carrier start function. Assuming that
the out-of-step blocking function has not operated, AND407 produces an output to initiate tripping following the coordina-
tion 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. Tripping,
as in all the schemes, is supervised by the Fault Detector at AND7, 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 ALPS via one of the contact converter inputs. The default, or As Shipped, contact converter 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.
The ground-directional overcurrent (GDOC) transient blocking logic consists of TL24, OR508, AND301, and OR302. After
an external fault, the GDOC blocking function starts the carrier and applies a blocking input to the comparator. If the exter-
nal fault persists for 25 ms, TL24 produces an output. At this point the GDOC blocking function is set up with an extended
dropout time so that the carrier is maintained and tripping is blocked at the comparator for at least 30 ms following the
clearing of the external fault.
The ground-distance and phase-distance transient blocking logic consists of OR20, AND503, TL25, and OR302. It oper-
ates in a similar manner to the GDOC transient blocking logic. Thus, if any of the overreaching zone tripping functions oper-
ate as a result of a current reversal or a fault-clearing transient, tripping is initiated because of the blocking output
maintained by the blocking function(s). TL24 never picks up on internal ground faults. On internal phase faults, the tripping
functions take priority over the blocking functions and prevent them from operating, or cause them to reset if an internal
fault occurs following an initial external fault.
1-30 ALPS Advanced Line Protection System GE Power Management