GE MULTILIN 269 MOTOR MANAGEMENT RELAY Series, Инструкция по эксплуатации

Страница: 88 / 142

3 SETUP AND USE
3-49
On the occurrence of a ground fault caused by insula-
tion breakdown, an unprotected motor will commonly
suffer severe structural damage and have to be re-
placed. The fault could also shut down the power sup-
ply bus to which the faulty motor is connected.
Ground faults can occur in otherwise good motors be-
cause of environmental conditions. Moisture or con-
ductive dust, which are often present in mines, can
provide an electrical path to ground thus allowing
ground fault current to flow. In this case, ground fault
protection should shut down the motor immediately so
that it can be dried or cleaned before being restarted.
For ground fault protection by the 269 relay, all three of
the motor conductors must pass through a separate
ground fault CT (see section 2.6). The CT may be ei-
ther GE Multilin’s 50:0.025A (2000:1 ratio) or 50:5 up to
1500:5 and is chosen in SETPOINTS mode, page 1.
Separate ground fault trip and alarm levels, and per-
sistence times (time delays) may also be set. The
ground fault trip can be instantaneous, or up to 20.0
seconds of time delay can be chosen to allow the 269
relay to be coordinated with other protective devices
and switchgear.
The amount of current that will flow due to a fault de-
pends on where the fault occurs in the motor winding.
A high current flow will result if a short to ground oc-
curs near the end of the stator winding nearest the ter-
minal voltage. A low ground fault current will flow if a
fault occurs at the neutral end of the winding since this
end should be a virtual ground. Thus a low level of
ground fault pickup is desirable to protect as much of
the stator winding as possible and to prevent the motor
casing from becoming a shock hazard. In resistance
grounded systems the ground fault trip level must be
set below the maximum current limited by the ground
resistor or else the relay will not see a large enough
ground fault current to cause a trip.
The ground fault trip level should be set as low as pos-
sible, although too sensitive a setting may cause nui-
sance trips due to capacitive current flow. If nuisance
trips occur with no apparent cause the trip level should
be increased; conversely if no nuisance trips occur a
lower fault setpoint may be desirable.
CAUTION: Care must be taken when turning on this
feature. If the interrupting device (circuit breaker or
contactor) is not rated to break ground fault current
(low resistance or solidly grounded systems), the trip
setpoint should be set to “OFF”. The feature may be
assigned to the AUX1 relay and connected such that it
trips an upstream device that is capable of breaking the
fault current.
3.12 Undercurrent Setpoints
These setpoints are found in SETPOINTS mode, page
1 and are normally used to detect a decrease in motor
current flow caused by a loss of, or decrease in, motor
load. This is especially useful for indication of loss of
suction for pumps, loss of airflow for fans, or a broken
belt for conveyors. When the current falls below the
setpoint value for the setpoint time, the relay assigned
to the undercurrent trip or alarm function will become
active.
If this feature is used for loss of load detection, the
"UNDERCURRENT ALARM LEVEL" or
“UNDERCURRENT TRIP LEVEL” setpoints should be
chosen to be just above the motor current level for the
anticipated reduced load condition. If the feature is not
desired, the alarm and trip levels should be set to
"OFF". The delay time setpoint, will then be ignored by
the relay.
If the motor is normally operated at a current level be-
low its rated full load current, this feature may be used
for a pre-overload warning. This is accomplished by
setting the "UNDERCURRENT ALARM LEVEL" to be
above the normal operating current of the motor but
below the rated full load current. In this way the under-
current function will cause the relay assigned to it to
become inactive if the motor current increases above
the Undercurrent setpoint level. This would indicate an
abnormal loading condition prior to an actual motor
overload.
The output relay assigned to the undercurrent function
will automatically reset itself when the motor stops (i.e.
when the phase current becomes zero) unless this relay
is programmed as latched (see "RELAY ALARM
LATCHCODE", SETPOINTS, page 5). The undercur-
rent trip function is always latched and a reset is re-
quired to clear the trip.
3.13 Rapid Trip / Mechanical Jam Set-
points
These setpoints are found in SETPOINTS mode, page
1 and are used to protect the driven mechanical system
from jams. If used, this feature is active only after the
motor has successfully started, and will cause relay
activation in the event of a stall while the motor is run-
ning.
A current surge of 150% to 600% of motor full load
from 0.5 to 125.0 seconds during motor operation, de-
pending on the setpoints chosen, will cause the relay
assigned to the Rapid Trip or Mechanical Jam alarm
functions to become active. To disable the Rapid Trip or
Mechanical Jam alarm functions, the "RAPID
TRIP/MECH. JAM TRIP LEVEL" or “MECHANICAL
JAM ALARM LEVEL” setpoints should be set to "OFF".
The "RAPID TRIP TIME DELAY" and “MECHANICAL