Ametek UPLC-II System Manual page 69

To generate the local and keying square waves, con-
ventional phase comparison systems use thresholds
equivalent to (or very near) the zero axis. As a
result, an internal fault with outfeed looks like an
external fault to those systems (see Figure 3–14).
The offset keying technique permits the relay sys-
tem to trip for internal faults with outfeed current
out at one terminal. While the outfeed condition is
very unusual, it presents difficult problems to the
great majority of pilot relaying systems when it
does occur. Outfeed can occur in any of the follow-
ing cases:
• Series-capacitor-compensated parallel lines.
• Weak-feed or zero-feed applications, partic-
ularly with heavy through load.
• Some multi-terminal applications.
• Series-compensated
sation) line with a source inductive reactance
smaller than series capacitor reactance.
• Some single-line-to-ground faults, occur ring
simultaneously with an open conductor,
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 sub-
system).
The offset keying technique allows the relay system
to work like a true current differential scheme. The
scheme takes advantage of the fact that, for the out-
feed 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 3–14,
where I G equals I F plus I H . While the two terminal
January 2016
currents may have any angular relationship with
one another, most outfeed conditions display a near-
ly out-of-phase relation ship. The out-of-phase con-
dition illustrated is the most difficult case for phase
comparison, as well as the most common outfeed
condition.
In the offset keying technique, the keying threshold
is displaced in the positive direction, away from the
zero axis. The local square wave thresholds are dis-
placed negatively. To maintain security, the local
thresholds are separated from each other, providing
"nesting" during external faults. Typical settings are
shown in Figure 3–15.
Figure 3–16 illustrates the square wave charac -
teristics of offset keying for normal internal faults,
external faults, and internal faults with outfeed.
(line-end compen-
The segregated Phase Comparison scheme incorpo-
rates a high degree of security. Its design is based
on extensive field experience and the model line
tests for the very long, series capacitor-compensat-
ed EHV lines.
Output trip signals are supervised by an arming
input and a number of security checks (see
Figure 3–14). Phase arming is performed by a cur-
rent rate-of-change detector that responds to sudden
increases, decreases, or angular shifts in current. 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 secondary.
Security checks to comparison AND (see Figure 3–
14) include (1) low channel signal blocking, (2)
lockout for sustained low channel signal, (3) chan-
nel noise clamp, and (4) receive guard block. For
the phase subsystems, a trip signal occurs if com-
parison AND has an output for more than 3ms (4ms
for the ground subsystem).
Chapter 3. applications
Page 3–23
3