Harmonics Reduction and Reactive Power Injection in Wind Generation Systems

Round 1

Reviewer 1 Report

• Equation (3).  Regrading transformations, e.g. from line to dq-components, refering to IEC 62428 "Modal components in AC systems" might be helpful. 
• Line 44-64. The text "can be up to 30%" is misleading because this does not mean a limit. It is true most systems are designed for +- 1/3 which leads to a speed range of 1:2.
• Line 61 - 63. Switching between rated and nominal powers is not understood.
• Line 101-110. Controling active AND reactive power is not a new idea.
• Section 2 Grid-side converter model. Equations (9) - (11) are not understandable and require revision. Fig. 1 is not sufficient as a model. If the intermediate circuit is modeled by a capacitor only, there is no difference between input and output voltage, and a small signal analysis can insofar not be made. (Note that (1) is for the AC filter only.) Definition of the symbols seems to be incomplete. It was noted that in (11) there is a mismatch of dimensions on left and right side.
• Section 3 Harmonic analysis ... Splitting up quantities in (12) - (14) is unusual in literature. Besides, in (13) two different equations are given for the same symbol quantity.

Author Response

1^st Reviewer

Comment 1: Equation (3).  Regarding transformations, e.g. from line to dq-components, refering to IEC 62428 "Modal components in AC systems" might be helpful.

Thank you for the observation, Changes were made to said equations to accurately represent the transformations made to the observed signals

Comment 2: Line 44-64. The text "can be up to 30%" is misleading because this does not mean a limit. It is true most systems are designed for +- 1/3 which leads to a speed range of 1:2. Depending on the manufacturer.

Agree on the misleading data of 30%, by omitting to mention that it depends on the manufacturer, and mentioning this rating may vary in this type of machine having a maximum slip around the 30%. So, the mentioned text lines were changed to:

“This scheme takes advantage on the wide slip range of the machine, which for most manufacturers can be around a 30% above the synchronous speed (hypersynchronous operation) or below the synchronous speed (subsynchronous operation) [2]. The back-to back converter allows the implementation of a vector control scheme to control speed and generated power of the machine using the rotor voltages and currents, these particular characteristics are applied when it is required to inject power through the rotor during subsynchonous operation, or to generate power through the rotor during hypersynchronous operation, where the maximum injected or extracted power is commonly a 30% of the rated power [2].”

Additionally, the next fragment is modified to mention the rating of the converter is proposed as a 30% of the rated power:

“The GSC is part of a “Back-to-back” converter in the wind generation system, it is shown by Figure 1, this system uses a Doubly Fed Induction Generator (DFIG), where the converter is designed with a proposed maximum active power (rated) 30% of the rated power of the DFIG [1], [2].”

 

Comment 3: Line 61 - 63. Switching between rated and nominal powers is not understood.

Agree on the ambiguous use of the term rated and nominal where the GSC should be dimensioned with a maximum active power (rated) of at least 30% the rated power of the DFIG, to maintain a safe margin, and the modified fragment has the following form:

“The GSC is part of a “Back-to-back” converter in the wind generation system, it is shown by Figure 1, this system uses a Doubly Fed Induction Generator (DFIG), where the converter is designed with a proposed maximum active power (rated) 30% of the rated power of the DFIG [1], [2].”

Comment 4: Line 101-110. Controlling active AND reactive power is not a new idea.

Thank you for your question, just to clarify the comment, the term “active power” is not discussed in this paragraph, however, agree on the control of reactive power being an incremental improvement to this type of systems, considering some works where grid voltage is regulated using reactive power, with their proposed control loops using diverse techniques for example droop control, but these schemes are more suitable for microgrids with a reduced number of sources and loads, the environment where a DFIG is used commonly, is as part of a wind farm connected to the main grid where the reactive power control should be coordinated between all of the wind farm. This is why the regulation voltage is left to the grid operator.

Comment 5: Section 2 Grid-side converter model. Equations (9) - (11) are not understandable and require revision. Fig. 1 is not sufficient as a model. If the intermediate circuit is modeled by a capacitor only, there is no difference between input and output voltage, and a small signal analysis can insofar not be made. (Note that (1) is for the AC filter only.) Definition of the symbols seems to be incomplete. It was noted that in (11) there is a mismatch of dimensions on left and right side.

Agree with the reviewer, this particular section had problems when condensing the information to the length of this article. Figure 2 has been added to offer a graphical representation of the DC link model, also a correction in the variables names was done to avoid ambiguity when identifying each one, also, a previous model was mentioned and compared with the presented, to differentiate the model shown. The following fragment was added:

“To model the DC link external loop a relation between the DC side and the grid side is established, according to figure 2 where said power can be approximately the same, when the converter losses are neglected. Previous works use only the capacitor model to estab-lish the DC bus transfer function [10]. However, a transfer function can also be stablished around the permanent regime set point, to obtain a better rejection of perturbations from the load.”

Figure 2. Single phase equivalent model of the GSC and the DC link, including the power relationship.

Comment 6: Section 3 Harmonic analysis Splitting up quantities in (12) - (14) is unusual in literature. Besides, in (13) two different equations are given for the same symbol quantity.

Agree with the comment, the mentioned equations corresponding to the pairs of adjacent harmonics were condensed in a single one using

 

Reviewer 2 Report

 On the 190th line, you should change the pairs of harmonics 6k-1 in 1-6k.  On the next lines 240,249, 250,252,253, some variables should be italicized.  The diagram of Bode is shown by Figure 2 (not by Figure 3). Figure 3 is missing. From Figure 2 all figures must be renumbered.

Author Response

2^nd Reviewer

Comment 1: On the 190th line, you should change the pairs of harmonics 6k-1 in 1-6k.

Agree with the reviewer. The term “pairs of harmonics” was changed to “adjacent harmonics”, grouping the harmonics with the indexes 6k+1 and 6k-1 located on the sides of every harmonic multiple of six (6k)

Comment 2: On the next lines 240,249, 250,252,253, some variables should be italicized.

The issue was resolved by changing the font and italicizing al equations in all the document that weren’t in format, a full revision was done in all equations in the manuscript

Comment 3: The diagram of Bode is shown by Figure 2 (not by Figure 3). Figure 3 is missing. From Figure 2 all figures must be renumbered.

Thank you for the correction, a new figure was added attending the comments of the first reviewer corresponding to figure 2, and the figure correspondence reviewed, so the numbering is now correct.

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear authors

the topic is interesting but I don't see a value added reading this work. Literature is not able to identify the novelty of your contribution and all structure of the work is not simple to follow. There are some good results but it is missing the relationship with methods used (why do you have used this approach) what are the assumptions? why this work is necessary in a general context).

Sorry, but I must reject it.

Author Response

3^rd Reviewer

Comment 1: the topic is interesting but I don't see a value added reading this work. Literature is not able to identify the novelty of your contribution and all structure of the work is not simple to follow. There are some good results but it is missing the relationship with methods used (why do you have used this approach) what are the assumptions? why this work is necessary in a general context).

Sorry, but I must reject it.

Allow me to thank you for reviewing the manuscript, we would ask you to reconsider your decision, we would like to state that the original version of the manuscript the contribution was difficult to identify due to the lack of clarity as well as a point of the contribution was not stated at all. We would like to list the contributions as the following:

This work presents a method for tunning the PI+Resonant controller with a damping factor considering a simple L filter and the delays introduced by the converter and discretization from the D/A converters, implemented in the synchronous reference frame (dq). whereas in the works mentioned. The first one presents ideal resonant controllers at dq frame without presenting a tuning method and an analysis of the controller stability[1]. And the next work presents a method where the tunning relies on the extra poles from a LCL filter, and this method only considering a PR controller with its respective degrees of freedom[2]. Considering these a more practical and robust implementation is presented. 

With regards of the reactive power control on the GSC a reactive power control loop was added to this scheme, allowing the GSC to generate and inject reactive power to the grid, also the total reactive power of the WGS can be controlled on the DFIG stator as well. Combined the WGS can control a higher reactive power when the reactive power generated by the machine is insufficient to supply the demand. As responded to the first reviewer, agree on the control of reactive power being an incremental improvement to this type of systems, considering some works where grid voltage is regulated using reactive power, with their proposed control loops using diverse techniques for example droop control, but these schemes are more suitable for microgrids with a reduced number of sources and loads, the environment where a DFIG is used commonly, is as part of a wind farm connected to the main grid where the reactive power control should be coordinated between all of the wind farm. This is why the regulation of the grid voltage is left to the grid operator.

 

 

[1] Liserre, M.; Teodorescu, R.; Blaabjerg, F. Multiple harmonics control for three-phase grid converter systems with the use of PI-RES current controller in a rotating frame. IEEE Transactions on Power Electronics 2006, 21, 3.

[2] Schiesser, M.; Wasterlain, S.; Marchesoni, M.; Carpita, M. A Simplified Design Strategy for Multi-Resonant Current Control of a Grid-Connected Voltage Source Inverter with an LCL Filter. Energies 2018, 11, 609.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Line 41: As was already commented to version 1, most manufacturers choose the speed range 1:2, meaning 1/3 above and 1/3 below the synchronous speed.

Line 135: Write tuning (not tunning).

Line 190: Equation (11) needs a revision, as requested in the review to version 1. A simple check of the dimensions reveals that, in SI units, on the left side we have Volt/Amp=Ohm, and on the right side  Volt/Siemens=Volt*Ohm. This inconsistency must be removed. This requires  revising the deduction in the foregoing paragraphs and equations .

Author Response

1^st Reviewer

Comment 1: Line 41: As was already commented to version 1, most manufacturers choose the speed range 1:2, meaning 1/3 above and 1/3 below the synchronous speed.

The fragment in lines 41 to 43 was edited as following:

“This scheme takes advantage of the wide slip range of the machine, which most manufacturers design to be around a 30% above the synchronous speed (hypersynchronous operation) and below the synchronous speed (subsynchronous operation) [2].”

Comment 2: Line 135: Write tuning (not tunning).

Thank you very much for the correction a revision on the manuscript was done for this misspelling and other mistakes

Comment 3: Line 190: Equation (11) needs a revision, as requested in the review to version 1. A simple check of the dimensions reveals that, in SI units, on the left side we have Volt/Amp=Ohm, and on the right side  Volt/Siemens=Volt*Ohm. This inconsistency must be removed. This requires  revising the deduction in the foregoing paragraphs and equations.

Thank you very much for the observation, the mismatch in units in the transfer function of eq (11) was due to an error in the mathematical analysis which affected equations (27) and (28), these equations were corrected. In regards to the results, they correspond to the current loop (Inner loop), whereas the voltage loop response is not featured in this paper, regardless, it can be reported that the calculation still yielded a stable response with a minor difference in the desired speed and damped response, leaving the current loop results unaffected. It is worth mentioning the existence of different models of the DC link which result in different formulas with varying results, some designers use numerical analysis tools such as SISOtool in Matlab to perform this task as well, leaving in all cases a stable system.

Reviewer 3 Report

Dear authors,

with pleasure I change my previous assessment. The work is significantly improved. I ask to improve the following points:
1. what is the role of green energy towards an economic growth after/during covid 19? It is useful to introduce the relevance of this renewable source (wind) within sustainability

2. the novelty of the paper can be justified better according to recent works.

3. methodology is ok

4. results can be improved through a comparison with literature. what are the differences, what are similar results?

5. last point of reflection. this system is able to improve only efficacy or also efficiency?

Author Response

3^rd Reviewer

Dear authors,

with pleasure I change my previous assessment. The work is significantly improved. I ask to improve the following points:

1. what is the role of green energy towards an economic growth after/during covid 19? It is useful to introduce the relevance of this renewable source (wind) within sustainability

The following text has been added to the manuscript which reflects on the question by the reviewer:

“For many nations, maintaining an energy mix with a wide diversity of sources is a necessary key to achieve energy independence, to allows nations avoid the import energetics (Such as fuel and electric power transmission from neighboring countries), creating risky situations when the global energy supply chain fails, as the demand increases again as activities resume under the “new normality” resulting from the COVID-19 global pandemic [1], as well the unexpected fail of transport routes [2], affecting the population and local economy causing price volatility in consumer goods, and running the risk of black-outs in countries dependent of external sources, wind energy is a solution for any country with potential for generating this type of energy.”

2. the novelty of the paper can be justified better according to recent works.

In lines 91 to 103 a review of recent works is done with the sources [14]-[16] cited, where related works, with different solutions are reviewed and compared with the presented work and their main characteristics where reviewed.

3. methodology is ok

Thank you for your observation, a second review was greatly appreciated

4. results can be improved through a comparison with literature. what are the differences, what are similar results?

A results comparison was realized with a classical control scheme replicated in the laboratory, although a comparison with other different techniques is done for qualitive characteristics, the quantitative comparison is more difficult, as some results presented in the literature were realized in different conditions, and leave some conditions unknown, as well as the use of different equipment, also, the proper limitations in the equipment available, made difficult to replicate the results presented when certain experimental conditions should be replicated (Grid voltage with a specific harmonic content, as well as the same parameters).

5. last point of reflection. this system is able to improve only efficacy or also efficiency?

In terms of efficacy the resonant current controller achieves the reduction of current harmonics bellow the maximum levels required by the most recent codes, with an easy implementation, efficiency is achieved through the reduction of power losses caused by distortion and reactive power consumption. Additionally, the calculations required by this type of controller are executed efficiently by any DSP platform

Round 3

Reviewer 3 Report

Congratulations.