Review Reports - Absolutely Selective Single-Phase Ground-Fault Protection Systems for Bunched Cable Lines

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper has potential, but requires major revision before reconsideration.

1) The manuscript lacks any statistical characterization of protection performance. Key metrics such as false-trip rates, missed-trip rates, sensitivity, and selectivity are not quantified. Without such data, the overall reliability of the proposed protection scheme remains unsubstantiated.

2) The study does not provide a probabilistic analysis of ΔI behavior under realistic operating variations, including differing fault resistances, parameter drift, or measurement noise. This omission raises concerns about the robustness of the proposed logic in non-ideal conditions.

3) Relay thresholds and dead-zone mitigation strategies are chosen purely heuristically. The paper would benefit from a more rigorous approach, such as applying supervised or unsupervised machine-learning methods to optimize threshold selection and improve detection reliability.

4) The protection strategy is entirely static and does not incorporate any adaptive or online learning capabilities. Given that network characteristics evolve over time, the absence of a data-driven mechanism for updating thresholds limits the long-term effectiveness of the method.

5) The manuscript repeatedly states that the proposed scheme is “almost the only one” capable of achieving absolute selectivity and cable-level fault identification. However, these claims are not supported by evidence, comparative analysis, or a systematic review of alternative approaches.

6) The work does not include experimental benchmarking against widely used industry protections, such as directional zero-sequence current relays or open-delta voltage-based methods. Without baseline comparisons, the claimed advantages of the proposed approach cannot be validated.

7) Although the paper references recent machine-learning-based SGF identification methods, it does not compare its own performance to these more advanced techniques. A quantitative evaluation against existing ML-based solutions would be necessary to justify the proposed method’s superiority.

Author Response

Comments 1: The manuscript lacks any statistical characterization of protection performance. Key metrics such as false-trip rates, missed-trip rates, sensitivity, and selectivity are not quantified. Without such data, the overall reliability of the proposed protection scheme remains unsubstantiated.

Response 1: Taking into account this comment of the reviewer, the following paragraph is inserted into the text of the article between lines 198 and 199:

" In 2006, an absolutely selective protection system against single-phase ground faults in bunched cable lines was mounted at the Aksu Ferroalloy Plant in the communication line between the main switchgear (MSG-10) of the main step-down substation (SDS-1) and switchgear SG-1. No false operation of this protection was reported by the maintenance personnel."

Comments 2: The study does not provide a probabilistic analysis of ΔI behavior under realistic operating variations, including differing fault resistances, parameter drift, or measurement noise. This omission raises concerns about the robustness of the proposed logic in non-ideal conditions.

Response 2: Since this protection has a dead zone ldz, a similar protection system should be mounted at the end of a protected line to cover the dead zone. This enables supporting the reliability of the protection under non-ideal operating conditions upon possible fluctuations ΔI under actual operating conditions, including resistance to failures or parameter drifts.

However, taking into account this comment of the reviewer, the above explanation is inserted in the text of the article between lines 196 and 197.

Comments 3: Relay thresholds and dead-zone mitigation strategies are chosen purely heuristically. The paper would benefit from a more rigorous approach, such as applying supervised or unsupervised machine-learning methods to optimize threshold selection and improve detection reliability.

Response 3: The proposed article provides a fairly simple definition of the maximum value of the " dead-zone ". The authors believe that this is sufficient, taking into account the inclusion of a response to remark 2 in the article.

Comments 4: The protection strategy is entirely static and does not incorporate any adaptive or online learning capabilities. Given that network characteristics evolve over time, the absence of a data-driven mechanism for updating thresholds limits the long-term effectiveness of the method.

Response 4: Indeed, the protection strategy is completely static and does not provide for any adaptation or online learning opportunities. However, the inclusion of a response to the expert's comment 2 in the article makes it possible not to take into account changes in network characteristics that lead to a change in the size of the " dead-zone".

Comments 5: The manuscript repeatedly states that the proposed scheme is “almost the only one” capable of achieving absolute selectivity and cable-level fault identification. However, these claims are not supported by evidence, comparative analysis, or a systematic review of alternative approaches.

Response 5: According to the authors, the proposed scheme is indeed the only one capable of providing absolute selectivity. This is confirmed by the patents obtained by the authors [17, 18], as well as the systematic search for alternative approaches to solving this problem. This search was carried out not only according to the list of references [8,12 and 16] given in the manuscript, but also all the information available to the authors on domestic and foreign relay protection. The expression "almost the only one" was added to the manuscript in case something similar does exist.

Comments 6: The work does not include experimental benchmarking against widely used industry protections, such as directional zero-sequence current relays or open-delta voltage-based methods. Without baseline comparisons, the claimed advantages of the proposed approach cannot be validated.

Response 6: The proposed protection principles are absolutely selective. That is, they are triggered only when SGF is on the protected line. All other known protections against SGF have limited selectivity and can be triggered by SGF on other lines. That is, the proposed protections have clear advantages that do not need additional confirmation.

Comments 7: Although the paper references recent machine-learning-based SGF identification methods, it does not compare its own performance to these more advanced techniques. A quantitative evaluation against existing ML-based solutions would be necessary to justify the proposed method’s superiority.

Response 7: The proposed protection principles are absolutely selective. That is, they have a property that is missing from all other known protection against SGF. That is, they have new properties and, consequently, a certain superiority, which, according to the authors, does not need additional evaluation.

Reviewer 2 Report

Comments and Suggestions for Authors

Absolutely Selective Single-Phase Ground-Fault Protection Systems for Bunched Cable Lines

Review

A technique for protecting a line of two cables against single phase ground fault in a network with an isolated neutral is developed. The technique is based on measuring the difference in the magnitudes of zero-sequence currents in these cables. The design of selective protection is developed. The techniques and devices are based on zero-sequence current transformers and ring measuring converters for building protection systems for single-phase ground faults. These devices can provide protection without the use of a zero-sequence voltage transformer.

The topic is interesting and important.

The selected references are relevant.

The work involves mathematical simulation, computations and results obtained from measurements.

Some suggestions for clarifications are below:

The manuscript should enrich the review of the state of the art and the relevant literature.
The novelty of this paper must be clearly formulated.
A description of the software used for computations must be added.
A real application of the proposed system is necessary to validate the study. If possible, real tests are also necessary to estimate the fault or prevent it.
In the Conclusions are summarized the previous ideas of the manuscript. However, further advancements with applications to real bunched cable lines systems should be proposed.

Author Response

Comments 1: The manuscript should enrich the review of the state of the art and the relevant literature.

Response 1: When writing the manuscript, the authors made an in-depth review of all available domestic and foreign literature on the search for additional information on the field in question in relay protection. Everything that could be found is given in [8,12 and 16], as well as the patent [17]. An additional search of the relevant literature in this field by the authors revealed the appearance of a new patent, which is placed in the manuscript under the number [18].

Comments 2: The novelty of this paper must be clearly formulated.

Response 2: In order for the novelty of this article to be more clearly formulated, a sentence should be added to the paragraph on stock 86-87:

" Thus, the design of new absolutely selective SPG protection devices capable of detecting a damaged cable is relevant."

Comments 3: A description of the software used for computations must be added

Response 3: A fairly simple and well-known Electronics Workbench circuit modeling system was used as software to calculate the SGF currents in a line of two cables, described in [28-30]. In the manuscript, Figure 2 shows the scheme by which the calculation is implemented in this system and provides a detailed calculation of the elements necessary to simulate currents in this system. It is not possible to add anything else to the manuscript.

Comments 4: A real application of the proposed system is necessary to validate the study. If possible, real tests are also necessary to estimate the fault or prevent it

Response 4: Taking into account this comment of the reviewer, the text of the article has been corrected. The following paragraph is inserted between lines 198 and 199:

"In 2006, an absolutely selective protection system against single-phase ground faults in bunched cable lines was mounted at the Aksu Ferroalloy Plant in the communication line between the main switchgear (MSG-10) of the main step-down substation (SDS-1) and switchgear SG-1. No false operation of this protection was reported by the maintenance personnel"

Comments 5: In the Conclusions are summarized the previous ideas of the manuscript. However, further advancements with applications to real bunched cable lines systems should be proposed.

Response 5: Taking into account the reviewer's wishes, the following paragraph is placed in the text of the manuscript between lines 338 and 339:

"The further improvement of real protection systems for bunched cable lines could consists in refusal to use electromechanical power directional relays, which consume a significant amount of electricity and require a zero-sequence voltage source for operation. In addition, this refusal can reduce the protection dead zone. "

Reviewer 3 Report

Comments and Suggestions for Authors

The paper proposes an absolutely selective single-phase ground-fault protection system for bundled cable lines in 6–10 kV networks with an isolated neutral. It points out that traditional protection methods, such as simple zero-sequence current protection and directional protection, suffer from relative selectivity issues. These methods cannot accurately identify the faulty cable and are significantly influenced by system parameters. By comparing the differences in zero-sequence currents and determining their direction, the authors designed protection schemes suitable for both two-cable and multi-cable lines. The system employs a combination of ring-type current transformers and relays to achieve fault detection and location. Simulation results demonstrate that the proposed method can effectively identify the faulty cable, and it features no dead zone and high reliability.

Suggestions:

The simulation assumes ideally consistent cable parameters. In practice, differences in cable batches and installation methods may lead to parameter variations. Could this affect the selectivity of the protection?
The length of the dead zone is related to the system’s ground capacitance. In actual operation, the ground capacitance may change with network topology. Does the system have the capability to adaptively adjust its thresholds?
The study relies entirely on simulation. It is recommended to provide actual field test or laboratory validation data.

Author Response

Comments 1: The simulation assumes ideally consistent cable parameters. In practice, differences in cable batches and installation methods may lead to parameter variations. Could this affect the selectivity of the protection?

Response 1: Differences in the parameters of cables of the same line and their installation methods cannot affect the absolute selectivity of protection. These differences can only lead to a slight increase in the "dead zone".

Comments 2: The length of the dead zone is related to the system’s ground capacitance. In actual operation, the ground capacitance may change with network topology. Does the system have the capability to adaptively adjust its thresholds?

Response 2: Indeed, when the topology of an electrical network changes, the capacity of its phases changes relatively. This leads to a change in the dead zone. In this regard, the manuscript suggests installing a similar protection kit at the end of the protected line. Thus, the insensitivity zone is blocked.

Comments 3: The study relies entirely on simulation. It is recommended to provide actual field test or laboratory validation data.

Response 3: Taking into account this comment of the reviewer, the text of the article has been corrected. The following paragraph is inserted between lines 198 and 199:

"In 2006, an absolutely selective protection system against single-phase ground faults in bunched cable lines was mounted at the Aksu Ferroalloy Plant in the communication line between the main switchgear (MSG-10) of the main step-down substation (SDS-1) and switchgear SG-1. No false operation of this protection was reported by the maintenance personnel."

Reviewer 4 Report

Comments and Suggestions for Authors

This paper presents a protection method for bunched cable lines. There are a few areas which need to be improved:

1- Some of the explanations aren’t clear or not very well-organised and it's a bit hard to follow the paper. Before giving enough context or basic background, the authors have presented complicated circuit details. It should also be described in the paper how single-phase ground faults flow in these systems or how bunched cables behave

2- the literature review should clarify why existing protection methods are not sufficient

3- the last paragraph of the introduction, which should outline the structure of the paper , is missing

4- The results section is very weak. There should be some numerical tests, real experiments, and comparisons with state-of-the-art

5- the authors have made strong claims in the abstract and conclusions but not all of them are actually supported by proper theoretical analysis or results

Thanks.

Author Response

Comments 1: Some of the explanations aren’t clear or not very well-organised and it's a bit hard to follow the paper. Before giving enough context or basic background, the authors have presented complicated circuit details. It should also be described in the paper how single-phase ground faults flow in these systems or how bunched cables behave

Response 1: When laying cables of the same line in a trench or overpass, they are not twisted, but laid side by side. In this regard, the flow of SGF currents in a damaged cable does not depend on the nearby cable. Therefore, the SGF currents from a damaged cable and undamaged network cables are calculated in a generally accepted manner. In this regard, the authors had a question whether any additions or clarifications should be made to the text of the manuscript.

Comments 2: the literature review should clarify why existing protection methods are not sufficient

Response 2: An additional analysis of the literature review showed that the existing protection methods do not allow the development of protection lines of two cables from SGF, which have absolute selectivity and the function of detecting damaged cables.

Comments 3: the last paragraph of the introduction, which should outline the structure of the paper , is missing

Response 3: According to the reviewer's suggestion, the last paragraph of the introduction will look like:

"The structure of this paper is as follows. Section 2 is devoted to the design of an absolutely selective SGF protection for a two-cable bunched line, which is capable of detecting a damaged cable in this line. Protection devices and techniques for calculating fault currents in the cables depending on the fault location are described, and currents in the measuring elements of the protection and dead zone size are determined in this section. Section 3 considers features of the protection design for a line in the form of a bunch of more than two cables, simulates fault currents in the cables of this line, and analyzes the operation of the measuring element of the protection. Section 4 describes the results of our work and discusses them. Final Section 5 provides the conclusions drawn during this work."

Comments 4: The results section is very weak. There should be some numerical tests, real experiments, and comparisons with state-of-the-art

Response 4: Taking into account this comment of the reviewer, the text of the article has been corrected. The following paragraph is inserted between lines 198 and 199:

"In 2006, an absolutely selective protection system against single-phase ground faults in bunched cable lines was mounted at the Aksu Ferroalloy Plant in the communication line between the main switchgear (MSG-10) of the main step-down substation (SDS-1) and switchgear SG-1. No false operation of this protection was reported by the maintenance personnel."

Comments 5: the authors have made strong claims in the abstract and conclusions but not all of them are actually supported by proper theoretical analysis or results

Response 5: Judging by this proposal, under the serious statements in the summary and conclusions, the reviewer understands that the proposed protection has absolute selectivity and the function of detecting a damaged cable. As it seems to the authors, no additional theoretical analysis is required to confirm this. As for the practical confirmation, it is given in the response to the reviewer's suggestion No. 4.