Mechanism Analysis and Suppression Strategy of Continuous High-Frequency Oscillation in MMC-HVDC System
Round 1
Reviewer 1 Report (Previous Reviewer 1)
Dear Authors, The following are the comments
1. The introduction is not satisfactory. There are many methods for achieving high frequency oscillation suppression. They are not presented and the importance of the chosen strategy is not verified
2. The works lacks experimental results. Why has this not been performed?
3. How the parameters of the controller been selected? How can they be ensured as optimum values?
4. A comparison of the proposed work with those existing has to be presented for validation of novelty. But this is missing?
5. The chosen strategy appears to simple to be novel. It basically reduces to the design of a proper filter.
Author Response
Thank you for your valuable comments, which have greatly improved the quality of our paper. After discussion with all our authors, we have made a lot of amendments and supplements to the paper and answered your comments in detail. Thank you again for your help with our paper.
Author Response File: 
Author Response.pdf
Reviewer 2 Report (Previous Reviewer 2)
The paper presents a solution to the stabilization of the control loop and response of a MMC converter in relation to the operation in a HVDC link. The paper has problems of clarity here and there and some explanations are missing.
1) Line 174,175. Sentence is not well structured and explained: i) do you verify the correspondence of a model just checking the frequency and amplitude of oscillation? and not the whole frequency response in the interval of interest?
ii) there is no verb, "closer" is an adjective or an adverb.
iii) "take into account the accuracy...": either the approximation (that is a mathematical expression) fulfills a criterion of accuracy (rms error, absolute error, etc.) or not; how can it take into account the "accuracy of the delay"?
2) Figure 8 and Figure 9 are hardly readable. Figure 9 shows colors for harmonics (what is the meaning?) and does not clarify where these harmonics were taken, nor why there are 3 insets (3 phases? of what?)
3) Line 337: "is" not needed, maybe "to" ; Line 348: on "the" voltage.
4) Line 353: sentence begins with "Where", as for following an equation, but after a figure and its caption the sentence should be standalone. "As shown in Figure 14, ..." should replace "Where".
5) Figure 15 unreadable.
6) Section 4.3.2. You speak here of different filters, Figure 17 speaks of low-pass, band-pass and band-stop filters, but it is not clear at all how their frequency responses were selected and where in this paper they are described (which parameters they have, which frequency response they have).
7) Line 399. After speaking of low-pass and band-pass filters, you say "the filter": which one?
8) Figure 18. Hardly readable and does not have the measuring units on the vertical axes.
Author Response
Thank you for your valuable comments, which have greatly improved the quality of our paper. After discussion with all our authors, we have made a lot of amendments and supplements to the paper and answered your comments in detail. Thank you again for your help with our paper.
Author Response File: Author Response.pdf
Reviewer 3 Report (New Reviewer)
The article presented for review describes the high-frequency oscillations characteristic of MMC-HVDC transmission systems. The authors undertook a comprehensive analysis, starting from the description of the phenomenon through the impedance model, and ending with the proposal to use a non-linear suppression filter. The article, after introducing corrections suggested to the original version, reflects the essence of the issue quite well and meets the publication requirements in terms of content. You can have some reservations about the editorial side, and these are comments such as:
* spaces between punctuation marks (dots, brackets) and the rest of the text (e.g. lines 26, 28),
* detailed captions under figures and an indication of their characteristic elements, which would facilitate the perception of the content presented on them (e.g. figures 2, 4, 19),
* some two-element formulas presented inline should be divided into two lines or these elements should be separated in a legible way (e.g. equation 1),
* some references point to incorrect figures (e.g. a reference on line 150 pointing to figure 5, which should be 6),
* standardization of drawing sizes to be considered (e.g. increasing the size of figure 7),
* standardizing the formatting of the coefficients (straight or italic font, e.g. line 426 t = 1 s).
These are, of course, comments that do not significantly affect the substantive value but should nevertheless be taken into account.
Author Response
Thank you for your valuable comments, which have greatly improved the quality of our paper. After discussion with all our authors, we have made a lot of amendments and supplements to the paper and answered your comments in detail. Thank you again for your help with our paper.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report (Previous Reviewer 1)
Dear Authors,
The revision is satisfactory but needs a small change: The conclusion should not have numbered points
Author Response
Thank you for your valuable comments, which have greatly improved the quality of our paper. After discussion with all our authors, we have made a lot of amendments and supplements to the paper and answered your comments in detail. Thank you again for your help with our paper
Author Response File: Author Response.pdf
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Dear Author,
A strategy has been proposed for suppressing oscillations. The following are the comments
1. The main drawback appears to be the lack of experimental validations to justify the approach. They should take some practical network and implement their strategy.
2. The author has presented a comparison with the low pass filter but there are other strategies available in the literature. They should attempt to compare with some recent ones.
3. A comparison table has be provided before the conclusion section and justify the novelty of the work.
4. Sections have been incorrectly numbered. Section 2 has been numbered twice
5. Abstract section has to mention some of the quantitative results.
6. Why should adopt this approach when I have other practical methods for oscillation suppression?
Reviewer 2 Report
The paper proposes a modification to the low-pass filter of a MMC-HVDC converter control, to explicitly include delay effects.
1) First of all English form needs to be revised as there are many errors spread over the paper and in particular in the Introduction. For example:
line 29: at worldwide
line 32: a trend of
line 36: the system
line 42: the safety
line 48: to be applied -> to apply
line 50, 52, 53, 54, 65, etc.: "authors" is treated as single person and the verb is thus singular (ending "s"): incorrect
line 60: which is difficult to meet
line 63: may not be obvious or invalid -- no meaning
line 108: oscillates -> past tense!
2) The Introduction should discuss references in more detail (ex. [9], [10], [11]) and adding others, to show the effectiveness of the past attempts to control or remediate instability, setting the reference to evaluate performance and novelty of the proposed. (see comment on References at the end)
3) Figure 5. What is the meaning of the dashed arrow and the minus sign down below?
Since you mention soon after an AC side, you should clearly identify it in the figure itself.
4) Sec. 2.2. You speak of delay, but you have not clarified the physical reason for such delay. Later you speak of control delay, but then it seems also that the HVDC line contributes to delay. Please, clarify since the beginning what are all the elements causing relevant delay, which one you can compensate, and in the Introduction highlight which methods were proposed in the past to this aim.
5) Section 2.2.1 The title has no sense, maybe System delay modeling.
6) Line 168. You selected fourth order based on a complexity issue (not so important with modern computational power). But what is the accuracy of this Pade 4th-order approximation, that is instead relevant to the overall problem?
7) Table 1. Many entries are wrongly formatted: Ac, Dc, Kv, etc.
8) Figure 7. The RTDS points are needed especially around the resonance to evaluate accuracy, whereas the peak is fully skipped.
What is then the quantitative estimate of the fitting accuracy?
9) Line 219. Inherent delay. Please clarify? Is it the delay in the MMC you mentioned before, or are there other delay terms? If the latter, they should have been described and discussed before.
10) Line 225. Figure x.
11) Line 227. Above 180° phase rotation there is no risk, but certainty of instability. You should introduce a little better the considerations on the phase rotation for the chance of instability. Don't you use phase margin?
12) Line 232. The pole frequency is related to delay? How you calculate it? Don't you use the Pade approximation?
13) Line 249. What you mean with "tracked". If the control has a delay, it does not track something, maybe lags. I don't understand.
14) Line 242-243. What does it mean "will be quickly followed in the ideal state"? followed by whom/what? which ideal state?
15) Figure 15 and 16 are too small and difficult to read.
16) Line 364. Can you show clearly in the figures where the negative damping phenomenon is in the 500-1000 Hz interval if the more traditional low-pass filter is used?
17) Line 390. Why capital letters?
18) Line 397. You say "delay link", but probably is "link delay". It is part of the delay elements that you have not discussed so clearly. If I guess well, the DC link (that may be many many km long) causes delay: please, clarify it at the beginning, showing why a long transmission line modeling does not represent well this delay. Or it does?
You should justify the Pade approximation and the selected 4th order, knowing that a transmission line model could provide the same delay representation.
19) References.
First, a problem of formatting not in line with MDPI style.
Then, for a subject that has been thoroughly discussed in the last ten years or so, the 18 references are a little undersized, considering that some are on more general methods rather than applications to HVDC and MMC-HVDC cases. Here you see other 10 collected for your perusal, with the intention of focusing on additional examples and studies, to consider for novelty and performance of the proposed.
a) Large-Signal Stability Analysis for VSC-HVDC Systems Based on Mixed Potential Theory, 10.1109/TPWRD.2019.2957270
b) Criterion for the Electrical Resonance Stability of Offshore Wind Power Plants Connected Through HVDC Links, 10.1109/TPWRS.2017.2663111
c) Extended Nodal Admittance Matrix Based Stability Analysis of HVDC Connected AC Grids, 10.1109/ACCESS.2022.3177232
d) Self-Synchronization of Wind Farm in an MMC-Based HVDC System: A Stability Investigation, 10.1109/TEC.2017.2661540
e) Comparative Study on Small-Signal Stability of LCC-HVDC System With Different Control Strategies at the Inverter Station, 10.1109/ACCESS.2019.2904395
f) Generalized Impedance Analysis and New Sight at Damping Controls for Wind Farm Connected MMC–HVdc, 10.1109/JESTPE.2020.3017896
g) REVIEW OF SUPPRESSION STRATEGIES FOR HIGH-FREQUENCY OSCILLATIONS IN MMC-HVDC SYSTEMS, 10.1049/icp.2021.2278
h) Improved Control Strategy of MMC–HVDC to Improve Frequency Support of AC System, 10.3390/app10207282
i) Interaction Assessment and Stability Analysis of the MMC-Based VSC-HVDC Link, 10.3390/en13082075
j) Hybrid Modulated Model Predictive Control in a Modular Multilevel Converter for Multi-Terminal Direct Current Systems, 10.3390/en11071861
