KLF Loss-of-Field Relay
41-748P
13
C. The desired undervoltage unit setting is com-
puted by:
Setting = V
AT
= 1.5 V
AN
where V
AN
is phase-to-neutral voltage.
Note:
An electrical check of this particular set-
ting is outlined in this instruction leaflet,
under the heading “Acceptance Check”.
5.5 TIME DELAY CONSIDERATIONS
It may be conservatively stated that the rotor struc-
ture and stator heating, as a result of a shorted field
can be tolerated for 10 seconds on a conductor-
cooled machine and 25 seconds for a conventional
machine. This time may be as low as 5 seconds for
an open field (as opposed to a field closed through a
field discharge resistor or an exciter armature) and
as high as one minute where the concern is protec-
tion of an adjacent tandem compound unit against
partial loss-of-excitation in the faulted machine.
In view of the above considerations, it is often desir-
able to use an external timer in conjunction with the
KLF Relay. The following examples are applications
where an external timer would be desirable.
1.
Cross-compound units, with undervoltage
unit setting of 90 volts, should use an exter-
nal timer to assure tripping before thermal
damage can result. The timer is energized at
the alarm output and should be set for 10
seconds for a cross-compound conductor
cooled machine. For a conventionally cooled
cross-compound machine, the external timer
should be set for 25 seconds.
As an alternative to this, the KLF with
shorted undervoltage contacts may be
applied and the alarm feature not used. With
this arrangement, tripping takes place after
the 200 ms time delay provided by the X unit
in the KLF relay.
2.
Machines connected to a common high volt-
age bus may be protected against loss of
voltage due to loss-of excitation in a adjacent
machine by using a one minute timer driven
by the alarm output of the loss-of-field relay.
3.
In some critical applications 2-zone loss-of-
field protection may be desirable. In this
case, the Zone-1 KLF impedance circle
should be small and fully offset in the nega-
tive reactance region. The long-reach should
be set above synchronous reactance, Xd.
The short-reach should be set equal to one-
half transient reactance, X
d
/2. The trip circuit
should be energized directly, with no time
delay. The alarm circuit should operate a
timer which may be set from 0.25 to 1.0 sec-
onds, depending on user preference. If the
condition persists, this timer permits tripping.
The second-zone KLF may be set with a
larger impedance characteristic and will
detect partial loss-of-field conditions. A typi-
cal setting would be to just allow the machine
to operate at maximum hydrogen pressure
and 0.95 per unit voltage. If a low voltage
condition occurs, it is recommended that trip-
ping be accomplished through a timer set for
0.8 seconds. Added to the X unit dropout
time of 0.2 seconds, this gives an overall
time of 1.0 second. If the voltage is main-
tained, then the alarm circuit should start a
“last-ditch” timer. This timer may be set any-
where from 10 seconds to one minute
depending on machine type and user prefer-
ence.
5.6 PERFORMANCE DURING REDUCED
FREQUENCY
During major system break-ups, it is possible that the
generators may be called upon to operate at reduced
frequency for long periods of time. During this condi-
tion the loss-of-field relay should be secure and not
over-trip for load conditions. The KLF relay has a
favorable characteristic during this condition, since
this tripping characteristic becomes more secure dur-
ing reduced frequencies, as shown Figure 7, page
15.
6.0
SETTING THE RELAY
The type KLF relay requires a setting for each of the
two compensators T
A
and T
C
, for each of the two
autotransformers, primaries S
A
and S
C
, and for the
undervoltage unit.
6.1 COMPENSATOR (T
A
AND T
C
)
Each set of compensator taps terminates in inserts
which are grouped on a socket and form approxi-
mately three quarters of a circle around a center
insert which is the common connection for all the
taps. Electrical connections between common insert
and tap inserts are made with a link that is held in
place with two connector screws, one in the common
and one in the tap.
