GE 239 Instruction Manual page 74

4–38
Courtesy of NationalSwitchgear.com
and is common in industrial environments. Causes can include increased resistance in
one phase due to a pitted or faulty contactor, loose connections, unequal tap settings
in a transformer or non-uniformly distributed three phase loads. The incoming supply
to a plant may be balanced but varying single phase loads within the plant can cause
a voltage unbalance at the motor terminals. The most serious case of unbalance is
single phasing which is the complete loss of one phase of the incoming supply. This
can be caused by a utility supply problem or by a blown fuse in one phase and can
seriously damage a three phase motor.
Under normal balanced conditions the stator current in each of the three motor
phases is equal and the rotor current is just sufficient to provide the turning torque.
When the stator currents are unbalanced, a much higher current is induced in the
rotor because it has a lower impedance to the negative sequence current component
present under unbalanced conditions. This current is normally at about twice the
power supply frequency and produces a torque in the opposite direction to the
desired motor output. Usually the increase in stator current is small (125 to 200%) so
that timed overcurrent protection takes a long time to trip. However the much higher
induced rotor current can cause extensive rotor damage in a short period of time.
Motors can tolerate different levels of current unbalance depending on the rotor
design and heat dissipation characteristics.
• UNBALANCE TRIP PICKUP: Unbalance protection is recommended at all times. Motor
data is rarely provided and direct measurement of rotor temperature is impractical so
setting the unbalance level is empirical. For a known balanced situation, a pickup level
of 10% and time delay of 5 seconds is recommended as a starting point. The pickup
level can be decreased until nuisance tripping occurs. Similarly the time delay may be
increased if necessary.
To prevent nuisance trips/alarms on lightly loaded motors when a much larger
unbalance level will not damage the rotor, the single phase detection will
automatically be defeated if the average motor current is less than 30% of the full
load current (I ) setting. Unbalance is calculated as:
FLC
I ≥ I :
av FLC
I < I :
av FLC
where: I = average phase current
av
I = current in a phase with maximum deviation from I
m
I = motor full load current setting
FLC
• UNBALANCE ALARM: The operation of this feature is identical to the operation of the
unbalance trip feature.
• UNBALANCE ALARM PICKUP: The operation of this feature is identical to the operation
of the unbalance trip pickup feature.
• UNBALANCE DELAY: If phase current unbalance increases above
PICKUP or UNBALANCE TRIP PICKUP
delay programmed in this setpoint, the respective relay will activate and the
respective warning message will be displayed.
I I
–
m av
-------------------- -
× 100%
I
av
I I
–
m av
-------------------- - × 100%
I
FLC
setpoint value and remains this way for the time
239 MOTOR PROTECTION RELAY – INSTRUCTION MANUAL
CHAPTER 4: PROGRAMMING
av
UNBALANCE ALARM