Pulsar TCF-10B, Системное руководство, Страница: 38 / 249

other wave, which has positive output during the
negative half-cycle of the sequence current
network, is compared to the receiver’s space
output in a second comparison circuit.
On internal faults, the positive half-cycle of the
local square wave lines up with the received mark
output to provide an AND-1 output (see
Figure 2-4). On the negative half-cycle, this local
square wave lines up with the received space
output to provide an AND-2 output. If an arming
signal is received (FD
2
and/or 21P) and either
AND-1 or AND-2 output exists for 4 ms, an input
to the trip flip flop initiates breaker tripping. The
same operation occurs at both terminals, tripping
breakers 1 and 2 simultaneously on either half-
cycle of fault current.
For tripping, both the mark and space frequencies
must be transmitted through the internal fault via
power line carrier channels. If these frequencies
are not received, the receiver detects a loss of
channel and clamps both the mark and space
outputs to a continuous positive state. This loss of
channel clamp enables both comparison circuits,
allowing the system to trip on the local square
wave input only. After 150 ms, the system output
clamps these to the zero state. At this point, the
system cannot trip and is locked out. An alarm
indicates loss of channel.
For external faults, the reversal of current at one
end shifts the square waves essentially 180°. As a
result, neither AND-1 nor AND-2 has the
sustained output required to operate the 4 ms timer
(see Figure 2-4). No trip occurs at either line
terminal.
2.1.5 Segregated Phase Comparison
System
The Segregated Phase Comparison system has
been developed to improve pilot relay protection,
particularly for the long EHV series capacitor-
compensated transmission lines. Long EHV series
capacitor-compensated lines are a source of
significant transients during the fault period.
Under these circumstances, sequence current
networks designed to operate at normal system
frequency may present a problem. The experience
with these Phase Comparison systems has,
however, been remarkably good. Directional
Comparison systems, on the other hand, are
subject to mis-operation on series capacitor-
compensated lines, particularly if the capacitor
gaps do not short the capacitors on faults.
Segregated phase comparison systems, which are
current-only, are independent of the following
phenomena:
• Power system frequency and wave form
• Effects of impedance unbalance between
the power system phase circuits.
• Maximum load/minimum fault current
margin.
The segregated phase comparison system can be
divided into two types: a two-subsystem scheme
and a three-subsystem scheme. In the two-
subsystem scheme, one subsystem operates from
delta current (I
a
-I
b
) for all multi-phase faults, and
a ground (3I
0
) current subsystem operates for all
ground faults. The three-subsystem scheme has a
subsystem for each phase (I
a
, I
b
, and I
c
). Each
subsystem consists of one channel (TCF–10B)
and one Phase Comparison relay.
Both segregated Phase Comparison systems
incorporate “offset keying,” enabling them to trip
for internal high-resistance ground faults and
internal faults with outfeed at one terminal. No
other system can clear these types of faults
without extra logic or channels. On a 500 kV line
with a 2,000:5 current transformer ratio, for
example, the three-subsystem scheme will operate
for ground-fault resistances up to about 100 ohms
primary impedance. Under the same conditions,
the two-subsystem scheme will operate up to
about 200 ohms primary fault resistance.
The two-subsystem package is suitable for all
applications except single-pole tripping, where the
three-subsystem package must be applied. The
basic operation of the scheme is illustrated in
Figure 2-5. Each current is fed through a nonin-
ductive resistor, supplying a voltage output to the
squaring amplifier (SA) that is exactly propor-
tional to the primary currents. The output of these
amplifiers is used to key the individual channels
and, through the local delay timers (LDT), to
provide the local square waves for comparison.
The timers are adjustable between 2 and 20 ms to
Page 2–6 December 1996
TCF–10B System Manual
Technologies, Inc.