Abstract
Voltage sag is one of the most common power quality disturbances in industry, which causes huge inrush currents in stator windings of induction motors, and adversely impacts the motor secure operation. This paper firstly introduces a 2D Time-Stepping multi-slice finite element method (2D T-S multi-slice FEM) which is used for calculating the magnetic field distribution in induction motors under different sag events. Then the paper deduces the transient analytical expression of stator inrush current based on the classical theory of AC motors and presents a separation method for the positive, negative and zero sequence values based on instantaneous currents. With this method, the paper studies the influences of voltage sag amplitude, phase-angle jump and initial phase angle on the stator positive- and negative-sequence peak currents of 5.5 kW and 55 kW induction motors. This paper further proposes a motor protection method under voltage sag condition with the stator negative-sequence peak currents as the protection threshold, so that the protection false trip can be avoided effectively. Finally, the calculation and analysis results are validated by the comparison of calculated and measured stator peak value of the 5.5 kW induction motor.
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