HIL Simulation of a Tram Regenerative Braking System

Round 1

Reviewer 1 Report

This paper presents a Hardware-in-the-loop (HIL) simulation of a tram regenerative braking system performed on a scaled model. The offline simulations are performed using a measured speed profile in order to validate the tram, super capacitor, and power grid model, as well as the energy control algorithm. The results are verified in the real time HIL simulation in which the tram and power grid are emulated using a three-phase converter and LiFePO4 batteries.

The paper topic is interesting, and the manuscript is very comprehensive. The results seem coherent and described with sufficient clarity. However, it is not very clear what is the scientific contribution of the paper and the impact of the results on knowledge in the field. The authors should describe what is the real contribution and originality of this paper compared with most recent research papers. Then, the author must show some criticism on some of the existing research and prove they bring something new.

The conclusions of the paper are not consistent.  The conclusions are in fact a short summary of the article. These should be highlighting the contributions of the paper. Based on the results obtained some generic and fundamental conclusions need to be drawn.

Author Response

We appreciate your time and effort in reviewing our paper. All your suggestions are included in the revised paper.

Our responses to your suggestions are as follows:

Response to Reviewer #1

1. The paper topic is interesting, and the manuscript is very comprehensive. The results seem coherent and described with sufficient clarity. However, it is not very clear what is the scientific contribution of the paper and the impact of the results on knowledge in the field. The authors should describe what is the real contribution and originality of this paper compared with most recent research papers. Then, the author must show some criticism on some of the existing research and prove they bring something new.

Authors’ response: Thank you for your suggestions about the coherence and clarity of the paper. Per your suggestion, the introduction has been expanded, as well as rewritten, in order to clarify and provide more criticism on the existing research.

2. The conclusions of the paper are not consistent.  The conclusions are in fact a short summary of the article. These should be highlighting the contributions of the paper. Based on the results obtained some generic and fundamental conclusions need to be drawn.

Authors’ response: Following the reviewer’s suggestion, the conclusion has been entirely rewritten in order to accentuate the contributions of the paper.

Reviewer 2 Report

Dear Authors,

Thank you for sending your paper to the journal Electronics. Your topic is fitting to the journal scope. Also, the tram regenerative braking system is an interesting topic, mainly because of the renaissance of such a system in Europe. Comparing to the heavy railway, trams have some additional constraint for using the regenerative braking system. I want to propose some improvement to the current version of the paper:

1. For abbreviations, always state the whole meaning first, followed by the abbreviation.
2. It is not good to use the so-called chain citations (for example line 41 and 42). Each reference deserves two to three sentences.
3. Try not to start a paragraph with: “on a figure or in the table”.
4. A crucial part of your research is missing the methodology part. Also, add a block diagram of your research methodology. It is hard to follow your idea.
5. It seems that different authors write a different part of the paper. This is good but needs to leverage the whole text to be straightforward for reading.
6. Check chapter 2 first paragraph. You are mixing rail and tram vehicle.
7. Figure 1 is not precise. The title is rail vehicle and on the figure is tram vehicle. You need to explain all symbols and abbreviations.
8. You have stated: “In this work, the electrical model of the supercapacitor module is needed to obtain information about the accumulated energy within the module and the magnitude of the voltage.” This sentence is clear, but before and afterwards this sentence, you explain that the tram vehicle has an onboard supercapacitor module. If so, this part needs to be explained.
9. In subchapter 3.2. there is no analytical writing of this part. In the first part, you are mentioning Zagreb Electric Tram and afterwards TMK2200. It will be easier than you explain step by step. I suppose that Zagreb Electric Tram is using TMK2200.
10. Can you check the explanations in figure 7 and 8?
11. Please check table 2.
12. Figure 18 is of poor quality.
13. The missing part is the discussion chapter.
14. The conclusion is written in a promotional way. Usually, conclusions need to send a clear message of your research and future research steps.
15. Check all technical mistakes in the current version of the PDF file.

After carefully reading your paper is seems that you rush to publish your results. Please keep in mind that the paper needs to be compact and send a scientific message to an audience.

The global idea is OK.

Regards,

Author Response

Thank you for your effort in reviewing our paper. Below is a point by point response to your comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

There some major issues with the proposed work.

• Is it not clear why HIL is really needed to evaluate the system since it appears that only simulated models have been used. The rationale for using HIL is its capability to test the real prototype of the application. A mere comparison between two simulated models is not scientifically sound.
• The HIL technique does not allow for proper real-time testing, even though it is very fast, accurate, and reliable. This is because additional infrastructure is required for the Matlab/Simulink interfacing that may introduce some bottlenecks in the processing. It is suggested to remove the "real-time" statements.
• The figures have very low quality. Please provide vectorial format pictures or higher quality raster ones.
• There are no actual comparisons in the same plots of the simulated and the HIL models. A proper HIL analysis should include a 3-plots figure: Simulated, Hardware, Error (usually the difference between the outputs).

Author Response

Thank you for your valuable comments. The following text contains a point-by-point response to your review.

• Is it not clear why HIL is really needed to evaluate the system since it appears that only simulated models have been used. The rationale for using HIL is its capability to test the real prototype of the application. A mere comparison between two simulated models is not scientifically sound.

Authors’ response: The HIL simulation experiment is based on using a measured tram speed profile and the resulting currents and voltages (grid, supercapacitor, drive) are obtained through physical measurements on the emulated regenerative braking system. In this way, the operational feasibility of the proposed energy flow control algorithm is verified. In the proposed HIL verification, the real time control elements are also taken into account.

• The HIL technique does not allow for proper real-time testing, even though it is very fast, accurate, and reliable. This is because additional infrastructure is required for the Matlab/Simulink interfacing that may introduce some bottlenecks in the processing. It is suggested to remove the "real-time" statements.

Authors’ response: We partially agree with your comment and we are aware that the ideal verification platform would be a tram control board with the energy flow control system implemented in MCU and everything else done in HIL platform. But by looking at the proposed realization of the HIL system in our paper it can be thought of as real-time execution of control algorithms. Therefore, we didn't entirely delete word real-time in the paper, because there is real-time execution of the energy flow control algorithm and PI controllers of hardware emulation part of laboratory HIL setup on dSpace target. Inductor currents are measured and input currents of all three DC/DC converters are calculated based on measured voltages and currents of respective converters and are fed to the energy flow control algorithm and PI controllers.

• The figures have very low quality. Please provide vectorial format pictures or higher quality raster ones.

Authors’ response:Per the reviewer’s suggestion, some figures has been replotted and redrawn in order to increase their quality and visibility.

• There are no actual comparisons in the same plots of the simulated and the HIL models. A proper HIL analysis should include a 3-plots figure: Simulated, Hardware, Error (usually the difference between the outputs).

Authors’ response:Following the reviewer’s suggestion, the mentioned plots have been added to the paper, specifically the Figure 23.

Round 2

Reviewer 2 Report

Dear Authors,

Thank you for an updated version of your paper.

Regards,

Reviewer 3 Report

The manuscript is ready for acceptance in its form.