Permissive Overreach Transfer Trip (Pott1); Permissive Overreach Transfer Trip With Blocking Functions (Pott2) - GE LPS-D Instruction Manual

1.8 PROTECTION SCHEME DESCRIPTIONS 1 PRODUCT DESCRIPTION

1.8.4 PERMISSIVE OVERREACH TRANSFER TRIP (POTT1)

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Figure 1–12: PERMISSIVE OVERREACH TRANSFER TRIP (POTT1) on page 1–31 is the logic diagram for
the POTT1 scheme. The POTT1 scheme requires receipt of a tripping signal from the remote end(s) to permit
tripping at the local end. The channel equipment is generally a frequency-shift (FSK) type. When a power-line
carrier channel is used, it is possible that an internal fault may attenuate the carrier signal sufficiently to pre-
clude receipt of the trip signal. For such cases, an unblocking channel that provides a time window of trip per-
mission for an attenuated signal caused by an internal fault should be considered.
For any multiphase fault on the protected line, one or more of the overreaching zone variable-Mho functions
operates at each terminal and applies one of the inputs of AND407, via OR2 and TL4. The output from OR2
also keys the transmitter to the trip frequency via OR5. The receiver produces a trip output at each terminal of
a two-terminal line. This received permissive trip signal is supplied to the relay logic via one of the contact con-
verters (CC1, as shipped). Assuming that the out-of-step blocking function has not operated, an AND407 out-
put results in a trip output via OR3, TL1, OR4, AND3, OR13, AND13, OR7, and AND7.
The same sequence of operations occurs for an internal ground fault when an overreaching zone ground-dis-
tance variable-mho function or the ground-directional-overcurrent function (or both) operates at each line ter-
minal. Note that if the fault-current contribution at one end is insufficient to pick up the overreaching trip
function there, then neither end can trip via the POTT1 logic. A hybrid scheme is preferable for such a weak- or
zero-in-feed condition.
Timer TL1 allows the relay to ride through spurious outputs that might be produced from the channel during
external faults within the reach of the overreaching trip functions. Timer TL4, in conjunction with timer TL1, pre-
vents a possible misoperation when a fault-current reversal occurs as a result of sequential clearing of a fault
on a parallel line. Note that tripping is supervised by the Fault Detector at AND7, thus confirming that tripping
occurs only after a fault on the power system.
The above description assumes a two-terminal line. When a POTT1 scheme is applied on a three-terminal
line, each terminal has two receivers and one transmitter, with each frequency-shift transmitter operating at a
different frequency. Now the trip signal must be received from each of the two remote terminals, as indicated
by an output from AND21.
On a line protected by a POTT1 scheme, a problem arises if the line is operated with the breaker at one end
open, but the breaker(s) at the other end(s) closed. For this condition, the relay at the closed end(s) cannot
operate for a fault on the line unless the transmitter at the open end is keyed to the trip frequency. A 52/b con-
tact from the breaker is used to key the transmitter continuously to the trip frequency when the breaker is open.
The default, or As-Shipped, contact converter for the first breaker is CC5; if two breakers are involved, as in a
ring bus or breaker-and-a-half bus arrangement, then CC6 is combined with CC5 at AND5 to indicate that the
line is open.
The sequence of operations is similar for an internal ground fault. Ground-distance, ground-directional-over-
current, 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 block-
ing circuits.

1.8.5 PERMISSIVE OVERREACH TRANSFER TRIP WITH BLOCKING FUNCTIONS (POTT2)

Figure 1–13: POTT WITH BLOCKING FUNCTIONS (POTT2) LOGIC DIAGRAM on page 1–32 is the logic dia-
gram for the POTT2 scheme. The POTT2 scheme performs exactly the same as the POTT1 scheme for all
internal faults. In the POTT2 scheme, the Zone 4 distance functions are set as reverse looking blocking units.
The Zone 4 distance functions provide transient blocking for when a fault-current reversal occurs as a result of
sequential clearing of a fault on a parallel line. This logic is the same as that used in the Hybrid scheme.
1- LPS-D Line Protection System GE Power Management
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