41-748P
KLF Loss-of-Field Relay
10
For example, from Figure 8 page 15, an output of 0.6
per unit kW on 30# hydrogen pressure curve is -0.4
per unit reactive kVA. Therefore,
= 0.721 per unit
and,
Converting to the impedance curve:
|Z| = 1.39 per unit
Since the angle remains the same, the impedance
plot conversion is:
, as shown in Figure 9, page
Continue this process with other points from curves
such as those of figure 8 until a complete curve like
that of figure 9 is obtained. After plotting the steady-
state stability limit and the machine capability curves
on the R-X diagram, plot the impedance unit circle
between the stability limit and the capability curve.
(Note in Figure 9 the relay circle cannot be plotted
within the 60# -V
T
= 0.95 curve, since the machine is
beyond the steady-state stability limit for these condi-
tions.) This plot defines the desired reach Z
A
and
radius R of the relay circle. Then use the following
procedure to select tap settings.
(2)
where
Z
base
= one per unit primary ohms/as seen from
the relay
kV
= rated phase-to-phase voltage of the
machine
kVA = rated kVA of the machine
R
C
= the current transformer ratio
R
V
= the potential transformer ratio
The actual settings, Z
A
and Z
C
, are:
Z
A
= (Z
A
per unit) X (Z
base
)
(3)
Z
C
= (Z
C
per unit) X (Z
base
) =
(2R-Z
A
) X (Z
base
)
(4)
where R = radius of circle in per unit.
The tap-plate settings are made according to
equations:
(5)
where:
T
= compensator tap value
S
= autotransformer primary tap
M
= autotransformer secondary tap value
(M is a per-unit value determined by taking the sum
of the values between the L and the R leads. The
sign is positive when L is above R and acts to lower
the Z setting. The sign is negative when R is above L
and acts to raise the Z setting).
The following procedure should be followed to obtain
an optimum setting of the relay:
1.
Select the lowest tap S which give a product
of 18.6S
A
greater than desired Z
A
and a
product of 6S
C
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 relay is to be applied to the fol-
lowing machine:
3-phase, 60 hertz, 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 9 is used:
Z
A
per unit =
1.68
Z
C
per unit =
2R - Z
A
= 2(0.94) -1.68 = 0.20
(The relay circle in Figure 9 was obtained by trial an
error using a compass to get the desired radius and
offset.)
kVA
(
)
C
0.6
(
)
2
0.4
–
(
)
2
+
=
θ
Tan
1
0.4
0.6
--------
–
–
33.6
°
–
=
=
Z
Vt
2
kVA
(
)
c
------------------
1.0
2
0.721
---------------
=
=
Z
1.39
33.6
°
–
∠
=
Z
base
1000
kV
( )
2
R
C
kVA
(
)
R
V
----------------------------------------
=
Z
A
or
Z
C
(
)
TS
1
M
±
--------------
=
M
TS
Z
------
1.
–
=
