Modified natural frame control of single‐phase cascaded H‐bridge multilevel converter under distorted grid voltage

Abstract

Cascaded H-bridge multilevel converter is widely applied in flexible AC transmission and grid connection of renewable energy, but its current quality decreases as the grid voltage distortion continues to deteriorate. A modified natural frame control method to enhance the harmonic elimination ability of converters is proposed in this paper. A single-phase cascaded delayed signal cancellation operator is applied to deal with the distorted grid voltage signals. Benefit from a system structure without any feedback loops or integral items, the transient response of single-phase cascaded delayed signal cancellation operator is faster than commonly-used second-order generalised integrator. The settling time of single-phase cascaded delayed signal cancellation operator facing grid voltage phase jump is less than one grid period. After immunising the synchronisation reference against harmonic distortion by using the single-phase cascaded delayed signal cancellation operator, the modified natural frame control method can then generate high-quality sinusoidal current. Phase-locked loop and coordinate transformation are unnecessary for the proposed method, which decreases the system complexity. Various simulations and experimental tests based on a down-scaled platform demonstrate the performance of the proposed modified natural frame control method under distorted grid voltage.