Direct Transfer-Trip Systems - Ametek UPLC CU44-VER04 Applications Manual

systems when it does occur. Outfeed can occur in
any of the following cases:
• Series-capacitor-compensated
lines.
• Weak-feed or zero-feed applications, par-
ticularly with heavy through load.
• Some multi-terminal applications.
• Series-compensated (line-end compen-
sation) line with a source inductive reac-
tance smaller than series capacitor reac-
tance.
• Some single-line-to-ground faults, occur-
ring simultaneously with an open conduc-
tor, where the fault is on one side of the
open conductor.
• Some single-line-to-ground faults with
high fault resistance and heavy through
load (such conditions can cause outfeed
only in the faulted phase current, not in the
ground subsystem).
The offset keying technique allows the relay sys-
tem to work like a true current differential scheme.
The scheme takes advantage of the fact that, for
the outfeed condition, the current into the line is
greater in magnitude than the current out of the
line for the internal fault.
This relationship is illustrated in Figure 3–9,
where I G equals I F plus I H . While the two termi-
nal currents may have any angular relationship
with one another, most outfeed conditions display
a nearly out-of-phase relationship. The out-of-
phase condition illustrated is the most difficult
case for phase comparison, as well as the most
common outfeed condition.
In the offset keying technique, the keying thresh-
old is displaced in the positive direction, away
from the zero axis. The local square wave thresh-
olds are displaced negatively. To maintain securi-
ty, the local thresholds are separated from each
other, providing "nesting" during external faults.
Typical settings are shown in Figure 3–10.
Figure 3–11 illustrates the square wave charac-
teristics of offset keying for normal internal faults,
external faults, and internal faults with outfeed.
May 2012
The segregated Phase Comparison scheme incor-
porates a high degree of security. Its design is
based on extensive field experience and the model
parallel
line tests for the very long, series capacitor-com-
pensated EHV lines.
Output trip signals are supervised by an arming
input and a number of security checks (see
Figure 3–9). Phase arming is performed by a cur-
rent rate-of-change detector that responds to sud-
den increases, decreases, or angular shifts in cur-
rent. It operates on current changes of 0.5 A or
more, with an operating time of 2 ms. Ground
arming is 3I magnitude—typically 0.8 A second-
ary.
Security checks to comparison AND (see
Figure 3–9) include (1) low channel signal block-
ing, (2) lockout for sustained low channel signal,
(3) channel noise clamp, and (4) receive guard
block. For the phase subsystems, a trip signal
occurs if comparison AND has an output for more
than 3ms (4ms for the ground subsystem).
3.2 Direct Transfer-Trip
Systems
Direct transfer-trip systems provide circuit-break-
er tripping at remote or receiver terminals, without
any supervision by fault detectors. The most
important consideration in a direct transfer-trip
system is the type of channel applied. The com-
munications equipment must carry the total bur-
den of system security and dependability.
Direct transfer-trip systems are applied for:
• Line protection with non-permissive under
reaching transfer-trip systems.
• Transformer protection where there is no
circuit breaker between the transformer and
transmission line.
• Shunt reactor protection.
• Remote breaker failure protection.
A sample schematic is shown in figure 3-12.
Chapter 3. Applications
Page 3–11
3