Application - ABB 615 series Technical Manual

1MAC059074-MB A
4.4.4.6
4.4.4.7
615 series ANSI
Technical Manual
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.

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.
Signals
Table 502: 46M Input signals
Name Type
I SIGNAL
2
BLOCK BOOLEAN
Default Description
0 Negative sequence current
0=False Block signal for activating the blocking mode
Section 4
Protection functions
607