Modeling and Validation of the Switching Techniques Applied to Back-to-Back Power Converter Connected to a DFIG-Based Wind Turbine for Harmonic Analysis

Round 1

Reviewer 1 Report

Authors can find a review of their article in the attached file.

Comments for author File: Comments.pdf

Author Response

Thank you very much for your general comments about this work.

Author Response File: Author Response.pdf

Reviewer 2 Report

It is a well organised paper regarding three switching techniques for a low power DFIG inverter system. Models have been well analysed and the implementation methods have been well elaborated. This work is a very good comparative study reference for engineers and researchers in the practical designs.

The main issues with the work is it is lack of new contributions - all are known methodologies and similar studies have been done previously. The low power level of the experiment has limited the complexities and challenges of real-world DFIMs.

If possible, it is recommended to have colour printed waveforms in the simulation and measurement results. The THD components in three schemes can be drawn in a bar chart which would be much convenient for getting the comparison ideas.

Author Response

We have added a couple of contributions from our work (this information has been inserted into Page 4, line 158–163 of the document) that differ from other research works, mainly the contribution of validating the results obtained from the proposed model with the results obtained from the experimental prototype at low power. It is worth mentioning that most of the works reported in the literature contain limited information about the aspects necessary for the interconnection of the DFIG with the electrical grid, its synchronization elements as well as the analysis of the results. Furthermore, this model can be extrapolated at higher powers, for example: 10, 50, 500 and up to 2250 kW.

The low power level of the experiment has limited the complexities and challenges of real–world DFIM

We agree with your observation. For this reason, we decided to increase the power of the DFIG from 3 kW to 1.6 MW, since this power is characteristic of large wind turbines in different wind farms around the world. We have obtained the generator, converter and electrical grid parameters to carry out the pertinent simulation. The results of the proposed model in steady state provide important and interesting information about the harmonic and inter–harmonic content in the wind turbine output currents, which are injected into the electrical grid. Unfortunately, this case study was only carried out without the validation of an experimental prototype because we do not have such large machines in our laboratory. This information has been inserted into Page 23–26 of the document.

It is recommended to have color printed waveforms in the simulation and measurement results.

We have added colored figures to better recognize the results of the proposed analysis. This information has been inserted into Page 25, line 820 of the document.

The THD components in three schemes can be drawn in a bar chart which would be much convenient for getting the comparison ideas

Thank you very much for your recommendation. This information has been inserted into Page 25, line 820 of the document.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper presents the analysis of the harmonic frequencies generated by the DFIG interfacing with back-to-back power converters. The topic is not new. Below are points of weakness.

1) What's the contribution of this paper?

2) English needs to be polished.

3) Both Figure 15 and Figure 19 are rotor voltage, what's the difference?

Author Response

What's the contribution of this paper?

We have added a couple of contributions from our work (this information has been inserted into Page 4, line 158–163 of the document) that differ from other research works, mainly the contribution of validating the results obtained from the proposed model with the results obtained from the experimental prototype at low power. It is worth mentioning that most of the works reported in the literature contain limited information about the aspects necessary for the interconnection of the DFIG with the electrical grid, its synchronization elements as well as the analysis of the results. Furthermore, this model can be extrapolated at higher powers, for example: 10, 50, 500 and up to 2250 kW. In addition, the results of the different study cases are validated by means of a low-power experimental prototype resulting in a clear and concise model of the harmonic and inter–harmonic analysis of a DFIG connected to the electrical grid (this information has been inserted into Page 23–26 of the document).

English needs to be polished

Thank you very much for your comments. The entire document has been revised again and several grammars and spelling corrections have been made.

Both Figure 15 and Figure 19 are rotor voltage, what's the difference?

This error has been corrected. Thanks for pointing it out.

Author Response File: Author Response.pdf

Reviewer 4 Report

In this article a model of a DFIG connected to the B2B power converter is proposed to which different switching techniques are implemented for inter–harmonic propagation studies. The topic is interesting, and the writing is well. However, several problems should be solved. As said in this paper, the harmonic currents in the rotor appear at the following frequencies: f=(6h+/-1)sf. Please explain the difference between this frequency and mirror coupling frequency, which is reported through soc-based droop coefficients stability region analysis of the battery for stand-alone supply systems with constant power loads. Comparison simulation results should be added in the revised version.

Author Response

Some problems should be solved. As said in this paper, the harmonic currents in the rotor appear at the following frequencies: f=(6h+/-1)sf. Please explain the difference between this frequency and mirror coupling frequency, which is reported through soc-based droop coefficients stability region analysis of the battery for stand alone supply systems with constant power loads.

The difference between the DFIG currents at harmonic frequencies and the mirror coupling frequency is that the latter applies a type criterion to assess the stability of the power electronics converter with the bidirectional power flow. Now, to deal with the mirror frequency coupled characteristic, 2 methods are proposed: the apparent impedance analysis method and the harmonic stability analysis method which are outside the scope of this study. Furthermore, the Generalized Nyquist Criterion (GNC) method is complex and inappropriate for the design of the AC system in this study. Although, if you want to solve this problem, it is advisable to use several criteria of forbidden region to eliminate the tedious process of drawing the generalized Nyquist curve in turn and thus be able to solve other problems such as artificial conservatism and the subsystem stability and dynamic characteristics of the single inverter itself [1–5];

1. Rygg and M. Molinas, “Apparent Impedance Analysis: A Small-Signal Method for Stability Analysis of Power Electronic-Based Systems,” IEEE J. Emerg. Sel. Topics Power Electron., vol. 5, no. 4, pp. 1474-1486, Dec. 2017.

2. X. Wang and F. Blaabjerg, “Harmonic Stability in Power Electronic- Based Power Systems: Concept, Modeling, and Analysis,” IEEE Trans. Smart Grid, vol. 10, no. 3, pp. 2858-2870, May 2019.

3. Z. Liu, J. Liu, W. Bao and Y. Zhao, “Infinity-Norm of Impedance-Based Stability Criterion for Three-Phase AC Distributed Power Systems with Constant Power Loads,” IEEE Trans. Power Electron., vol. 30, no. 6, pp. 3030-3043, June 2015.

4. A. Riccobono and E. Santi, “Comprehensive Review of Stability Criteria for DC Power Distribution Systems,” IEEE Trans. Ind. Appl., vol. 50, no. 5, pp. 3525-3535, Sept.-Oct. 2014.

5. X. Li, X. Ruan, Q. Jin, M. Sha and C. K. Tse, “Small-Signal Models with Extended Frequency Range for DC-DC Converters with Large Modulation Ripple Amplitude,” IEEE Trans. Power Electron., vol. 33, no. 9, pp. 8151-8163, Sept. 2018.

Comparison simulation results should be added in the revised version.

A Figure has been added that shows the comparison of the different THD indices for each of the switching techniques in the 3 kW–DFIG as well as in the 1.6 MW–DFIG.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The reviewer has no further comments.

Reviewer 4 Report

the comments have been responded