Analysis of ANPC Turn-Off Overvoltage Considering Stray Inductance

Active Neutral-Point Clamped (ANPC) converters have been extensively applied to wind power, solar power, grid interconnection of new energy sources, and other fields. ANPC converters with Integrated Gate Commutated Thyristors (IGCTs) characterized by high current-carrying and voltage withstanding capabilities will face a significant current change rate in the circuit during the commutation process. This high di/dt induces substantial overvoltage across the stray inductance in the commutation loop, posing challenges to the safe operation and cost control of the equipment. Hence, there is an urgent need to investigate the overvoltage induced by stray inductance during commutation. This paper first demonstrates the principles for calculating stray inductance in ANPC converters. Next, using the finite element simulation software ANSYS, the paper extracts the partial inductance matrix of the commutation loop and the associated stray inductance for a directly series-connected ANPC three-level converter. Based on this stray inductance analysis, a simplified circuit model of the ANPC commutation circuit taking into account stray inductance is developed. Later on, this paper analyzes the transient behavior during the ANPC circuit’s turn-off process on the basis of the simplified commutation circuit, and calculates the overvoltage spikes caused by stray inductance. Finally, a double-pulse simulation circuit is constructed on the PSCAD platform to validate the analytical and calculation process, as well as the accuracy of the simplified circuit model through simulation.