## 6 Conclusions  
This paper proposes an improved control method aimed at enhancing the performance of the UPQC DC-side voltage control and verifies its application in a renewable energy grid-connected system. Through comparative experiments with LADRC, IVA-LADRC, SVA-LADRC, and VAV-LADRC methods, the superiority of the proposed approach in voltage stability and harmonic suppression is validated, with notable advantages in dynamic response and control accuracy. To further evaluate the performance of the VAV-LADRC control method, the photovoltaic power-generation system was integrated into the system, and the stability of the system was simulated under dynamic light intensity variations. Experimental results show that the proposed control strategy can effectively maintain the stability of the DC-side voltage in a dynamically changing light environment, significantly reducing voltage fluctuations, and performing well in harmonic suppression, with all indicators meeting the system's technical requirements. This further validates the stability and robustness of the method in complex grid-connected environments.  
In conclusion, the proposed improved control method not only achieves excellent control performance in UPQC DC-side voltage regulation but also demonstrates strong adaptability and stability in photovoltaic grid-connected systems, showing promising prospects for engineering applications. As the proportion of renewable energy integrated into the grid continues to increase, distributed energy systems and power electronics technologies will play a more significant role in improving power quality. The application of the proposed control strategy in UPQC systems provides valuable insights for future power-quality management in smart grids and microgrids. Future research can further optimize the method to address more complex grid environments and explore its integration and application in large-scale photovoltaic, wind energy, and other renewable energy grid-connected systems.  
Conceptualization, K.S.; Methodology, K.S. and N.L.; Software, N.L.; Validation, K.S. and C.L.; Formal analysis, C.L. and H.L.; Investigation, Z.L. and H.L.; Resources, J.S. and S.J.; Data curation, C.L. and S.J.; Writing--original draft preparation, K.S. and N.L.; Writing--review and editing, B.H. and K.S.; Visualization, K.S.; Supervision, N.L.; Project administration, B.H.; Funding acquisition, B.H. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China under grant number 61601140, and the Postdoctoral Initiation Foundation of Heilongjiang under grant number 68641400.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.  
The authors sincerely thank the National Natural Science Foundation of China and the Postdoctoral Initiation Foundation of Heilongjiang for their financial support. We are also very grateful to the anonymous reviewers for their valuable comments and suggestions for the improvement of this paper.  
The authors declare no conflicts of interest.