Large-Scale Electric Propulsion Systems in Ships Using an Active Front-End Rectifier
Round 1
Reviewer 1 Report
This reviewer finds the paper interesting in some points. However, there are some concerns as follows:
1) Although the authors have tried to provide some backgrounds for this work, the motivations are still not clear. This reviewer suggests rewriting the first two paragraphs of the Introduction in such a way that the motivations become clear and organized. Please avoid unnecessary explanations and be to the point.
2) Abstract also should be revised in such a way that the motivations, contributions, and significant numerical results are described clearly. Now, the abstract is unorganized and written carelessly. Please avoid any typo and grammatical error in the abstract.
3) This reviewer finds the literature review insufficient. There are several newly published research papers in this area and the authors should carefully distinguish the new contributions of their work form the existing studies A comprehensive literature review is necessary to address this issue. Try to provide a literature survey in an organized fashion. Note that it is not sufficient to just summary some the related works. Please clearly mention the shortcomings of the existing works that are addressed in this paper.
4) Additionally, please clearly describe the contributions of the paper. Now, some points that mentioned in the Introduction as contributions. However, they are not really contributions. They are just newly used approaches/techniques. Please clearly distinguish new contributions and mention the advantages/reasons to propose them.
Author Response
Reviewers' comments:
Reviewer 1
This reviewer finds the paper interesting in some points. However, there are some concerns as follows:
Response: We are very grateful for your line-by-line comments across the manuscript. Your comments are valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We have studied the comments carefully and have revised the manuscript accordingly, which we hope meet with approval.
We put our responses to your comments on the original manuscript as well as the main corrections in the paper and the responses to your official comments are as follows.
All changes were highlighted.
1) Although the authors have tried to provide some backgrounds for this work, the motivations are still not clear. This reviewer suggests rewriting the first two paragraphs of the Introduction in such a way that the motivations become clear and organized. Please avoid unnecessary explanations and be to the point.
RESPONSE: We carefully modified the contents for motivation of this paper in accordance with your guidance on the original manuscript.
“As environmental pollution has become a global issue, the International Maritime Organization(IMO) has been strengthening regulations on emissions of sulfur oxides, nitrogen oxides, and carbon dioxide from ships[1-2]. As a result of that various researches are being carried out in order to cope with environmental regulations that are strengthening internationally in the shipbuilding and shipping industries[3]. And the electric propulsion system of vessels with propulsion motors is also one of emerging countermeasures[4-7]. As shown in Fig. 1, the order of environment friendly electric propulsion ship is dramatically increased on 2017 World Fleet Resister by Clarkson’s Research[8].”
2) Abstract also should be revised in such a way that the motivations, contributions, and significant numerical results are described clearly. Now, the abstract is unorganized and written carelessly. Please avoid any typo and grammatical error in the abstract.
RESPONSE: Following your suggestion, we modified abstract with more clear contents in the revised manuscript.
“In case of electric propulsion system on the vessel, DFE rectifiers have been applied for large-sized ships and AFE rectifiers have been utilized for small and medium-sized ships as a part of system. In this paper, we design a large electric propulsion ship system using AFE rectifier using the proposed phase angle detector and verify the feasibility of the system by simulation. The phase angle derived from the proposed phase angle detection method is applied to the control of the AFE rectifier instead of the zero crossing method used to detect the phase angle in the control of the conventional AFE rectifier. We compare and analyze the speed control, DC-link voltage, harmonic content and measurement data of heat loss by inverter switch obtained from the simulation of the electric propulsion system with the 24 pulse DFE rectifier, the conventional AFE rectifier and the proposed AFE rectifier. As a result of the simulation, it was confirmed that the proposed AFE rectifier derives a satisfactory result similar to that of a 24 pulse DFE rectifier with a phase shifting transformer installed according to the speed load of the ship, and it can be designed and applied as a rectifier of a large-sized vessel.”
3) This reviewer finds the literature review insufficient. There are several newly published research papers in this area and the authors should carefully distinguish the new contributions of their work form the existing studies A comprehensive literature review is necessary to address this issue. Try to provide a literature survey in an organized fashion. Note that it is not sufficient to just summary some the related works. Please clearly mention the shortcomings of the existing works that are addressed in this paper.
RESPONSE: Following your suggestion, we added literature review in the revised manuscript.
4) Additionally, please clearly describe the contributions of the paper. Now, some points that mentioned in the Introduction as contributions. However, they are not really contributions. They are just newly used approaches/techniques. Please clearly distinguish new contributions and mention the advantages/reasons to propose them.
RESPONSE: We highly appreciate your valuable comment. As per your comments, we added contribution of this research with difference between proposed AFE and conventional AFE method.
Author Response File: 
Author Response.docx
Reviewer 2 Report
In the paper there are some criticism with must be solved.
Some comments:
1)Line 119, 144, 169, 174, etc...- Delete "Error! Reference source not found"
2) Line 236- "ed is arbitrarly set to be the active power.....". This is not clear. The product ed x id is the active power (less than the
coefficient 1.5) if and only if the eq x iq is equal to zero.
3) The novelties of the control is not clear.
4) According to the high voltage and power necessary for the application, why it is not used a multilevel structure for the AFE ?
5) Some data of the propulsion motor are not shown (power, rated angular speed, rated frequency, etc...).
6) Fig.24: It seems tht the dynamic of the standard and proposed method is the same.
7) FIg.8a) why are there two voltag peaks (near 0.6 and 0.8 s )? This peaks can be due to a load variation (which is not presentd in the other simulations).
8) Heat losses: Which is the temperature used for the simulations? Is the IGBT parameters variation taken into account?
Author Response
Reviewers' comments:
Reviewer 2
This reviewer finds the paper interesting in some points. However, there are some concerns as follows:
In the paper there are some criticism with must be solved.
Response: We are very grateful for your line-by-line comments across the manuscript. Your comments are valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We have studied the comments carefully and have revised the manuscript accordingly, which we hope meet with approval.
We put our responses to your comments on the original manuscript as well as the main corrections in the paper and the responses to your official comments are as follows.
All changes were highlighted
Some comments:
1)Line 119, 144, 169, 174, etc...- Delete "Error! Reference source not found"
RESPONSE: We have modified the errors.
2) Line 236- "ed is arbitrarly set to be the active power.....". This is not clear. The product ed x id is the active power (less than the coefficient 1.5) if and only if the eq x iq is equal to zero.
RESPONSE: We carefully reviewed your comments on the manuscript.The proposed phase angle detection technique converts the 3-phase AC power supply voltage to the d-q axis on the synchronous rotation coordinate system, and obtains the ed and eq voltages having the DC quantity which is easy to control. In this case, ed is set as active power and eq is set as reactive power so that the voltage of eq is controlled to be 0 as in Fig.xx, so that the power is controlled to be influenced only by ed, which indicates d-axis voltage.
3) The novelties of the control is not clear.
RESPONSE: We have considered the opinion of the reviewer, and based on the opinion, in this paper, it is assumed that the adjustment of the phase angle, which is necessary to control the AFE rectifier, is not instantaneous due to the instantaneous oscillation of the generator output voltage due to the load variation occurring in the operation of various propulsion electric motor of vessels.
In the simulation, the generator output voltage is instantaneously changed and the output results of the conventional AFE rectifier with phase angle detector and the proposed phase angle detection technique are compared and analyzed. The simulation results show that the proposed phase angle detection technique outputs superior control results.
4) According to the high voltage and power necessary for the application, why it is not used a multilevel structure for the AFE ?
RESPONSE: Thank you for your detailed feedback. We can also use a multilevel structure for AFE rectifiers. When a multi-level structure is applied, a better DC link output voltage can be obtained. However, since the number of semiconductor switches increases, the controller design becomes complicated and the heat loss of the switching element will be increased.
Therefore, in this paper, a rectifier of two level structure is applied, and in the case of a large electric propulsion ship, a semiconductor switching element is arranged in series in a two-level structure against the influence of high voltage and high power.
5) Some data of the propulsion motor are not shown (power, rated angular speed, rated frequency, etc...).
RESPONSE: According to your opinion, we have placed data of the propulsion motor
6) Fig.24: It seems that the dynamic of the standard and proposed method is the same.
RESPONSE: Fig. 24 shows the propulsion motor speed response characteristics of the DFE rectifier, the conventional AFE rectifier, and the electric propulsion system using the proposed AFE rectifier. Although the front end of the electric propulsion system is differently applied, the speed response graph of the propulsion motor shows a similar result because indirect vector control is applied to the rear end inverters.
The results of this speed response characteristic are described in order to judge whether the circuit configuration of each electric propulsion system constituted is appropriate. The simulation circuit design of each system is considered appropriate based on the speed response characteristic output.
7) FIg.8a) why are there two voltag peaks (near 0.6 and 0.8 s )? This peaks can be due to a load variation (which is not presentd in the other simulations).
RESPONSE: Thank you for your detailed feedback. In case of a real ship, the adjustment of the phase angle necessary for controlling the AFE rectifier may not be instantaneous due to the variation of the generator output voltage when the loads (BOW thruster, various kinds of large pump operation, deck device operation, etc.) have been changed. In this paper, the simulation was performed by generating a state where the output voltage of the generator is instantly fluctuating randomly in the simulation. In the conventional AFE rectifier, the phase angle is detected by applying the zero crossing method. However, as shown in the simulation result, the phase angle error occurs instantaneously due to the fluctuation of the generator output voltage in the vicinity of 0.6s and 0.8s, peak output voltage in DC link has been occurred.
8) Heat losses: Which is the temperature used for the simulations? Is the IGBT parameters variation taken into account?
RESPONSE: We added some clarification as below.
Parameters including the electrical characteristic curves (Vce (sat) vs Ic, Eon vs Ic, Eoff vs Ic, etc.) of the IGBT elements according to the operating temperature were entered by referring to the data sheet of the IGBT element used in the inverter. In order to measure the heat generated by the IGBT, an electric circuit including heat resistance that interferes with heat transfer is shown in Fig. xx, and the heat loss generated by the heat sink was measured.
Thank you
Author Response File: Author Response.docx
Reviewer 3 Report
The paper addresses the replacement of the normally preferred Direct Front End (DFE) rectifier by the Active Front End (AFE) type in large ships. Furthermore, to address the issue of higher AFE rectifier losses, the use of a phase angle detector for phase-locked loop (PLL) method is presented through power systems software, PSIM, simulations. The results of the different architectures are presented and concludes that the PLL improved AFE is a possible future replacement for the DFE as the semiconductor switching devices continue to improve.
Overall the main comment is that the paper needs a better review from the authors before submitting to journal publication.
Author Response
Reviewers' comments:
Reviewer 3
This reviewer finds the paper interesting in some points. However, there are some concerns as follows:
Response: We are very grateful for your line-by-line comments across the manuscript. Your comments are valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our research. We have studied the comments carefully and have revised the manuscript accordingly, which we hope meet with approval.
We put our responses to your comments on the original manuscript as well as the main corrections in the paper and the responses to your official comments are as follows.
All changes were highlighted
The paper addresses the replacement of the normally preferred Direct Front End (DFE) rectifier by the Active Front End (AFE) type in large ships. Furthermore, to address the issue of higher AFE rectifier losses, the use of a phase angle detector for phase-locked loop (PLL) method is presented through power systems software, PSIM, simulations. The results of the different architectures are presented and concludes that the PLL improved AFE is a possible future replacement for the DFE as the semiconductor switching devices continue to improve.
Overall the main comment is that the paper needs a better review from the authors before submitting to journal publication.
RESPONSE: We agree with the reviewer’s opinions. We have revised the entire manuscript carefully. Appreciated your valuable comment.
“As environmental pollution has become a global issue, the International Maritime Organization(IMO) has been strengthening regulations on emissions of sulfur oxides, nitrogen oxides, and carbon dioxide from ships[1-2]. As a result of that various researches are being carried out in order to cope with environmental regulations that are strengthening internationally in the shipbuilding and shipping industries[3]. And the electric propulsion system of vessels with propulsion motors is also one of emerging countermeasures[4-7]. As shown in Fig. 1, the order of environment friendly electric propulsion ship is dramatically increased on 2017 World Fleet Resister by Clarkson’s Research[8].
Figure. 1. Annual new-building contract of ships
The components of the conventional large-sized electric propulsion ship are generally composed of generator, DFE rectifier with phase shifting transformer, inverter and propulsion motor, and it is possible to design the size of engine room with some margin[9-11]. In an electric propulsion ship, when the switching of inverters are occurred, a harmonic current is generated in a power system[12]. Thus, large and small problems occur in the generator, transformer, and propulsion motor. Various methods for reducing harmonics have been studied. In the case of large electric propulsion systems, phase shifting transformer has been adopted as the most common method of installing a transformer on the output side of a generator[3,13]. There are various methods of harmonics reduction of the DFE rectifier using a phase shifting transformer such as multi-pulse of the rectifier output[14-16], active filter installation[17], and improvement of the transformer connection method[18-20].
However, when the phase shifting transformer is installed, there is a disadvantage as installation space and cost increase. Moreover, it is difficult to apply it to a small and medium sized ship having a small space. AFE rectifiers have been mainly applied to small and medium sized electric propulsion ships[21-22], but recently, as the technologies of power semiconductors with high capacity and high speed switching characteristics have been developed, so that it is possible to model a large-sized electric propulsion system using AFE rectifier[23]. The AFE rectifier must be designed with a control circuit that can control the semiconductor switch, and it is especially necessary to accurately detect the phase angle of the power supply voltage. Zero crossing technique that can detect the phase angle quickly is simple and has no special control method[24-27]. However, due to the fluctuation of generator output voltage in case of high load such as propulsion motor or bow thruster. The detection of the phase angle may not be performed momentarily. Various methods have been studied to overcome the severe disadvantage of this zero crossing technique.
In large-sized commercial vessels, it is crucial to secure the space for cargo transportation as much as possible[3,9]. However, to reduce the harmonics contained in the ship power system, the DFE rectifiers with large sized phase shifting transformer have disadvantage to load cargo. And the AFE rectifier using the existing zero crossing technique also has various problems[28-29].
In this paper, AFE rectifier using PLL method is applied to large electric propulsion system instead of phase angle detection method using zero crossing method[30-33]. We used the power analysis program, PSIM, to model an AFE rectifier that uses the PLL method. A comparison simulations were performed for large-scale electric propulsion systems with the conventional DFE as well as proposed AFE rectifiers; based on this simulation, the resulting speed of the propulsion motor, DC output of the DC link, and harmonic output characteristics of the input power supply were analyzed based on the type of rectification. In addition, the thermal loss in the switching element, which is present in the inverter when AFE rectifiers are used as well as its stability were evaluated. Based on these results, the characteristics of the DFE and AFE rectifiers in a large-scale electric propulsion system were compared to confirm the higher effectiveness of using the PLL-method-based AFE rectifiers in large-scale electric propulsion systems compared with the use of conventional DFE rectifiers.”
Thank you
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Congratulations!
Reviewer 2 Report
No comments
Reviewer 3 Report
The submitted paper has significantly improved from the initial submission. This reviewer considers that it should be accepted in the present form.
