I.L. 41-490H
6
The three-phase unit is also inherently directional
and does not require a separate directional unit.
If a solid three-phase fault occurs right at the relay lo-
cation, the entire voltage triangle collapses to zero a
balance point condition, as shown by the relay char-
acteristic in Figure 11 which passes through the ori-
gin. However, since the YZ voltage also drops to ze-
ro, the relay would be unable to determine whether
an internal or external fault existed. To correct this
condition, a resonant circuit is added to the C-B volt-
age circuit of the relay which allows the ZY voltage
to determine whether the fault is inside the protected
line section or behind the relay.
4.4
Sensitivity: KD-10, 3-Phase-Unit
The unit will operate with the correct directional
sense for zero voltage three-phase faults when nor-
mal voltage exists at the relay terminals prior to the
fault. This operation occurs due to memory action as
described above. The unit will have zero torque or
perhaps a slight opening torque if there is zero volt-
age at the relay prior to the fault or after the memory
action has subsided. For medium and long reach re-
lays with an impedance setting of 5.8 ohms the
three-phase unit will directionally operate for faults
which produce 2 volts line-to-line and 1.0 ampere at
the relay terminals.
Sensitivity with 2 volts line-to-line for any tap is de-
fined by Equation 7:
(7)
For short reach relays (0.2-4.5 ohms) with an imped-
ance setting of 1.23 ohms the three-phase unit will
directionally operate for faults which produce 0.5
volts line-to-line and 2.7 ampere at the relay termi-
nals.
Sensitivity with 0.75 volts line-to-line for any tap is
defined by Equation 8:
(8)
The KD-10 relay may be set without regard to possi-
ble overreach due to dc transients.
4.5
Distance Characteristic:
KD-11, 3-Phase Unit
The three-phase unit of the KD-11 relay has a char-
acteristic circle which includes the origin as shown in
Figure 13.
A single turn current coil on the cylinder unit provides
for current-only torque and is small compared to the
many turns of the T Max. setting of the compensator
and has very little influence on the overall settings.
However, as the compensator setting is reduced, the
single turn current coil becomes larger by compari-
son and has more and more effect on the overall set-
tings.
For 1.3–36.7 ohms range the reach and maximum
torque angle will vary approximately as follows:
For .75–21.2 ohms range the reach will vary approx-
imately as follows:
NOTE:
When setting KD-11 Relays disregard
the change in T-Value, but
include the percentage error into test
current values.
The .2–4.5 ohms range KD-11 relays have no over-
reach regardless of the tap being used. The maxi-
mum torque angle will stay constant at 60
°
. The
I
5.8
T
--------
amperes
=
I
3.4
T
--------
amperes
=
T Nominal
T Actual
%
Overreach
MTA
Equiv.
Reverse T
10
7.02
5.0
3.51
2.50
2.00
1.50
10.1
7.13
5.12
3.64
2.64
2.14
1.656
1.0
1.5
2.4
3.7
5.6
7.2
10.4
75
76
79
82
83
85
87
.13
.13
.12
.12
.11
.11
.11
T Nominal
T Actual
%
Overreach
MTA
Equiv.
Reverse T
5.8
4.06
2.90
2.03
1.45
1.16
.87
5.92
4.18
3.036
2.17
1.615
1.33
1.05
2.2
3.4
7.6
5.9
8.3
12
17
79
80
82
85
89
91
98
.13
.13
.12
.12
.12
.11
.11