Study on Grid-Connected Strategy of Distribution Network with High Hydropower Penetration Rate in Isolated Operation
Round 1
Reviewer 1 Report
The paper is well written, well structured and gives a good overview of the issues arising from the disconnection of a small- hydropower microgrid from the main grid. However, in the reviewer's opinion some minor issues need to be addressed before the publication.
Comments and recommendations are as follows:
1. The Introduction should be improved. The current state-of-art must be clearly presented in order to better point out the contribution to the scientific literature and provide a better rationale for using it. More properly, the authors must introduce the research area to unfamiliar readers. Info must be provided for the concept of smart microgrids, highlighting the benefits that these systems may lead to both to the utility grid and end-users, i.e. promote the integration of DERs, the electrification of rural areas, guarantee a continuity in power supply, and so on. In this sense, an overview of main “pilot projects for smart microgrids” developed around the world may be useful. Then, the main issues arising from the realization of a microgrid from distributed energy sources already installed into current distribution networks as well as the main remedial actions should be clearly defined. In fact, it must be considered that most of these resources have been designed for back-up or stand-alone applications. Therefore, their electrical configuration as well as their controls and synchronization algorithms must be modified before to integrate these sources into a microgrid structure along with other sources. Useful could be the paper “A simple circuit model for the islanding transition of microgrids” and the others related to it.
2. The flow chart shown in Fig. 6 should be described step by step.
3. For clarity purposes, it could be better to provide the time-domain behavior of the microgrid frequency.
Author Response
Point 1: The Introduction should be improved. The current state-of-art must be clearly presented in order to better point out the contribution to the scientific literature and provide a better rationale for using it. More properly, the authors must introduce the research area to unfamiliar readers. Info must be provided for the concept of smart microgrids, highlighting the benefits that these systems may lead to both to the utility grid and end-users, i.e. promote the integration of DERs, the electrification of rural areas, guarantee a continuity in power supply, and so on. In this sense, an overview of main “pilot projects for smart microgrids” developed around the world may be useful. Then, the main issues arising from the realization of a microgrid from distributed energy sources already installed into current distribution networks as well as the main remedial actions should be clearly defined. In fact, it must be considered that most of these resources have been designed for back-up or stand-alone applications. Therefore, their electrical configuration as well as their controls and synchronization algorithms must be modified before to integrate these sources into a microgrid structure along with other sources. Useful could be the paper “A simple circuit model for the islanding transition of microgrids” and the others related to it.
Response 1: First of all, thank you very much for your suggestions for revision, which is of great benefit to the improvement of the paper. According to your suggestion, we have made the following changes to the paper.
1) In order to facilitate the unfamiliar readers to understand the content of the paper, the introduction of the microgrid and the operation mode of the distributed power supply is further described. Supplementary description was mad about the technical requirements of distributed generator to form a microgrid. Microgrid system needs to be designed to provide the real and reactive power requirements of the loads within the island serve the range of load operation conditions. The stable operation of the microgrid system requires its frequency manage equipment, voltage regulator equipment, protection devices and load management devices to meet the requirements of both grid connection operation and island operation. Microgrids require good design and adequate construction funding.
2) The introduction further describes the current situation of small hydropower access to the power grid in the mountainous areas of southern China. Due to the underdeveloped economy and other reasons, the investment in the distribution network is insufficient, so that the local small hydropower does not have the preconditions for the operation of the microgrid. And the current treatment measures for island hydropower are not ideal. This paper proposes further optimization measures.
3) In the fourth part of this paper, the electrical configuration, control and synchronization algorithm needed by this method are added on the basis of the existing distribution network.
Point 2: The flow chart shown in Fig. 6 should be described step by step
Response 2: Sorry to the unclear enough in the manuscript. According to your valuable suggestion, we have revised the paragraph and added the description. The different situations in the flow chart are described in the form of “Case I”, “Case II”, “Case III”. Please refer to the red label in this chapter. In addition, Fig. 5 is added as an auxiliary explanation.
Point 3: For clarity purposes, it could be better to provide the time-domain behavior of the microgrid frequency.
Response 3:
The time-domain behavior of the microgrid frequency has been provided in Fig.8.
In order to make the readers who are not familiar with the field easy to read this paper, the paper is further strengthened, mainly including the following aspects.
4)The original title “2.2 Method basis of the proposed fast grid connection strategy” was changed to "2.2 Method basis of the proposed fast grid connection strategy", and the method was described in detail in Chapter 3 for easy reading.
5)At the beginning of the "III Solution method" section, the theoretical basis of the method is added to the transition to the method of this paper.
6)In addition to the above-mentioned questions mentioned by the experts, we have further refined the wording of the paper, including the unified expression of the name to facilitate reading, and the addition of charts to help readers understand.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Here is a series of improvements to improve the paper:
206. Change "Initial amplitude-frequency characteristics of small hydropower islands" by problem description.
254. Change "Fast Connection Strategy of Isolated Small Hydropower" by Solution Method or Proposal Method
307. Change "Simulation Analysis of Influence of Fast Grid on Power Grid" by "Results and Discussion"
Figure 3. Error in the description " M ain"
245. "isolatedSHPto"
Figure 6. delete Chinese letters
288. enumerate the equation
Update all references according to journal standards
As far as research is concerned, I find the description of the problem and the proposed method very poor. It would be necessary to describe more deeply how the connection / disconnection of the switches is carried out, how are the electrical parameters monitored, what hardware is used, how do we always ensure the proper connection to the grid, ... As well as if it is possible to explain it with a real case.
Author Response
Point 1: How are the electrical parameters monitored, what hardware is used, how do we always ensure the proper connection to the grid.
Response 1:
Thanks for the suggestions. According to the expert opinions, the actual application scenarios, the methods of this paper and the solution of the problems are enriched, including the following parts.
1) Thank you for your valuable comments. The original description of the hardware is not clear.
Based on your suggestion, a detailed description of Figure 6 is added in the third part” Solution method” of the paper, which is shown in red. In order to more clearly describe the small hydropower rapid grid-connecting strategy expressed in Fig.6, Fig.5 and the corresponding text are added for further explanation. The figure 5 is as follows. For the added text, please see the red font section in the third part of this paper.
Thanks for the suggestion to add hardware descriptions. The original application scenario of Figure 5 is moved to the content of Chapter 4, and the original Figure 5 adds descriptions of electrical parameters monitored, hardware configuration, and proposed method.
The fourth part of this paper adds detailed descriptions of the hardware flow of the device hardware, device software, the amount of electrical energy collected by the device, and the signal output by the device. The original Figure 6 is a schematic diagram of the cost method. The added content is as follows. For detailed modifications, please refer to the red label in this chapter.
Point 2: If it is possible to explain it with a real case.
Response 2:
Thank you for your valuable comments. In response to your suggestion to increase a real case, this paper has been modified accordingly. The fourth chapter adds a section "Recorded wave" that describes a real case. In the real case, due to the tie line fault, the tie line tripped and and the hydropower was in isolation operation. Isolated hydropower was successfully connected to the distribution grid via BATS equipment in a short period of time. The case logogram gives a record of the actions of the BATS in the actual case. Figure 10 shows the voltage waveforms of the hydropower after the off-grid and after the grid connection, the connection line and the current signal of the standby line.
Compare the oscillography of the real case (Fig.10) with the simulation results of SHP's rapid grid connection. The results show that the real case is consistent with the simulation results, which confirms the effectiveness of the small grid hydropower grid-connected strategy. For details, please see the red bold content in the paper.
Point 3: I find the description of the problem and the proposed method very poor.
Response 3:
In view of the lack of clarity of this method, the third chapter of Figure 5 is a general description of the method. In Figure 5, the rapid integration of hydropower is summarized as 1Tie line trips, 2generator and load keep running, 3BATS close the standby breaker. For additional text descriptions, see the red font section of Chapter 3.
In order to explain in detail the hydropower rapid grid connection strategy, Figure 6 is combined with Figure 5 to explain the small hydropower rapid grid connection strategy according to different hydropower access conditions. Case I: SHP fast reconnected to the grid. Case II: SHP outage. Case III: Isolated SHP does not meet fast grid conditions.
Point 4: 206. Change "Initial amplitude-frequency characteristics of small hydropower islands" by problem description.
Response 4: The changes have been made as requested.
Point 5: 254. Change "Fast Connection Strategy of Isolated Small Hydropower" by Solution Method or Proposal Method
Response 5: The changes have been made as requested.
Point 6: 307. Change "Simulation Analysis of Influence of Fast Grid on Power Grid" by "Results and Discussion"
Response 6: The changes have been made as requested.
Point 7: Figure 3. Error in the description " M ain"
Response 7: Thank you for your reminder that this spelling error has been corrected.
Point 8: 245. "isolatedSHPto"
Response 8: This spelling error has been corrected.
Point 9: enumerate the equation
Response 9: The formulas have been enumerated as required.
Point 10:
Response 10: Change "2.2 Proposal of fast grid connection strategy" to "Method basis of the Proposed fast grid connection strategy" to avoid ambiguity.
In order to make the readers who are not familiar with the field easy to read this paper, the paper is further strengthened, mainly including the following aspects.
11)The original title “2.2 Method basis of the proposed fast grid connection strategy” was changed to "2.2 Method basis of the proposed fast grid connection strategy", and the method was described in detail in Chapter 3 for easy reading.
12)At the beginning of the "III Solution method" section, the theoretical basis of the method is added to the transition to the method of this paper.
13)In addition to the above-mentioned questions mentioned by the experts, we have further refined the wording of the paper, including the unified expression of the name to facilitate reading, and the addition of charts to help readers understand.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Thank you very much for taking into account my comments, then I detail some small errors that I have found:
Place table 1 on one page, is a little table for cut
Figure 6.
Delete Chinese characters and improve the quality of blue text.
dalay --> delay??
384. Figure 10 does not exist reference should be made to figure 9.
Figure 9. I do not understand the meaning of numbering:
Why are there two points 1?
Why different colors?
Author Response
Point 1: Place table 1 on one page, is a little table for cut
Response 1: Thank you very much for your suggestions. Table 1 has been modified as per your requirements.
Point 2: Figure 6. Delete Chinese characters and improve the quality of blue text.
Response 2: The Chinese characters in Figure 6 have been modified to English characters. Improve the clarity of blue text.
Point 3: dalay --> delay??
Response 3: This spelling mistake has been changed.
Point 4: 384. Figure 10 does not exist reference should be made to figure 9.
Response 4: This mistake has been revised.
Point 5: Figure 9. I do not understand the meaning of numbering: Why are there two points 1?
Why different colors?
Response 5: The number in Figure 9 is the serial number of the software output signal channel and has no practical meaning. For this problem, Figure 9 has been further modified. These numbers with nothing actual meaning have been removed.
The reason why waveforms have different colors is that it is convenient to distinguish waveforms.
Author Response File: Author Response.docx
