New SOGI-FLL Grid Frequency Monitoring with a Finite State Machine Approach for Better Response in the Face of Voltage Sag and Swell Faults
Round 1
Reviewer 1 Report
- All the references listed should be sequentially cited in order in the text.
- Is the variable of ω0 in (1)(2) the same as that in Fig. 1?
- Following Fig. 18, an additional time-response due to the presence of the noise in the voltage source should be illustrated. This can help us realize more the performance.
Author Response
Hello Dear
Thank you for your comment and spending time on my paper,
- All the references listed should be sequentially cited in order in the text.
Response:
Thank you for this comment. We have checked the sequential order of references and have corrected it when necessary. Additionally, in order to enrich the paper, we have chosen to include some additional references.
All the changes and the new references are highlighted in yellow colour for easy identification.
Comment
- Is the variable of ω0 in (1)(2) the same as that in Fig. 1?
Response:
Yes, it is. Note that Fig. 1 appropriately distinguishes between two conceptually different quantities, namely, ω and ω0. On the one hand, the signal at the output of the FLL block in Fig. 1 is labelled as ω, and as defined in page 2, line 76, it “is the FLL estimated grid frequency”. On the other hand, in line with the classical Control Theory definitions of LP and BP filters shown in equations (1)-(2), their center frequency is denoted as ω0 and labelled correspondingly at the center frequency input of the SOGI block in Fig. 1. This center frequency ω0 is here supplied by the FLL grid frequency estimate ω. Thus, the FLL ω tunes the SOGI ω0.
Comment
- Following Fig. 18, an additional time-response due to the presence of the noise in the voltage source should be illustrated. This can help us realize more the performance.
Response:
Thank you for your comment. Following your suggestion, a new figure labelled as “Figure 19” has been added, showing the SOGI-FLL-FSM response to a voltage sag in presence of a 5-th harmonic with 4% amplitude in the grid voltage. It can be checked that the SOGI-FLL-FSM behaviour is similar to that shown in Fig. 18, subplot d), but with harmonic distortion now present in the estimated frequency response.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The work presented for review is interesting. Well prepared. This is a theoretical work. No significant scientific element was identified. The authors do not indicate the possibility of practical application. They also do not present practical, industrial results - a pity.
Substantive remark:
L 156 -158 The work concerns, in a sense, control theory. In my opinion, it is worth using tools related to this field of automation. To minimize the duration of the time response, no trial and error procedure is used. Specific optimization methods and indicator methods are used. I think that additional explanations or at least a quote are needed here.
The work contains some minor technical errors:
L13 FFL abbreviation not explained. There is an explanation in L9 but applies to SOGI-FLL. Due to the frequent use of FFL, I suggest to explain in brackets.
L81 change "defined as:" to "Defined as:"
L81, L83 patterns (1-3) incorrectly formatted (patterns 4 and 5 - correct), incorrect order of citations; see: https://www.mdpi.com/journal/energies/instructions (Microsoft Word template)
The summary did not indicate the participation of individual authors.
Author Response
Hello Dear
Thank you for your comment, and spending time on the paper
The work presented for review is interesting. Well prepared. This is a theoretical work. No significant scientific element was identified. The authors do not indicate the possibility of practical application. They also do not present practical, industrial results - a pity.
Response
Thank you for your comment. We agree with your remarks at this regard. The work that we are here submitting relates to the PhD research performed by the corresponding author Sajad Abdali Nejad. At the current state of development of his PhD, we are in position of advancing the results we are now presenting. We expect to continue progressing and moving in the direction that you appropriately have identified.
Comment:
Substantive remark:
L 156 -158 The work concerns, in a sense, control theory. In my opinion, it is worth using tools related to this field of automation. To minimize the duration of the time response, no trial and error procedure is used. Specific optimization methods and indicator methods are used. I think that additional explanations or at least a quote are needed here.
Response:
We thank your remark and agree with it. It is true that it can be found in literature methods for optimizing the parameters of the SOGI-FLL and that it would be a desirable objective for us to apply a more formal technique for the SOGI-FLL-FSM tuning than merely trial-and-error. Nevertheless, this is a not at all a straightforward task. In our approach, the added nonlinearity introduced by the inclusion of saturation and the large signal transient nature of the voltage sags and swells analysed perturbations, may cause the misoperation of classical tuning methods based on small signal SOGI-FLL linearized models. For this reason, we have initially solved the tuning problem in an empirical way, by extensive simulation for all the different types and depths of the considered perturbations, putting the emphasis on the novelty of the FSM approach for improving the response to voltage sags and swells. No doubt, it would be a very interesting problem to be addressed in the future as a derived research line. In addition, and according to your suggestion of introducing additional explanations, we have added some lines in this regard at Section 4. Discussion to acknowledge this limitation and identify this as future research beyond the scope of this work.
Comment:
The work contains some minor technical errors:
L13 FFL abbreviation not explained. There is an explanation in L9 but applies to SOGI-FLL. Due to the frequent use of FFL, I suggest to explain in brackets.
We thank your suggestion. According to it, the first appearance of the abbreviation “FLL” in line 9 of page 1 of the revised version of the manuscript has been explained by previously including its definition. The change has been highlighted in yellow colour for easy identification.
L81 change "defined as:" to "Defined as:"
Thank you for identifying this error. After revising the whole sentence in line 81, the final wording reads: “[…] with their transfer functions defined as […]”, avoiding the former syntax inconsistency. The change has been highlighted in yellow color for easy identification.L81, L83 patterns (1-3) incorrectly formatted (patterns 4 and 5 - correct), incorrect order of citations; see: https://www.mdpi.com/journal/energies/instructions (Microsoft Word template)
We thank your advice at this regard. The different patterns and other format issues have been checked according to the instructions for authors in the Microsoft Word template found at the site of the journal. All the changes have been highlighted in yellow colour for easy identification.
The summary did not indicate the participation of individual authors.
Thank you for identifying this error. The corresponding changes have been introduced at the paragraph beginning at line 389 and highlighted in yellow colour for easy identification.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
This version could be acceptable for publication.
