Multilin 469 Motor Management Relay ... - GE Digital Energy

CHAPTER 5: SETTINGS
If the undervoltage alarm feature is enabled, an alarm will occur once the magnitude of
either Vab, Vbc, or Vca falls below the pickup level while running or starting for a period of
time specified by the delay (note that pickup levels are multiples of motor nameplate
voltage). The running pickup level also applies when the motor is stopped and the U/V
ACTIVE ONLY IF BUS ENERGIZED settings is programmed to “No”.
Undervoltage trips can be set for single-phase or three-phase conditions. If undervoltage
tripping is enabled, and the UNDERVOLTAGE TRIP MODE is set for “3-Phase”, a trip will occur
only when the magnitude of all three phases falls below the pickup level while running or
starting for a period of time specified by the time delay. On the other hand, if undervoltage
trip is enabled, and the UNDERVOLTAGE TRIP MODE is set for “1-Phase”, a trip will occur
once the magnitude of either Vab, Vbc, or Vca falls below the pickup level while running or
starting for a period of time specified by the time delay. Note that pickup levels are
multiples of motor nameplate voltage. The running pickup level also applies when the
motor is stopped, and the U/V ACTIVE ONLY IF BUS ENERGIZED settings is programmed to
“No”.
An undervoltage on a running motor with a constant load results in increased current. The
relay thermal model typically picks up this condition and provides adequate protection.
However, this settings may be used in conjunction with the time delay to provide additional
protection that may be programmed for advance warning by tripping.
Attempting to start a large motor when the supply voltage is down may also be
undesirable. An undervoltage of significant proportion that persists while starting a motor
may prevent the motor from reaching rated speed. This may be especially critical for a
synchronous motor. As such, this feature may be used in with a time delay to provide
protection for undervoltage conditions before and during starting.
In the event of system problems causing asymmetrical voltage conditions where at least
one voltage remains above pickup, an Alarm condition will occur, indicating that the
voltage on at least one phase is below acceptable levels. The trip relay will not be
energized unless the UNDERVOLTAGE TRIP MODE is set to “1-Phase”. The factory default
setting for UNDERVOLTAGE TRIP MODE is “1-Phase”.
To prevent for nuisance undervoltage trips due to VT Fuse Failure, set the UNDERVOLTAGE
TRIP MODE to “3-Phase”. The alarm relay will be energized in the event of a single-phase
undervoltage, which can also be an indication of a potential VT fuse failure. Typically a fuse
failure is detected when there are significant levels of negative-sequence voltage,
indicating voltage unbalance due to the loss of one phase, without correspondingly
significant levels of negative-sequence current, indicating current unbalance, measured at
the output CTs.
If the conditions for Fuse Failure exist, an alarm will occur after a time delay due to an
undervoltage condition in at least one phase. If the motor is running, the voltage in the
faulted phase will be zero, and the measured load current should not indicate a significant
amount of negative or unbalance currents. Therefore the motor can be kept in service until
the opportunity to replace the faulty fuse is available.
If the alarm is caused by an abnormal system conditions, a significant amount of
unbalance current will be present. If the condition is not detected on time, the unbalance
function or the underpower element will trip the motor.
469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL 5–77