Pulsar TCF-10B, System Manual, Page: 37 / 249

Because the trip fault detectors (P) do not operate for external
faults, underreaching transfer trip systems do not require external
fault-clearing coordination circuits (transient blocking) and are,
therefore, inherently simpler than any of the other schemes. You
obtain maximum security if you use additional permissive fault
detectors. These schemes also provide minimum operating times
for many faults that are tripped directly, without using the
channel.
December 1996 Page 2–5
Chapter 2. Applications and Ordering Information
2
Figure 2–4. Basic Operation of the Dual
Phase Comparison Pilot Relaying System.
2.1.4 Dual Phase
Comparison
Unblocking
Systems
Dual comparison systems require a
duplex channel: one frequency for
each line terminal. The TCF–10B
frequency-shift channel equipment
is available for this purpose;
normally used in an unblocking
system. Continuous channel moni-
toring is also provided, because
either a mark or space carrier
signal is always transmitted.
The transmitter is keyed to its
mark frequency when the square
wave from the filter goes positive,
and is keyed to its space frequency
when the square wave is at zero.
There are two outputs at the
receiver: the “mark” output is a
square wave that goes positive
when a mark frequency is
received; the “space” output goes
positive when a space frequency is
received.
The basic operation of the Dual
Phase Comparison system is
shown in Figure 2-4. For internal
faults, the single phase outputs of
the sequence current networks are
essentially in phase, although such
output represents currents 180°
apart in the power system. The
network output goes through a
squaring amplifier that keys the
frequency shift transmitter. An
adjustable delay circuit delays the
local square wave by a time equal
to the channel delay time.
The network output is then used to
develop two complementary
square waves. One wave, which
has a positive state during the
positive half-cycle of the sequence
current network, is compared with
the receiver’s mark output. The