ABB Relion 611 Series, Technical Manual

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depends on the value of the negative-sequence current. If the sum reaches zero
without a fault being detected, the accumulation stops and the timer is reset.
• If the reset time set through the Cooling time setting elapses without a fault being
detected, the timer is reset.
The reset period thus continues for a time equal to the Cooling time setting or until the
operate time decreases to zero, whichever is less.
4.4.4.6 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.
4.4.4.7 Signals
Table 257: MNSPTOC Input signals
Name Type Default Description
I
2
SIGNAL 0 Negative sequence current
BLOCK BOOLEAN 0=False Block signal for activating the blocking mode
1MRS757454 D
Section 4
Protection functions
611 series 249
Technical Manual