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KLF-1 Loss-of-Field Relay
1. Select the lowest tap S which gives a product
of 18.6S
greater than desired Z
A
product of 6S
greater than desired Z
C
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.
5.3
SAMPLE CALCULATIONS
Assume that a KLF-1 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 (page 23)
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
=
----------------------------------- -
( kVA )R
base
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 S
A
Z
= 27.6 is 2. Set S
A
and a
A
.
C
TS
.
M
=
------- - 1
–
Z
R
= 150/1
V
2
×
(
)
×
1000
18
1400
C
=
----------------------------------------------------- -
,
×
183 500
150
)
(
) 16.45
(
)
=
1.68
base
)
(
) 16.45
(
)
=
0.20
base
tap for 18.6 S
greater than
A
in tap 2.
A
Step 2:
T
nearest to
A
Set TA in 15.8 tap.
T
S
A
A
Step 3:
------------ - 1
M
=
–
A
Z
1.145 -1 = + .145
Set M = + .15. Place R lead in 0, L lead in upper .06.
The relay setting is now:
T
S
15.8
A
A
Actual
Z
=
------------- -
=
-------------------- -
±
A
1
1
M
This is 99.7% of the desired setting.
To set Z
= 3.29 ohms:
C
Step 1:
The lowest S
3.29 is S
= 1
C
Set S
= 1
C
Step 2:
T
nearest to
C
Set T
in 3.64 tap.
C
T
S
C
C
Step 3:
------------- - 1
M
=
C
Z
C
= 1.107 -1 = +.107
Hence, the nearest M
C
in 0.03 tap and L lead in the upper .06 tap.
(Since M
has plus sign, lead L must be over R).
C
T
S
C
C
Then,
Z
=
---------------------- -
(
)
C
1
+
M
C
98.8% of the desired value.
5.4
UNDERVOLTAGE UNIT
A. The undervoltage unit is usually set to a value
corresponding to the minimum safe system volt-
age 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 sys-
tem voltage, or 53 volts line-neutral for relays
rated 69 Vac and 90 Vac for relays rated 120
Vac.
41-748.31B
27.6
---------- -
= 13.8 is T
= 15.8.
A
2
×
15.8 2
-------------------- - 1
=
–
27.6
×
2
31.6
27.5
=
---------- -
=
0.15
1.15
+
tap for 6S
greater than
C
C
3.29
---------- -
=
3.29 is 3.64
1
×
3.64
1
-------------------- - 1
–
=
–
3.29
value is +.12. Now set R lead
×
3.64 1
=
-------------------- -
=
3.25
ohms, or
·
1
.12
+
7
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