Dry Pass, Wet Fail: Ground Impedance Testing of Field-Aged PV Modules—Implications for Repowering/Revamping Within 5–10 years and for Environmental Sustainability

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

General assessment

The manuscript presents a systematic comparison of dry and IEC 61215 MQT-15 wet insulation resistance measurements on 37 field-aged crystalline-silicon PV modules, with clear implications for safety, O&M strategies, and sustainability-oriented repowering/revamping. The topic is relevant and timely for large PV fleets, and the dataset is larger and more statistically meaningful than many prior studies.

Materials and Methods 

Thirty-seven field-aged crystalline-silicon PV modules from utility-scale plants were tested. Dry insulation resistance was measured at 1000 V DC under controlled laboratory conditions, followed by IEC 61215 MQT-15 wet leakage testing using a saltwater bath. Dry and wet resistance values were compared to the IEC 40 MΩ·m2 criterion, and a dry-test threshold was evaluated as a screening indicator for wet failure.

this work is good, however see below:

1-Within the text I propose giving more details on Table 1.
2-For (𝑅wet×𝐴≥40 MΩ⋅m2.), give more interpretation on that.
3-I rpose ref on (𝑅ratio=𝑅dry/𝑅wet,)expression.
4-Increase the quality of Figure 4

Author Response

Reviewer 1

1- Within the text I propose giving more details on Table 1.
Response: We expanded the Table 1 caption and surrounding text to clearly define units, the “Cat.” category (R_wet ≈ 0 Ω), and how such modules are handled in ratio/statistics.

2- For (?_wet×?≥40 MΩ⋅m2.), give more interpretation on that.
Response: We added an explicit interpretation of the area-normalized IEC criterion, provided equivalent wet-resistance limits for representative module areas (A = 1.6/1.8/2.0 m²), and translated these limits into approximate leakage-current levels at 1000 V.

3- I propose ref on (?ratio=?dry/?wet,) expression.
Response: We clarified the definition and usage of the ratio in Section 2.4 as an analysis metric (with consistent units) and use it to characterize the magnitude of dry-to-wet drop; as it is a simple derived quantity, we did not treat it as requiring a specific literature reference.

4- Increase the quality of Figure 4
Response: Figure 4 was updated to improve clarity and legibility, including the categorical binning that explicitly captures catastrophic wet failures as a separate class.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript entitled “Dry Pass, Wet Fail: Ground Impedance Testing of Field-Aged PV Modules - Implications for Repowering/Re-1 vamping within 5-10 years and for Environmental Sustainability” contributes to the sustainability of photovoltaic (PV) modules by analyzing the insulation resistance degradation of aged modules installed at 37 utility-scale PV power plants. With the IEC wet-condition testing requirements, the overall structure of the manuscript is logical and clearly organized. However, several arguments require stronger technical justification, and additional considerations are needed to enhance the scientific contribution.

1. Table of contents would be recommended.

2. According to the MDPI journal guidelines, the formatting of third-level headings would be revised to ensure consistency.

3. The key message appears to make strengthening the existing IEC 61215 MQT 15 requirements to mitigate PV module degradation under wet environmental conditions. Please discuss the potential drawbacks, limitations, and unintended consequences that may arise from reinforcing IEC wet-condition testing. A balanced discussion would significantly improve the technical understanding.

4. The manuscript claims that wet environmental conditions are the primary reason of the degradation in insulation resistance. But this is not sufficiently justified by the results presented in Section 3.1. Please provide clearer and more quantitative evidence by addressing the following aspects. The surrounding environmental conditions of the 37 PV plants (e.g., humidity-prone locations, coastal or high-condensation environments), the representativeness of the selected aged modules (e.g., installation year, operational duration, and degradation history), and please clarify the moisture penetration mechanism across the 37 sites.

5. Please classify the moisture penetration mechanisms identified in the aged modules across the 37 locations. With these different classification, a framework could be effectively extended to propose practical strategies of repowering phtovoltaics (PV).

Author Response

Reviewer 2

1. “Table of contents would be recommended.”
   Response: We respectfully did not add a Table of Contents, as it is not required in the MDPI article format and the manuscript is already clearly navigable via numbered sections and headings.
2. “According to the MDPI journal guidelines, the formatting of third-level headings would be revised to ensure consistency.”
   Response: We revised the formatting of all third-level headings to ensure consistent MDPI-compliant styling and numbering throughout the manuscript.
3. “Please discuss the potential drawbacks, limitations, and unintended consequences that may arise from reinforcing IEC wet-condition testing.”
   Response: We added a dedicated discussion of drawbacks and unintended consequences in Section 4.4, including practical implementation burdens, boundary-condition dependence, potential false alarms, and possible material/design trade-offs.
4. “Please provide clearer and more quantitative evidence… environmental conditions… representativeness… moisture penetration mechanism across the 37 sites.”
   Response: We clarified the sample context (utility-scale plants in a moderate Central European climate, Slovakia) in Section 2.1 and strengthened quantitative evidence in Section 3 (dry vs. IEC-wet distributions, pass/fail and dry-pass/wet-fail counts, and Rdry/Rwet statistics). Because sites and suppliers are anonymized, we cannot disclose site-specific microclimate histories, but we expanded the mechanistic explanation of moisture-assisted leakage under dew/wet operation in Section 4.1.
5. “Please classify the moisture penetration mechanisms identified in the aged modules across the 37 locations.”
   Response: We introduced a pragmatic, action-oriented leakage-behavior classification framework in Section 4.1.1 and Table 4, linking electrical signatures (R_dry, R_wet, and R_dry/R_wet) to indicative moisture-related pathways and recommended O&M/repowering actions.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

1. Consider revising “5- - 10 years” to “5–10 years” for correct typographical formatting.

2. Briefly clarify how this study fills the gap left by prior works (e.g., Roy et al.) regarding field-aged module statistics and real-world morning dew conditions.

3. Include module ages and backsheet types if available, as these factors influence wet insulation behavior.

4. Ensure axis labels and legends are fully visible in the final version; Figure 3 currently appears as a placeholder.

5. Expand slightly on how the proposed dry-test threshold (≈55.5 GΩ) could be adjusted for different module sizes or climates.

6. Consider condensing the five bullet points into 3–4 broader statements to enhance readability.

7. Check consistency in formatting (e.g., DOI links, journal abbreviations) across all entries.

Author Response

Reviewer 3

1. “Consider revising ‘5- - 10 years’ to ‘5–10 years’…”
   Response: Corrected to “5–10 years” throughout the manuscript (including the title).
2. “Briefly clarify how this study fills the gap left by prior works (e.g., Roy et al.)…”
   Response: We revised the Introduction to state the gap more clearly (limited field-aged module statistics and dew-relevant wet screening in prior work) and to position our contribution as IEC-MQT15 dry/wet testing on N = 37 field-aged modules.
3. “Include module ages and backsheet types if available…”
   Response: Detailed module ages and backsheet types were not consistently available for all anonymized field samples, and we clarified this limitation while strengthening the discussion of backsheet-dependent wet-leakage behavior with supporting literature (see Introduction and Section 4.1).
4. “Ensure axis labels and legends are fully visible… Figure 3 currently appears as a placeholder.”
   Response: Figure 3 has been updated to the final plot export and adjusted so that axis labels and legend elements are fully visible.
5. “Expand slightly on how the proposed dry-test threshold (≈55.5 GΩ) could be adjusted…”
   Response: We added a first-order area scaling of the dry-test threshold consistent with the IEC area-normalized criterion (Equation (5)) and noted a practical approach for climate/fleet calibration via wet-testing a representative subset (Section 3.3).
6. “Consider condensing the five bullet points into 3–4 broader statements…”
   Response: We consolidated and streamlined the key-point statements to improve readability while preserving the main quantitative takeaways.
7. “Check consistency in formatting (e.g., DOI links, journal abbreviations)…”
   Response: We checked the reference list and standardized formatting (DOIs, journal abbreviations, and overall consistency) across all entries.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript entitled "When 'Dry' Passes but 'Wet' Fails: IEC 61215 MQT 15 Wet Ground Impedance of Field-Aged PV Modules and Implications for Repowering/Revamping within 5–10 Years and for Environmental Sustainability" has improved the logical flow and is supported by experimental evidence.