ABB RELION REX640 Technical Manual page 848

Protection functions
The reset period thus continues for a time equal to the
the operate time decreases to zero, whichever is less.
4.4.5.7 Application
In a three-phase motor, the conditions that can lead to unbalance are single
phasing, voltage unbalance from the supply and single-phase fault. The negative
sequence current damages the motor during the unbalanced voltage condition,
and therefore the negative sequence current is monitored to check the unbalance
condition.
When the voltages supplied to an operating motor become unbalanced, the
positive-sequence current remains substantially unchanged, but the negative-
sequence current flows due to the unbalance. For example, if the unbalance is
caused by an open circuit in any phase, a negative-sequence current flows and it is
equal and opposite to the previous load current in a healthy phase. The combination
of positive and negative-sequence currents produces phase currents approximately
1.7 times the previous load in each healthy phase and zero current in the open phase.
The negative-sequence currents flow through the stator windings inducing
negative-sequence voltage in the rotor windings. This can result in a high rotor
current that damages the rotor winding. The frequency of the induced current is
approximately twice the supply frequency. Due to skin effect, the induced current
with a frequency double the supply frequency encounters high rotor resistance
which leads to excessive heating even with phase currents with value less than the
rated current of the motor.
The negative-sequence impedance of induction or a synchronous motor is
approximately equal to the locked rotor impedance, which is approximately one-
sixth of the normal motor impedance, considering that the motor has a locked-rotor
current of six times the rated current. Therefore, even a three percent voltage
unbalance can lead to 18 percent stator negative sequence current in windings. The
severity of this is indicated by a 30-40 percent increase in the motor temperature
due to the extra current.
4.4.5.8 Signals
MNSPTOC Input signals
Table 770: MNSPTOC Input signals
Name
I3P
BLOCK
ST_EMERG_ENA
848
Type Default
SIGNAL -
BOOLEAN 0=False
BOOLEAN 0=False
1MRS759142 F
Cooling time setting or until
Description
Three-phase currents
Block signal for acti-
vating the blocking
mode
Enable emergency
start to disable lock
of start of motor
REX640
Technical Manual