Application of Single-Phase Supply AC-DC-AC VFD for Power Factor Improvement in LED Lighting Devices Loaded Power Distribution Lines

Round 1

Reviewer 1 Report

1. Abstract is unclear. The first authors deal with frequency drives, compensation, and then LED...

The abstract needs to be rewritten, and more apparent.

2. Introduction is clear but it is not enough. What is the paper's benefit? what are paper contributions? What is the difference between previously published studies?

3. Paper organization add in Introduction

4. Fig 4 - add literature

5. There are a lot of mathematical equations. Are these equations novel, or they are taken from the literature?

6. Please add in Appendix Matlab codes for calculating displacement using Fourier series and results from Table 1.

7. Fig. 11 Waveforms of I for C=23.5uF is unclear

8. Fig A2 is unclear.

9. Figs A3-A6 - vector diagram is defined for first harmonic, or not?

10. Have you calculated power distortion?

11. the reference list is poor - must be added a lot of references.

Author Response

Reviewer's note 1. Abstract is unclear. The first authors deal with frequency drives, compensation, and then LED...The abstract needs to be rewritten, and more apparent.

Response to the note 1. Dear reviewer. I rewrote the abstract to consistently explain the relationship between LEDs, reactive power compensation, and VFDs.

Reviewer's note 2. Introduction is clear but it is not enough. What is the paper's benefit? what are paper contributions? What is the difference between previously published studies?

Response to the note 2. Dear reviewer. I supplemented the Introduction section with reference to the benefits and contributions of this paper. I also pointed out the differences between previously published studies

Reviewer's note 3. Paper organization add in Introduction

Response to the note 3. Dear reviewer. I supplemented the Introduction section with paper organization.

Reviewer's note 4. Fig 4 - add literature

Response to the note 4. Dear reviewer. According to your comment, we have included a reference to the cited literature.

Reviewer's note 5. There are a lot of mathematical equations. Are these equations novel, or they are taken from the literature?

Response to the note 5. Dear reviewer. Equations 1-7 are used to describe the operation of a diode rectifier and to find the angle of displacement in steady state. After calculating the angle of displacement, the transient process in the rectifier is described by the Duhamel's integral, considering the inductance and resistance of the power distribution lines. Equation 8 and equation 10 are taken from the literature [14] to calculate Duhamel's integral and Equations 11 to 18 are derived to describe the instantaneous value of the VFD diode rectifier current.

Reviewer's note 6. Please add in Appendix Matlab codes for calculating displacement using Fourier series and results from Table 1.

Response to the note 6. Dear reviewer. We added Matlab codes for calculating displacement using Fourier series and results from Table 1 in to Appendix A.

Reviewer's note 7. Fig. 11 Waveforms of I for C=23.5uF is unclear

Response to the note 7. Dear reviewer. We supplemented the paper with the following explanation of the current waveform.

The inductance and resistance of the power distribution lines with the capacitance of the VFD DC circuit form a corresponding resonant frequency with a sequential oscillating circuit. When the instantaneous value of the voltage in the power distribution lines becomes higher than the voltage in the capacitor of the DC circuit, the capacitor charges. Depending on the value of the capacitance in the DC circuit, and the value of the resistance and inductance of the power distribution lines, the charging of the capacitor may occur aperiodic and periodic, with fading oscillations during charging. This is the situation we see in the current waveform at C = 23 uF. With a larger capacity of the DC circuit, charging takes place aperiodically and the oscillations do not occur, as we see in other current waveforms.

Author Response File: Author Response.pdf

Reviewer 2 Report

The article under review is devoted to the study of the idea proposed by the authors of reactive power compensation in 0.4 kV power distribution lines by varying the capacity of the DC circuit of variable frequency drives of asynchronous motors.

In the Introduction and literature review, the prerequisites for conducting research are considered in sufficient detail, and the purpose of the paper is formulated. The main parts of the paper describe the idea proposed by the authors. Theoretical foundations are outlined, and the results of experimental studies on a laboratory setup are presented.

After reading the paper I would like to note:

1) In my opinion, the idea proposed by the authors is not innovative. Currently, most frequency converters of asynchronous drives to rectifiers do not use diodes, but transistors, the operation of which allows both to provide and to draw out reactive power. However, despite this, the solutions proposed by the authors have practical significance under certain operating conditions. Nevertheless, the authors should provide a comparative analysis of their proposed solutions with standard modern solutions in the Introduction and literature review.

2) The question remains: why do the authors propose to change the capacitance value in the DC circuit of frequency converters, and not in the DC circuit of the rectifier of LED devices?

3) lease explain what is the value of multi-stage experiments carried out in accordance with the diagrams shown in Figures 7, 13 and 15? It seems to me that it would be enough to conduct experiments in accordance with the diagram presented in Figure 17. This would allow us to present the research material more concisely.

4) In addition, the References list cannot be considered sufficient.

Author Response

Reviewer's note 1. In my opinion, the idea proposed by the authors is not innovative. Currently, most frequency converters of asynchronous drives to rectifiers do not use diodes, but transistors, the operation of which allows both to provide and to draw out reactive power. However, despite this, the solutions proposed by the authors have practical significance under certain operating conditions. Nevertheless, the authors should provide a comparative analysis of their proposed solutions with standard modern solutions in the Introduction and literature review.

Response to the note 1. Dear reviewer. The novelty of the idea presented in this article is that it uses already installed VFDs to compensate for reactive power, which, like most classic VFDs, have a factory fitted supply side diode rectifier. You are right in a global perspective - the modern VFD is factory fitted with IGBT or MOSFET rectifiers. Direct matrix AC-AC frequency converters are also used. These VFDs have the ability to improve the power factor without additional upgrades.

Reviewer's note 2. The question remains: why do the authors propose to change the capacitance value in the DC circuit of frequency converters, and not in the DC circuit of the rectifier of LED devices?

Response to the note 2. Dear reviewer. The proposal to change the capacitance value in the DC circuit of the frequency converters, and not in the DC circuit of the rectifier of the LED devices is based on the fact that one VFD has the ability to compensate for the reactive power produced by a large amount of LED. As a result, the number of upgrades offered would be lower. Another reason is that most frequency converters are installed in automation cabinets, which usually have extra space for upgrading components. There would be no extra space in the LED lighting devices housings.

Reviewer's note 3. Please explain what is the value of multi-stage experiments carried out in accordance with the diagrams shown in Figures 7, 13 and 15? It seems to me that it would be enough to conduct experiments in accordance with the diagram presented in Figure 17. This would allow us to present the research material more concisely.

Response to the note 3. Dear reviewer. The experimental studies presented in Figures 7, 13 and 15 were performed to ensure the consistency of the study. The experiment presented in Figure 7 was aimed at analyzing reactive power only in rectifier supply-side device loaded power distribution lines, excluding the influence of other components. The experiment presented in Figure 13 aimed to analyze the possibility of a rectifier supply-side device to compensate for the reactive power produced by an LED without the influence of an inverter and a motor. The experiment presented in Figure 15 was intended to analyze only the nature of the reactive power produced by the VFD. This experiment also confirmed the direction of the reactive power shown in Figure 2 (b).The experiment presented in Figure 17 summarized and confirmed the previous assumptions and the results of previous experiments.

Reviewer's note 4. In addition, the References list cannot be considered sufficient.

Response to the note 4. Dear reviewer. I supplemented the reference section by using additions to highlight the benefits and contributions of this paper, and also pointed out the differences between previously published studies.

Author Response File: Author Response.pdf

Reviewer 3 Report

The Authors stated, that in  the  article   an innovative approach to conventional AC-DC-AC variable frequency drives in order   to use these drives as a reactive power compensation device is presented. The proposed method  is based on using of  the interaction between the power distribution lines inductivity and the DC circuit capacitance of the variable frequency drive. It has been shown that the power factor can be controlled by varying the capacity of the DC circuit. The proposed approach is simple in application but it has many practical limitations. The control  of power factor is not continuous but gradual. The method requires access to the intermediate circuit of the frequency converter. Not all converter manufacturers allow it, especially during the warranty period.

The literature presents many methods of reactive power compensation that ensure continuity of compensation and more precise control. The article significantly reduces the number of loads connected to the considered network. The frequently used non-controlled inductive loads were omitted. Despite the relatively high power of LED loads, only 1-phase power and frequency converter power supply systems are considered.

The article shows that the light emitting diode lighting devices are non-linear electrical receivers. Nevertheless, the principles of the linear theory of electric circuits were used to compensate for reactive power. It would also be interesting to use the modern theory of instantaneous power in the analysis (the works of Akagi and team). The impact of the use of line filters and motor filters has been taken into account to a small extent in the analyzes. Sometimes filters are  also used in DC link of VFD.

Many critical remarks concern the mathematical relationships presented in Section 2 of the article. No simplifying assumptions made for theoretical analysis were given. The equations shown are not mathematically exact. Integrals of time functions are presented, but the adopted limits of integration are given as angles. In many equations there is a dimensional mismatch between the components or sides of the equation.

The Rap resistance in Equation (3) should be defined and described.

On page 6, L.157 it is given, that:  The resistance of the circuit consisting of the 1kW VFD inverter and the induction motor is equal to 80Ω. This is quite great value and it should be explained.

P.8, Table 1.    Units are not given for many variables.

Author Response

Reviewer's note 1. The Authors stated, that in  the  article   an innovative approach to conventional AC-DC-AC variable frequency drives in order   to use these drives as a reactive power compensation device is presented. The proposed method  is based on using of  the interaction between the power distribution lines inductivity and the DC circuit capacitance of the variable frequency drive. It has been shown that the power factor can be controlled by varying the capacity of the DC circuit. The proposed approach is simple in application but it has many practical limitations. The control  of power factor is not continuous but gradual. The method requires access to the intermediate circuit of the frequency converter. Not all converter manufacturers allow it, especially during the warranty period.

Response to the note 1. Dear reviewer. Thank you for listing the shortcomings of this method. The authors of the study fully agree with your comments. Based on your comments, we have expanded the discussion section.

Reviewer's note 2. The literature presents many methods of reactive power compensation that ensure continuity of compensation and more precise control. The article significantly reduces the number of loads connected to the considered network. The frequently used non-controlled inductive loads were omitted. Despite the relatively high power of LED loads, only 1-phase power and frequency converter power supply systems are considered.

Response to the note 2. Dear reviewer. I agree with your remark that there is equipment that controls reactive power in power distribution lines more precisely. However, the installation of this equipment requires significant investment and financial costs. In the example given with the sports arena LED screen just the lack of investment is holding back the power factor improvement. The method proposed in this paper would allow a result to be achieved at a lower cost. Most of the VFDs installed in the observed sports arena are single-phase supply side. They are used to power and control the motors of pumps, fans and other engineering equipment. Therefore, the single-phase supply VFD is investigated in this study. The application of three-phase VFD to reactive power compensation is planned in further studies. We noted this in the discussion section.

Reviewer's note 3. The article shows that the light emitting diode lighting devices are non-linear electrical receivers. Nevertheless, the principles of the linear theory of electric circuits were used to compensate for reactive power. It would also be interesting to use the modern theory of instantaneous power in the analysis (the works of Akagi and team). The impact of the use of line filters and motor filters has been taken into account to a small extent in the analyzes. Sometimes filters are also used in DC link of VFD.

Response to the note 3. Dear reviewer. Nonlinear LED light devices connected to power distribution lines create the first harmonic phase shift and at the same time generate the full spectrum of higher harmonics. Only the compensation of the reactive power produced by the fundamental first harmonic, which can be both lagging and leading, is considered in this study. The influence of other harmonics is not considered in this study, because their compensation is in principle different from the compensation of the reactive power produced by the first harmonic. Publications related to the interaction between a diode rectifier and power distribution lines focus on higher harmonics and the problems they cause. As for line filters and motor filters, this study focused on the influence of capacitors in the VDF DC circuit and their interaction with PDL inductance. The influence of filters was not analyzed. This is the subject of future detailed studies, although initial experiments have shown that Schneider electric VFD DC circuit inductive filters affect reactive power.

Reviewer's note 4. Many critical remarks concern the mathematical relationships presented in Section 2 of the article. No simplifying assumptions made for theoretical analysis were given. The equations shown are not mathematically exact. Integrals of time functions are presented, but the adopted limits of integration are given as angles. In many equations there is a dimensional mismatch between the components or sides of the equation.

Response to the note 4. Dear reviewer. We've made changes to Equations (1) - (4) based on your comment.

Reviewer's note 5. The Rap resistance in Equation (3) should be defined and described.

Response to the note 5. Dear reviewer. We made changes to Figure 4 to make the purpose of Rap clear.

Reviewer's note 6. On page 6, L.157 it is given, that:  The resistance of the circuit consisting of the 1kW VFD inverter and the induction motor is equal to 80Ω. This is quite great value and it should be explained.

Response to the note 6. Dear reviewer. In the theoretical evaluations, we used the equivalent inverter in induction motor resistance of 80om based on the simplified calculations below. The practical DC circuit voltage is 285V, with a nominal voltage of 230V for the power distribution lines and an estimate of the voltage drop across the lines and the VFD supply side. At a VFD power of 1000W, a current of 1000/285 = 3.5A flows. In this case, the equivalent motor resistance of the inverter in induction Rap = 285/3.5 = 80Ω.

Reviewer's note 7. P.8, Table 1.    Units are not given for many variables.

Response to the note 7. We supplemented the table with units for variables.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors answered all of my questions.

Reviewer 2 Report

Comments have been considered. The article in this version can be accepted.