A Novel Control Strategy for Grid-Connected Inverter Based on Iterative Calculation of Structural Parameters

Round 1

Reviewer 1 Report

1. The paper's literature survey is lacking. Please see the following reference, which is a survey paper and you can put more relevant references from there.

DOI: 10.1109/MIE.2015.2478920}

2. Why are Kd and Kq going to be varying? PWM carriers are fixed in amplitude and therefore, therefore, there values are fixed, typically normalized to 1.

3. Given the fact that the proposed scheme estimates parameters to improve tracking, a thorough comparison should be provided when the inverter is controlled using PI/PR regulators and with the proposed parameter estimation augmented MPC. This needs to be done for both simulation and experimental results to actually show if the proposed method actually improves anything drastically.

Author Response

Point 1: The paper's literature survey is lacking. Please see the following reference, which is a survey paper and you can put more relevant references from there. DOI: 1
0.1109/MIE.2015.2478920}

Response 1: Thank you very much for provide me with the article.

I have added it as a reference and gotten a lot of relevant references from it to enrich my manuscript.

In fact, I have almost rewritten this part from Line 75 to Line 94, and the references form Line 476 to Line 490. Please review it again, thank you.

Point 2: Why are Kd and Kq going to be varying? PWM carriers are fixed in amplitude and therefore, therefore, there values are fixed, typically normalized to 1.

Response 2: Thank you for the question.

Before this research, I also thought the PWM-bridge gain is absolutely fixed if the carriers are fixed in amplitude. But I was surprised to find that the gain is actually varying slightly, what is more, it is different in DQ axis, so I defined Kd and Kq to represent the actual gains. However, even though they are varying, the range is very small, and they can be approximated as a fixed value which was defined as the center value K in the manuscript.

Some original data of simulation experiment is shown, we can see that Kd and Kq are actually varying slightly. Now I am working on explaining this phenomenon, so I did not put it in the manuscript.

Point 3: Given the fact that the proposed scheme estimates parameters to improve tracking, a thorough comparison should be provided when the inverter is controlled using PI/PR regulators and with the proposed parameter estimation augmented MPC. This needs to be done for both simulation and experimental results to actually show if the proposed method actually improves anything drastically.

Response 3: Thank you for your suggestion.

I have added both simulation and experimental results to present the comparison with PI controller to prove the improvement of the dynamic performance. And reorganized these two parts. Please review it again. The simulation comparison is from Line 299 to Line 333. And the experimental comparison is from Line 404 to Line 424.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript presents a novel control strategy for grid-connected inverter based on iterative calculation of structural parameters. The subject may attract interest to the readers.

The Abstract in its current from is an alternative Introduction, it should clearly describe the scope with more focusing on the proposed approach and results of the study.

Introduction should be improved; it is not clear which is the novelty contribution of the paper. I'd suggest to better point out what is the proposal with more references: new methodology proposed by the authors for the first time.

On the experimental results in section 4; the case study is poorly presented with no sufficient data to analyse or replicate the results. In general, experimental results section is conducted to evaluate, analyse, or verify something. The organization of the experimentation section should be: 1) objective or aim, 2) result, 3) conclusion (outcomes and claims.) The section should be reorganized, otherwise this section is make no sense.

+ The objective of the experimentation should be clearly stated at the beginning of the section.

+ There are few explanations on tables connected to sections 2 and 3. The readers can not understand what the authors want to claim. The main role of figures is to guide logic or theory of the paper, and finally prove the claim. Suitable explanations should be added.

Author Response

The manuscript presents a novel control strategy for grid-connected inverter based on iterative calculation of structural parameters. The subject may attract interest to the readers.

Point 1: The Abstract in its current from is an alternative Introduction, it should clearly describe the scope with more focusing on the proposed approach and results of the study.

Response 1: Thank you. I have revised the Abstract to focus more on the proposed strategy and results of the study, from Line 11 to Line 13, and from Line 19 to Line 22.

Point 2: Introduction should be improved; it is not clear which is the novelty contribution of the paper. I'd suggest to better point out what is the proposal with more references: new methodology proposed by the authors for the first time.

Response 2: Thank you for your suggestion!

I have improved the Introduction. I have added a lot of explanation and description in the Introduction, from Line 32 to Line 51, and from Line 56 to Line 71, and form Line 75 to Line 92.

In fact, I have almost rewritten this part to better point out what is the proposal with more references. Please review it again. Thank you.

Point 3: On the experimental results in section 4; the case study is poorly presented with no sufficient data to analyse or replicate the results. In general, experimental results section is conducted to evaluate, analyse, or verify something. The organization of the experimentation section should be: 1) objective or aim, 2) result, 3) conclusion (outcomes and claims.) The section should be reorganized, otherwise this section is make no sense.

+ The objective of the experimentation should be clearly stated at the beginning of the section.

Response 3: Yes, thank you.

I have reorganized the simulation and experimental parts according to your advice. I have separated the simulation part in three sections, each section only described and analyzed one simulation. And at the beginning of each section, I have pointed out the objective of this simulation clearly, from Line 224 to Line 225, and from Line 272 to Line 273, and from Line 300 to Line 301. Then the simulation results and their analysis followed the objectives, all in red in the manuscript. In the end of each section, I have added the conclusions to express clearly what was proved in the simulation, from Line 263 to Line 270, and from Line 289 to Line 298, and from Line 326 to Line 333.

The experiment part was reorganized in the same form as the simulation part. The objectives are from Line 347 to Line 348, and from Line 375 to Line 376, and from Line 405 to Line 407. The conclusions are from Line 366 to Line 368, and from Line 400 to Line 403, and from Line 421 to Line 424. The other lines in red are the results and analysis.

Thank you again. I do feel these two parts are clearer after reorganized.

Point 4: + There are few explanations on tables connected to sections 2 and 3. The readers can not understand what the authors want to claim. The main role of figures is to guide logic or theory of the paper, and finally prove the claim. Suitable explanations should be added..

Response 4: Yes, thank you. I have added some explanations to describe the connection between Section 2 and Section 3, from Line 205 to Line 209, and from Line 214 to Line 216.

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript points out a new control strategy dedicated to grid-connected inverters. The proposed method is based on iterative calculations of their structural parameters.

Even if the results are really interesting, it is very difficult to read the scientific and technical contributions of the manuscript. The context of the study is not clearly explained. The methods and results are not adequately described. Finally, English changes are required.

Please find below my comments.

1) The introduction must be improved. In particular:

  - The context of this study is brief and fails to detail why "the control strategies of grid-connected inverters are currently the hotspot in renewable energy development".

  - Line 31: please explain in detail the objectives of the dq and αβ transformations.

  - The literature review is really interesting, but key information are missing:

    * From Line 31 to Line 43: the coauthors listed many published papers. However, the achievements of each paper are not clearly described. What about the advantages and drawbacks of each classical grid-current control strategy?

    * From Line 44 to Line 54: the authors must explain the main achievements for each paper related to droop or damping control strategy.

  - From Line 68 to Line 77, what about the novelties of the proposed control strategy in comparison with the MPC method? I think that the proposed strategy strongly reduces the computation time. Moreover, the coauthors must better explain the contributions of the manuscript.

2) Regarding the section named "The Proposed Novel Control Strategy", it is really too difficult for readers to understand the main ins and outs of the proposed method.

3) The section named "3. Simulation Results" must be improved. In particular:

  - Figure 4 shows the simulation model. It is very difficult to understand the main objectives. A block diagram will help readers to better understand this section of the manuscript.

  - Figure 4 is not discussed in the text of the section named "3.1. Simulation Model".

  - Table 1 gives the parameters of the simulation. However, the authors do not discuss the values. Why these values?

  - From all simulation results, what about the effectiveness of the proposed control strategy?

4) The section named "4. Experimental Results" is really interesting, because the proposed experimental results prove the relevance of the proposed control strategy. However, this section of the manuscript must be improved. In particular:

  - The coauthors explain that the inverter is the core of the experimental platform. However, it is of utmost importance to explain the key features of this converter.

  - The values of the resistance and inductance of the AC-grid are not the same as Table 1. Could you better explain the differences? Could you discuss the methods to extract these experimental values?

  - Could you explain the method to calculate the THD-parameter?

Author Response

This manuscript points out a new control strategy dedicated to grid-connected inverters. The proposed method is based on iterative calculations of their structural parameters.

Even if the results are really interesting, it is very difficult to read the scientific and technical contributions of the manuscript. The context of the study is not clearly explained. The methods and results are not adequately described. Finally, English changes are required.

Please find below my comments.

1) The introduction must be improved. In particular:

Point 1:  - The context of this study is brief and fails to detail why "the control strategies of grid-connected inverters are currently the hotspot in renewable energy development".

Response 1: I have revised this part in my manuscript in Line 28 and Line 29.

Thank you.

Point 2: - Line 31: please explain in detail the objectives of the dq and αβ transformations.

Response 2: Thank you.

I have added detailed explanation of the objectives of dq and αβ transformations in the manuscript, according to the references, from Line 32 to Line 35.

Point 3:  - The literature review is really interesting, but key information are missing:

Response 3: Thanks for your interests in this paper.

I have added some key information into the review.

In fact, I have almost rewritten the introduction. Please review it again.

Point 4: * From Line 31 to Line 43: the coauthors listed many published papers. However, the achievements of each paper are not clearly described. What about the advantages and drawbacks of each classical grid-current control strategy?

Response 4: Thank you for your suggestion.

I have added some description to make the achievements of each paper clearly and explain the advantages and drawbacks of each classical grid-current control strategy in my manuscript from Line 36 to Line 51.

Point 5: * From Line 44 to Line 54: the authors must explain the main achievements for each paper related to droop or damping control strategy.

Response 5: Thank you.

I have added some description to explain the main achievements for each paper related to droop or damping control strategy in my manuscript, from Line 56 to Line 59, and From Line60 to Line 73.

Point 6: - From Line 68 to Line 77, what about the novelties of the proposed control strategy in comparison with the MPC method? I think that the proposed strategy strongly reduces the computation time. Moreover, the coauthors must better explain the contributions of the manuscript.

Response 6: Yes, you are right.

The proposed strategy strongly reduces the computation time, compared with MPC. I mentioned it in the abstract, at the end of instruction and in the conclusions, but not in this part.

I have added something to explain the contributions of the manuscript, from Line 75 to Line 90, and from Line 92 to Line 94.

Thank you.

Point 7: 2) Regarding the section named "The Proposed Novel Control Strategy", it is really too difficult for readers to understand the main ins and outs of the proposed method.

Response 7: Thank you very much for your patient reading.

It may be difficult to be understood, because it is a novel control strategy first proposed. Even though you thought it is difficult to understand the ins and outs, I think you have already got the key idea of my strategy. Because the control strategy can be divided into two parts, shown in Figure.2 the two boxes in the yellow dotted box, the outs of one part are just the ins of the other one. The whole strategy is a circle, like a snake biting its own tail. So, it is really difficult to distinguish ins and outs. And this is the main idea or key innovation of the strategy.

I have added some detailed explanation in the manuscript, from Line 171 to Line 175, and from Line 178 to Line 180, and from Line 188 to Line 193.

3) The section named "3. Simulation Results" must be improved. In particular:

I have replaced the name as “Simulation Analysis” and reorganized and improved the whole section. Please review it again, thank you.

Point 8: - Figure 4 shows the simulation model. It is very difficult to understand the main objectives. A block diagram will help readers to better understand this section of the manuscript.

Response 8: The block diagrams are shown in Figure.2, and Figure.3. I put the simulation model in the manuscript with the aim of helping the readers to understand the ins and outs of the strategy.

I have added some explanation in the manuscript, from Line 205 to Line 209, and Line 214 to Line 216.

Thank you.

Point 9: - Figure 4 is not discussed in the text of the section named "3.1. Simulation Model".

Response 9: It was discussed just below the figure in Line 212 to Line 216. Thank you.

Point 10: - Table 1 gives the parameters of the simulation. However, the authors do not discuss the values. Why these values?

Response 10: An inverter product was modified to build the experimental platform, shown in Figure7. And most simulation parameters are based on actual values. Udc is the value of series PV cells. Lg and Rg are not actual values but estimated ones. Lg and LR are the parameters we cannot set, if a control strategy is all right, it should be fit for all the reasonable Lg and LR values.

I have added this part in the manuscript, from 219 to 222. Thank you.

Point 11: - From all simulation results, what about the effectiveness of the proposed control strategy?

Response 11: Yes. Thank you for your suggestion.

I have reorganized this part and added the conclusions of every simulation. From the conclusions, the effectiveness of the proposed control strategy is revealed.

The conclusions are added from Line 263 to Line 270, and from Line 289 to Line 298, and from Line 326 to Line 333.

4) The section named "4. Experimental Results" is really interesting, because the proposed experimental results prove the relevance of the proposed control strategy. However, this section of the manuscript must be improved. In particular:

Point 12: - The coauthors explain that the inverter is the core of the experimental platform. However, it is of utmost importance to explain the key features of this converter.

Response 12: Thank you.

I have added the explanation in the manuscript in Line 340 and Line 341.

Point 13: - The values of the resistance and inductance of the AC-grid are not the same as Table 1. Could you better explain the differences? Could you discuss the methods to extract these experimental values?

Response 13: Yes. I used different grid impedance mainly because of the limitations of experimental conditions. The inductances in the platform were designed for 50kVA grid-connected inverter. I used a 5kVA inverter to do the experiment, the current is too low for the inductance, so the actual inductance value may be different from its design value. In practice, the grid impedance cannot be set artificially. A good control strategy should be fit for all the reasonable impedance. So, I did not design another inductance strictly following the simulation parameters just to prove the strategy can be apply into every situation.

All the inverter parameters are actual values used in the product. I can explain the parameter design of an inverter, but I think it is not necessary in this article. The main idea of this article is to propose a novel control strategy, not to design the filter, or choose the carry frequency of an inverter.

Thank you.

Point 14: - Could you explain the method to calculate the THD-parameter?

Response 14: I read it directly from a handheld power analyzer phase by phase. Thank you.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

No further comments on the paper itself, but I would like to make a couple of suggestions regarding paper writing:

Please mark revised/added text in a different color so that the reviewer can easily locate the changes, for added figures and tables, you may change the color of the captions.

Regarding the PI regulator, the oscillations are generally not encountered in a well tuned PI regulator. There are several references that talk about grid converter control with PI regulators. But for the sake of comparison in this paper, the presented results will do.

Author Response

Thank you very much. 

I have added two references about PI controller design from Line 317 to Line 319, and from Line 497 to Line 501. These added parts are in red.

Thank you. I do feel the paper has improved a lot with your help. 

Author Response File: Author Response.docx

Reviewer 2 Report

This revision has addressed all raised issues in the original version and can be acceptable for publication.

Author Response

Thank you for your affirmation.

Thank you very much.

Author Response File: Author Response.docx

Reviewer 3 Report

The context and objectives of the manuscript are now clearly described.

The contributions of the article are now clearly detailed.

However, the section related to the simulation results can be improved. In particular, it is better to have a block diagram to explain the simulation model (see Figure 4).

Finally, moderate English changes are still required.

I propose to accept the manuscript after minor revision.

Author Response

Thank you for your affirmation.

Point 1:   It is better to have a block diagram to explain the simulation model (see Figure 4).

Response 1: Figure 2 and Figure 3 are just the blocks you mentioned to explain Figure 4.

I have added some words to explain their relationship, from Line 207 to Line 208, and from Line 214 to Line 216.

Point 2: moderate English changes are still required.

Response2: I have checked the full text and corrected several grammatical mistakes in red. 

Thanks again.

Author Response File: Author Response.docx