I.L. 41-748.1F
11
dropout voltage of the undervoltage unit
should be set for 58 V
L-N
.
3. For all applications where the alarm function
is not to be used, the undervoltage unit con-
tact should be short circuited by means of a
jumper wire.
4. For industrial applications, with two or more
generators on the same bus, the undervolt-
age unit contact should be short circuited
and the alarm circuit not used.
5. For synchronous condenser and large motor
applications, the undervoltage unit contact
should, in general, be short circuited and the
alarm circuit not used. In general cases the
machine may be treated as in 2, above.
6. for gas turbine unites, with high generator
impedance, the undervoltage unit may not
operate. For these applications the under-
voltage contacts should be short circuited.
In cases where each generator is equipped with its
own transformer (unit connected) the standard fac-
tory setting of 53 VL-N (corresponding to 92.5 VL-L)
is usually satisfactory for the undervoltage unit. It
should operate at a level commensurate with the
minimum safe voltage for system stability.
NOTE: An electrical check of this particular set-
ting is outlined in this instruction leaflet,
under the heading “Acceptance Check”.
6. 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 conduc-
tor-cooled machine and 25 seconds for a conven-
tional machine. This time may be as low as 5
seconds for an open field (as opposed to a field
closed through a field discharged resistor on an
exciter armature) and as high as one minute where
the concern is protection of an adjacent tandem com-
pound 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-1 Relay. The following examples are applica-
tions where an external timer would be desirable:
1.
Cross-compound units, with undervoltage
unit setting of 58 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 alterna-
tive to this, the KLF-1 with shorted underolt-
age contacts may be applied and the alarm
feature not used. With this arrangement, trip-
ping takes place after the 250 ms time delay
provided by the X unit in the KLF-1 relay.
2. Machines connected to a common high volt-
age bus may be protected against partial
loss of voltage due to loss-of-excitation in an
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-1 impedance circle should be
small and fully offset in the negative reactance
region. The long-reach should be set equal to
synchronous reactance, Xd. The short-reach
should be set equal to one-half transient reac-
tance, Xd1/2. The trip circuit should trip
directly, with no time delay. The alarm circuit
should operate a timer which may be set from
1/4 - 1.0 seconds, depending on use prefer-
ence. If the condition persists, this timer per-
mits tripping.
The second-zone KLF-1 may be set with a
larger impedance characteristic and will detect
partial loss-of-field conditions. A typical setting
would be to just allow the machine to operate
at maximum hydrogen pressure and .95 per
unit voltage. If a low voltage condition occurs, it
is recommended that tripping be accomplished
through a timer set for 3/4 second. Added to
the X unit dropout time of 1/4 second, this
gives an overall time of 1.0 second. If the volt-
age is maintained, then the alarm circuit
should start a “last ditch” timer. This timer may
be set anywhere from 10 seconds to one
minute, depending on machine type and use
preference.
