Systematic Development and Hardware-in-the-Loop Testing of an IEC 61850 Standard-Based Monitoring and Protection System for a Modern Power Grid Point of Common Coupling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have presented a good case study. But it’s not clear what the main difference is between this article and other available ones. It has been mentioned that a complete and integrated protection framework is one of the main contributions. It’s essential to answer this question, that only integrating the available methods is the only contribution of this research, or if they have modified different parts of the research.  So, it’s a major issue, and it’s necessary to highlight the main contributions. Since they have presented a good case of real-time simulation tests and so on, it’s good to give a chance to revise the article.

Also, the authors should respond to the following review comments carefully:

• The paper’s title should be revised. Currently, the paper’s title is so generic. It’s not clear what the main contribution is. Indeed, the paper’s title should reflect the main contribution clearly. Also, if it’s possible to shorten the paper’s title, it would be helpful.
• The abstract should be revised. Addressing the gaps in the literature in the abstract is recommended. From different viewpoints (theoretical and industrial ones), addressing the gaps in the literature emphasizes the paper's advantages.
• It’s needed to explain the test setup clearly in the abstract. Simulating the current and load sources, and the protection testing device should be explained briefly in the abstract.
• Adding some quantitative advantages of this study to the abstract is suggested.
• It’s not clear to me the relation between busbar protection and contents in the abstract. Do load conditions affect the busbar protection? Or will the busbar protection performance be affected due to external faults and short circuits?
• The second listed contribution is so generic. Doing the power flow before the short circuit analyses could be a good note, but not one of the main contributions. Other listed contributions have the same issue. It’s needed to revise the listed contributions and remove unnecessary ones.
• It’s needed to give a photo of the test setup. Fig. 35 is good, but not enough.
• As it’s shown in Fig. 35, different relay testers have been used, as well as RTDS and relays. Which relay testers have been used? In Fig. 35, Omicron CMC relay testers have been shown?
• How have the CT and PT performances been considered?
• 40 should be explained in more detail. Other comtrade files should be explained and discussed in detail.
• The impacts of IEC 61850, communication, and protocols should be discussed in more detail.

Comments for author File: Comments.pdf

Author Response

General note

Once again, we sincerely thank the reviewer for the constructive feedback. Please note that all newly added text, discussions, and quantitative test results mentioned in the replies below are highlighted in orange in the revised manuscript to clearly show the changes made.

 

Comment 1

The paper’s title should be revised. Currently, the paper’s title is so generic. It’s not clear what the main contribution is. Indeed, the paper’s title should reflect the main contribution clearly. Also, if it’s possible to shorten the paper’s title, it would be helpful.

 

Reply to comment 1

We have carefully considered this point during the revision process. We respectfully opted to retain the current title, “Systematic Development and Hardware-in-the-Loop Testing of an IEC 61850 Standard-Based Monitoring and Protection System for the Point of Common Coupling in Modern Power Grids”, because it was intentionally formulated to capture the core novelty and scope of this work, namely:

• the systematic development process (covering instrument transformer design, relay logic configuration, and IEC 61850 dataset binding),
• the hardware-in-the-loop (HIL) validation on a real-time digital simulator with physical SEL relays, and
• its specific application at the Point of Common Coupling (PCC) in a modern wind-integrated transmission-level grid.

 

These elements together form the central contribution of the study, and the title directly mirrors this combined methodological and application-focused novelty.

 

We also note that technical journals in this field (IEC 61850-based protection studies) often use descriptive titles to help readers identify the exact application domain and testing methodology, which improves discoverability and indexing. Shortening the title by omitting key elements such as “HIL Testing” or “IEC 61850” would risk obscuring its distinctive focus and making it appear similar to prior work.

 

For these reasons, and after careful consideration, we believe the current title best represents the contribution and scope of the manuscript while remaining aligned with field conventions.

 

Comment 2

The abstract should be revised. Addressing the gaps in the literature in the abstract is recommended. From different viewpoints (theoretical and industrial ones), addressing the gaps in the literature emphasizes the paper's advantages.

 

It’s needed to explain the test setup clearly in the abstract. Simulating the current and load sources, and the protection testing device should be explained briefly in the abstract.

 

Reply to comment 2

We carefully considered this point and have revised the abstract to better reflect the identified research gap, the experimental setup, and the paper’s contribution from both theoretical and practical perspectives.

 

Specifically, the revised abstract now:

• Highlights the gap in the literature by stating that most prior IEC 61850-based protection studies focused on isolated functions or distribution-level networks, while this study addresses the lack of integrated monitoring and protection schemes validated at a transmission-level PCC with a large-scale wind power plant, thereby clarifying the paper’s theoretical and industrial relevance.
• Briefly describes the test setup, stating that a real-time digital simulator (RTDS) running an IEEE 9-bus system integrated with a large-scale wind power plant was used to emulate the power system and current/load sources, and that the proposed low-impedance busbar differential scheme was implemented and validated on a physical SEL-487B protection relay interfaced via IEC 61850 GOOSE messaging.

 

We also note that similar feedback on strengthening the abstract was previously raised by Reviewer 4 (Comment 1), who recommended adding quantitative performance metrics (trip times, operate/restraint currents, and GOOSE delays). This has been done, and these values are now included in the abstract to demonstrate the system’s measured performance.

 

Additionally, the concern about clearly highlighting the contribution and uniqueness of this work was raised by Reviewer 2 (Comment 1), and the abstract was updated to better align with the detailed contributions described in the Introduction.

 

With these revisions, the abstract now (i) contextualizes the study within the existing literature, (ii) clearly states the test setup, and (iii) highlights the novel contribution and validated performance of the proposed IEC 61850-based protection system.

 

Comment 3

Adding some quantitative advantages of this study to the abstract is suggested.

 

It’s not clear to me the relation between busbar protection and contents in the abstract. Do load conditions affect the busbar protection? Or will the busbar protection performance be affected due to external faults and short circuits?

 

Reply to comment 3

Regarding the suggestion to include quantitative advantages in the abstract, we have now revised the abstract to explicitly state the key measured performance metrics of the proposed system, including:

• Differential operate/restraint currents of 0.32/4.38 PU under initial load demand and 1.96/6.20 PU under increased load demand, confirming stable relay operation across loading conditions,
• Trip times consistently below 300 ms, with full end-to-end fault clearance measured at 606.667 ms (initial load) and 706.667 ms (increased load), and
• IEC 61850 GOOSE communication delays consistently below 1 ms with no packet losses.

 

We note that this point was also raised by Reviewer 4 (Comment 1), and the requested quantitative metrics have been incorporated into the abstract to emphasise the validated performance and practical advantages of the proposed scheme.

 

Regarding the relation between busbar protection and load conditions, we have clarified this link in the abstract and discussed it in Section 6.2. In our system, the busbar differential protection logic uses adaptive setting groups (Group 1 for initial load and Group 2 for increased load). Changing load conditions influence the magnitude of through-currents and CT saturation risk, which can affect the stability margin of busbar differential elements if not properly accounted for. The implemented adaptive setting group logic ensures that busbar protection performance remains stable and secure across different loading conditions, even during external faults or short circuits, and this contribution has now been explicitly stated in the abstract to strengthen clarity.

 

With these additions, the abstract now both presents the quantitative advantages of the system and clearly explains how load conditions are incorporated into the busbar protection design and performance evaluation.

 

Comment 4

The second listed contribution is so generic. Doing the power flow before the short circuit analyses could be a good note, but not one of the main contributions. Other listed contributions have the same issue. It’s needed to revise the listed contributions and remove unnecessary ones.

 

Reply to comment 4

While the initial load flow assessment before short-circuit analyses was included to demonstrate methodological thoroughness and ensure network stability before applying protection tests, we acknowledge that it may not represent a primary scientific contribution on its own. To address this, we have revised the contributions list in the Introduction by:

• Removing overly generic points (such as the initial load flow assessment) as standalone contributions, and
• Emphasising the central contributions, namely:
• the systematic development of a low-impedance IEC 61850 busbar differential protection scheme implemented on a physical SEL-487B relay at a transmission-level PCC,
• its real-time hardware-in-the-loop (HIL) validation using an RTDS and IEC 61850 GOOSE communication,
• the adaptive relay setting group logic based on changing load conditions, and
• the formal CT/VT selection and IEC 61850 configuration framework designed for scalability and interoperability.

 

We note that this point about clarifying and strengthening the uniqueness of the contributions was also raised by Reviewer 2 (Comment 1), who requested clearer positioning and comparison to recent IEC 61850/HIL studies. The revised contributions list now directly reflects these distinctive aspects and removes any elements that could appear as routine procedural steps rather than novel contributions.

 

This ensures the listed contributions are concise, non-generic, and focused on the key novel outcomes of the study.

 

Comment 5

It’s needed to give a photo of the test setup. Fig. 35 is good, but not enough.

 

As it’s shown in Fig. 35, different relay testers have been used, as well as RTDS and relays. Which relay testers have been used? In Fig. 35, Omicron CMC relay testers have been shown?

 

Reply to comment 5

To enhance the clarity of our test setup, we have revised the schematic of the testbench (previously Figure 35, now Figure 18) and included an actual photograph of the complete hardware-in-the-loop (HIL) testbench as Figure C1 in Appendix C of the revised manuscript. This photo shows the full laboratory arrangement, including:

• the Real-Time Digital Simulator (RTDS) rack running the IEEE 9-bus system integrated with the large-scale wind power plant (LSWPP),
• Giga-Transceiver Analogue Output (GTAO) and Giga-Transceiver Front Panel Interface (GTFPI) modules used to interface the RTDS signals with the protection relays,
• the SEL-487B Protection Automation Control relay, configured to perform the low-impedance busbar differential protection function,
• the SEL-751A feeder protection relay, which acted as the circuit breaker Intelligent Electronic Device (IED) receiving GOOSE-based trip commands from the SEL-487B, and
• Omicron CMS 156/356 voltage and current amplifiers used to boost the ±10 V analogue outputs from the RTDS to the nominal secondary current and voltage levels required by the SEL relays.

We also clarify that no Omicron CMC relay test sets were used in this work. The reviewer’s impression may have arisen from the physical appearance of the Omicron CMS 156/356 amplifiers, which are dedicated signal amplifiers and not protection relay testers.

 

As much as we have included a photo of the test bench setup in the appendix, we also wish to kindly make the reviewer aware that our laboratory, where the overall setup is located, is extremely small, which made it difficult to capture a clear and properly aligned photograph suitable for this manuscript. We would gladly follow the editor’s or reviewer’s recommendation should they prefer that this photo be moved to the supplementary materials rather than appearing within the main manuscript.

 

Comment 6

How have the CT and PT performances been considered?

 

Reply to comment 6

The performance of the current transformers (CTs) and potential transformers (PTs) used in this study was carefully considered during the development phase to ensure accurate and dependable operation of the proposed protection system. As described in Section 3.2.1 (Instrument Transformer Selection and Configuration), the CT and PT ratios, burden, and accuracy classes were selected in accordance with IEEE C57.13 and IEC 61869 standards, ensuring that:

• the CTs provide sufficient accuracy and saturation margin under maximum through-fault and load currents, and
• the PTs provide stable and precise voltage signals to the SEL-487B relay during both steady-state and fault conditions.

 

Furthermore, their configuration within the SEL-487B relay was verified during the HIL tests to confirm that the measured operate and restraint currents (0.32/4.38 PU under initial load and 1.96/6.20 PU under increased load) were consistent with expected values, indicating that the instrument transformers were performing correctly and did not introduce measurement errors that could compromise differential protection.

 

We also note that the importance of including standards-based CT/VT design and configuration as part of the contribution of this work was highlighted by Reviewer 4 (Comment 3), who pointed out that many previous IEC 61850-based protection studies did not explicitly consider instrument transformer performance.

 

This feedback led to the explicit mention of the CT/VT design methodology as part of the scientific contribution in Section 6.4 (Comparative Contributions) and Table 9, which contrasts this study with prior works.

 

This integration of CT/PT performance considerations ensures accurate signal acquisition, secure differential protection operation, and reproducibility of the developed framework.

 

Comment 7

40 should be explained in more detail. Other comtrade files should be explained and discussed in detail.

 

Reply to comment 7

In the revised manuscript, the content previously shown as Figure 40 now appears as Figures 23–26 following the restructuring of the results section. We have expanded the accompanying discussion in Section 5.2.2 (Protection Performance) to provide a clearer explanation of these figures and their associated COMTRADE waveform files.

Specifically, the revision now includes:

• A description of the fault type, location, and applied loading condition for each COMTRADE capture,
• An explanation of the signals shown in the figures (operate and restraint currents, differential element pickup, trip command status, and breaker operation flags), and
• A discussion of how these signals verify correct operation of the busbar differential scheme and the adaptive Group 1 to Group 2 switchover logic.

 

We also added text explaining how the COMTRADE files were captured from the SEL-487B relay during HIL testing via the RTDS/RSCAD and GTFPI interfaces, and how they were analysed to extract:

• Operate/restraint current values (0.32/4.38 PU under initial load; 1.96/6.20 PU under increased load),
• Fault detection-to-trip times (< 300 ms), and
• GOOSE-based breaker operation confirmation delays (< 1 ms).

 

This improvement addresses this reviewer’s concern and also complements the earlier feedback from Reviewer 4 (Comment 4), who noted that these waveform plots (previously Figures 36–43) required more quantitative analysis and interpretation.

 

With these additions, the COMTRADE-based results are now thoroughly explained and supported by clear numerical indicators, making their interpretation straightforward for readers.

 

Comment 8

The impacts of IEC 61850, communication, and protocols should be discussed in more detail.

 

Reply to comment 8

In the revised manuscript, we have expanded the discussion of IEC 61850 communication and protocols to more clearly articulate their functional impact on the proposed monitoring and protection system. This has been incorporated primarily in Section 3.5 (IEC 61850 Communication Integration) and further discussed in Section 6.4 (Comparative Contributions).

Specifically, the revised text now explains that:

• The system uses IEC 61850 GOOSE messaging for high-speed, peer-to-peer delivery of protection trips and process measurements between the RTDS model and the SEL-487B/SEL-751A relays, achieving deterministic latencies consistently below 1 ms with no packet losses, which is essential for time-critical protection.
• IEC 61850 logical nodes, data sets, and control blocks were systematically configured in the ICD/SCD files to create a vendor-neutral and modular architecture that enables interoperability between different IEDs.
• The adoption of IEC 61850 standard data models decouples the protection logic from vendor-specific implementations, allowing the framework to be scaled to multivendor environments and additional PCCs without modifying the core logic.
• The use of standardized sampled value (SV) and GOOSE services ensures time-coherent delivery of current and voltage measurements and trip signals, directly affecting the dependability and speed of the protection functions.

 

We also note that Reviewer 4 (Comment 3) raised a related concern that previous studies often omitted interoperability and standards-based instrument transformer design, and this prompted us to explicitly discuss the role of IEC 61850-based communication in ensuring interoperability and scalability in Section 6.4 and the new Section 6.5 (Scalability and Interoperability Considerations).

 

Through these revisions, the manuscript now clearly articulates the operational and architectural impacts of IEC 61850 communication and protocols on the developed framework, addressing the reviewer’s concern.

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

Enhance positioning and uniqueness
Though there is little discussion of originality in comparison to more recent publications, the research offers a thorough HIL protection architecture based on IEC 61850. What distinguishes this study from current IEC 61850 and HIL protection studies might be better highlighted by including a clearer comparison (e.g., adaptive differential schemes, advanced interoperability solutions, or grid-forming inverter integration).

Extend your quantitative analysis
The majority of the outcomes are qualitative (trip confirmations, waveforms). Including quantitative performance indicators, such comparing error margins under CT saturation, GOOSE signal transmission delay, or defect detection time in milliseconds, will strengthen the proof of the system's dependability.

A discussion about interoperability and scalability
Though replicability is mentioned, the approach's scalability to bigger networks and handling of multivendor interoperability issues—both of which are crucial in actual IEC 61850 substations—are not examined in the study. Adding more context to this conversation would make the research more useful.

Improve the course of future work
Potential enhancements (adaptive logic, cybersecurity, inverter functionality) are briefly listed in the future development section. To demonstrate how this study might develop into a more comprehensive framework, a roadmap with particular test cases (such as cyberattack resilience scenarios, interoperability benchmarks, and fault ride-through with inverter-rich systems) could be proposed.

Comments for author File: Comments.pdf

Author Response

General note

Once again, we sincerely thank the reviewer for the constructive feedback. Please note that all newly added text, discussions, and quantitative test results mentioned in the replies below are highlighted in red in the revised manuscript to clearly show the changes made.

 

Comment 1: Enhance positioning and uniqueness
Though there is little discussion of originality in comparison to more recent publications, the research offers a thorough HIL protection architecture based on IEC 61850. What distinguishes this study from current IEC 61850 and HIL protection studies might be better highlighted by including a clearer comparison (e.g., adaptive differential schemes, advanced interoperability solutions, or grid-forming inverter integration).

 

Reply to comment 1

In the revised manuscript, we have strengthened the positioning of our study by explicitly contrasting it with recent IEC 61850-based protection studies. While earlier works mainly addressed isolated aspects such as interoperability [6], backup subscription schemes [5], composite sequence current techniques [12], or overcurrent protection testing [17], our study uniquely integrates multiple advanced features that have not been demonstrated together before. Specifically, it presents:

• A low-impedance IEC 61850 busbar differential protection scheme implemented on a physical SEL-487B relay at a transmission-level PCC within an IEEE 9-bus system integrated with a large-scale wind power plant,
• Real-time HIL validation on RTDS hardware using physical SEL-487B and SEL-751A relays,
• Adaptive relay setting logic that automatically switches between Group 1 and Group 2 settings based on system loading conditions,
• Pre-fault load flow assessment to verify network stability before applying protection tests,
• Fully documented configuration procedures (including CT/VT sizing, relay alias tables, and IEC 61850 ICD/SCD datasets) to support reproducibility, and
• A modular and scalable IEC 61850 framework that forms a foundation for future interoperability, cyber-resilience, and inverter-integration studies.

 

This combined hardware-based validation, adaptive configuration, and realistic transmission-level context establishes a novel and replicable protection framework, which forms the core contribution of this work.

 

Comment 2: A discussion about interoperability and scalability
Though replicability is mentioned, the approach's scalability to bigger networks and handling of multivendor interoperability issues—both of which are crucial in actual IEC 61850 substations—are not examined in the study. Adding more context to this conversation would make the research more useful.

 

Reply to comment 2

We have now expanded the discussion section to include the scalability and interoperability aspects of the proposed framework. The revised manuscript clarifies that the system was intentionally built using standardized IEC 61850 logical nodes, GOOSE-based peer-to-peer messaging, and modular ICD/SCD configuration files, which allow additional feeder bays, busbar zones, or PCCs to be incorporated without altering the underlying logic. This modular architecture supports scaling the framework to multi-bus or multi-substation networks and facilitates future integration of multivendor IEDs, addressing interoperability challenges highlighted in [6].

 

Comment 3: Extend your quantitative analysis
The majority of the outcomes are qualitative (trip confirmations, waveforms). Including quantitative performance indicators, such comparing error margins under CT saturation, GOOSE signal transmission delay, or defect detection time in milliseconds, will strengthen the proof of the system's dependability.

 

Reply to comment 3:

We have incorporated quantitative performance indicators into Section 5.2.2 and Section 6 of the revised manuscript to substantiate the dependability, speed, and security of the proposed protection system. Specifically, we now report:

• Fault detection and trip times: The SEL-487B relay consistently detected internal faults and issued trip signals within less than 300 ms from fault inception to trip issuance, while the measured end-to-end fault clearance times (relay + breaker) were 606.667 ms under initial load and 706.667 ms under increased load.
• GOOSE-based breaker confirmation delays: The associated breaker operation confirmations over IEC 61850 GOOSE were consistently received within < 1 ms, confirming deterministic, low-latency peer-to-peer communication.
• CT saturation as future work: Although CTs were carefully selected and configured to minimise saturation effects (Section 3.2.1), the current study did not explicitly test the system under intentional CT saturation conditions. This aspect has been identified as part of the planned future enhancements to further assess and improve the security margin of the protection scheme.

 

These integrated quantitative metrics demonstrate that the developed IEC 61850-based protection system achieves fast (< 300 ms) fault detection, secure operation under stressed conditions, and reliable GOOSE-based trip confirmation, validating its suitability for transmission-level PCC protection in renewable-integrated networks.

 

Comment 4: Improve the course of future work
Potential enhancements (adaptive logic, cybersecurity, inverter functionality) are briefly listed in the future development section. To demonstrate how this study might develop into a more comprehensive framework, a roadmap with particular test cases (such as cyberattack resilience scenarios, interoperability benchmarks, and fault ride-through with inverter-rich systems) could be proposed.

 

Reply to comment 4:

The future work section (Section 7) has been substantially expanded to present a clear roadmap of targeted enhancements and staged test cases aimed at evolving the current system into a more comprehensive IEC 61850-based protection and monitoring platform for renewable-integrated power grids. The roadmap now includes:

• Adaptive logic refinement: Enhancing the automatic switchover between Group 1 and Group 2 relay settings, including automatic reversion to Group 1 once loading returns to normal, tested through staged load variation, feeder switching, and islanding/reconnection scenarios.
• Grid-forming inverter (GFM) integration: Incorporating GFM models in the RTDS environment to evaluate their contribution to fault ride-through capability, system inertia, and frequency support under high renewable penetration.
• Cybersecurity and resilience testing: Assessing the robustness of the IEC 61850-based framework by injecting delayed or corrupted GOOSE and sampled value messages to simulate cyberattacks and verifying the protection system’s resilience and intrusion detection strategies.

 

These planned activities provide a structured pathway for expanding the developed framework beyond monitoring and protection-only functions toward a fully integrated monitoring–operation–protection (MOP) platform suitable for future renewable-dominated power grids.

 

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript primarily demonstrates the implementation of standard tools, that is, SEL-487B, RTDS, IEC 61850, and HIL. In the literature, similar studies have already been published. The authors are recommended to explicitly highlight what is new compared to prior work.

In this work, the results are primarily qualitative. No quantitative performance metrics, such as: trip times (ms) under different conditions, and fault detection sensitivity limits, are reported.

The results section confirms correct operation but does not critically analyze limitations or compare with state-of-the-art methods, for example, transient-energy-based, sequence component, or frequency-based schemes. A benchmarking comparison would significantly improve impact.

Figures 25–33, relay configuration screenshots, are too detailed for the main text; consider moving them to supplementary material.

The study relies on a single relay model (SEL-487B) and a small test system (IEEE 9-bus). How generalizable are the results to other relays or vendors?

The paper is very long (59 pages), the authors should condense it to improve readability.

Some tables and results are repetitive; for instance, differential element settings could be summarized more concisely.

The authors should comment on the practical limitations of their approach.

Many typos need to be fixed.

 

 

 

 

Comments on the Quality of English Language

Many typos need to be fixed.

Author Response

General note

Once again, we sincerely thank the reviewer for the constructive feedback. Please note that all newly added text, discussions, and quantitative test results mentioned in the replies below are highlighted in green in the revised manuscript to clearly show the changes made.

 

Comment 1

The manuscript primarily demonstrates the implementation of standard tools, that is, SEL-487B, RTDS, IEC 61850, and HIL. In the literature, similar studies have already been published. The authors are recommended to explicitly highlight what is new compared to prior work.

 

Reply to comment 1

We have explicitly clarified the novelty of this study in the Introduction and Section 6.4 (Comparative Contributions), supported by a new Table 9.

 

We now emphasise that, unlike previous IEC 61850-based protection studies that focused on isolated aspects (e.g., overcurrent protection [17], backup subscription schemes [5], interoperability [6], or composite sequence-current methods [12]), our work uniquely integrates:

• a low-impedance IEC 61850 busbar differential scheme implemented on a physical SEL-487B relay at a transmission-level PCC within an IEEE 9-bus system integrated with a large-scale wind power plant (LSWPP),
• real-time HIL validation using RTDS hardware and physical SEL-487B/SEL-751A relays,
• adaptive setting group logic that switches between Group 1 and Group 2 based on dynamic load conditions, and
• a standards-based CT/VT design and IEC 61850 dataset framework enabling scalability and interoperability.

 

This clarification directly addresses this comment and also aligns with similar feedback from Reviewer 2 (Comment 1) and Reviewer 4 (Comment 3).

 

Comment 2

In this work, the results are primarily qualitative. No quantitative performance metrics, such as: trip times (ms) under different conditions, and fault detection sensitivity limits, are reported.

 

Reply to comment 2

We have integrated key quantitative performance metrics throughout the results, particularly in Section 5.2.2 (Protection Performance) and Section 6.2 (Discussion).

 

Specifically, we now report:

• Operate/restraint currents of 0.32/4.38 p.u. under initial load and 1.96/6.20 p.u. under increased load,
• Fault detection + trip issuance times consistently of 300 ms, with total end-to-end clearance of 606.667 ms (initial load) and 706.667 ms (increased load), and
• GOOSE-based breaker operation confirmation delays consistently < 1 ms with no packet losses.

 

These additions directly address this comment and are also aligned with similar requests from Reviewer 1 (Comment 3), Reviewer 2 (Comment 3), and Reviewer 4 (Comments 1 and 4).

 

Comment 3

The results section confirms correct operation but does not critically analyze limitations or compare with state-of-the-art methods, for example, transient-energy-based, sequence component, or frequency-based schemes. A benchmarking comparison would significantly improve impact.

 

Reply to comment 3

We have added a new comparative analysis subsection (Section 6.4) and Table 9, which benchmarks this work against selected recent IEC 61850/HIL protection studies.

 

This subsection highlights that while prior works mainly focused on isolated aspects (overcurrent, backup subscription, interoperability), our work is the first to combine real-time IEC 61850-based monitoring, adaptive low-impedance busbar differential protection on a physical relay, and formal CT/VT design within a transmission-level PCC setting.

 

This benchmarking directly addresses the reviewer’s concern and builds on similar feedback from Reviewer 2 (Comment 1) and Reviewer 4 (Comment 3).

 

Comment 4

Figures 25–33, relay configuration screenshots, are too detailed for the main text; consider moving them to supplementary material.

 

Reply to comment 4

The detailed relay configuration screenshots (previously Figures 25–33) have been removed from the main text and relocated to Appendix B: IEC 61850 Communication and HIL Setup Screenshots, while only one summarising schematic of the setup remains in the main body.

 

Sections 3.2 and 4.2 were also condensed to include only key logic and references to the appendices.

This change improves readability and aligns with a similar recommendation from Reviewer 4 (Comment 2).

 

Comment 5

The study relies on a single relay model (SEL-487B) and a small test system (IEEE 9-bus). How generalizable are the results to other relays or vendors?

 

Reply to comment 5

We have addressed this by adding a new Section 6.5 (Scalability and Interoperability Considerations).

 

Here, we explain that the framework was designed using modular IEC 61850 logical nodes, GOOSE-based peer-to-peer messaging, and standardized ICD/SCD files, which makes it vendor-neutral and easily expandable to other IEDs, feeder bays, busbar zones, and PCCs without altering core logic.

 

This addition addresses this comment and also aligns with similar concerns from Reviewer 2 (Comment 2) and Reviewer 4 (Comment 6).

 

Comment 6

The paper is very long (59 pages), the authors should condense it to improve readability.

 

Reply to comment 6

To improve readability, we have condensed lengthy procedural content in Sections 3.2 and 4.2, moved detailed parameter tables to Appendix A, and relocated stepwise screenshots to Appendix B.

 

This restructuring reduces the main text’s length and shifts the focus from descriptive setup content to the scientific results and discussion.

 

This revision also aligns with Reviewer 4 (Comment 2), who raised the same concern.

 

Comment 7

Some tables and results are repetitive; for instance, differential element settings could be summarized more concisely.

 

Reply to comment 7

In the revised manuscript, repetitive settings tables have been consolidated into a single summary table (Table 8), while full parameter listings are now only provided in Appendix A.

 

This change reduces repetition and improves clarity, while still preserving reproducibility for future researchers.

 

Comment 8

The authors should comment on the practical limitations of their approach.

 

Reply to comment 8

In response, we have added an explicit paragraph on practical limitations at the end of Section 6 (Discussion).

 

This paragraph acknowledges that the current framework:

• has only been validated on a single relay model (SEL-487B) and an IEEE 9-bus system,
• has not yet been tested under extreme conditions such as intentional CT saturation, cyberattacks, or very high penetration of inverter-based resources, and
• will require multi-vendor interoperability testing before field deployment.

 

We also cross-reference how these aspects are planned to be addressed in Section 7 (Future Work), which outlines adaptive logic refinement, grid-forming inverter integration, and IEC 62351-compliant cybersecurity testing.

 

This addition addresses this comment, which is unique to Reviewer 3 and had not been previously requested by Reviewers 1, 2, or 4.

 

Comment 9

Many typos need to be fixed.

 

Reply to comment 9

The revised manuscript has undergone a full language and technical editing pass, and all typographical, grammatical, and formatting errors identified during review have been corrected.

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript is technically rich, presenting the systematic development and HIL validation of an IEC 61850-based monitoring and protection system at the point of common coupling (PCC) in a modern wind-integrated grid. The work is relevant and aligned with current research trends in renewable-integrated power systems. The technical detail is impressive, and the use of the SEL-487B relay in a real-time digital simulation environment provides practical value. 

Major concerns rising from the current version of the work are:

1. The abstract should better highlight the quantitative results (trip time, accuracy).
2. The manuscript is overly descriptive in sections (especially relay configuration details, aliasing tables, etc.). Many of the HIL section reads like a relay setup manual. While this proves reproducibility, it overwhelms the scientific message. This could be streamlined into appendices or supplementary material. Long figures and screenshots - Figures 19–34 are very detailed but distract from the main flow. Consider summarizing the procedure in the main text and moving screenshots to supplementary files. Keep focus on insights and outcomes.
3. The scientific contribution is less emphasized. Many results are presented as “the system worked” without comparative or quantitative evaluation. Try to compare your system’s performance to other recent IEC 61850/HIL protection studies (some are cited in the Introduction but not discussed). As example, authors cite [5], [6], [10], [12], [17], etc., but do not explicitly compare how their scheme performs vs. those.
4. Figures 36–43 (monitoring/fault detection results) are useful, but they lack quantitative analysis. They mostly show waveforms without extracting fault detection times, CT saturation behavior or latency.
5. Figures 7–9, 19–34 have the text too small to read; authors should improve clarity.

6. The HIL setup uses a 9-bus + one WPP model, which is a simplified test grid. This raises the question: Would the proposed scheme scale to larger, real-world grids with multiple PCCs and multivendor devices? Please address this limitation.

7. Authors correctly list IEC 62351 cybersecurity in future work, but the HIL platform could also be used to inject faulty GOOSE/SV messages, delays, or dropouts. At least mention that your HIL testbed is capable of being extended for cyber-attack resilience testing.

Author Response

General note

Once again, we sincerely thank the reviewer for the constructive feedback. Please note that all newly added text, discussions, and quantitative test results mentioned in the replies below are highlighted in grey in the revised manuscript to clearly show the changes made.

 

Comment 1

The abstract should better highlight the quantitative results (trip time, accuracy).

 

Reply to comment 1

Reviewer 1 noted that “the abstract should better highlight the quantitative results (trip time, accuracy).” We agree with this point. In response, we have revised the abstract to explicitly include the key quantitative performance metrics that demonstrate the accuracy, speed, and reliability of the developed IEC 61850-based monitoring and protection system. These metrics were already measured and discussed in the results section, and they are now summarised in the abstract as follows:

• Differential operate/restraint currents: 0.32/4.38 PU under initial load demand and 1.96/6.20 PU under increased load demand, confirming stable operation across different loading conditions.
• Trip times: Fault detection plus breaker operation consistently completed within less than 300 ms, with total fault clearance measured at 606.667 ms (initial load) and 706.667 ms (increased load) in the HIL setup.
• GOOSE communication delays: Deterministic peer-to-peer messaging with latency consistently below 1 ms and no packet losses, confirming reliable high-speed signalling.

 

By explicitly stating these results in the abstract, the revised version now highlights the quantitative validation of the system’s performance, directly addressing Reviewer 1’s concern and strengthening the overall scientific contribution of the study.

 

Comment 2

The manuscript is overly descriptive in sections (especially relay configuration details, aliasing tables, etc.). Many of the HIL section reads like a relay setup manual. While this proves reproducibility, it overwhelms the scientific message. This could be streamlined into appendices or supplementary material. Long figures and screenshots - Figures 19–34 are very detailed but distract from the main flow. Consider summarizing the procedure in the main text and moving screenshots to supplementary files. Keep focus on insights and outcomes.

 

Reply to comment 2

This concern about the manuscript being overly descriptive was also raised by other reviewers. We acknowledge that several parts of the original submission, especially the detailed relay configuration steps, aliasing tables, and numerous stepwise screenshots, made the work read more like a technical setup manual than a scientific article. These details were initially included to ensure full reproducibility of the study, as implementing an IEC 61850-based monitoring and protection system and integrating it with an HIL testbench can be challenging for new researchers.

 

In line with your comment, we revised the manuscript to better align it with the purpose and scope outlined in the abstract, which emphasises presenting a systematic approach to the development and validation of the proposed IEC 61850-based monitoring and protection system (rather than serving as a configuration guide). Specifically, we have:

• Relocated the highly descriptive content, including detailed configuration procedures, aliasing tables, and parameter listings, to Appendix A SEL-487B Configuration Details for reference only.
• Moved stepwise screenshots of the IEC 61850 communication and HIL setup to Appendix B, and kept only one summarising schematic of the overall testbench in the main text.
• Condensed Sections 3.2 and 4.2 to present only the key configuration logic, design decisions, and their rationale, while using cross-references to the appendices for full details.

 

This restructuring maintains reproducibility for readers who wish to replicate the setup, but ensures the main text is now focused on the scientific contributions and experimental outcomes, consistent with what is stated in the abstract.

 

Comment 3

The scientific contribution is less emphasized. Many results are presented as “the system worked” without comparative or quantitative evaluation. Try to compare your system’s performance to other recent IEC 61850/HIL protection studies (some are cited in the Introduction but not discussed). As example, authors cite [5], [6], [10], [12], [17], etc., but do not explicitly compare how their scheme performs vs. those.

 

Reply to comment 3

We have substantially revised Section 6 (Results and Discussion) to clearly emphasise the scientific contribution of this study and to distinguish it from previous IEC 61850/HIL-based protection studies cited in the introduction (e.g., [5], [6], [10], [12], [17]).

 

Firstly, we restructured the discussion into two clearly defined case studies to present our results more explicitly:

• Case Study 1: Validates the real-time monitoring of electrical and mechanical signals from WTGU13 using IEC 61850 GOOSE messaging. This demonstrated accurate transmission of process measurements from the RTDS model to the physical relays, with no packet losses. We also state that these monitored signals (with others to be added) will form the basis of our next research stage to develop a coordinated monitoring–operation–protection (MOP) framework.
• Case Study 2: Evaluates the protection performance of the low-impedance busbar differential scheme under both initial and increased load demand conditions, confirming correct operation of the adaptive Group 1 to Group 2 switchover logic and fault detection + breaker operation within < 300 ms, with full end-to-end fault clearance measured at 606.667 ms and 706.667 ms, respectively.

 

Secondly, we added a new comparative contribution subsection (Section 6.4), which highlights that prior studies mainly addressed isolated aspects — such as overcurrent protection [17], backup subscription schemes [5], or interoperability [6] — and often omitted real-time monitoring or standards-based instrument transformer design. In contrast, this work uniquely combines:

• real-time IEC 61850-based monitoring,
• adaptive low-impedance differential protection implemented on a physical SEL-487B relay and validated using RTDS, and
• formal CT/VT selection and configuration in accordance with IEEE/IEC standards to ensure accurate measurements and dependable relay operation at a transmission-level PCC.

 

Finally, we included a new Table 9 in Section 6.4 comparing this work’s key features and performance (real-time HIL validation, IEC 61850-based monitoring, adaptive logic, CT/VT design, and < 300 ms trip time) with selected recent studies.

These additions make the scientific contribution and novelty of our study explicit and well contextualised against existing IEC 61850-based protection literature, directly addressing this reviewer’s comment.

 

Comment 4

Figures 36–43 (monitoring/fault detection results) are useful, but they lack quantitative analysis. They mostly show waveforms without extracting fault detection times, CT saturation behavior or latency.

 

Reply to comment 4

In response to the reviewer’s request for more quantitative analysis of the waveform results, we have revised the discussion of Figures 23–26 (previously Figures 36–43) and added a new summary paragraph at the end of Section 5.2.2, presenting the measured performance values.

 

The updated discussion now explicitly reports that:

• The selected monitoring signals were transmitted via IEC 61850 GOOSE with deterministic latency consistently below 1 ms and no packet losses, confirming reliable and time-coherent communication between the RTDS model and the physical relays.
• The SEL-487B relay detected internal faults and issued trip signals within less than 300 ms from fault inception to trip issuance, with total end-to-end fault clearance (relay + breaker operation) measured at 606.667 ms under initial load and 706.667 ms under increased load.
• The associated GOOSE-based breaker operation confirmations were received within 1 ms after the trip command.

 

In addition, the captions of Figures 23–26 have been updated to mention these measured values so that the waveform plots are directly supported by quantitative data.

These additions ensure that the waveform-based results are now backed by clear numerical performance indicators, directly addressing this reviewer’s comment.

 

Comment 5

Figures 7–9, 19–34 have the text too small to read; authors should improve clarity.

 

Reply to comment 5

We have improved the font size of the text on the figures.

 

 

 

 

Comment 6

The HIL setup uses a 9-bus + one WPP model, which is a simplified test grid. This raises the question: Would the proposed scheme scale to larger, real-world grids with multiple PCCs and multivendor devices? Please address this limitation.

 

Reply to comment 6

In response to this comment, we have added a dedicated paragraph in Section 6 (Discussion) under the new subsection “6.5 Scalability and Interoperability Considerations” to explicitly address the scalability and interoperability potential of the proposed framework.

 

Although the present HIL setup is based on an IEEE 9-bus system integrated with a single large-scale wind power plant, the framework was intentionally designed using modular IEC 61850 Logical Node structures, GOOSE-based peer-to-peer communication, and standardized ICD/SCD configuration files. These design attributes allow the system to be expanded with additional feeder bays, busbar zones, and PCCs without altering the underlying protection logic.

 

Furthermore, because the system is built on vendor-neutral IEC 61850 data models, it can be extended to multivendor intelligent electronic devices (IEDs), which is essential for realistic substation environments.

 

This clarification has been incorporated to demonstrate how the framework can be scaled to larger and more complex networks in future studies, directly addressing the reviewer’s concern.

 

Comment 7

Authors correctly list IEC 62351 cybersecurity in future work, but the HIL platform could also be used to inject faulty GOOSE/SV messages, delays, or dropouts. At least mention that your HIL testbed is capable of being extended for cyber-attack resilience testing.

 

Reply to comment 7

Thank you for this valuable suggestion. We have updated the Future Work section (Section 7.3) to explicitly state that the current HIL testbed will be extended to evaluate the resilience of IEC 61850-based systems against cyberattacks.

 

Specifically, we now note that future work will involve using the RTDS and the IEC 61850 IED configuration platform to inject faulty or delayed GOOSE and sampled value (SV) messages, message dropouts, and denial-of-service conditions to assess the system’s response under compromised communication scenarios.

 

This addition clarifies that the proposed platform can support IEC 62351-compliant cybersecurity and resilience testing in future studies, directly addressing the reviewer’s concern.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have tried to respond to review comments.

Author Response

We sincerely thank the reviewer for dedicating time to going through the manuscript. Their comments have truly helped us to improve the way we present our work.

Reviewer 3 Report

Comments and Suggestions for Authors

The paper is too long (72 pages). As requested in the last revision, many parts of the paper could be reduced. 

Author Response

Reply to the comment

We sincerely thank you for this valuable observation. We have now substantially reduced the overall length (from 72 to 53) of the manuscript by streamlining the main text and removing repeated or less critical descriptions. The essential results and discussions have been retained to preserve the technical depth, while detailed configuration data, step-by-step relay settings, and extended tables/figures have now been moved to the appendices for quick reference. Furthermore, additional explanatory material (such as full QuickSet screenshots, extended aliasing tables, and detailed dataset mappings) has been prepared as supplementary material, available on request, to ensure full reproducibility without overextending the main manuscript. This restructuring has reduced the main body length while still ensuring that all necessary details are transparently documented.

 

We trust that this revised format now addresses the reviewer’s concern and improves the readability of the paper.

Reviewer 4 Report

Comments and Suggestions for Authors

The revised version includes all the requested changes. These are justified and correctly inserted into the manuscript.
I consider that in this form, the article can be accepted for publication.

Author Response

We also appreciate the reviewer for his or her dedication to our work. It is through their reading with understanding that helped us improve from the first revision of our manuscript.

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Overall, the manuscript is much improved and can be accepted.