Permissive Overreach Transfer Trip (Pott1) - GE ALPS Instruction Manual

1 PRODUCT DESCRIPTION 1.8 PROTECTION SCHEMES

1.8.4 PERMISSIVE OVERREACH TRANSFER TRIP (POTT1)

1
Figure 1–13: 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 preclude receipt of the trip signal. For such cases, an
unblocking channel that provides a time window of trip permission 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 to the comparator, 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 converters (CC1, as shipped).
Assuming that the out-of-step blocking function has not operated, an AND407 output results in a trip output via OR3, TL1,
OR4, AND3, OR13, AND13, OR7, and AND7.
This same sequence occurs for an internal ground fault when an overreaching zone ground-distance variable-Mho function
or the ground-directional-overcurrent function (or both) operates at each line terminal. Note that if the fault-current contribu-
tion 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, prevents a possible misopera-
tion 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 termi-
nal 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 contact 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-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.
1.8.5 POTT WITH BLOCKING FUNCTIONS (POTT2)
Figure 1–14: POTT WITH BLOCKING FUNCTIONS (POTT2) on page 1–32 is the logic diagram 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 block-
ing 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.
GE Power Management ALPS Advanced Line Protection System 1-27