Leakage Current Reduction in Single-Phase Grid-Connected Inverters—A Review

Round 1

Reviewer 1 Report

This is a review paper on leakage current reduction in single-phase grid-connected inverters. The paper is well written and suggested changes are:

1. pg 3 Please explain VDE-AR-N 4105 and VDE-0126-1-1
2. pg.4 line 124, 35% of their nominal voltage, please state where is this figure come form? Is this figure country specific?
3.  pg 10 and 11, the diagram is too small to see, consider redraw them.
4.  please check equation2 to make sure all the symbols used are correct
5. Conclusion is not written well, perhaps need to rewritten

Author Response

Reviewer 1. This is a review paper on leakage current reduction in single-phase grid-connected inverters. The paper is well written and suggested changes are:

• pg 3 Please explain VDE-AR-N 4105 and VDE-0126-1-1

Thank you for your comments. We agree with your statement. In the new version, the indicated sentence was changed to: “The VDE-AR-N 4105 and VDE 0126-1-1 standards establish the rules for grid-connected systems”. This sentence is a preamble to Section 2.1 where each standard is explained in detail.

• 4 line 124, 35% of their nominal voltage, please state where is this figure come form? Is this figure country specific?

Thank you for your comments. We agree with your statement. The sentence was rewritten as follows: “Generally, there are protections that disconnect the system if its voltage supply is reduced by 35% of its nominal value.”

• pg 10 and 11, the diagram is too small to see, consider redraw them.

Thank you for your comments. We agree with your statement. The diagrams were drawn for best quality, each figure was enlarged, including Figure 12.

• please check equation2 to make sure all the symbols used are correct

Thank you for your comments. The meaning of V_PV and P_PV was included, and the correct writing of the equation was corroborated from [44].

“Where: ∆v is the voltage ripple, P_MPP and V_MPP are the power and voltage at maximum power point (MPP), P_PV and V_PV are the power and nominal voltage of the panel, α and β are the coefficients which describe the second–order Taylor approximation of the current and K_PV is the utilization ratio which is given from the rate of average generated power to the theoretical MPP power.”

• Conclusion is not written well, perhaps need to rewritten

A: Thank you for your comments. We agree with your statement. The conclusions were rewritten to highlight the contribution of the work in a clearer way for the reader.

Reviewer 2 Report

The paper require extensive modifications to be published

• The abstract does not contain any numerical values.
• Abstract should be numerically quantized  as a scientific paper. 
• No actual experimental proof was shown in this paper.
• No meaningful conclusions are presented in this paper.

Author Response

Reviewer 2. The paper require extensive modifications to be published.

• The abstract does not contain any numerical values.
• Abstract should be numerically quantized  as a scientific paper. 
• No actual experimental proof was shown in this paper.
• No meaningful conclusions are presented in this paper.

We appreciate your comments in order to enrich the scientific content of our work. We consider not including numerical values in the abstract since it is a review of several topologies. Experimental results are not included for the same reason. Finally, the conclusions were rewritten.

The paper presents a general review of the state-of-the-art of grid-connected inverters with leakage current reduction. Moreover, the main standards of the PV modules and inverters are presented. The behavior of the CMV, its origin and effect in transformerless grid-connected inverters are analyzed. Also, a comparative analysis of the most common topologies is performed. Finally, the main challenges and research trends within this topic are highlighted. Hopefully, this review will lead to increased efforts in research and investigation of leakage current reduction in transformerless inverters.

Reviewer 3 Report

The paper presents a review of the state of the art of grid connected transformer less PV inverters for leakage current reduction.

The main strength of the paper is the fact that the paper summaries and describes the standards that must be met by the devices that inject power into the grid. The standards are separated by stage. The paper goes next to comparing and analyzing most used PV topologies in relation to the leakage current generated by each.

The main weakness of the paper is that the authors do not described in what inverters the selected topologies are used (for example it is known that H5 topology is used in Sunny Boy inverter family from SMA). This information would improve the quality of the paper validating the importance of the selected topologies. Also would justify the importance of choosing different topologies in future projects.

The paper is well written and easy to follow with a very clear train of thought. Though small mistakes where found:

1. Figure 1 is unproportionate. For example the green rectangle is less than half the grey rectangle but it represents 2228 GW from the total of 2378 GW.
2. In Figure 2 the Multilevel category should be connected to one of the above categories (maybe voltage source)
3. The phrase “In this scheme, two crucial elements are raised…” should be reformulated to “In this scheme, two crucial issues are raised…”
4. The reviewer could not understand the meaning of the phrase “Generally, coils that disconnect the system are used in their supply voltage is reduced to a value below 35% of their nominal voltage.” The phrase should be reformulated.
5. Equation 1 has a typo (d²P_PV)
6. Equation 3 has a typo (sen(wt) should be sin(wt))
7. In Figure 6 V_CM-DM is not defined.

Author Response

Reviewer 3. The paper presents a review of the state of the art of grid connected transformer less PV inverters for leakage current reduction.

The main strength of the paper is the fact that the paper summaries and describes the standards that must be met by the devices that inject power into the grid. The standards are separated by stage. The paper goes next to comparing and analyzing most used PV topologies in relation to the leakage current generated by each.

The main weakness of the paper is that the authors do not described in what inverters the selected topologies are used (for example it is known that H5 topology is used in Sunny Boy inverter family from SMA). This information would improve the quality of the paper validating the importance of the selected topologies. Also would justify the importance of choosing different topologies in future projects.

Thanks for your suggestions. As part of an investigation derived from the current work, it is intended to delve further into specific applications for each topology. However, we did not intend to include it in the present work in order to focus only on the technical aspects.

The paper is well written and easy to follow with a very clear train of thought. Though small mistakes where found:

• Figure 1 is unproportionate. For example the green rectangle is less than half the grey rectangle but it represents 2228 GW from the total of 2378 GW.

Thank you for your comments. We agree with your statement. Figure 1 was redesigned, with a better proportion of its indicators.

• In Figure 2 the Multilevel category should be connected to one of the above categories (may be voltage source).

Thank you for your comments. We agree with your statement. Indeed, Figure 2 was corrected.

• The phrase “In this scheme, two crucial elements are raised…” should be reformulated to “In this scheme, two crucial issues are raised…”

Thank you for your comments. The text was changed as you suggested.

“Figure 4 shows a general connection scheme for grid-connected PV systems. In this scheme, two crucial issues are raised, the minimum voltage control and the frequency at the grid connection point. Generally, there are protections systems that disconnect the system if its voltage supply is reduced by 35% of its nominal value.”

• The reviewer could not understand the meaning of the phrase “Generally, coils that disconnect the system are used in their supply voltage is reduced to a value below 35% of their nominal voltage.” The phrase should be reformulated.

Thank you for your comments. The paragraph was rewritten as shown in the previous point.

• Equation 1 has a typo (d²P_PV)

Thank you for your comments. Equation 1 was corrected.

• Equation 3 has a typo (sen(wt) should be sin(wt))

Thank you for your comments. Equation 3 was corrected.

• In Figure 6 V_CM-DM is not defined.

Thank you for your comments. The term was defined on line 176 as follows:

“Where: V_CM−DM indicates the influence of the differential mode voltage to the common-mode voltage”

Round 2

Reviewer 1 Report

All the changes are implemented based on the reviewer's comments, so no further changes are required.

Author Response

Reviewer 1. All the changes are implemented based on the reviewer's comments, so no further changes are required.

Thank you, we appreciate your comments.

Reviewer 2 Report

The paper has been modified up to some extent. The following modifications are required further.

• Numerical values should be added to abstract.
• Conclusion is not suumary of work. It should represent findings.
• Conclusion should be modified by referring some good journals

Author Response

Reviewer 2. The paper has been modified up to some extent. The following modifications are required further.

• Numerical values should be added to abstract.

We appreciate your comments to enrich the scientific content of our work. Numerical values were included in the abstract. In this way, the reader will be able to contextualize the ideas that are addressed in our work. For example:

…It is estimated that 21% of the global electricity generation capacity from renewable sources is supplied by photovoltaic systems.

This idea highlights the importance of studying photovoltaic panels today. It provides a statistic about the amount of energy generated by these systems.

...However, the connection standards for photovoltaic inverters establish a maximum total harmonic distortion of 5%.

The asseveration notes the obligation of PV systems to comply with specific values for connection to the grid.

... It is important to mention that analyzed topologies present a variation of the leakage current between 0 to 180 mA.

The sentence gives the reader a clear overview of the leakage current values of the analyzed topologies, allowing a straightforward comparison with the maximum leakage current allowed (300 mA).

• Conclusion is not summary of work. It should represent findings.

Thank you, we appreciate your comments. The conclusions were modified. The last three paragraphs were rewritten to give the reader clearer ideas.

…The rise in renewable energy has caused an increase in the use of inverters. These devices are used as an interface between the power source and the grid. Moreover, it is common the use of transformers to have electrical isolation between the input and output. However, these elements are expensive, bulky, heavy, and have magnetic losses. Transformerless inverters have been developed to avoid the aforementioned limitations. One of the main drawback in this type of topologies is the presence of a leakage current between the terminals of the photovoltaic cell and the physical ground of the grid.

The paper presents a general review of the state-of-the-art of grid-connected inverters with leakage current reduction. Moreover, the main standards of the PV modules and inverters are presented. The behavior of the CMV, its origin and effect in transformerless grid-connected inverters are analyzed. Also, a comparative analysis of the most common topologies is performed. Finally, the main challenges and research trends within this topic are highlighted. Hopefully, this review will lead to increased efforts in research and investigation of leakage current reduction in transformerless inverters.

In this paper, it is concluded that grid-connected systems have to comply with specific standards for each region or country. A THD less than 5\% and a leakage current less than 300 mA are some of the most important standards to consider. Moreover, the output of the PV requires particular attention, since it presents a voltage ripple that do not damage the PV cell, but it reduces the available power. Furthermore, the unbalance in the inductors of the output filter increases the leakage current. The unbalance in the inductors is due to a dispersion in the capacity, aging rate and temperature behavior of the output inductors. These factors must be considered in the design of the controller to mitigate the effects of this undesirable phenomenon.

Within the analyzed topologies, it should be noted that the H5 topology shows a good compromise in terms of reduction in the leakage current, number of components, and current quality. This topology and other schemes, connect the negative terminal of the PV to the neutral point of the grid using passive components and switches. Furthermore, another commonly used strategy is the disconnection of the AC side and the PV during free wheeling times of inductors in current-source inverters.

Finally, it was highlighted that the modulation strategy is a critical factor in reducing the leakage current. Bipolar modulations offer the greatest reduction of this current in PV applications. However, its high complexity makes it difficult to use. Otherwise, the unipolar modulation strategy is widely used for its simplicity. In the works reviewed in this paper, only one of the seventeen articles presented the bipolar modulation.   

• Conclusion should be modified by referring some good journals.

Thank you very much for your feedback. In the conclusion section, we did not attempt to include references to establish our statements and close the paper by giving to the reader our expectations based on the state-of-the-art of the previous sections. Then, in other sections, the reviews are cited that can complement the information of the work. For example:

…In [58] it is presented a comparative table, which also highlights the maximum average current of the switches and the overall efficiency of the system. The research [111] also shows a study with structures similar to transformerless single-phase single-stage grid-tied flying inductor inverters.”

[58] Vosoughi, N.; Hosseini, S.H.; Sabahi, M. Single-phase common-grounded transformer-less grid-tied inverter for PV application. IET Power Electronics 2020, 13, 157–167.

Azary, M.T.; Sabahi, M.; Babaei, E.; Meinagh, F.A.A. Modified single-phase single-stage grid-tied flying inductor inverter with MPPT and suppressed leakage current. IEEE Transactions on Industrial Electronics 631 2017, 65, 221–231.

Additionally, we included 66 references from prestigious journals, displayed as:

20-IEEE Transactions on Power Electronics

22-IEEE Transactions on Industrial Electronics

7-IET Power Electronics

4-IEEE transactions on industry applications

1-IEEE Transactions on Applied Superconductivity

1-IEEE Transactions on Circuits and Systems II: Express Briefs

1-IEEE Transactions on Electromagnetic Compatibility

7-Energies MDPI

4-Electronics MDPI

1-Sustainability MDPI

1-Applied Sciences MDPI

1-Journal of Energy and Power Engineering

8-IEEE Access

Reviewer 3 Report

I have no further comments to this paper

Author Response

Reviewer 3. I have no further comments to this paper.

Thank you, we appreciate your comments.

Round 3

Reviewer 2 Report

Authors have modified the paper and  now it is ready to publish.