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I.L. 41-748.1F
2. Select a value of M that will most nearly
make it equal to:
If the sign is negative, then the M taps are connected
with the R lead above the L lead to raise the setting.
5.2. SAMPLE CALCULATIONS
Assume that a KLF relay is to be applied to the fol-
lowing machine:
3-phase, 60 cycles, 3600 rpm, 18 kv, rated at 0.9 pf,
183,500 KVA at 45# H
.
2
R
= 1400/1
C
If the recommended setting from figure 8 is used:
The relay circle needed for a particular set of
machine capability curves may be obtained by trial
and error using a compass. The offset and radius of
the relay circle in figure 8 were drawn by this method.
Z
per unit = 1.68
A
Z
per unit = 2R -Z
= 2 x 0.94 -1.68 = 0.20
C
A
2
(
)
1000 kv
R
(1)
Z
=
---------------------------------- -
(
)R
base
kva
V
= 16.45 ohms
(
) Z
(
(2)
Z
=
Z
per unit
A
A
= 27.6 ohms
(
) Z
(
(3)
Z
=
Z
per unit
C
C
= 3.29 ohms
To set Z
= 27.6 ohms
A
Step 1:
The lowest tap S
Z
= 27.6 is 2. Set S
A
Step 2:
T
nearest to
A
Set TA in 15.8 tap.
T
S
A
A
Step 3:
M
=
-------------- 1
A
Z
1.145 -1 = + .145
Set M = + .15. Place R lead in O, L lead in upper .06.
The relay setting is now:
10
TS
----- - 1
.
M
=
–
Z
R
= 150/1
V
2
×
(
)
×
1000
18
1400
C
=
----------------------------------------------------- -
,
×
183 500 150
)
(
) 16.45
(
1.68
=
base
)
(
) 16.45
(
)
=
0.20
base
for 18.6 S
greater than
A
A
in tap 2.
A
27.6
= 13.8 is T
= 15.8.
---------- -
A
2
×
15.8 2
–
-------------------- - 1
–
=
27.6
T
Actual
Z
=
--------------
A
1
This is 99% of the desired setting.
To set Z
= 3.29 ohms:
C
Step 1:
The lowest tap S
3.29 is S
Set S
Step 2:
T
nearest to
C
Set T
Step 3:
M
C
= 1.107 -1 = +.107
Hence, the nearest M
in 0.03 tap and L lead in the upper .06 tap.
(Since M
has plus sign, lead L must be over R).
C
Then,
Z
=
-----------------------
(
C
98% of the desired value.
5.3. UNDERVOLTAGE UNIT
The undervoltage unit is usually set to a value corre-
)
sponding to the minimum safe system voltage for
stability. The voltage depends on many factors, but is
usually between 70 and 80 percent of normal system
voltage. The undervoltage unit is set at the factory for
77 percent of normal system voltage, or 53 volts
line-neutral. In applications where multiple units are
connected to the same bus, loss-of-field of one unit
may not depress the bus voltage of the point where
the undervoltage unit will operate if it has the stan-
dard setting. The following recommendations should
be considered:
1. For cross-compound turbine generator appli-
cations, the dropout (contact closure of
"back" contact of voltage unit, i.e., the con-
tact which is in the trip circuit and is shown
closed on schematic) voltage of the under-
voltage unit should be set for 58 V
alent to 100 V
2. For water wheel generator applications, with
multiple machines tied to a common bus, the
S
×
15.8 2
31.6
A
A
=
-------------------- -
=
---------- -
±
1
0.15
1.15
M
+
for 6S
C
C
= 1
C
= 1
C
3.29
3.29 is 3.64
---------- -
=
1
in 3.64 tap.
C
T
S
×
3.64 1
C
C
=
-------------- - 1
–
=
-------------------- - 1
3.29
Z
C
value is +.12. Now set R lead
C
T
S
×
3.64 1
C
C
3.25
=
-------------------- -
=
)
1
+
M
.12 ˙
1
+
C
).
L-L
27.5
=
greater than
–
ohms, or
(equiv-
L-N
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