Voltage-Based Droop Control of Electric Vehicles in Distribution Grids under Different Charging Power Levels
, Ruben Lliuyacc-Blas 1,2 and Peter Kepplinger 1,3,*
Round 1
Reviewer 1 Report
This manuscript presents a systematic study to evaluate the impacts of the voltage-based droop control method for EV charging on grid operation. The manuscript is well written. The overall methodology and the corresponding results are clearly demonstrated. Only several minor comments are listed below.
- In Line 231, it should be 80% for the 3.3kW (U3) charging rate. Please double check it.
- Compared to the conclusions obtained in [14], this research shows the opposite conclusion for an EV charging rate of 3.3kW at 100% penetration. The authors stated that the difference is due to the tighter reference points. Please elaborate this explanation in a more detailed way.
- The droop controller would lead to the longer charging time, which may be one limitation of this control method. Can the authors give some discussions on the possible solutions to mitigate this demerit?
Author Response
We appreciate the time and effort the reviewers spent on their valuable feedback on our manuscript. We have highlighted the changes within the manuscript. Here is a point-by-point response to the reviewers’ comments and concerns.
Point 1: In Line 231, it should be 80% for the 3.3kW (U3) charging rate. Please double check it.
Response 1: The authors would like to thank the reviewer for pointing out this and apologize for the mistake. This has been corrected in the revised manuscript.
Original text:
Line #230- #231
Specifically, at 50 % penetration in case of 11 kW (U1) followed by 6.6 kW (U2) at 70 % penetration, and 100 % for the 3.3 kW (U3) charging rate.
Revised text:
Line #234- #236
Specifically, at 50 % penetration in case of 11 kW (U1) followed by 6.6 kW (U2) at 70 % penetration, and 80 % for the 3.3 kW (U3) charging rate.
Point 2: Compared to the conclusions obtained in [14], this research shows the opposite conclusion for an EV charging rate of 3.3kW at 100% penetration. The authors stated that the difference is due to the tighter reference points. Please elaborate this explanation in a more detailed way.
Response 2: The authors agree with the reviewers and accordingly the manuscript has been revised to emphasize this point.
Original text:
Line #240- #243
In contrast, the results of this research show that the droop controlling assists in complying with the defined time limit at this charging rate, but with a set of reference points tighter than theirs in the droop characteristics.
Revised and new text:
Line #246- #252
In contrast, the results of this research show that the droop controlling assists in complying with the defined time limit at this charging rate. The reason for this discrepancy is the differences in the voltage reference points in the controller. The paper by Leemput et al. has a lower and upper voltage reference points of 0.85 p.u. and 0.90 p.u., respectively. In contrast, we use a more restricted set of reference points, with the lower and upper reference points being set at 0.92 p.u. and 0.96 p.u., respectively.
Point 3: The droop controller would lead to the longer charging time, which may be one limitation of this control method. Can the authors give some discussions on the possible solutions to mitigate this demerit?
Response 3: We have incorporated this suggestion in the conclusion section of the revised manuscript.
New text:
Line #317- #320
This limitation could be ameliorated by adding local PV production, which could recover the charging droops. Thus, we suggest to investigate the interaction between distributed generation by PV and the droop control of EVs.
Reviewer 2 Report
This paper presents voltage-based droop control of electric vehicles in distribution grids for different charging power levels. This is an interesting and useful work and the authors have considered basic concepts and relevant experiments through addressing sufficient and relevant references as well as comparing the different scenarios. The paper is well written, and simulation results have been addressed to validate the performance of the proposed method. However, the following items need to be clarified:
- Authors should provide more details about the proposed control mechanism. Using a flowchart can be a good option to present the novelty of the method and to flow the whole process of the applied control strategy.
- What are the main features of advanced metering infrastructure (AMI)?
- There is no proper explanation about EV usage behavior modeling. I would suggest reporting an overview of collected datasets as well as EV usage behavior modeling.
- What is the main uncertainty investigated in this study? How robust is the applied controller to properly restrain the existing uncertainties?
Author Response
We appreciate the time and effort the reviewers spent on their valuable feedback on our manuscript. We have highlighted the changes within the manuscript. Here is a point-by-point response to the reviewers’ comments and concerns.
Point 1: Authors should provide more details about the proposed control mechanism. Using a flowchart can be a good option to present the novelty of the method and to flow the whole process of the applied control strategy.
Response 1: The authors would like to thank the reviewer for this suggestion and Figure 1 in page 3, showing the overall concept is added in the revised manuscript.
Point 2: What are the main features of advanced metering infrastructure (AMI)?
Response 2: The relevant features for the implementation of the proposed method is highlighted in the revised manuscript and for the additional features a reference is provided.
Original text:
Line #113- #116
To facilitate the control mechanism, it is assumed that each EV charging infrastructure is equipped with a droop controller accompanied by an advanced metering infrastructure (AMI) at the point of connection.
Revised text:
Line #113- #116
To facilitate the control mechanism, it is assumed that each EV charging infrastructure is equipped with a droop controller accompanied by an advanced metering infrastructure (AMI) at the point of connection [19] capable of measuring the voltages. The general concept of the voltage droop mechanism is depicted in Figure 1.
Point 3: There is no proper explanation about EV usage behavior modeling. I would suggest reporting an overview of collected datasets as well as EV usage behavior modeling.
Response 3: We agree with the reviewer that this information would help to understand the method more clearly. Therefore, we have updated the manuscript by including a short overview of the EV usage behaviour modelling information in section 2.3.
New text:
Line #186- #188:
After filtering and cleaning the mobility data in the survey, 15320 weekday and 5696 weekend driving profiles were created and used as a library to model the daily EV usage behavior. These profiles were assigned for the EVs randomly.
Point 4: What is the main uncertainty investigated in this study? How robust is the applied controller to properly restrain the existing uncertainties?
Response 4: The method relies on the voltage measurements at the point of connection, and therefore relies on the uncertainties associated with the measurements. But within the scope of this paper we do not look into these uncertainties.
Reviewer 3 Report
Thank you for sharing your research.
I think thata the paper is useful for the researchers but also for the diffusion of the innovation, so the idea to use Energies is good.
I appreciated the paper, it is interesting, motivation is good and also the way in which issues have been faced.
I have no changes required, only I ask to describe better in 3.1 the standars, are little confusing.
I will appreciate a discussion also for this or a new paper, if you can define which are the weak points in grid model of figure 2, I suppose the more distant from transformers, but it can be useful to describe better, and if possible to add local green generation systems that can recover droops.
Go ahead!
Author Response
We appreciate the time and effort the reviewers spent on their valuable feedback on our manuscript. We have highlighted the changes within the manuscript. Here is a point-by-point response to the reviewers’ comments and concerns.
Point 1: I ask to describe better in 3.1 the standards, are little confusing.
Response 1: The authors would like to thank the reviewer for this suggestion and the Section 3.1 is updated to improve the clarity.
Original text:
Line #209- #212
The European standard EN50160 specifies that for 95 % of the time within a week, the 10-minutes mean rms supply voltage in LV distribution networks should not deviate more than 10 % of the nominal value.
Revised text:
Line #212- #215
The European standard EN50160 specifies that the 10-minutes rms value of the supply voltage in LV distribution networks should not deviate from the nominal value more than 10 % for 95 % of the time within a week.
Original text:
Line #217- #219
Figure 3 shows the maximum duration of the rms values of the nodal voltages below a deviation of -10 % and the violations of the -15 % voltage limit.
Revised text:
Line #221- #223
Figure 4 shows the maximum duration of the rms values of the nodal voltages exceeding -10 % of the nominal voltage value and the violations of the -15 % voltage limit.
Original text:
Line #222- #231
As illustrated in Figure 3, the voltage deviation exceeds the -10 %-limit in the benchmark scenario (without EVs). Nevertheless, the grid voltage is in compliance with the EN50160 voltage standards as the maximum duration below the limit is less than 5 % of the time. The compliance with time limit is met in all the scenarios up to 70 % penetration, however, utilization of the available voltage reserves grows steadily with increasing penetration for the uncontrolled scenarios (U*), most notably in the scenario U1 (uncontrolled 11 kW scenario). In contrast, the minimum voltage deviation exceeds the -15 %-limit already at lower penetrations, resulting in a violation of ENE50160 voltage standards. Specifically, at 50 % penetration in case of 11 kW (U1) followed by 6.6 kW (U2) at 70 % penetration, and 100 % for the 3.3 kW (U3) charging rate.
Revised text:
Line #225- #236
As illustrated in bottom plot of the Figure 4, the voltage deviation exceeds the -10 %-limit already in the benchmark scenario (without EVs). Nevertheless, the grid voltage is in compliance with the EN50160 voltage standards as the maximum duration below the -10 %-limit is less than 5 % of the time. The compliance with time limit is met in all the scenarios up to 70 % penetration as illustrated in the top plot of Figure 4 , however, utilization of the available voltage reserves grows steadily with increasing penetration for the uncontrolled scenarios (U*), most notably in the scenario U1 (uncontrolled 11 kW-scenario). In contrast, the minimum voltage deviation exceeds the -15 %-limit already at lower penetrations, resulting in a violation of the voltage limit defined in the ENE50160 standards. Specifically, at 50 % penetration in case of 11 kW (U1) followed by 6.6 kW (U2) at 70 % penetration, and 80 % for the 3.3 kW (U3) charging rate.
Original text:
Line #234- #236
At even higher penetration (90 % and above), the controller is not capable of eliminating the violation of the -15 %-limit anymore.
Revised text:
Line #239- #241
For the penetrations above 80 %, the controller is not capable of eliminating the violation of the -15 %-limit (voltage limit) anymore.
Original text:
Line #240- #249
In contrast, the results of this research show that the droop controlling assists in complying with the defined time limit at this charging rate, but with a set of reference points tighter than theirs in the droop characteristics. In addition, the results indicate that at high charging power (11 kW) and at high penetrations (at 90 % and above), although the controller does not contribute to the compliance with the minimum voltage limit, the compliance with the time limit is achieved. However, even in these cases the grid voltage is very close to the threshold limits. Furthermore, even the controlled scenarios with low charging power levels approach the minimum voltage limit at high penetrations, exhibiting a reduced voltage reserve.
Revised text:
Line #246- #259
In contrast, the results of this research show that the droop controlling assists in complying with the defined time limit at this charging rate. The reason for this discrepancy is the differences in the voltage reference points in the controller. The paper by Leemput et al. has a lower and upper voltage reference points of 0.85 p.u. and 0.90 p.u., respectively. In contrast, we use a more restricted set of reference points, with the lower and upper reference points being set at 0.92 p.u. and 0.96 p.u., respectively.
The results indicate that at high charging power (11 kW) and at high penetrations (at 90 % and above), although the controller does not contribute to the compliance with the minimum voltage limit i.e. -15 %, the compliance with the time limit is achieved. However, even in these cases the grid voltage is very close to the threshold limits. Furthermore, even the controlled scenarios with low charging power levels approach the minimum voltage limit at high penetrations, exhibiting a reduced voltage reserve.
Point 2: I will appreciate a discussion also for this or a new paper, if you can define which are the weak points in grid model of figure 2, I suppose the more distant from transformers, but it can be useful to describe better, and if possible to add local green generation systems that can recover droops.
Response 2: The authors appreciate the interesting suggestion made by the reviewer. And it would have been interesting to explore this aspect. However, we agree with the reviewer and believe that it would be more appropriate to consider it in another publication.
Round 2
Reviewer 2 Report
The authors have responded to questions and made the necessary changes to the paper.
