I.L. 41-748.1F
10
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
C
= 1400/1
R
V
= 150/1
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
A
per unit = 1.68
Z
C
per unit = 2R -Z
A
= 2 x 0.94 -1.68 = 0.20
(1)
= 16.45 ohms
(2)
= 27.6 ohms
(3)
= 3.29 ohms
To set Z
A
= 27.6 ohms
Step 1:
The lowest tap S
A
for 18.6 S
A
greater than
Z
A
= 27.6 is 2. Set S
A
in tap 2.
Step 2:
T
A
nearest to
= 13.8 is T
A
= 15.8.
Set TA in 15.8 tap.
Step 3:
=
1.145 -1 = + .145
Set M = + .15. Place R lead in O, L lead in upper .06.
The relay setting is now:
Actual
This is 99% of the desired setting.
To set Z
C
= 3.29 ohms:
Step 1:
The lowest tap S
C
for 6S
C
greater than
3.29 is S
C
= 1
Set S
C
= 1
Step 2:
T
C
nearest to
Set T
C
in 3.64 tap.
Step 3:
= 1.107 -1 = +.107
Hence, the nearest M
C
value is +.12. Now set R lead
in 0.03 tap and L lead in the upper .06 tap.
(Since M
C
has plus sign, lead L must be over R).
Then,
ohms, or
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
L-N
(equiv-
alent to 100 V
L-L
).
2. For water wheel generator applications, with
multiple machines tied to a common bus, the
M
TS
Z
------
1
–
=
Z
base
1000
kv
( )
2
R
C
kva
(
)
R
V
-----------------------------------
1000
18
( )
×
2
1400
×
183 500
150
×
,
------------------------------------------------------
=
=
Z
A
Z
A
per unit
(
)
Z
base
(
)
1.68
(
)
16.45
(
)
=
=
Z
C
Z
C
per
unit
(
)
Z
base
(
)
0.20
(
)
16.45
(
)
=
=
27.6
2
-----------
M
A
T
A
S
A
Z
--------------
1
–
=
15.8
2
×
27.6
---------------------
1
–
Z
A
T
A
S
A
1
M
±
--------------
15.8
2
×
1
0.15
+
---------------------
31.6
1.15
-----------
27.5
=
=
=
=
3.29
1
-----------
3.29 is 3.64
=
M
C
T
C
S
C
Z
C
---------------
1
–
3.64
1
×
3.29
---------------------
1
–
=
=
Z
C
T
C
S
C
1
M
C
+
(
)
-----------------------
3.64
1
×
1
.12
˙
+
---------------------
3.25
=
=
=
