Improved Topology and Control Strategies for DC Converters in Wind Power Full DC Systems

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 The paper ostensibly discusses a DC-DC converter for DC wind power plants, yet several issues cast doubt on its clarity and novelty:

 1.      Unclear Topic:

 The paper's title suggests a focus on a DC converter, but Figure 3 solely pertains to the DC converter, while the remainder details a DC-AC converter.

 2.      Lack of Novelty: 

Sections 3 and 4 present an analysis and control strategy for an AC-DC converter without offering any novel contributions. The DC-AC MMC topology, mathematical analysis, and circulating current suppression strategy were previously described in [1], [2], and [3], respectively.

[1] A. Lesnicar and R. Marquardt, "A new modular voltage source inverter topology," presented at EPE, Toulouse, France, 2003.

[2] A. Antonopoulos, L. Angquist, and H.-P. Nee, "On dynamics and voltage control of the Modular Multilevel Converter," in 13th European Conference on Power Electronics and Applications, EPE, 2009, pp. 1-10.

[3] Q. Tu, Z. Xu, and L. Xu, "Reduced Switching-Frequency Modulation and Circulating Current Suppression for Modular Multilevel Converters," IEEE Transactions on Power Delivery, vol. 26, no. 3, pp. 2009-2017, 2011.

 3.      Results and Simulation Issues:

 The results lack novelty and could be enhanced.

No information about the converter and simulation software is provided.

Figure 3 indicates initial oscillations, possibly due to simulation initialization rather than real circulating current.

 4.      Fig. 6 and 8 imply identical transfer functions for the circulating current model and MMC output model, which is incorrect.

5.      Fig. 8 suggests control of the second harmonic in the synchronous reference frame (SFR) but uses a PIR controller. Why do you use a resonant controller to control dc components?.

 6.      Missing Descriptions:

 The paper lacks details about the overall DC-DC converter control, DC voltage control, DC power, etc.

 7.      Lack of Comparative Analysis:

 The paper lacks a discussion on the advantages of a DC wind power plant compared to an AC power plant.

 

Comments on the Quality of English Language

While understandable,  English writing can be improved.

Author Response

Dear Reviewer,

Thank you for your insightful and constructive comments regarding our manuscript. We appreciate the opportunity to clarify and enhance our work based on your feedback. Here is our point-by-point response:

1. Unclear Topic: While Figure 4 in the manuscript illustrates an AC-DC converter, it's true that the topology of the DC-AC converter is quite similar, and thus was not included, which might have caused some misunderstanding. To address this, we will add the relevant section on the DC-AC converter topology to clarify this aspect.

2. Lack of Novelty: The main innovation of our paper lies in Figure 3, which presents a novel DC converter topology. The subsequent sections focus on the control and simulation of this converter. We believe that this contribution significantly enhances the novelty of our work.

3. Results and Simulation Issues: The simulations in our study were conducted using MATLAB's Simulink. We acknowledge the need for further elaboration on this aspect and will enhance the details and clarity regarding the simulation process and results.

4. Figures 6 and 8: Although the models in Figures 6 and 8 may appear similar, their transfer functions are distinct. We encourage a closer examination of the models, which will reveal the differences between them.

5. Use of PIR Controller: The choice of a PIR controller in our system effectively controls the DC components and suppresses the double-frequency circulating current, meeting our system's unique requirements. This approach is an integral part of our proposed solution.

6. Further Refinements: We are committed to further refining our manuscript to address any gaps and enhance its overall quality and clarity.

7. Comparative Analysis of DC and AC Wind Power Plants: We acknowledge the absence of a comparative analysis between DC and AC wind power plants in our study. As a future direction, we plan to incorporate such an analysis to strengthen the findings and implications of our research.

We are grateful for your valuable feedback and are dedicated to improving our manuscript for potential publication. Thank you for your guidance and support.

Sincerely,

Reviewer 2 Report

Comments and Suggestions for Authors

Dear authors, please accept my comments as suggestions only:

 

1. The objective of this study is well given (lines 45-46), although the literature analysis requires significant improvements to support the necessity of the suggested improvements. Also, volume and cost reduction approaches require better analysis and presentation. The presented further research is not that focused in this direction.

 

2. You may want to compare the isolated multilevel DC-DC topologies better, giving an argument about the choice of the DAB (fig. 2) converter.

 

3. The main focus in part 4 is the control system. However, some improvements could be suggested. For example, why was the Proportional-Integral-Resonant strategy chosen, and what advantages does this approach offer? The control system could be better introduced in the literature review, showing comparison and analysis.

 

4. Could you suggest the inverter power loss and efficiency? Developing the design procedure of the converter (as stated in line 292) must include all elements' definitions, thermal analysis, power loss, etc., which is currently not done. A case study could be developed to show the converter design at specific parameters.

 

5. Considering the high-voltage application and IGBT connected in series, do you take into account the necessary circuits for voltage equalisation and what their impact on the overall operation could be?

 

6. The simulation results are shown in part 6. In this part, you may want to cite the equations (1-20) used to execute the module, which are given in the previous part. In the current format, both parts look separated, not showing the consistency in your research.  

 

7. The paper will be significantly improved by adding an experimental verification. It could be developed with a physical model with reduced power, verifying the control strategy and stable operation. Presenting experimental data could enhance the research, taking it to the level of an impact factor journal.

 

Thank you for the interesting paper. I hope my comments will be helpful.

 

Author Response

Dear Reviewer,

Thank you for your insightful comments and constructive suggestions regarding our manuscript titled “[Your Manuscript Title]”. We greatly appreciate the time and effort you have invested in reviewing our work. Here are our responses to your points:

1. General Acknowledgment: We sincerely thank you for your feedback and assure you that we will thoroughly consider your suggestions to enhance our manuscript.

2. Detailed Comparison of Isolated Multilevel DC-DC Topologies: Following your advice, we have incorporated a more detailed comparison of isolated multilevel DC-DC topologies in our manuscript. This includes a comprehensive justification for our choice of the Dual Active Bridge (DAB) converter, highlighting its specific advantages and relevance to our study.

3. Control System and PIR Strategy: The choice of the Proportional-Integral-Resonant (PIR) strategy for our control system is based on its distinct advantages, which we have now elaborated in the revised manuscript. We believe that this detailed explanation will provide a clearer understanding of our approach.↳

4. Inverter Power Loss and Efficiency: Thank you for your suggestion. I will later consider developing a case study to demonstrate the converter design for specific parameters.

5. High-Voltage Application and IGBT Series Connection: We appreciate your input on this matter. Currently, we have not fully considered the aspect of voltage equalization for high-voltage applications with IGBTs connected in series. We will explore this area further.

6. Consistency in Simulation Results: To enhance the coherence between the theoretical framework and simulation results, we have revised Part 6 of our manuscript. We believe this modification will improve the overall consistency of our research presentation.

7. Development of a Physical Model: We are in the process of considering the development of a low-power physical model to verify the control strategy and stable operation. Presenting experimental data in the future will significantly enrich our research and its quality.

We are committed to improving our manuscript and look forward to your continued guidance. Once again, thank you for your valuable input.

Sincerely,

Reviewer 3 Report

Comments and Suggestions for Authors

1. In the first paragraph of the Introduction, the authors has stated that DC convertors are crucial components in DC systems, while DC/AC convertors are more important than only DC ones because their applications in converting DC power of PV to AC in the connection points of PVs to the main network. Also, such convertors are employed to convert the AC power of wind units to DC power for satisfying DC loads or charging DC batteries used in the network according to  https://doi.org/10.1016/j.apenergy.2023.121253 and https://doi.org/10.1109/TIA.2023.3330457. Therefore, the authors should explain this important issue in the Introduction by citing above-mentioned references.

 

2. The authors should review some recently published papers in the field of DC convertors application in the Introduction.

 

3. The main contributions and novelty of current research work should be mentioned at the end of Introduction section.

 

4.  Quality of Figure 17 and 18 should be improved for showing voltage and power variations more clearly.

 

 

5. The conclusion should be expanded to include the findings of the paper.

Author Response

Dear Reviewer,

Thank you for your valuable feedback on our manuscript. We appreciate your thorough review and constructive suggestions. Below, we address each of your comments:

1. Introduction - DC/AC Converters' Role:

   • We agree with your observation regarding the importance of DC/AC converters. As suggested, we have revised the introduction to more explicitly explain their crucial role, especially in the context of PV systems and wind power units, with the cited references (https://doi.org/10.1016/j.apenergy.2023.121253 and https://doi.org/10.1109/TIA.2023.3330457) now incorporated for better substantiation.

2. Review of Recent Papers in DC Converters Application:

   • Following your recommendation, we have included a review of recent papers in the field of DC converters in the introduction. This addition provides a comprehensive background and situates our work within the current state of research.

3. Highlighting Main Contributions and Novelty:

   • We acknowledge the importance of clearly stating our research's contributions and novelty. We have modified the end of the introduction section to emphasize the main contributions and the novelty of our research work, highlighting how it advances the field.

4. Improving Quality of Figures 17 and 18:

   • We have enhanced the quality of Figures 17 and 18 to more clearly show voltage and power variations. These improvements aim to provide a clearer visual representation and aid in better understanding the discussed concepts.

5. Expansion of Conclusion:

   • As suggested, we have expanded the conclusion section to more comprehensively include the findings of the paper. This revised conclusion now provides a detailed summary of our key findings and their implications.

We trust that these revisions address your concerns effectively. We are committed to improving our manuscript and are grateful for your guidance in this process.

Thank you again for your insightful comments.

Sincerely,

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

My questions were competently answered, and the paper was improved. I would recommend the article to be published in the journal.

Reviewer 3 Report

Comments and Suggestions for Authors

My concerns have been carefully addressed by the authors.