Multi-Input Ćuk-Derived Buck-Boost Voltage Source Inverter for Photovoltaic Systems in Microgrid Applications

Round 1

Reviewer 1 Report

The paper is well organized, the objective is of high importance, and a proposed solution is useful. Good experimental results as well. 

Comments for author File: Comments.pdf

Author Response

Reviewer #1

The authors provide sufficient simulation and clear experimental results. However, it was written in Abstract: “The numerical simulations are presented together with experimental results showing a good agreement” – there wasn’t presented comparison between the simulation and the experiment. The comparison was between the proposed converter and the existed ZSI, and the propose converter did not prevail in all outcomes. I would suggest improving the last parts of the paper by detailed explanations of the simulation and experimental results.

Response:  The authors would like to thank the reviewer for his valuable comments. The manuscript has been modified and a Discussion Section has been added providing detailed information about the simulation and experimental results.

From the numerical simulations and the experimental results in the previous sections, it can be observed a good matching between the waveforms of the capacitor voltage, load current and voltage. For example, the capacitor voltage is regulated to a peak voltage of 380 V in the numerical simulations as depicted in Fig. 9-10  and on the other hand in the experimental results shown in Fig. 14(b) and Fig. 16(a). Similarity, for the shadow effect on the PV power of one PV module, it can be observed that the other two inputs have no effect as shown in the numerical simulations shown in Fig. 11. The same apply to the experimental results shown in Fig. 17(a) .

Reviewer 2 Report

The figure shown in Fig. 1a     does not appear to be the same with the figure in ref [23].

The rearranged figure of Fig.     1b is not the same as Fig. 1a. Can the authors explain how the     re-arragnement proposed in Fig. 1b is the same with the other circuit?

The control of the proposed     converter should be explained. How is control of the capacitor voltage     achieved and which converter controls that voltage?

The proper function of the     MPPT should be demonstrated. A reduction in the sun leads to lower power     but how is actual operation at the MPP demonstrated in this work?

Is the grid frequency 50Hz or     60Hz?

The input power appears to     vary with a second harmonic. This would not be the case in a converter     with two stage tracking, or in fact any three phase converter. The authors     should explain that operation of the proposed converter.

A complete diagram of the     proposed control implementation should be provided. The gains for the     controllers used should also be provided.

Under partial shadow, the     currents of one module vary significantly, while the other two are almost     constant. Can the authors explain this?

The inductor currents should     be provided.

How is the dc voltage     decided. Why is it 380V in one case and 370V in the second case?

A photo of the experiment     must be provided.

In Fig. 14, the currents are     not properly balanced in the three phases and the results are clipped. The     figure must be revised.

The injected current from the     FFT of Fig. 15 has 8% dc component. This is also seen in the simulation     results and is not acceptable as most codes require a maximum dc current     of 0.5%. This should be explained and addressed.

Table 2, the inductor value     should be in mH, not mF.

Author Response

See attached PDF file 

Author Response File: Author Response.pdf

Reviewer 3 Report

See the attached review file

Comments for author File: Comments.pdf

Author Response

Response to Reviewer #3

1. In the introductory section, the main objectives of the work should be more clearly formulated
in response to the critical analysis of the current research stage, going through the next path: the
disadvantages of the existing systems à the research opportunity à the novel approach. In the same idea, the authors should refer more broadly to their published studies in the field, and
highlight the elements that differentiate the current work from the previous works.

Response:  Thanks for the reviewer for his comment and for providing interesting guidelines. The main objectives were formulated according to the provided path.

Regarding the published studies, a conference paper has been proposed on the performance analysis of single-input three-phase buck-boost inverter in [a]. However, a consistent performance evaluation of the Ćuk-derived BBVSI is still missing and requires and an investigation of the system behavior, especially, when operated as multi-input inverter. In addition, in our previous paper shown below presented in an IEEE conference, very few simulation results were presented without any experimental validation.

 E. Abdeen, M. A. Gaafar and M. Orabi, "Performance Analysis for Single-Stage Buck-Boost Inverter," 2019 International Conference on Innovative Trends in Computer Engineering (ITCE), IEEE. Aswan, Egypt, 2019, pp. 587-592

One of the important points that is still required more research is the increase of the voltage gain on multi-input three-phase inverters, so single PV module can be used from obtaining required output grid voltage.

2. The research methodology must be more clearly described; a schematic representation of the
research workflow could clarify the approach.

Response: Thank you very much for your comment. The research methodology followed in our paper starts from mathematical modeling, analysis and design, numerical simulations and finally experimentally validating the results. This has been summarized in the introduction.

3. The quality of some figures should be improved, mainly in terms of text/notation clarity - size.

Response:  Thank you very much for your suggestion. The quality of some figures has been improved as requested by the reviewer.

4. All the parameters used in the equations must be explained with the first use; check the entire
work in this regard.

Response:  Thank you for your comment. The parameters for the equations have been illustrated and explained in the manuscript just when they are first used.

5. The information presented in the 2nd and 3rd sections of the work is partly known from the
scientific literature, so that they should be presented in a more synthetic form (in addition, the
two sections could be integrated into a single one).

Response:  Thank you very much for your comment and suggestion. Section 2 mainly describes the basic operation of single input operation for clarity and simplicity of explanation. This is followed by the description of the operation of multi-input operation in Section 3. Sections 2 and 3 have been updated and merged in one section (Section 2) as requested by the reviewer. 

6. Indicate the measures units for the variation diagrams in all the figures (some of them are
missing).

Response: Thank you very much for your suggestion. The measures units in all the figures have been indicated. See below:

7. The conclusions section should be extended and improved by a more detailed discussion on the research findings, and providing future research directions in the field, in terms of research
opportunities opened by this work.

Response:  Thank you very much for your suggestion. A Discussion section were added where detailed information about the numerical simulation and experimental results is provided. Future Research was also proposed.

Round 2

Reviewer 2 Report

Thank you for the answers to the comments.

The derivation of the     three-phase circuit from the original single boost converter is still not     explained. The original converter shares the dc side of the system but in     your topology each phase is connected independently. This variation is not     explained in detail and is not trivial to the system.

Your experimental results     still remain from an RL load and not from the grid connected system of the     simulations.

Some more explanations on how     the M = D limitation applies to the converter and what it means for the     dc-side of the converter when connecting to the grid.

Can the authors provide the     capacitor current and the voltages of the PV modules in the simulation     results. I would also suggest presenting a figure with the individual PV     curves to show tracking of the MPPT.

Can the authors show (even in     the letter) the response of the converter for a change in the dc voltage     reference, for example from 380V to 420V.

Is 10kHz the switching     frequency of both the dc-dc converter and the inverter?

Author Response

Please see the attached file.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Thank you for the answers to the comments.

One minor question - given the variations in the PV voltage that you show in your current review, how exactly is MPPT achieved. These voltage waveforms differ substantially from what actual MPPT voltages look like in PV inverters.

Author Response

Thank you for the answers to the comments.

One minor question - given the variations in the PV voltage that you show in your current review, how exactly is MPPT achieved. These voltage waveforms differ substantially from what actual MPPT voltages look like in PV inverters.

Reply: Thank you very much for your efforts and comments to improve the readability of this paper.

We have added the following text  to make clear how the used MPPT controller Works. Please see red text from line 203 to line 212.

"The waveforms of the PV voltage strongly depend on the type of the MPPT controller used and the values of its parameters. If we restrict ourselves to a P&O MPPT controller, which is the one used in this paper, many variants of this algorithm exist. Namely, the output of the MPPT block could be a current reference that should be tracked by the PV current, a voltage reference that should be tracked by the PV voltage or it could be the duty cycle that must be directly applied to the converter. In the first two cases, PI compensators are used to process the error between the controlled variables and their references. In the third case, the duty cycle is applied without any compensation scheme.  Since the focus of the paper is not on this particular aspect, the simplest P&O MPPT controller providing directly the duty cycle is used in this paper. "