41-748P
KLF Loss-of-Field Relay
12
1)
= 16.48 ohms
2) Z
A
= Z
A
(per unit) Z
base
= (1.68)(16.48)
= 27.7 ohms
3) Z
C
= Z
C
(per unit) (Z
base
) = (0.20) (16.48)
= 3.29 ohms
To set Z
A
= 27.7
Step 1: The lowest tap S
A
for 18.6 S
A
greater than
Z
A
= 27.7 is 2. Set S
A
in tap 2.
Step 2: T
A
nearest to
= 13.8 is T
A
= 15.8
Set T
A
in 15.8 tap
Step 3:
Set M = +.15. Place R lead in 0, L lead in upper.06
(giving 0.15 between tap leads). The relay setting is
now:
Actual
This is 99.3% of the desired setting.
To set Z
C
= 3.29 ohms.
Step 1. The lowest tap S
C
for 6
SC
greater than 3.29
is S
C
= 1.
Set S
C
= 1
Step 2. T
C
nearest to
is 3.64
Set T
C
in 3.64 tap.
Step 3.
Hence, the nearest M
C
value is -0.12. Now set R
lead in 0.03 tap and L lead in the 0.06 tap (giving
0.03 between tap leads).
(Since M
C
has plus sign lead R must be over L.)
Then, ohms,
or
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. This voltage depends on many
factors, but is usually between 70 and 90 percent
of normal system voltage. The undervoltage unit
is set at the factory for 77% of normal system
voltage, or 92 V
L-L
(equivalent to 80 volts on the
undervoltage unit). In cases where each genera-
tor is equipped with its own transformer (unit con-
nected system) the standard factory setting is
usually satisfactory for the undervoltage unit.
B. In applications where multiple units are con-
nected to the same bus, loss of field of one unit
may not depress the bus voltage to the point
where the undervoltage unit will operate if it has
the standard setting. The following recommenda-
tions should be considered:
1.
For cross-compound turbine generator appli-
cations, the dropout voltage (i.e., the voltage
at which the back contact of the undervolt-
age unit closes) of the undervoltage unit
should be set for 87% of normal voltage
(equivalent to 90 volts on the undervoltage
unit.)
2.
For waterwheel generator applications, with
multiple machines tied to a common bus, the
dropout voltage of the undervoltage unit
should be set at 87%.
3.
For all applications where the alarm func-
tions is not to be used the undervoltage unit
contact should be jumped (shorted).
4.
For industrial applications, with two or more
generators on the same bus, the undervolt-
age unit contact should be jumped (shorted)
and the alarm circuit not used.
5.
For small synchronous condenser and large
motor applications, the undervoltage unit
contact should, in general, be jumped
(shorted), and the alarm circuit not used. In
special cases the machine may be treated
as in 2, above, where knowledge exists of
expected undervoltage level.
6.
For gas turbine units, with high generator
impedance, the undervoltage unit may not
operate on loss-of-field. For these applica-
tions the undervoltage contacts should be
short circuited.
Z
base
1000
kV
( )
2
R
C
kVA
(
)
R
V
-------------------------------------
1000
18
( )
2
1400
×
×
183 500
150
×
,
------------------------------------------------------
=
=
27.7
2
-----------
T
A
S
A
Z
--------------
1
–
15.8
x
2
27.7
------------------
1
–
1.145
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
=
T
C
S
C
Z
C
-------------
1
–
3.64
1
×
3.29
---------------------
1
–
1.07
1
–
0
107
–
=
=
=
Z
C
T
C
S
C
1
M
C
+
(
)
----------------------
3.64
1
0.12
+
---------------------
3.25
=
=
=