Bidirectional Interleaved DC–DC Converter for Supercapacitor Energy Storage Integration with Reduced Capacitance
Round 1
Reviewer 1 Report
Dear Authors,
the paper is interesting, however it requires significant improvements. In the abstract and title Authors declare, that a bi-directional converter is analyzed in their work but in the fact, only a charging process of the supercapacitor is described. Thus, results and analysis for supercapacitor discharging process should be also added.
In case of the the Figure 2 a theoretical currents and voltages operational waveforms of described converter should be also presented.
In the introduction a review and comparison with similar solutions should be added including advantages and disadvantages of compared approaches.
Equation (1) should be rewritten/modified to remove designations "floor".
In the Figure 24 legend caption must be corrected.
In the Figure 32 a scheme of used laboratory setup should be added. A table presenting main parameters of tested converter should be also included in the chapter 5.
A comprehensive experimental efficiency characteristics measured at supercapacitor charging and discharging mode should be also presented.
Best regards.
Author Response
We are very thankful to our respected reviewer for his guidance and valuable suggestions.
Point 1: The paper is interesting, however it requires significant improvements. In the abstract and title Authors declare, that a bi-directional converter is analyzed in their work but in the fact, only a charging process of the supercapacitor is described. Thus, results and analysis for supercapacitor discharging process should be also added.
Response 1: We agree with this recommendation, so we added a description of the discharging process of supercapacitors as well. We have presented the main differences between charging and discharging processes. If an SC circuit is charged from some initial voltage V1 to a voltage V2 at a certain time t and then discharged back to V1 at the same time t, then basically the charging and discharging processes are equivalent. For example, the discharging current is the same as the charging current, only with the opposite sign direction. However, one important fact was revealed that the charging efficiency is higher than discharging efficiency and this difference depends on both the charging/discharging time and the internal resistance R of the SC circuit. (During charging, the efficiency means the energy successfully stored in SC divided by the total energy transmitted to SC, while during discharging, the efficiency means the energy successfully reached the converter divided by the total energy discharged from SC).
Point 2: In case of the Figure 2 a theoretical currents and voltages operational waveforms of described converter should be also presented.
Response 2: Theoretical per-phase current waveforms have been added; other waveforms are very similar to conventional buck converter in a charge mode and boost converter in a discharge mode.
Point 3: In the introduction a review and comparison with similar solutions should be added including advantages and disadvantages of compared approaches.
Response 3: The introduction has been extended.
Point 4: Equation (1) should be rewritten/modified to remove designations "floor".
Response 4: We added an explanation of this function just under this Equation (1). “floor” is a math function that rounds down the result returning the largest integer that is less or equal to the result. That is an important part of this equation, and we cannot imagine other way to write it. It can be found that now and then this function “floor” is used in the expressions of the equations also in publications.
Point 5: In the Figure 24 legend caption must be corrected.
Response 5: Thank you for this remark, we checked and improved all figures. Besides, we have added many new figures, so the figure numbers have changed.
Point 6: In the Figure 32 a scheme of used laboratory setup should be added. A table presenting main parameters of tested converter should be also included in the chapter 5.
Response 6: The figure with used laboratory setup has been added.
Point 7: A comprehensive experimental efficiency characteristics measured at supercapacitor charging and discharging mode should be also presented.
Response 7: In the experimental part, we included a small description of the efficiency in case of charging. Although the oscilloscope can be set to mathematical functions, the results are not expected to be as accurate in terms of real-time efficiency of power and/or energy storing. We assume that the most rational way is to calculate the charging efficiency using the measured current values. An oscilloscope produces an array of numbers (current values) when measuring current and this array can be opened in Excel. There is a column or spreadsheet. It is known how long every single current value recorded in each cell of the column lasts, so the corresponding power loss can be calculated in the next cell, knowing the total resistance (SC total internal resistance + board trace resistance). Further, the sum of the power array of these losses is the total lost energy. The successfully stored energy can be calculated elementary if we know the SC voltage at the beginning and end of charging. Knowing these two mentioned energies, the total efficiency of charging can be calculated without any problems. Of course, it is also possible to calculate and get “real-time” values of power and/or energy storage efficiency in the same Excel file, but this is the case if we are interested in how the efficiency changed during charging not just the total value in the very end of charging. Regarding the energy storing efficiency diagram, the total efficiency of the charging process would correspond to the value of the very last point of the diagram.
Reviewer 2 Report
1) In the paper entitled: "Bi-directional interleaved DC-DC converter for supercapacitor energy storage integration with reduced capacitance" a certain strategy of control method of the multiphase interleaved DC-DC converter for supercapacitor energy storage is considered. As a result of the conducted research, it is expected to reduce the requirements for filtering the output signals, as well as to reduce the energy loss in the supercapacitor energy storage.
2) These issues were widely discussed. The results has been demonstrated and interpreted.
3) In the opinion of this reviewer, the issues discussed can be presented more clearly and legibly. Some specific regards:
Lines 46-47: The typical schematics … . Why was this type and polarity of MOS transistors chosen?.
Line 83: From the other side … . From the other hand ...?
Line 95, Equation (1): The meaning of the expression "floor(X)" is rather obvious here, but in opinion of this reviewer all variables used should – in principle - be explained.
Lines 112-113: By increasing number of phases … . Grammar check seems to be required for this sentence.
Lines 113-114: However, due to parasitic resistances … . This sentence seems to be incomplete.
Lines 121-122: The equations and graphs are correct … . Grammar check seems to be required for this sentence.
Line 130: The input of the converter … . A new paragraph starts here, so there's a new thread perhaps. From this point of view, the phrase: "... in this case." seems to be unclear.
Line 133: Cout(min) – minimum input capacitance. Why is "minimum input capacitance" marked as the output one (Cout)?
Lines 155-162: In practice, both stationary and … . […] … a certain constant voltage value at its output. In the opinion of this reviewer, too many thoughts crowded into just two sentences.
Line 190: 3.1. Description of theorethical model and charging time calculation. The SC model presented in this section is extremely simplified. More accurate models can be found in the literature. In opinion of this reviewer a concise discussion of the SC model used should be presented.
Line 212: … decreases, as is visible from Fig. 6. Is the Figure number correct here?
Line 572: 5. Discussion The article concerns the energy efficiency of a certain energy transmission strategy. Perhaps it would be good to supplement this paragraph with a concise discussion of energy losses in the converter.
4) In the opinion of this reviewer, the article "Bi-directional interleaved DC-DC converter for supercapacitor energy storage integration with reduced capacitance" can be published in the Electronics journal after minor revision. Notes formulated above can be useful in this revision.
Author Response
We are very thankful to our respected reviewer for his guidance and valuable suggestions.
Point 1: Lines 46-47: The typical schematics … . Why was this type and polarity of MOS transistors chosen?
Response 1: Thank you for noticing incorrect type of semiconductor devices in the Figure. We improved the circuit. The most commonly used semiconductor devices for low voltage are N-Channel Si MOSFETs (shown in the Figure), but in our experimental setup we have used e-mode GaN transistors to increase power density of the converter.
Point 2: Line 83: From the other side … . From the other hand ...?
Response 2: Corrected.
Point 3: Line 95, Equation (1): The meaning of the expression "floor(X)" is rather obvious here, but in opinion of this reviewer all variables used should – in principle - be explained.
Response 3: That expression “floor” is not a variable but a math function that rounds down the result returning the largest integer that is less or equal to the result. For understanding, we have added the explanation just under the equation.
Point 4: Lines 112-113: By increasing number of phases … . Grammar check seems to be required for this sentence.
Response 4: We added “the” before “number”, then it sounds clearer.
Point 5: Lines 113-114: However, due to parasitic resistances … . This sentence seems to be incomplete.
Response 5: Now the sentence is completed.
Point 6: Lines 121-122: The equations and graphs are correct … . Grammar check seems to be required for this sentence.
Response 6: Corrected
Point 7: Line 130: The input of the converter … . A new paragraph starts here, so there's a new thread perhaps. From this point of view, the phrase: "... in this case." seems to be unclear.
Response 7: We took out the phrase “in this case”.
Point 8: Line 133: Cout(min) – minimum input capacitance. Why is "minimum input capacitance" marked as the output one (Cout)?
Response 8: Corrected
Point 9: Lines 155-162: In practice, both stationary and … . […] … a certain constant voltage value at its output. In the opinion of this reviewer, too many thoughts crowded into just two sentences.
Response 9: We made more sentences out of those two.
Point 10: Line 190: 3.1. Description of theoretical model and charging time calculation. The SC model presented in this section is extremely simplified. More accurate models can be found in the literature. In opinion of this reviewer a concise discussion of the SC model used should be presented.
Response 10: We agree with this recommendation and as a result we added a discussion justifying the use of this so-called simple RC model. We have acknowledged that this is a simplified model the operation results of which might differ from those of a real SC due to various reasons. Hence, there are different models that are more precise referring to several literature sources. We found one article describing the results of the comparisons of the accuracies of RC, multibranch and two-branched models. By using these data, we clarified that the maximum error of the simple RC model in comparison to a real SC operation is not higher than 6%. This error is not that critically high. There are also other sources stating that RC has a good accuracy for fast dynamics, and it is generally used for the sizing of energy storage systems and calculating approximate efficiency of the storage system. Therefore, we decided to use a simplified RC model.
Point 11: Line 212: … decreases, as is visible from Fig. 6. Is the Figure number correct here?
Response 11: We corrected and modified the numbers of all figures. We have improved the entire paper and added many new figures. So now the figures from the previous version might have a different numbering.
Point 1: Line 572: 5. Discussion The article concerns the energy efficiency of a certain energy transmission strategy. Perhaps it would be good to supplement this paragraph with a concise discussion of energy losses in the converter.
Response 12: The discussion about energy efficiency has been added.
Reviewer 3 Report
The work analyses control method of the multiphase interleaved DC-DC converter for supercapacitor energy storage integration on the DC bus with reduced input and output filter size. It does not include the enough novelty. The written quality of this manuscript is too terrible. I recommend to reject it.
1. In these figures such as Fig. 6-18, the units of x-axes and y-axes should be noted.
2. The Figure's quality of Figure 18, 20 and 22 is terrible. The labels of x x-axes and y-axes are missing.
3. The references' format should be consistent. Some key references are missing. Only 17 references are given. No reference is given from page 4 to page 26.
4. In Page 3, ", multiple current sensors and closed- loop current regulators are used per-phase as in [15]– [19]". However, the total 17 references are given.
5. These equations and symbols are chaos. For example, what's the meaning of parameter "D" in Eq. (6)?
Author Response
We are very thankful to our respected reviewer for his guidance and valuable suggestions.
Point 1: In these figures such as Fig. 6-18, the units of x-axes and y-axes should be noted.
Response 1: Now we noted the units of both x-axes and y-axes. If y-axis units are sometimes not separately noted, then they are mentioned in the graph title because the title is usually about the y-axis values, and they are a function of the x. Since we added a lot of new figures, the figures are now numbered differently.
Point 2: The Figure's quality of Figure 18, 20 and 22 is terrible. The labels of x x-axes and y-axes are missing.
Response 2: Thank you for the remark, we agree that the quality of these figures was terrible because they were copy-pasted from another file thus distorting the figures and size of the letters making them blurry and hard to read. We have used text box to include texts and symbols in figures because then nothing can blurry them. As we mention in the previous response, these figures might be with different numbering now. Speaking of y-axis, any title of a diagram refers to the y-axis, for example, “Charging current (A)” – it means that the talk is about y-axis which is the axes charging current numerical values in amperes. X-axis is time axis, and the unit is seconds (sec). In these three Figures, where several parameters are shown changing simultaneously over the same time, we believe that it is quite sufficient to display the numerical values of the x-axis only under the lowest diagram where it is noted that x-axes units are seconds (sec).
Point 3: The references' format should be consistent. Some key references are missing. Only 17 references are given. No reference is given from page 4 to page 26.
Response 4: We added new references in page 6 and other pages. Now the total number of references has been increased.
Point 4: In Page 3, ", multiple current sensors and closed- loop current regulators are used per-phase as in [15]– [19]". However, the total 17 references are given.
Response 4: Thank you for noticing this mistake. It has been solved.
Point 5: These equations and symbols are chaos. For example, what's the meaning of parameter "D" in Eq. (6)?
Response 5: We improved the overview of the equations. (“D” means duty cycle). Explanation of any variable used in the equations can be found in the text.
Round 2
Reviewer 1 Report
Paper has been improved. I have no more questions. I think I may be accepted for publication.
Reviewer 3 Report
All my omments have been addressed. I recommend to accept it.
