I.L. 41-490H
4
As shown in Figure 19, the T compensator second-
ary is connected to modify the phase A voltage. With
a fault in the trip direction, the induced voltage in the
compensator secondary bucks the phase A voltage.
Vector diagrams in Figure 8 illustrate the operation
during 3-phase faults at four locations. The system
impedance and the compensator angle are as-
sumed to be at 90
°
for illustrative purposes only. Pre-
fault voltages are depicted by the large dashed trian-
gle. The smaller dashed triangle in each case is the
system voltages at the relay location during the fault.
This triangle is modified by the compensator voltage
-1.5I
A
T where 1.5T is the compensator mutual im-
pedance. The terminals of the tripping unit are des-
ignated: X, Y and Z. Phase A tripping unit voltage is:
V
X
= 1.5 V
AN
-1.5 I
A
T
(1)
Note that 3I
0
= 0 for 3-phase faults
(2)
Phase B and phase C tripping unit voltages are:
V
Y
= V
BN
(3)
V
Z
= V
CN
(4)
For a fault at A, beyond the relay operating zone, the
compensator voltage, -1.5I
A
T modifies the phase A
voltage, reducing the voltage triangle of the tripping
unit to X-Y-Z. With an X-Y-Z rotation the tripping unit
torque is in the restraining direction.
For a fault at B the current is larger than for a fault at
A, so that -1.5I
A
T is larger. The point X is in line with
points Y and Z. No torque is produced, since the
X-Y-Z triangle has a zero area.
For a fault in the operating zone, such as at C, point
X is below the YZ line. Now the rotation is X, Z, Y,
which produces operating 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 reversed polarity. Compensator voltage,
-1.5
A
T, increases the area of the bus voltage trian-
gle, A-B-C. Tripping unit voltage has an X-Y-Z rota-
tion which produces restraining torque.
A solid 3-phase fault at the relay location, tends to
completely collapse the A-B-C voltage triangle. The
area of the X-Y-Z triangle also tends to be zero un-
der these conditions. A memory circuit in the KD-10
relay provides momentary operating torque under
these conditions, for an internal fault. In the KD-11
relay the winding Z in the current circuit, in conjunc-
tion with the compensator voltage, produces a cur-
rent-only torque, which maintains operating torque
under the condition of zero potential. In the short
reach relay the offset is obtained by means of an ad-
ditional compensator T
BR
.
The P
3A
- R
3F
parallel resistor-capacitor combina-
tion in the compensated phase provides correct
phase-angle relation between the voltage across the
front and back coils of Z (3
φ
) and the current, similar
phase shift is produced in left and right hand coils by
capacitor C
3C
. The P
3A
-C
3A
combination also pro-
vides control of transients in the coils of the cylinder
unit.
3.2
Phase-to-Phase Unit
Compensator primaries of T
AB
and T
BC
are ener-
gized by I
A
, I
B
and I
C
as shown in Figure 19. Com-
pensator secondaries are connected to modify their
respective phase voltages (e.g., T
AB
modifies V
AB
).
With a fault in the trip direction, the induced voltages
i n t h e c o m p e n s a t o r s e c o n d a r i e s b u c k t h e
phase-phase voltages.
Vector diagrams in Figure 9 illustrate the operation
during phase B-C faults at four locations. The sys-
tem impedances and the compensator angle are as-
sumed to be at 90
°
, for illustrative purposes. Prefault
voltages are depicted by the large dashed triangles.
The smaller light triangle in each case is the system
voltages at the relay location during the fault. This tri-
angle is modified by the compensator voltages
-(I
A
-I
B
) Z
C
and -(I
C
-I
B
) Z
C.
Z
C
is the compensator mu-
tual impedance. In this case I
A
= O. The terminals of
the tripping unit are designated; X, Y, and Z. Tripping
unit voltages for phase B-C faults are:
V
XY
= V
AB
-(I
A
-I
B
)Z
C
(5)
V
ZY
= V
CB
-(I
C
-I
B
)Z
C
(6)
For a fault at A, in Figure 9 beyond the relay operat-
ing zone, the compensator voltages change the
A-B-C voltage sequence to the X-Y-Z sequence.
Voltages of this sequence applied to operating unit
produce restraining torque.
For a fault at B, the currents are larger than for a fault
at A, so that compensator voltages are larger. Points
