Unbalance Setpoints - GE 269Plus Instruction Manual

3 SETUP AND USE

UNBALANCE SETPOINTS

3.11 UNBALANCE SETPOINTS
Unbalanced three-phase supply voltages are a major cause of induction motor thermal damage. Unbalance is
caused by a variety of factors and is common in industrial environments. Causes can include increased resis-
tance in one phase due to pitted or faulty contactors, transformer faults, and unequal tap settings, or non-uni-
formly distributed three phase loads. The incoming supply to a plant may be balanced, but varying single
phase loads in the plant can cause voltage unbalance at the motor terminals. The most serious case of unbal-
ance 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.
Unbalance at the motor terminals means an increase in the applied negative sequence voltage. This results in
a large increase in the negative sequence current drawn by the motor due to the relatively small negative
sequence impedance of the rotor. This current is normally at about twice the power supply frequency and pro-
3
duces a torque in the opposite direction to the desired motor output. For small unbalances the overall output
torque will remain constant, but the motor will be developing a large positive torque to overcome the negative
sequence torque. These opposing torques and the high negative sequence current produce much higher rotor
losses and consequently greatly increased rotor heating effects. Stator heating is increased as well, but to a
much smaller extent. The amount of unbalance that a given motor can tolerate is therefore dependent on the
rotor design and heat dissipation characteristics.
Persistent, minor voltage unbalance can thus lead to rotor thermal damage while severe unbalance such as
single phasing can very quickly lead to a motor burnout.
For phase currents above 100% FLC, the 269Plus calculates the ratio of the negative to positive sequence cur-
rents ( I / I ) and uses this ratio in two seperate protective functions. It is used to bias the 269Plus thermal
n p
memory which represents the thermal capacity of the motor as a whole (enabled in Setpoints page 5). The
method of determining I / I is independent of actual line frequency or phase current lead/lag characteristics,
n p
and when enabled is used to bias the thermal memory. This negative sequence unbalance method provides
readings similar to the NEMA unbalance calculation but gives more realistic results for the thermal effect of
unbalance on the motor (for an unbalance example see Appendix A). For phase currents below 100% FLC, the
relay calculates the ratio of In to full load current ( I / I
) and uses this to provide protection. This avoids nuis-
n FLC
ance trips due to relatively high levels of In with lower levels of Ip that may create high U/B levels at low loads.
For unbalance protection, trip and alarm I / I ratios may be chosen along with appropriate persistence times
n p
(time delays) in Setpoints page 1. If no separate unbalance protection is desired, the trip and alarm levels
should be set to . The delay times will then be disregarded by the relay. Above 100% FLC, if an unbalance
O F F
of more than 30% persists for more than 4 seconds, a will result. Below 100% FLC, if motor
S I N G L E P H A S E T R I P
load is greater than 25%, and any one phase reads zero, this will also be considered a single phase condition.
The single phase time delay can be adjusted by contacting the factory.
If the is set to , single phase protection will be turned off.
U N B A L A N C E T R I P L E V E L O F F
NOTE
It should be noted that a 1% voltage unbalance typically translates into a 6% current unbalance. For example,
if the supply voltage is normally unbalanced up to 2%, the current unbalance seen by a typical motor would be
2 × × × × 6 = 12%. Set the alarm pickup at 15% and the trip at 20% to prevent nuisance tripping; 5 or 10 seconds is
a reasonable delay.
Other factors may produce unbalanced phase currents. Cyclic, pulsating and rapidly changing loads have
been observed to create unbalance in motors driving machines such as ball mill grinders, shredders, crushers,
and centrifugal compressors, where the load characteristics are constantly and rapidly changing.
Under such circumstances, and in order to prevent nuisance unbalance trips or alarms, the pickup level should
not be set too low. Also, a reasonable time delay should be set to avoid nuisance trips or alarms. It is recom-
mended that the unbalance input to thermal memory be used to bias the thermal model, thus accounting for
motor heating that may be caused by cyclic short term unbalances.
GE Power Management 269Plus Motor Management Relay 3-
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Courtesy of NationalSwitchgear.com