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.