Electric Vehicle Powertrains with Modular Battery Banks Tied to Multilevel NPC Inverters

Round 1

Reviewer 1 Report

The technical content of the paper is good for publication in this journal. However, the authors should respond to a few comments.

1. The title is confusing. The actual work incorporates the battery pack tied with NPC in the power train. But the title looks like a sentence. A good title has a maximum of 10 words.

2. The major contributions must be listed in the bulletin at the end of the introduction section. 

3.  Literature study is poor. There are plenty of related works in this field. So, the authors are suggested to improve the literature study by referring to newly published related works. 

4. So many grammatical and typos throughout the paper. 

5. On what basis the design scenarios have been considered in this study?

6. What are the challenges and demerits of including the battery pack with NPC?

7. How to overcome the challenges?

8. What are the future scopes?

9. Introduce a separate sub-section to summarize the findings from the simulation experiments. 

Author Response

Reviewer 1

Dear Reviewer,

We would like to thank you for your time and comments to improve the manuscript.

In the following, we present our response, specifying the changes included in the paper to answer the questions and requests. All the changes have been highlighted in green in the revised version of the manuscript.

Response to the Reviewer comments

The technical content of the paper is good for publication in this journal. However, the authors should respond to a few comments.

1. The title is confusing. The actual work incorporates the battery pack tied with NPC in the power train. But the title looks like a sentence. A good title has a maximum of 10 words.

Response: Thank you for pointing out that the title was not clear. We have changed it to “Electric Vehicle Powertrains with Modular Battery Banks tied to Multilevel NPC Inverters”. We tried to make it simpler and clearer.

2. The major contributions must be listed in the bulletin at the end of the introduction section.

Response: A new paragraph has been included in the introduction to clarify the procedure that has been carried out in the work, which can be considered as the main contribution. We would like to emphasize that the design proposal and comparison presented in this work is not found in the literature, to the best of our knowledge.

3. Literature study is poor. There are plenty of related works in this field. So, the authors are suggested to improve the literature study by referring to newly published related works.

Response: To address the request of the Reviewer, we have added 8 new references [5-6][22-23][27-29][31]. Our intention has been to include specific and useful references to support the work, regardless of the date of publication. However, we will be glad to incorporate any additional references that the Reviewer considers relevant to the work.

4. So many grammatical and typos throughout the paper.

Response: We have revised the paper thoroughly, trying our best to provide the most correct version of the manuscript. We will be glad to address any additional specific correction that the Reviewer may bring to our attention. We would also like to point out that, as usual, in case that the manuscript is finally accepted, MDPI will review the manuscript (grammar, spelling,...) before its publication.

5. On what basis the design scenarios have been considered in this study?

Response: In this work, the main goal is to show the potential benefits given by using a powertrain design approach based on combining a modular battery bank with multilevel NPC traction inverter topologies, in comparison to a conventional two-level powertrain design. To better illustrate the benefits of the proposed design approach, given its modularity and scalability, among other features, the design scenario has been defined to include several vehicles with different power ratings. To keep things simple, three vehicle types have been considered: a small-power electric vehicle (EV), a medium-power EV, and a high-power EV.

EV1 (rated power 50 kW) represents a small-power EV as, for instance, the Dacia Spring (33 kW), the Renault Twingo electric (60 kW), the Smart EQ fortwo (60 kW) or the Volkswagen e-Up (61 kW).

EV2 (rated power 100 kW) represents a medium-power EV as, for instance, the Nissan Townstar (90 kW), the Peugeot e-208 (100 kW), the Citroën ë-SpaceTourer (100 kW) or the Nissan Leaf (110 kW).

EV3 (rated power 150 kW) represents a high-power EV as, for instance, the Mini electric (135 kW), the Kia e-Soul (150 kW), the Subaru Solterra (160 kW) or the Tesla Model 3 Standard Plus (175 kW).

We realize that the number (3) and the rated power (50, 100, 150 kW) used to define the design scenario are somehow arbitrary. Nevertheless, in our opinion, this design scenario seems to be suitable to show the benefits of the proposed powertrain design approach. We honestly think that an additional number of cases or different rated power would not provide significant new information or different conclusions to the work.

To clarify this point, we have modified the text at the beginning of subsection 2.1. Please, find it highlighted in green in the new version of the manuscript.

6. What are the challenges and demerits of including the battery pack with NPC?

Response: While the proposed design approach benefits from being able to produce different vehicle powertrains from a basic and standard battery module and a basic standard power switch, it has the drawback of requiring multiple units of these basic elements that need to be efficiently and reliably assembled together. Therefore, the system complexity can be regarded to increase from this point of view. In addition, compared to a two-level inverter design approach where only two wires are needed to connect the battery to the inverter, in a multilevel NPC inverter, more than two wires are needed.

In the conclusions, some additional sentences have been added to clarify this point. Please, find them highlighted in green in the new version of the manuscript.

7. How to overcome the challenges?

Response: Regarding the need to assemble multiple battery modules and multiple power switches, an efficient and reliable assembly method should be conceived in order to achieve a robust assembly consuming the lowest cost, time and space.

Regarding the need to incorporate additional connections between the battery and the multilevel inverter, although the current rating of these additional wires is much smaller than the rating of the two basic outer wires, the distance between the battery system and the inverter should be reduced as much as possible. This will reduce the total wire length needed.

In the conclusions, some additional sentences have been added to clarify this point. Please, find them highlighted in green in the new version of the manuscript.

8. What are the future scopes?

Response: The proposed design approach introduces additional complexity since multiple units of a basic battery module and multiple units of a power switch are required. However, this opens an opportunity for standardization of these two basic elements and offers novel degrees of freedom to improve the performance of the battery and the inverter. In the battery, part of the battery management system functions can be integrated with the inverter, e.g. providing a lossless battery state-of-charge balancing of the battery modules. In the inverter, the use of multiple power switches enables the possibility to increase the efficiency, improve the distribution of losses and increase the power density, and improve the fault tolerance and reliability. Exploiting these opportunities is an interesting future research line.

In the conclusions, some additional sentences have been added to clarify this point. Please, find them highlighted in green in the new version of the manuscript.

9. Introduce a separate sub-section to summarize the findings from the simulation experiments.

Response: A new subsection “3.7. Summary of the simulation results” has been included in the manuscript to address the request of the Reviewer.

Author Response File: Author Response.pdf

Reviewer 2 Report

Alepuz et al. submitted the research article in Electronics. In this paper, the potential benefits achieved through a powertrain design approach based on combining a modular battery bank with multilevel NPC traction inverter topologies are analyzed, in comparison to a conventional two‐level powertrain design. Several aspects are analyzed: modularity, complexity, battery‐pack state‐of‐charge balancing, inverter loss, motor ac voltage harmonic distortion, motor common‐mode voltage, and reliability. The comparison results show that the proposed design approach, based on modular battery packs and multilevel technology, provides interesting advantages that will typically overcome the drawbacks.

Though the authors organized the data well and presented the results with enough explanation. However, it requires some clarifications. I request the authors to please go through the following comments and concerns and address them accordingly.

1.     Please add some quantifiable data in the abstract and conclusion sections. It seems too generic. For a scientific article, there must be some facts and figures in these sections. I also recommend modifying the title because it is too lengthy. The title should be eye-catching and concise.

2.     How did the authors choose the powertrain design approaches? Please comment on it.

3.     Can the authors further explain the fact, mentioned between lines#250-252? It needs further clarification.

4.     How could the failure analysis be conducted by the inverter meantime? Please explain.

5.     Please replace the parameter with parameters in Tables, wherever used. The font size in Figure 2 can be improved. Also, please check the size of the axis/ticks/legends in Figures 4 and 5.

Author Response

Reviewer 2

Dear Reviewer,

We would like to thank you for your time and comments to improve the manuscript.

In the following, we present our response, specifying the changes included in the paper to answer the questions and requests. All the changes have been highlighted in green in the revised version of the manuscript.

Response to the Reviewer comments

Alepuz et al. submitted the research article in Electronics. In this paper, the potential benefits achieved through a powertrain design approach based on combining a modular battery bank with multilevel NPC traction inverter topologies are analyzed, in comparison to a conventional two‐level powertrain design. Several aspects are analyzed: modularity, complexity, battery‐pack state‐of‐charge balancing, inverter loss, motor ac voltage harmonic distortion, motor common‐mode voltage, and reliability. The comparison results show that the proposed design approach, based on modular battery packs and multilevel technology, provides interesting advantages that will typically overcome the drawbacks.

Though the authors organized the data well and presented the results with enough explanation. However, it requires some clarifications. I request the authors to please go through the following comments and concerns and address them accordingly.

1. Please add some quantifiable data in the abstract and conclusion sections. It seems too generic. For a scientific article, there must be some facts and figures in these sections. I also recommend modifying the title because it is too lengthy. The title should be eye-catching and concise.

Response: We have changed the title to “Electric Vehicle Powertrains with Modular Battery Banks tied to Multilevel NPC Inverters”. We tried to follow the suggestion of the Reviewer to make it shorter, eye-catching and concise.

We have also added some quantitative data in the abstract and conclusions. In particular, the identified maximum percentage reduction in inverter power loss, total harmonic distortion and rms common mode voltage, valid for a specific design scenario at certain operating conditions. Please also note that we have added a new subsection “3.7. Summary of the simulation results”, with a simple summary of the main facts and figures of the results, although it is not a simple task because of the variability of the benefits given by design B in comparison to design A.

2. How did the authors choose the powertrain design approaches? Please comment on it.

Response: The proposed powertrain design approach (design approach B) was selected with the aim of being able to design a range of powertrains from a single basic standard battery module and a single standard power switch, just by combining several units of each. We believe that this is a promising approach and our expertise in multilevel NPC conversion techniques allowed us to envision it. Design approach A corresponds to the conventional design approach, where a custom battery and a custom power switch at the corresponding voltage rating is selected for each different powertrain. Design approach A serves as a reference for comparison.

In this work, the main goal is to show the potential benefits given by using a powertrain design approach based on combining a modular battery bank with multilevel NPC traction inverter topologies, in comparison to a conventional two-level powertrain design. To better illustrate the benefits of the proposed design approach, given its modularity and scalability, among other features, the design scenario has been defined to include several vehicles with different power ratings. To keep things simple, three vehicle types have been considered: a small-power electric vehicle (EV), a medium-power EV, and a high-power EV.

EV1 (rated power 50 kW) represents a small-power EV as, for instance, the Dacia Spring (33 kW), the Renault Twingo electric (60 kW), the Smart EQ fortwo (60 kW) or the Volkswagen e-Up (61 kW).

EV2 (rated power 100 kW) represents a medium-power EV as, for instance, the Nissan Townstar (90 kW), the Peugeot e-208 (100 kW), the Citroën ë-SpaceTourer (100 kW) or the Nissan Leaf (110 kW).

EV3 (rated power 150 kW) represents a high-power EV as, for instance, the Mini electric (135 kW), the Kia e-Soul (150 kW), the Subaru Solterra (160 kW) or the Tesla Model 3 Standard Plus (175 kW).

We realize that the number (3) and the rated power (50, 100, 150 kW) used to define the design scenario are somehow arbitrary. Nevertheless, in our opinion, this design scenario seems to be suitable to show the benefits of the proposed powertrain design approach. We honestly think that an additional number of cases or different rated power would not provide significant new information or different conclusions to the work.

To address this point, we have modified the text at the beginning of subsection 2.1. and we have added a new text after Table 2. Please, find it highlighted in green in the new version of the manuscript.

3. Can the authors further explain the fact, mentioned between lines#250-252? It needs further clarification.

Response: The inverter operation generates a common-mode voltage at the inverter ac terminals, which represents an average voltage between the motor stator windings and ground. Due to non-negligible parasitic capacitances, the impedance between the stator windings and ground is finite and then the common-mode voltage causes the circulation of a common-mode current through this impedance, that typically leads to undesirable effects. For instance, this common-mode current flows through the motor bearings, which causes bearing degradation. On the other hand, since this current flows through a loop including ground, it may cause interferences in other systems. Consequently, a reduced common-mode voltage improves the system operation and extends the lifetime of the motor drive.

We have extended the first paragraph in subsection 3.5. to clarify this. In our understanding, a more detailed explanation of the common-mode voltage effects on the motor drive is beyond the scope of the present work. However, we have also added a new reference [31]. This reference is a review paper which provides a detailed description and an extensive explanation of the common-mode voltage effects on the motor drive. We hope that the above explanations together with this new reference satisfies the clarification requested by the Reviewer.

31. Turzyński, M.; Musznicki, P. A Review of Reduction Methods of Impact of Common-Mode Voltage on Electric Drives. Energies, 2021, 14, doi:10.3390/en14134003.
32. How could the failure analysis be conducted by the inverter meantime? Please explain.

Response: We understand that the Reviewer is asking about the inverter mean time to failure (MTTF) in subsection 3.6. The inverter leg MTTF values shown in Table 5 have been calculated by means of Markov models (Markov chain diagrams) of two-level, three-level and four-level ANPC inverter legs, by using the method detailed in [32]. The MTTF is a figure-of-merit to characterize the reliability, and therefore the failure rate, of the inverter.

In summary, the method in [32] defines the Markov chain of an inverter leg, considering all possible partial failure states involving one or more failed power switches in the inverter leg. It is worth highlighting that ANPC multilevel converters, thanks to their intrinsic redundancies, can maintain operation in case of failure of one or more power switches. That is, a switch failure does not necessarily lead to a full converter shutdown.

Subsection 3.6 has been slightly extended to clarify this point. Please, find it highlighted in green in the revised version of the manuscript.

5. Please replace the parameter with parameters in Tables, wherever used. The font size in Figure 2 can be improved. Also, please check the size of the axis/ticks/legends in Figures 4 and 5.

Response: The word “parameter” has been replaced with the word “parameters” in Tables 1, 2 and 4.

Figure 2 has been enlarged, and the font has improved its readability. Figure 1 has also been enlarged.

We agree that axis/ticks/legends of Figures 4 and 5 were rather small. We have increased these figures as much as possible to make them larger and more readable. To make the axis/ticks/legends even larger, we would need to run the Matlab simulations and generate again all the figures, which requires a considerable amount of time. Unfortunately, we could not do it in the present revision due to a very tight agenda these days. If the Reviewer considers that it is necessary to make them larger than they are now, we will gladly do it in the next revision round. To further help the reader, we have indicated the colors used for each power loss in the captions of Figures 4 and 5.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have made an effort to address the comments raised by the reviewer. Now, the title looks better, and it conveys the required message to the readers. The literature study also improved. The responses provided for testing scenarios are convincing. 

Reviewer 2 Report

It can be accepted now.