Experimental Testing of Amplified Inertia Response from Synchronous Machines Compared with Frequency Derivative-Based Synthetic Inertia

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. Low inertia is one of the key issues that the current power system urgently needs to address. This paper focuses on the inertia analysis and management of a specific area and proposes a solution. Based on engineering practice, this paper has significant social benefits, but its scientific attributes are relatively weak.

2. In the introduction section, the author elaborately presents the origin of the research content of this article, which stems from the low inertia situation that occurred in the Nordic electrical power grid region. This approach is quite eye-catching as genuine scientific issues typically arise from practical engineering problems. However, the author is overly focused on the engineering problem itself and lacks the textual refinement to distill the scientific question. The solution is directly derived from the problem encountered in the engineering practice, with insufficient references to relevant literature and a lack of scientific articulation of the engineering problem. It is recommended that the author enhance these aspects.

3. In the second part of this article, namely the Method section, the author directly presents three relatively common methods for enhancing inertia. However, the reasons why these three types of enhancement measures were chosen have not been well argued or explained, which is one of the impacts of not summarizing the references well in the Introduction section. It is suggested that the author strengthen the beginning of the Method section, detailing the methods to be adopted in this paper and their advantages.

4.The simulation results of this paper well demonstrate that the proposed method has a significant effect on inertia enhancement. However, compared with similar methods, the author lacks comparison on the advantages of the method proposed in this paper. It is suggested to add some comparison content to make the advantages and innovation of the proposed scheme more prominent.

Author Response

1. Low inertia is one of the key issues that the current power system urgently needs to address. This paper focuses on the inertia analysis and management of a specific area and proposes a solution. Based on engineering practice, this paper has significant social benefits, but its scientific attributes are relatively weak.

The authors thank for the feedback. Several changed have been made in order to make the paper more cohesive, which includes the removal of one implementation that has not been sufficiently tested. Authors also reframe the language used as to be clear that the investigated methodology, although presenting the desired characteristics for grid support, presents drawbacks that make it less desirable than some alternatives. The authors believe these changes tackle the issue presented by the reviewer, while still being able to present the novel method which has not been presented before in literature.

2. In the introduction section, the author elaborately presents the origin of the research content of this article, which stems from the low inertia situation that occurred in the Nordic electrical power grid region. This approach is quite eye-catching as genuine scientific issues typically arise from practical engineering problems. However, the author is overly focused on the engineering problem itself and lacks the textual refinement to distill the scientific question. The solution is directly derived from the problem encountered in the engineering practice, with insufficient references to relevant literature and a lack of scientific articulation of the engineering problem. It is recommended that the author enhance these aspects.

The authors agree and thank for the reviewer feedback. Section 1.2 has been greatly improved as a response, adding new sources and with further detailing of each, contextualizing the paper on the literature. 

3. In the second part of this article, namely the Method section, the author directly presents three relatively common methods for enhancing inertia. However, the reasons why these three types of enhancement measures were chosen have not been well argued or explained, which is one of the impacts of not summarizing the references well in the Introduction section. It is suggested that the author strengthen the beginning of the Method section, detailing the methods to be adopted in this paper and their advantages.

The authors agree that further examining the other types of inertia enhancement was needed, which is once again covered in section 1.2 alongside the references. The authors also note that the method presented on this paper is relatively novel rather than common, and regret the confusion caused. In order to further differentiate the proposed method from the common inertia enhancement methods, a new terminology was used. The authors now refer to the proposed method as “Amplified Inertia Response (AIR)”, replacing the more generic term used in the previous version of the paper “Enhanced Inertia (EI)”. 

4.The simulation results of this paper well demonstrate that the proposed method has a significant effect on inertia enhancement. However, compared with similar methods, the author lacks comparison on the advantages of the method proposed in this paper. It is suggested to add some comparison content to make the advantages and innovation of the proposed scheme more prominent.

The authors thank the reviewer for the feedback. The authors also note that the experimental results do compare our proposed method “Amplified Inertia Response” to derivative-based “Synthetic Inertia” method. Albeit there is agreement that a further and more comprehensive comparison scenario would be ideal, the authors understand that such comparisons are out of scope and would distract from the paper contribution which is presenting this new form of inertia enhancement.

Reviewer 2 Report

Comments and Suggestions for Authors

The topic of the paper is important because the wide spread of decentralized renewable sources such as PV energy systems disturbs the stability of AC grid by lowering the inertia of the grid. The main solution to stabilize a grid frequency is additional resources, which should be connected to compensate it. The authors described several approaches to increase the grid inertia, which are already well-known and tried to compare them in simulation mode with micro-grid and grid-level test cases. But the paper in the current representation of the paper has some weaknesses, which authors should improve:

1. The review is not sufficient. The state-of-the-art solutions and current findings of other researchers in the topic of the paper should be done. The authors of the paper just mentioned the methods (Synthetic inertia, enhanced inertia), which are already known, so it would be useful to review similar papers, where the efficiency of these methods and comparison is already done. 

2. The scientific soundness presented is not enough. The authors made the conclusion based only on two simulation experiments (at least, only two are shown in the paper). Will the findings be similar for more general cases? In terms of the power level - where is the border between "smaller" and "larger grids  (line 321)? Also, the paper has no quantitative comparison of the control methods, just qualitative analysis with general phrases such as "a much higher", "faster response", etc. It will be very useful to use some quantitative indicators for the comparison of methods.

3.  Authors should improve the representation of the test system (Fig.5 b) by using arrows and descriptions.

4. It is better to have a description of the paper’s content after the introduction, which gives the reader more details about the paper's structure before the main content.

Author Response

The topic of the paper is important because the wide spread of decentralized renewable sources such as PV energy systems disturbs the stability of AC grid by lowering the inertia of the grid. The main solution to stabilize a grid frequency is additional resources, which should be connected to compensate it. The authors described several approaches to increase the grid inertia, which are already well-known and tried to compare them in simulation mode with micro-grid and grid-level test cases. But the paper in the current representation of the paper has some weaknesses, which authors should improve:

1. The review is not sufficient. The state-of-the-art solutions and current findings of other researchers in the topic of the paper should be done. The authors of the paper just mentioned the methods (Synthetic inertia, enhanced inertia), which are already known, so it would be useful to review similar papers, where the efficiency of these methods and comparison is already done. 

The authors agree and thank for the reviewer feedback. Section 1.2 has been greatly improved as a response, adding new sources and with further detailing of each, contextualizing the paper on the literature.  The authors also note that the method presented on this paper (previously called enhanced inertia) is relatively novel rather than common, and regret the confusion caused. In order to further differentiate the proposed method from the common inertia enhancement methods, a new terminology was used. The authors now refer to the proposed method as “Amplified Inertia Response (AIR)”, replacing the more generic term used in the previous version of the paper “Enhanced Inertia (EI)”. 

2. The scientific soundness presented is not enough. The authors made the conclusion based only on two simulation experiments (at least, only two are shown in the paper). Will the findings be similar for more general cases? In terms of the power level - where is the border between "smaller" and "larger grids  (line 321)? Also, the paper has no quantitative comparison of the control methods, just qualitative analysis with general phrases such as "a much higher", "faster response", etc. It will be very useful to use some quantitative indicators for the comparison of methods.

The authors thank the reviewer for the comment, and agree with the issue. Usually definitions of smaller/larger or more commonly weaker/stronger grids are not precisely defined. In this work, we mean smaller grids as microgrids with low inertia, while larger grids reference to the Nordic grid with relative high inertia when compared to our laboratory setup. Due to equipment and time constrains, experimental tests with multiple grid sizes would not be feasible.

Regardless, authors do agree that the support for the conclusion was not as strong as previously implied, and worked on reframing the language for a clearer message.

3. Authors should improve the representation of the test system (Fig.5 b) by using arrows and descriptions.

The author thanks the reviewer for the comment. Labels were added to Figure 5 to further clarify each part.

4. It is better to have a description of the paper’s content after the introduction, which gives the reader more details about the paper's structure before the main content.

The authors thank for the reviewer comment. An additional subsection was added on the Introduction (Section 1.3 Paper Structure) to address the issue.

Reviewer 3 Report

Comments and Suggestions for Authors

The paper compared two approaches for delivering inertia-like grid services. The reviewer has some comments:

1. What if there is an imbalance in the three-phase grid? Are there any experimental results?
2. How did you choose all the parameters in Table Ⅰ?
3. More relevant methods should be incorporated and compared in the manuscript. What are the advantages of your proposed method over the previous AI-based methods? ([1]doi: 10.1109/TPEL.2023.3299979),  The author should discuss these articles in the literature review.
4. What are the main contributions and motivations of your method? The contributions should be highlighted in your introduction
5. The execution of different algorithms should be added.

Comments on the Quality of English Language

 The English could be improved to more clearly express the research.

Author Response

The paper compared two approaches for delivering inertia-like grid services. The reviewer has some comments:

1. What if there is an imbalance in the three-phase grid? Are there any experimental results?

The authors thank the reviewer for the feedback. The frequency transients that are affected by grid inertia occurs usually when either three-phase generation or three-phase loads are disconnected to the grid. The imbalances between the three phases are not substantive enough, and are thus usually disregarded from such analysis in literature.

1. How did you choose all the parameters in Table Ⅰ?

The authors appreciate the valuable comment! In agreement with the reviewer, a new introductory text was added as Table 1 described, explaining the main values shown in the table.

1. More relevant methods should be incorporated and compared in the manuscript. What are the advantages of your proposed method over the previous AI-based methods? ([1]doi: 10.1109/TPEL.2023.3299979),  The author should discuss these articles in the literature review.

The authors thank for the reviewer comment. The literature review section was updated in response, further including methods used in literature. The aforementioned paper, however, is considered out of scope by the authors, once it pertains to converter stability and not grid stability. The method suggested by the mentioned paper does not tackle questions of grid stability, and is not proposed as an alternative to the methods cited in our work.

1. What are the main contributions and motivations of your method? The contributions should be highlighted in your introduction

The authors appreciate the reviewer feedback. The main contribution of the proposed method relies on enhancing inertia to amplifying the response of synchronous generation, thus avoiding the noise amplification and delay issues caused by derivative-based methods. This clarification was added in the paper (lines 104-109) when placing the proposed method on the literature context.

1. The execution of different algorithms should be added.

The authors thank the reviewer for comment, and although agree overall that more algorithms would be optimal, it is believed that presenting the novel method and its grid support characteristics, as well as its limitations and drawbacks, are the main scope of the project.

Comments on the Quality of English Language

 The English could be improved to more clearly express the research.

The authors thank the reviewer for the comment. The text was reviewed and rewritten to better convey the desired message.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

all commences have been duly addressed.

Author Response

1. Low inertia is one of the key issues that the current power system urgently needs to address. This paper focuses on the inertia analysis and management of a specific area and proposes a solution. Based on engineering practice, this paper has significant social benefits, but its scientific attributes are relatively weak.

The authors thank for the feedback. Several changed have been made in order to make the paper more cohesive, which includes the removal of one implementation that has not been sufficiently tested. Authors also reframe the language used as to be clear that the investigated methodology, although presenting the desired characteristics for grid support, presents drawbacks that make it less desirable than some alternatives. The authors believe these changes tackle the issue presented by the reviewer, while still being able to present the novel method which has not been presented before in literature.

2. In the introduction section, the author elaborately presents the origin of the research content of this article, which stems from the low inertia situation that occurred in the Nordic electrical power grid region. This approach is quite eye-catching as genuine scientific issues typically arise from practical engineering problems. However, the author is overly focused on the engineering problem itself and lacks the textual refinement to distill the scientific question. The solution is directly derived from the problem encountered in the engineering practice, with insufficient references to relevant literature and a lack of scientific articulation of the engineering problem. It is recommended that the author enhance these aspects.

The authors agree and thank for the reviewer feedback. Section 1.2 has been greatly improved as a response, adding new sources and with further detailing of each, contextualizing the paper on the literature. 

3. In the second part of this article, namely the Method section, the author directly presents three relatively common methods for enhancing inertia. However, the reasons why these three types of enhancement measures were chosen have not been well argued or explained, which is one of the impacts of not summarizing the references well in the Introduction section. It is suggested that the author strengthen the beginning of the Method section, detailing the methods to be adopted in this paper and their advantages.

The authors agree that further examining the other types of inertia enhancement was needed, which is once again covered in section 1.2 alongside the references. The authors also note that the method presented on this paper is relatively novel rather than common, and regret the confusion caused. In order to further differentiate the proposed method from the common inertia enhancement methods, a new terminology was used. The authors now refer to the proposed method as “Amplified Inertia Response (AIR)”, replacing the more generic term used in the previous version of the paper “Enhanced Inertia (EI)”. 

4.The simulation results of this paper well demonstrate that the proposed method has a significant effect on inertia enhancement. However, compared with similar methods, the author lacks comparison on the advantages of the method proposed in this paper. It is suggested to add some comparison content to make the advantages and innovation of the proposed scheme more prominent.

The authors thank the reviewer for the feedback. The authors also note that the experimental results do compare our proposed method “Amplified Inertia Response” to derivative-based “Synthetic Inertia” method. Albeit there is agreement that a further and more comprehensive comparison scenario would be ideal, the authors understand that such comparisons are out of scope and would distract from the paper contribution which is presenting this new form of inertia enhancement.

Reviewer 2 Report

Comments and Suggestions for Authors

The paper was improved in comparison with the initial version in terms of 1st, 3rd, 4th and 5th comments. The part of the 2nd comment "... the paper has no quantitative comparison of the control methods, just qualitative analysis with general phrases such as "a much higher", "faster response", etc. It will be very useful to use some quantitative indicators for the comparison of methods" was about quantitative analysis, unfortunately it was not presented in the modified manuscript. 

Author Response

The topic of the paper is important because the wide spread of decentralized renewable sources such as PV energy systems disturbs the stability of AC grid by lowering the inertia of the grid. The main solution to stabilize a grid frequency is additional resources, which should be connected to compensate it. The authors described several approaches to increase the grid inertia, which are already well-known and tried to compare them in simulation mode with micro-grid and grid-level test cases. But the paper in the current representation of the paper has some weaknesses, which authors should improve:

1. The review is not sufficient. The state-of-the-art solutions and current findings of other researchers in the topic of the paper should be done. The authors of the paper just mentioned the methods (Synthetic inertia, enhanced inertia), which are already known, so it would be useful to review similar papers, where the efficiency of these methods and comparison is already done. 

The authors agree and thank for the reviewer feedback. Section 1.2 has been greatly improved as a response, adding new sources and with further detailing of each, contextualizing the paper on the literature.  The authors also note that the method presented on this paper (previously called enhanced inertia) is relatively novel rather than common, and regret the confusion caused. In order to further differentiate the proposed method from the common inertia enhancement methods, a new terminology was used. The authors now refer to the proposed method as “Amplified Inertia Response (AIR)”, replacing the more generic term used in the previous version of the paper “Enhanced Inertia (EI)”. 

2. The scientific soundness presented is not enough. The authors made the conclusion based only on two simulation experiments (at least, only two are shown in the paper). Will the findings be similar for more general cases? In terms of the power level - where is the border between "smaller" and "larger grids  (line 321)? Also, the paper has no quantitative comparison of the control methods, just qualitative analysis with general phrases such as "a much higher", "faster response", etc. It will be very useful to use some quantitative indicators for the comparison of methods.

The authors thank the reviewer for the comment, and agree with the issue. Usually definitions of smaller/larger or more commonly weaker/stronger grids are not precisely defined. In this work, we mean smaller grids as microgrids with low inertia, while larger grids reference to the Nordic grid with relative high inertia when compared to our laboratory setup. Due to equipment and time constrains, experimental tests with multiple grid sizes would not be feasible.

Regardless, authors do agree that the support for the conclusion was not as strong as previously implied, and worked on reframing the language for a clearer message.

3. Authors should improve the representation of the test system (Fig.5 b) by using arrows and descriptions.

The author thanks the reviewer for the comment. Labels were added to Figure 5 to further clarify each part.

4. It is better to have a description of the paper’s content after the introduction, which gives the reader more details about the paper's structure before the main content.

The authors thank for the reviewer comment. An additional subsection was added on the Introduction (Section 1.3 Paper Structure) to address the issue.

Reviewer 3 Report

Comments and Suggestions for Authors

No comments

Author Response

Comment: "The paper was improved in comparison with the initial version in terms of 1st, 3rd, 4th and 5th comments. The part of the 2nd comment "... the paper has no quantitative comparison of the control methods, just qualitative analysis with general phrases such as "a much higher", "faster response", etc. It will be very useful to use some quantitative indicators for the comparison of methods" was about quantitative analysis, unfortunately it was not presented in the modified manuscript."

Reply: The authors thank the reviewer for the comment, and agree with the issue. To overcome it, such comparisons between the results and methods were reviewed in text and either addressed numerically (as in line 294, and 324-325) or rewritten to statements that can be more intuitively supported (lines 341-343, 354 - 362).