I
S
Set
Start value
I
r
Set
Rated current
Figure 330: MNSPTOC Inverse Curve B
If the fault disappears, the negative-sequence current drops below the
Start
value setting and the
START
output is deactivated. The function does not reset
instantaneously. Resetting depends on the equation or the
Cooling time setting.
The timer is reset in two ways:
• When the negative sequence current drops below start value the subtraction in
the denominator becomes negative and the cumulative sum starts to decrease.
The decrease in the sum indicates the cooling of the machine and the cooling
speed 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
1MRS757644 H
Protection functions
620 series
Technical Manual
627
