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Y and Z coincide now and the area of the X-Y-Z tri-
angle is zero. No torque is produced.
For a fault in the operating zone, such as at C , the
compensator voltages reverse the rotation of trip-
ping unit voltages to X-Z-Y sequence. Voltages of
this sequence applied to operating unit produce op-
erating torque.
For a fault behind the relay at D, restraining torque is
produced. Since the fault is behind the relay, the cur-
rent is of reverse polarity and tripping unit voltage
has an X-Y-Z rotation. This rotation produces re-
straining torque.
Note that this unit does not require memory action,
since the sound-phase voltage reacts with the com-
pensator voltage to produce a strong restraining or a
strong operating torque, depending upon the fault lo-
cation. This is true even for a complete collapse of
t h e f a u l t e d p h a s e - t o - p h a s e v o l t a g e . T h e
phase-to-phase unit is identical in the KD-10 and
KD-11 relays.
Similar vector diagrams apply for a fault between
phases A and B or between phases C and A. Each
of the three phase-to-phase fault combinations sub-
jects the cylinder unit to a similar set of conditions.
4.
CHARACTERISTICS
4.1
Distance Characteristics
Phase-to Phase Unit
This unit responds to all phase-to-phase faults and
most two-phase-to-ground faults. It does not re-
spond to load current, synchronizing surges, or
out-of-step conditions. While a characteristic circle
can be plotted for this unit on the R-X diagram, as
shown in Figure 10, such a characteristic circle has
no significance except in the first quadrant where re-
sistance and reactance values are positive. A small
portion of the fourth quadrant, involving positive re-
sistance values and negative reactance values,
could have some significance in the event that the
transmission line includes a series capacitor. The
portion of the circle in the first quadrant is of interest
because it describes what the relay will do when arc
r e s i s t a n c e i s i n v o l v e d i n t h e f a u l t . T h e
phase-to-phase unit operating on an actual trans-
mission system is inherently directional and no sep-
arate directional unit is required.
Courtesy of NationalSwitchgear.com
An inspection of Figure 10 indicates that the circle of
the phase-to-phase unit is dependent on source im-
pedance Z
. However the circle always goes
S
through the line balance-point impedance. The
reach at the compensator (and line) angle is con-
stant, regardless of the system source impedance.
The broadening out of the characteristic circle with a
relatively high source impedance gives the
phase-to-phase unit the advantageous characteris-
tic hat for short lines, it makes a greater allowance
for resistance in the fault. Stated another way, the
characteristics approach that of a reactance relay
more and more closely as the line being protected
becomes shorter and shorter with respect to the
source impedance back of the relaying location.
4.2
Sensitivity: Phase-to-Phase Unit
A plot of relay reach, in percent of tap block setting,
versus relay terminal voltage and current sensitivity
is shown in Figure 12. The unit will operate with the
c o r r e c t d i r e c t i o n a l s e n s e f o r z e r o v o l t a g e
phase-to-phase faults. For this condition the fault
current must be not less than 0.015 relay amperes
with an ohm setting of 5.8 with rated voltage on the
unfaulted phase. Pick up current is proportionately
higher in S=2 and S=3 taps.
The KD-10 relay may be set without regard to possi-
ble overreach due to dc transients. Compensators
basically are insensitive to dc transients which at-
tend faults on high angle systems. The long time
constant of a high angle system provides a minimum
rate of change in flux-producing transient current
with respect to time, and therefore induces a mini-
mum of unidirectional voltage in the secondary.
Asymmetrical currents resulting from faults on
low-angle systems having a short time constant can
induce considerable voltage in the secondary, but
for the first half cycle, the transient-derived voltage
subtracts from the steady-state value. This transient
decays so rapidly that it is insignificant during the
second half cycle when it adds to the steady-state
value.
4.3
Distance Characteristic–
KD-10, 3-Phase Unit
The three-phase unit has a characteristic circle
which passes through the origin as shown in Figure
11. This circle is independent of source impedance.
I.L. 41-490H
5
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