Linear Active Disturbance Rejection Control and Stability Analysis for Modular Multilevel Converters Under Weak Grid

The modular multilevel converters(MMCs) are popularly used in high-voltage direct current(HVDC) transmission systems. However, for the direct modulation based MMC, its complex internal dynamics and the interaction with the grid impedance would induce the frequency coupling effect, which may lead to instability issues, especially in the case of weak grid. To effectively suppress the sub-and super-synchronous oscillations, this paper proposes a linear active disturbance rejection control(LADRC) based MMC control strategy. The LADRC mainly consists of the linear extended state observer(LESO) and the linear state error feedback(LSEF). And it is a potential method to enhance the system stability margin, attributing to its high anti-interference capability and good tracking performance. Thereupon, the system small-signal impedance model considering frequency coupling is established. And the effect of the introduction of the LADRC on the system stability is further investigated using the Nyquist criterion. Particularly, the influences of key control parameters on the stability are discussed in detail. Meanwhile, the impact of LADRC on the transient performance is explored through closed-loop zero poles. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are verified via electromagnetic simulations.

Expression patterns and functional divergence of homologous genes accompanied by polyploidization in cotton(Gossypium hirsutum L.)

Naturally allotetraploid cotton has been widely used as an ideal model to investigate gene expression remodeling as a consequence of polyploidization.However,the global gene pattern variation during early fiber development was unknown.In this study,through RNA-seq technology,we comprehensively investigated the expression patterns of homologous genes between allotetraploid cotton(G.hirsutum)and its diploid progenitors(G.arboreum and G.raimondii)at the fiber early development stage.In tetraploid cotton,genes showed expression level dominance(ELD)bias toward the A genome.This phenomenon was explained by the up-/downregulation of the homologs from the nondominant progenitor(D genome).Gene ontology(GO)enrichment results indicated that the ELD-A genes might be a prominent cause responsible for fiber property change through regulating the fatty acid biosynthesis/metabolism and microtubule procession,and the ELD-D genes might be involved in transcription regulation and stress inducement.In addition,the number and proportion of completely A-and D-subfunctionalized gene were similar at different fiber development stages.However,for neofunctionalization,the number and proportion of reactivated D-derived genes were greater than those of A at 3 and 5 DPA.Eventually,we found that some homologous genes belonging to several specific pathways might create novel asymmetric transcripts between two subgenomes during polyploidization and domestication process,further making the fiber property meet the human demands.Our study identified determinate pathways and their involved genes between allotetraploid cotton and their progenitors at early fiber development stages,providing new insights into the mechanism of cotton fiber evolution.

Impedance Modeling and Stability Factor Assessment of Grid-connected Converters Based on Linear Active Disturbance Rejection Control

—With the increase of converter-based renewable energy generation connected into the power grid, the interaction between renewable energy and grid impedance has introduced lots of new issues, among which the sub-and super-synchronous oscillation phenomenon makes a big concern. The linear active disturbance rejection control(LADRC) is a potential way to improve the damping characteristics of the grid-connected system, but the key factors and influencing mechanism on system stability are unknown. This paper establishes the equivalent impedance and coupling admittance models of a typical three-phase grid-connected converter. Then, the influence of the key factors such as the bandwidth of the LADRC and grid impedance on the stability and frequency coupling effect is assessed in detail. Finally, the theoretical analysis results are verified by simulations and experiments.