A Study on Optimal Power System Reinforcement Measures Following Renewable Energy Expansion

Round 1

Reviewer 1 Report

The paper assesses the options for grid strengthening in Korea to accommodate renewable power increases based on the Korean national plan. The strengthening options are assessed in a number of different ways to include static and dynamic elements. Whilst the premise of the paper is of interest, the quality of the paper is not high enough. In particular, it is not made clear at any point what the novel contribution of the paper is, the literature review is lacking and does not include key papers, the methodology is not described in enough detail to allow a proper assessment to be made and the results are presented in a confusing way including many tables, which are insufficiently described.

Major comments

Introduction: Needs to be expanded and improved to make a case for the need for and novelty of the work carried out. I would expect a much more thorough review of the academic literature here, as well as an indication of the novelty of the paper and it's major contributions.

Methodology: Many different types of analysis are carried out, but they are not described in sufficient detail to fully understand the work being done. Lots of flow charts and analysis types are named, but not sufficiently described, e.g. with equations. The descriptions are frequently confusing and unclear, and a thorough, detailed proof reading is required. For example, Table 2 is not described in detail, and it is difficult to work out exactly what it represents, figure 5 is not well described and it’s unclear what it is showing. Section 2.2.3. DB Stabilization seems to be a repeat of 2.2.2. Dispatch with very slight changes

Results: Too many tables are used, and are frequently described without sufficient detail to allow them to be interpreted. Figure 10 has no units on either axis and is impossible to interpret.

Conclusions: these should not include additional tables and figures.

Minor comments

Figures 2 and 3 don’t seem to add anything to the paper and are difficult to read.

Figure 4 and description would be better placed in the methodology

Not all figures and tables are referred to in the main text. This needs correcting.

Units are frequently in error (e.g. line 165 refers to “an 8 km impedance”)

Author Response

To enhance the readability of this paper, we have modified as follows:
(1) Academic literature and report about project applied to grid are added and explained in the text.
(2) To clarify the purpose of this paper, we have described the methodology for deriving the reinforcement based on scenario development of 63.8 GW and 9 GW, renewable modeling based on grid code, and power system assessment.
(3) All tables and figures are referenced and explained in the text.
(4) The number of tables are decreased from 26 to 20.
(5) Some tables are merged or re-arranged.
(6) The name of tables and figures is changed.
(7) The units of graph added.
(8) We will be proofread on the MDPI site.

Author Response File: Author Response.docx

Reviewer 2 Report

No comments.

Author Response

To enhance the readability of this paper, we have modified as follows:
(1) Academic literature and report about project applied to grid are added and explained in the text.
(2) To clarify the purpose of this paper, we have described the methodology for deriving the reinforcement based on scenario development of 63.8 GW and 9 GW, renewable modeling based on grid code, and power system assessment.
(3) All tables and figures are referenced and explained in the text.
(4) The number of tables are decreased from 26 to 20.
(5) Some tables are merged or re-arranged.
(6) The name of tables and figures is changed.
(7) The units of graph added.
(8) We will be proofread on the MDPI site.

Reviewer 3 Report

The paper requires a lot more clarification.
Moreover, there is no proper literature review.
What are other researchers doing in that field?
What about research work being done in other regions of the world?
What models are they using?

From what I saw, none of the tables (except 8) is referenced and explained in the text.
This makes it hard for the reader to understand what they actually show.
Figures 1, 2, 6, 10, 12, A1 are not referenced or explained in the text either.

The model is not explained properly in Section 2.
What is the definition of "load levels"?
In Figure 5 (b), a "new method" and an "existing method" are depicted what does that mean? What does the figure actually show?

The diamonds in the flow charts in Figures 4 and 8 should contain questions such that it becomes clear what the "yes" and "no" branches mean.

What do the values in Tables 3, 4, 5, 7, 15, 16 and 17 mean?
What do "O" and "X" mean in Tables 9, 11, 12, 13, 21, 23, 24 and 25?

General comments:
- Instead of "26-year" and "31-year", it should simply say 2026 and 2031.
- There is no Figure 11.
- The appendix only contains a figure, but no explanation to it.
- The word "figure" is sometimes abbreviated, sometimes it is not.
- It is better to use the word "to" instead of dashes for ranges. It's sometimes confusing in the paper.

Author Response

To enhance the readability of this paper, we have modified as follows:
(1) Academic literature and report about project applied to grid are added and explained in the text.
(2) To clarify the purpose of this paper, we have described the methodology for deriving the reinforcement based on scenario development of 63.8 GW and 9 GW, renewable modeling based on grid code, and power system assessment.
(3) All tables and figures are referenced and explained in the text.
(4) The number of tables are decreased from 26 to 20.
(5) Some tables are merged or re-arranged.
(6) The name of tables and figures is changed.
(7) The units of graph added.
(8) We will be proofread on the MDPI site.

Author Response File: Author Response.docx

Reviewer 4 Report

The introduction must be enriched by at least 15-20 papers regarding "optimal power system reinforcement measures". Both the introduction and paper reviews are weak. 

Author Response

The authors have added several references. Academic literature and report about project applied to grid are added and explained in the text.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Thankyou to the authors for their response to the previous comments. I appreciate the effort to respond, but the following concerns I have remain unaddressed. Please see the following comments.

Major Comments

The authors construct scenarios (I think representing different generation/demand scenarios, like an extended min/max analysis), run the different analysis of these using power flow analysis for static and PSSE for dynamic, from these to determine where limits are violated. The authors then try some different reinforcements and run the analyses again to see what difference is made. The authors need to describe what the novelty of this approach is, and is anything done here generally applicable to other power systems/studies? What is the key conclusion from this? Even after reinforcement, some constraints are violated. What do the authors suggest?

Use of English: The manuscript needs to be checked and corrected by a fluent English speaker, as there are numerous points where the meaning is not clear. I’m not sure if this should be left for editorial staff to do, as I as a reviewer cannot understand what is meant on numerous occasions. I think the authors need to make their meaning clear. Example: “Dividing into three stages by the connection capacity enabled the composition to increase the interconnected bus and interconnection line as the connection capacity increases.” I can not work out what the authors are saying here.

The methodology, and particularly the use of the database needs to be described much more clearly. It is not at all clear to me exactly what the database consists of and how it is used. Examples of this are given in the comments below.

Additional Comments

Abstract: Include key results.

Figure 2 Table and figure not necessary - this information doesn't add anything to the paper and can be described briefly in the text. Figure is too small to make out clearly anyway.

Literature review: The literature need to include more specific examples of previous work to provide background for this paper. A clear statement on the novelty of the approach adopted here and the significance of the work should also be included.

Use of a Database (DB): The significance of the DB used is not clear. For example the authors state: “DB is built based on the static and dynamic models of renewable energy, the dispatch of non-variable sources, and the convergence w.r.t power flow calculation of DB.” But this is very unclear as to what the actual database is. The DB is partly based on the convergence w.r.t power flow calculation of DB? This doesn’t make much sense to me.

Further “After that, the power system assessment is performed based on the DB configuring the inter-connection measures.” How does the DB configure the inter-connection measures? More clarity is needed on points such as this.

Figure 3 and figure 8 have many similar points- what is the difference between the two methodologies shown here? I think the authors need to be much clearer initially about the work being carried out in this paper.

P5 The authors state “the renewable energy outputs in other regions have been set to have an output of 13.3 GW, based on the average output, which is 25.2% of the capacity factor for wind power and 29.7% for PV.” What is the justification for this? How much effect would the demand/generation in other regions affect the analysis? The authors should discuss

P5 The authors state “For the region of uninterest, the modeling was conducted by internal transformer via aggregation, while the Sinan region, the region of interest, is modeled by constructing an external transformer.” I don’t think internal transformer and external transformer. Are common terms in power systems analysis. The authors should clarify their meaning.

Fig 4b What justification is there for "New Method" vs "Existing Method"? more explanation is needed in the text. Text in figure is hard to read

2.2.2 Dispatch. My understanding of this section is that the authors schedule the generation for each region (i.e. the whole power network) via merit order dispatch, but it really isn’t clear and I’m unsure I’ve interpreted correctly. The authors state “the convergence of power flow analysis is secured to ensure overall stability of the DB” but I’ve no idea what they mean by this, as they have never clearly explained exactly what the database is and what it does. The clarity of this section (and most of the rest of the methodology) needs to be greatly improved.

Fig 5) I think this would be better in results section than here.

3.1 Connection measure. Why is the DC line in figure 7 connected to a different bus (Boryeong)? In this scenario, is there still an AC line connected to Shin Hwa Soon.

3.2 Reinforcement: The authors state: “to analyze the point in time when the power system weakens after linking renewable energy, in order to select the point in time”. What "point in time" is being referred to here? Are they modelling different years? Or do they just mean the renewable penetration level at which this happens?

3.2 Reinforcement: The authors state: “… a reinforcement proposal is selected based on the sensitivity analysis first.” How is it selected? This seems a key point in the paper, but is not made clear anywhere.

4.1.3 Contingency analysis: What is meant by divergence here? Should this rather be non-convergence? i.e. the power flow doesn’t converge?

4.1.3 Contingency analysis: Authors state “The contingency analysis was conducted by composing for 22 main lines”. Meaning is not clear here.

4.1.4 Short-circuit analysis: Authors state “To reduce the violations, busbar separation, new breaker installation and CLR installation should be considered.” Are these included in this paper as reinforcement measures? Authors should clarify.

4.1.6 Robustness analysis: Authors state “For the 2031 year, it can be seen that minimum CSCR determined at 4 GW.” From the table, the minimum appears to be at 7 GW

4.2.1 Active power recovery analysis Authors state: “As shown in Figure 10(a), the problem of divergence occurs when the contingency is removed. To solve this problem, the parameters of dynamic simulation changed.” Which parameter and how are they changed? ; “and the parameter for active power recovery time was recovered for each stage as shown Figure 10(c).” What stages are being referred to here? It is not clear ; “In addition, due to instability on the 154 kV renewable energy connection bus, FACTS with small capacity was installed for the bus with a low CSCR value.” What was the effect of this?

Fig 10 a and b, text is too small to read.

4.2.2. Voltage profile analysis; Authors refer to interface lines. What do they mean by this?

4.2.3 Frequency Stability analysis Authors state: “Static reserve denotes the between active power generation and maximum power generation, while dynamic reserve denotes the between active power generation and maximum output of turbine-governor.” I think there are words missing from this sentence

Figure 11 – the text is too small to read

4.3 Establishment of a reinforcement measure. The authors state “In 2026, expansion and replacement were made on the connections with 1 GW (an increase of Uiryeong Current Limiting Reactor capacity), 2 GW (replacement of Shin-Onyang-Chungyang line type upon connection), and 6 GW (Shingangjin-Gwangyang line type replacement). In 2031, expansion and replacement occurred on the connections of 1 GW (Uiryeong Current Limiting Reactor capacity increase, Shinonyang-Chungyang line type replacement) and 5 GW (Shingangjin-Gwangyang line type replacement).” What was the basis for these reinforcement decisions? How did the authors choose these over others? More justification is needed.

Even after the reinforcement measures are put in place, e.g. in table 19 for 80% and 60 % demand, neither measure seems to work. What is the solution then? Should other reinforcements be made? More discussion is needed.

Figure 12, axis values are very small to read and units are not included.

There are 2 figure 12’s. For the second figure 12 more description is needed. Does a filled in block indicate that a reinforcement measure was needed? If so this should be stated clearly. I’d like to see more detail on what the reinforcement measures were as well.

Conclusions: The authors state the optimal reinforcement plan was derived. My reading is that two reinforcement plans were compared, but the choice of these reinforcement plans is not described in detail. Are the authors confident that the reinforcement plan here is the optimal one?

Author Response

Recently, Korea is facing rapid changes in the power system such as increasing renewable energy, expanding HVDC, expanding STATCOM/SVC, and installing TCSC. In addition, a system interconnection plan for offshore wind power and large-scale solar power complexes is being established. However, even though new equipment is supplied to the power system, it is analyzed without considering renewable energy in the power system analysis process. Similarly, for the power system analysis application in EMS, renewable energy is operated as a negative load model. In order to compensate for these problems and maintain the stability of the power system, this paper describes the assessment of the stability according to the expansion of renewable energy and the establishment of an adequate reinforcement procedure for the grid through transmission lines and HVDC. This paper introduces the following.

• First, a power system DB containing a total of 63.8 GW of renewable energy is constructed. Scenarios with renewable energy capacities of 1 to 9 GW are established in the Sinan region.
• Second, renewable energy was modeled according to the Korean Grid Code standard. Renewable energy Aggregation, reactive power supply capability, dynamic model, and LVRT standard are modeled in detail
• Third, static and dynamic stability analysis were performed to examine the stability of renewable energy in connection with the power system. Static analysis performed power flow analysis, contingency analysis, short-circuit analysis, and robustness analysis. Dynamic analysis performed transient stability, frequency stability and LVRT analysis.
• Fourth, adequate grid reinforcement procedures were established according to the connection of renewable energy. After performing static and dynamic stability analysis for each scenario, grid reinforcement procedures based on transmission lines and HVDC were established. When selecting a grid reinforcement location, sensitivity analysis was performed and then reinforced in a substation with high sensitivity.
• After applying the reinforcement, some violations cleared that the ESS and synchronous condenser were installed at the Shinan region.

The renewable energy connection analysis and reinforcement plan establishment procedure proposed in this paper have the following differentiating factors from previous studies.

• First, in general, when reviewing the power system connection of renewable energy, only the region of interest is modeled. However, this paper reflects the characteristics most similar to the future power system, including renewable energy in the whole of Korea.
• Second, an integrated analysis was performed through various static and dynamic stability analysis when examining the connection of renewable energy to the power system.
• Third, we have reasonably established the grid reinforcement procedure according to the expansion of renewable energy. Since reinforcement plan is highly related to economic and social factors, effective reinforcement procedures were established through minimal installation.

As of 2020, in Korea, 18GW of renewable energy has been linked to the power system, mostly small-capacity renewable energy. Korea is planning to connect large-scale renewable energy such as Saemangeum solar power complex and offshore wind power on the west coast. Based on the method proposed in this paper, Korea has a plan to apply the procedures of connecting large-scale renewable energy to power system and reinforcing grid.

After applying the reinforcement proposed in this paper, some violations occurred for dynamic analysis such as transient stability and frequency stability. In order to solve this problem, we are additionally considering the installation of ESS and synchronous condenser. In future research, dynamic modeling of ESS, optimal placement of ESS and synchronous condenser, and operation strategy of ESS are planned.

Author Response File: Author Response.docx

Reviewer 3 Report

While the introduction now includes a short literature review, it lacks details on what the referenced papers dealt with and where the novelty and contributions of this paper lie.

The reference to Germany is unnecessary in my opinion.

The number of scenarious could be reduced as this expands the results by quite some extent.
On the other hand, the conclusion could be extended with a greater summary of the results.

A language check is necessary.

Author Response

Recently, Korea is facing rapid changes in the power system such as increasing renewable energy, expanding HVDC, expanding STATCOM/SVC, and installing TCSC. In addition, a system interconnection plan for offshore wind power and large-scale solar power complexes is being established. However, even though new equipment is supplied to the power system, it is analyzed without considering renewable energy in the power system analysis process. Similarly, for the power system analysis application in EMS, renewable energy is operated as a negative load model. In order to compensate for these problems and maintain the stability of the power system, this paper describes the assessment of the stability according to the expansion of renewable energy and the establishment of an adequate reinforcement procedure for the grid through transmission lines and HVDC. This paper introduces the following.

• First, a power system DB containing a total of 63.8 GW of renewable energy is constructed. Scenarios with renewable energy capacities of 1 to 9 GW are established in the Sinan region.
• Second, renewable energy was modeled according to the Korean Grid Code standard. Renewable energy Aggregation, reactive power supply capability, dynamic model, and LVRT standard are modeled in detail
• Third, static and dynamic stability analysis were performed to examine the stability of renewable energy in connection with the power system. Static analysis performed power flow analysis, contingency analysis, short-circuit analysis, and robustness analysis. Dynamic analysis performed transient stability, frequency stability and LVRT analysis.
• Fourth, adequate grid reinforcement procedures were established according to the connection of renewable energy. After performing static and dynamic stability analysis for each scenario, grid reinforcement procedures based on transmission lines and HVDC were established. When selecting a grid reinforcement location, sensitivity analysis was performed and then reinforced in a substation with high sensitivity.
• After applying the reinforcement, some violations cleared that the ESS and synchronous condenser were installed at the Shinan region.

The renewable energy connection analysis and reinforcement plan establishment procedure proposed in this paper have the following differentiating factors from previous studies.

• First, in general, when reviewing the power system connection of renewable energy, only the region of interest is modeled. However, this paper reflects the characteristics most similar to the future power system, including renewable energy in the whole of Korea.
• Second, an integrated analysis was performed through various static and dynamic stability analysis when examining the connection of renewable energy to the power system.
• Third, we have reasonably established the grid reinforcement procedure according to the expansion of renewable energy. Since reinforcement plan is highly related to economic and social factors, effective reinforcement procedures were established through minimal installation.

As of 2020, in Korea, 18GW of renewable energy has been linked to the power system, mostly small-capacity renewable energy. Korea is planning to connect large-scale renewable energy such as Saemangeum solar power complex and offshore wind power on the west coast. Based on the method proposed in this paper, Korea has a plan to apply the procedures of connecting large-scale renewable energy to power system and reinforcing grid.

After applying the reinforcement proposed in this paper, some violations occurred for dynamic analysis such as transient stability and frequency stability. In order to solve this problem, we are additionally considering the installation of ESS and synchronous condenser. In future research, dynamic modeling of ESS, optimal placement of ESS and synchronous condenser, and operation strategy of ESS are planned.

Author Response File: Author Response.docx

Reviewer 4 Report

No Cooments

Author Response

Recently, Korea is facing rapid changes in the power system such as increasing renewable energy, expanding HVDC, expanding STATCOM/SVC, and installing TCSC. In addition, a system interconnection plan for offshore wind power and large-scale solar power complexes is being established. However, even though new equipment is supplied to the power system, it is analyzed without considering renewable energy in the power system analysis process. Similarly, for the power system analysis application in EMS, renewable energy is operated as a negative load model. In order to compensate for these problems and maintain the stability of the power system, this paper describes the assessment of the stability according to the expansion of renewable energy and the establishment of an adequate reinforcement procedure for the grid through transmission lines and HVDC. This paper introduces the following.

• First, a power system DB containing a total of 63.8 GW of renewable energy is constructed. Scenarios with renewable energy capacities of 1 to 9 GW are established in the Sinan region.
• Second, renewable energy was modeled according to the Korean Grid Code standard. Renewable energy Aggregation, reactive power supply capability, dynamic model, and LVRT standard are modeled in detail
• Third, static and dynamic stability analysis were performed to examine the stability of renewable energy in connection with the power system. Static analysis performed power flow analysis, contingency analysis, short-circuit analysis, and robustness analysis. Dynamic analysis performed transient stability, frequency stability and LVRT analysis.
• Fourth, adequate grid reinforcement procedures were established according to the connection of renewable energy. After performing static and dynamic stability analysis for each scenario, grid reinforcement procedures based on transmission lines and HVDC were established. When selecting a grid reinforcement location, sensitivity analysis was performed and then reinforced in a substation with high sensitivity.
• After applying the reinforcement, some violations cleared that the ESS and synchronous condenser were installed at the Shinan region.

The renewable energy connection analysis and reinforcement plan establishment procedure proposed in this paper have the following differentiating factors from previous studies.

• First, in general, when reviewing the power system connection of renewable energy, only the region of interest is modeled. However, this paper reflects the characteristics most similar to the future power system, including renewable energy in the whole of Korea.
• Second, an integrated analysis was performed through various static and dynamic stability analysis when examining the connection of renewable energy to the power system.
• Third, we have reasonably established the grid reinforcement procedure according to the expansion of renewable energy. Since reinforcement plan is highly related to economic and social factors, effective reinforcement procedures were established through minimal installation.

As of 2020, in Korea, 18GW of renewable energy has been linked to the power system, mostly small-capacity renewable energy. Korea is planning to connect large-scale renewable energy such as Saemangeum solar power complex and offshore wind power on the west coast. Based on the method proposed in this paper, Korea has a plan to apply the procedures of connecting large-scale renewable energy to power system and reinforcing grid.

After applying the reinforcement proposed in this paper, some violations occurred for dynamic analysis such as transient stability and frequency stability. In order to solve this problem, we are additionally considering the installation of ESS and synchronous condenser. In future research, dynamic modeling of ESS, optimal placement of ESS and synchronous condenser, and operation strategy of ESS are planned.

Round 3

Reviewer 1 Report

Thanks to the authors for their detailed response to the comments. I just have a small number of further minor comments.

P9 -  define CSCR and SCR before first use

How does fig 7 relate to fig 5? I can’t see gye-ryong on fig 5. A clearer explanation is needed

I can’t see that ESS is defined anywhere

Author Response

We modified Figure 5.

Author Response File: Author Response.docx

Reviewer 3 Report

Thank you for incorporating my comments.

Author Response

We modified Figure 7 and some definition.