Evaluation of Photovoltaic Inverters According to Output Current Distortion in a Steady-State and Maximum Power Point Tracking
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsDear colleagues, Your work is of potential interest. I must confess that the idea of purchasing commercial devices and making some measurements doesn’t sound very scientific, but more from the engineering or the educational point of view. Still, the study is interesting for the educational and academic field. I will provide some comments which may help you improve it.
The introduction is a bit long. There is always more information that may be included, but there are topics that don´t need to be covered, for example, the paragraph “The fundamental component of a PV system is a PV cell…”. Sounds that may be of interest to people outside the field of inverter design and tests.
First of all, please avoid non-conservative statements that make the article sound even less scientific, like: “These inverters … generate significant harmonic distortions”. I am not sure we can say inverters generate significant harmonic distortions. It depends on its design. They are designed to pass the regulation tests and to be as cheap as possible. Designers may make better inverters, and they do it whenever regulations or tests change, and they actually design inverters (for example, in China) for different countries. Inverters for countries with fewer regulations (for example, developing countries) are cheaper, but they cannot be sold in countries with more strict regulations (for example, developed countries).
Another example is, “One key finding was that the PV inverters generate significant current distortion, and the total harmonic distortion of the current exhibits more significant variations during the transitions from sunrise to sunset.” maybe instead of saying “significant”, you can provide data like how much is significant. I am not saying it is not true; it just doesn´t sound scientific. Furthermore, your tests look like the hard start of an inverter; the hard start happens in one second, while the MPPT happens with slow changes during the hall day; the change from the maximum power to the shutdown taker takes hours, not seconds.
You mentioned some relevant standards; it would be better also if you could describe the way they test the standard in different countries; then, you may find out if you are doing the same. Finally, you are suggesting adjusting the standard based on your study; I would avoid that along with the other non-conservative statements in the full article. Standards are established and adjusted based on significant considerations such as grid safety, environmental impacts, and the minimization of externalities like electrical disturbances or pollutant emissions. A measurement or the observation that a specific inverter may not fully comply with certain standards (particularly if you are not doing the test by the regulatory authority) does not provide sufficient grounds for modifying these regulations. I would accept if such findings should be positioned as part of a broader discussion that encourages further investigation. In other words, if the standard doesn´t define a particular consideration, it is possible that that consideration is not important, and if we want to consider it, we must investigate why that consideration is important instead of pointing out that there are considerations that are not established.
I encourage you to refine the narrative of your article. To ensure that the claims align with the scope of your work without overstating its implications. This adjustment will strengthen the impact and credibility of your manuscript.
Author Response
Dear Reviewer 1,
First of all, the authors would like to thank you for your comments that helped to improve this manuscript. Your guidelines were extremely helpful. Thank you. In the revised manuscript, content modifications made in line with your comments are highlighted in yellow. You may consider the manuscript to be technically correct now. Specific responses to your comments are as follows.
General comments of Reviewer 1: “Dear colleagues, Your work is of potential interest. I must confess that the idea of purchasing commercial devices and making some measurements doesn’t sound very scientific, but more from the engineering or the educational point of view. Still, the study is interesting for the educational and academic field. I will provide some comments which may help you improve it.”
Response to the general comments of Reviewer 1: The authors would like to thank Reviewer 1 for highlighting the engineering and educational significance of the results obtained in this study. The authors responsibly claim that this study, in addition to engineering and educational contributions, also contains relevant scientific contributions. This may not have been adequately highlighted in the first version of the manuscript, but it is in the revised version. Thank you for the time you have allocated for evaluating and discussing our manuscript.
Comments 1 of Reviewer 1: “The introduction is a bit long. There is always more information that may be included, but there are topics that don´t need to be covered, for example, the paragraph “The fundamental component of a PV system is a PV cell…”. Sounds that may be of interest to people outside the field of inverter design and tests.”
Response to Comments 1 of Reviewer 1: In this regard, the introduction has been shortened as requested. Specifically, the third paragraph of the previous introduction and References [6-8] have been removed. Accordingly, the ordinal numbers of all the references have been modified consistently.
Comments 2 of Reviewer 1: “First of all, please avoid non-conservative statements that make the article sound even less scientific, like: “These inverters … generate significant harmonic distortions”. I am not sure we can say inverters generate significant harmonic distortions. It depends on its design. They are designed to pass the regulation tests and to be as cheap as possible. Designers may make better inverters, and they do it whenever regulations or tests change, and they actually design inverters (for example, in China) for different countries. Inverters for countries with fewer regulations (for example, developing countries) are cheaper, but they cannot be sold in countries with more strict regulations (for example, developed countries).”
Response to Comments 2 of Reviewer 1: The sentence that was singled out here as non-conservative has been modified as follows: “These inverters represent low-inertia sources in the utility grid, which, depending on the inverter design, control strategy, and output filters, generate certain harmonic distortions.” In addition, a more conservative style of presentation with more precise statements has been used when revising the manuscript.
Comments 3 of Reviewer 1: “Another example is, “One key finding was that the PV inverters generate significant current distortion, and the total harmonic distortion of the current exhibits more significant variations during the transitions from sunrise to sunset.” maybe instead of saying “significant”, you can provide data like how much is significant. I am not saying it is not true; it just doesn´t sound scientific. Furthermore, your tests look like the hard start of an inverter; the hard start happens in one second, while the MPPT happens with slow changes during the hall day; the change from the maximum power to the shutdown taker takes hours, not seconds.”
Response to Comments 3 of Reviewer 1: The sentence that was singled out as another example of a non-conservative statement has been modified as follows: “One of the key findings was that PV inverters generate a total harmonic distortion in the output current ranging from 2.94% to 327.67%.” Furthermore, the manuscript has been carefully reviewed and all non-conservative sentences have been identified and modified as requested.
Comments 4 of Reviewer 1: “You mentioned some relevant standards; it would be better also if you could describe the way they test the standard in different countries; then, you may find out if you are doing the same. Finally, you are suggesting adjusting the standard based on your study; I would avoid that along with the other non-conservative statements in the full article. Standards are established and adjusted based on significant considerations such as grid safety, environmental impacts, and the minimization of externalities like electrical disturbances or pollutant emissions. A measurement or the observation that a specific inverter may not fully comply with certain standards (particularly if you are not doing the test by the regulatory authority) does not provide sufficient grounds for modifying these regulations. I would accept if such findings should be positioned as part of a broader discussion that encourages further investigation. In other words, if the standard doesn´t define a particular consideration, it is possible that that consideration is not important, and if we want to consider it, we must investigate why that consideration is important instead of pointing out that there are considerations that are not established.”
Response to Comments 4 of Reviewer 1: The authors would like to thank Reviewer 1 for these useful remarks. The importance of standards and the fact that they are established on the basis of many years of practice and careful considerations are inevitable. That is why the relevant standards were used as a starting point in this research. Regardless of the results of this research and the conclusions drawn, the standards have changed and will continue to change. Therefore, it is reasonable to believe that standards can be changed for practical, economic, engineering, or legal reasons, as well as for scientific ones. All this served as the basis for the conducted research. The obtained results are quantitative, objective, and repeatable. The method used is described in detail and precisely, while the discussion of the results is based on concrete magnitudes. The authors hope that now the scientific validity of the applied method is achieved and can be more easily recognized. In this regard, the last sentences of the abstract and the conclusion have been modified in an appropriate way. The last sentence of the abstract, namely: “These results suggest that the standard should be amended to specify conditions under which the output current distortion is measured." has been replaced by the following sentence: "The presented results suggest that further research on the dependence of the output current distortion from PV inverters on their input power and voltage is needed.” In addition to this, the last sentence of the conclusion, specifically: "The results presented in this paper suggest that the standard should specify the conditions, primarily the input voltage and power, under which the power quality parameters prescribed by the standard will be measured." has been replaced by the following sentence: " The results presented in this paper suggest that a further work is needed to encourage additional research on the dependence existing between the output current distortions from different PV inverters and the associated input powers and voltages."
Comments 5 of Reviewer 1: “I encourage you to refine the narrative of your article. To ensure that the claims align with the scope of your work without overstating its implications. This adjustment will strengthen the impact and credibility of your manuscript.”
Response to Comments 5 of Reviewer 1: In connection with this comment, the authors have attempted to improve the narrative throughout the manuscript to the best of their own linguistic ability.
Sincerely Yours,
Prof. Dr. Dardan Klimenta
On behalf of all the authors.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript is well elaborated with clear objectives.
The findings may impact future normative requirements on inverters for PV.
A few comments are applicable:
Line 28, where it is written: "the growing energy production, which relies on fossil fuels.", substitute by:
"anthropogenic greenhouse gas emissions from fossil fuels".
Line 30, where it is written: "find new and cleaner energy sources.", substitute by: "expand renewable energy". Reason: PV is not longer new, it is a very mature technology.
Line 37, where it is written: "optional", substitute by: "battery".
Line 84, delete: "and aimed".
Line 409, clarify in the manuscript if low solar irradiance would produce less current distortion on the inverter if backed up by a battery in a DC-coupled system. Clarify if there should be two classes of PV inverters depending on the application: inverters for AC-coupled systems with low current distortion, and inverters for DC-coupled systems supported by batteries, allowing higher current distortion.
Author Response
Dear Reviewer 2,
First of all, the authors would like to thank you for your comments that helped to improve this manuscript. Your guidelines were extremely helpful. Thank you. In the revised manuscript, content modifications made in line with your comments are highlighted in green. You may consider the manuscript to be technically correct now. Specific responses to your comments are as follows.
General comments of Reviewer 2: “The manuscript is well elaborated with clear objectives. The findings may impact future normative requirements on inverters for PV. A few comments are applicable:”
Response to the general comments of Reviewer 2: The authors would like to thank Reviewer 2 for highlighting the significance of the results obtained in this study. Thank you for the time you have allocated for evaluating and discussing our manuscript.
Comment 1 of Reviewer 2: “Line 28, where it is written: "the growing energy production, which relies on fossil fuels.", substitute by: "anthropogenic greenhouse gas emissions from fossil fuels".”
Response to Comment 1 of Reviewer 2: In the first paragraph of Section 1, the sentence "Climate change is primarily caused by the growing energy production, which relies on fossil fuels." has been replaced by the following sentence: "Climate change is primarily caused by anthropogenic greenhouse gas emissions from fossil fuels.".
Comment 2 of Reviewer 2: “Line 30, where it is written: "find new and cleaner energy sources.", substitute by: "expand renewable energy". Reason: PV is not longer new, it is a very mature technology. ”
Response to Comment 2 of Reviewer 2: In the first paragraph of Section 1, the sentence "As a result, significant efforts are being made to find new and cleaner energy sources." has been replaced by the following sentence: "As a result, significant efforts are being made to expand the use of renewable energy sources.".
Comment 3 of Reviewer 2: “Line 37, where it is written: "optional", substitute by: "battery". ”
Response to Comment 3 of Reviewer 2: In the second paragraph of Section 1, the sentence "A typical photovoltaic (PV) system includes a PV array, a DC/AC converter, called also PV inverter, and optional energy storage." has been replaced by the following sentence: "A typical photovoltaic (PV) system includes a PV array, a DC/AC converter, called also PV inverter, and a battery energy storage system.".
Comment 4 of Reviewer 2: “Line 84, delete: "and aimed".”
Response to Comment 4 of Reviewer 2: In the sixth paragraph of Section 1, the sentence "The model was developed using MATLAB/Simulink and aimed to investigate power quality when the microgrid operates connected to the conventional grid." has been replaced by the following sentence: "The model was developed in MATLAB/Simulink to investigate power quality when the microgrid operates connected to the conventional grid.". In the revised version of the manuscript, the aforementioned paragraph is the fifth in order.
Comment 5 of Reviewer 2: “Line 409, clarify in the manuscript if low solar irradiance would produce less current distortion on the inverter if backed up by a battery in a DC-coupled system. Clarify if there should be two classes of PV inverters depending on the application: inverters for AC-coupled systems with low current distortion, and inverters for DC-coupled systems supported by batteries, allowing higher current distortion.”
Response to Comment 5 of Reviewer 2: In connection with Comment 5, the last paragraph of Section 5 which read: "It is a trivial fact that solar irradiance depends on the time of day, the day of the year, and variable climatic factors. For these reasons, for a significant part of the operating time of the PV inverter, the input power will be less than the nominal one. Since the distortion of the output current of the PV inverter depends on the input power, which is the main conclusion of this research, it is expected that the current distortion will be different in various time periods. This assumption indicates the need for further research in this direction."
has been replaced by the following paragraph:
“In the DC-coupled configuration, if a battery does not back up the PV inverter, the input power is only supplied from the PV array, as in the measurements conducted. It is a trivial fact that solar irradiance depends on the time of day, the day of the year, and variable climatic factors. For these reasons, for a significant part of the operating time of the PV inverter, the input power is less than the nominal one. Since the distortion of the output current from the PV inverter depends on its input power, which is the main conclusion of this research, it is expected that the current distortion differs in various periods. Moreover, lower solar irradiance is expected to produce less current distortion from the PV inverter if it is backed up by a battery in a DC-coupled configuration, because the battery compensates for the input power deficit. In addition to the DC-coupled configuration, there is also an AC-coupled configuration. For both coupling configurations, during the one-day operation, the battery charging and discharging periods can be distinguished [36]. In the case of battery discharge, the input power of the PV inverter is equal to the total power of the PV array and the battery for the DC-coupled configuration, or only to the PV array power for an AC-coupled configuration [36]. In the case of battery charging, the input power of the PV inverter is the PV array power [36]. Since the distortion in the output current from each coupling configuration depends on the input power of the PV inverter, it is obvious that the current distortion must differ in various operating intervals. The observations presented here indicate the need for further research in this direction.”
Specifically, the last paragraph of Section 5 has been modified and expanded in accordance with Comment 5. To confirm the validity of the given clarification, Reference [36] has also been used in this paragraph. Reference [36] is as follows: 36. Sandelic, M.; Sangwongwanich, A.; Blaabjerg, F. Reliability evaluation of PV systems with integrated battery energy storage systems: DC-coupled and AC-coupled configurations. Electronics 2019, 8(9), 1059. https://doi.org/10.3390/electronics809105
Sincerely Yours,
Prof. Dr. Dardan Klimenta
On behalf of all the authors.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsMy comments have been addressed. Thank you.
