By James Robinson – Business Manager, VPS Power.

This paper explores the importance of transformer oil testing before and after energisation of new assets going into service. It highlights key diagnostic methods, their role in operational reliability, and how they align with insurance and warranty requirements. By implementing a structured testing regime, asset managers can mitigate risks, extend transformer life, and ensure compliance with industry standards.

1. Introduction
Transformers are critical assets within power systems and their reliability hinges significantly on the condition of their insulating oil. Transformer oil serves not only as an insulator, but also as a coolant. Over time, its properties can degrade due to thermal, electrical, and chemical stresses. Pre- and post-energisation checks for new assets going into service are essential to ensure the oil has maintained its protective qualities and there are no thermal or electrical faults before and soon after energisation. These checks are not only best practices and recommended by international standards, but are often mandated by insurance and warranty agreements.

2. Technical Recommendations and Standards Compliance
Transformer oil testing must align with international standards to ensure reliability, safety, and compliance with insurance and warranty requirements. The following recommendations are based on current IEC standards and industry best practices:

2.1 Classification of Oil Use
Once insulating oil has come into contact with internal transformer materials, it can no longer be classified as ‘unused.’ This distinction is critical for interpreting test results and determining maintenance actions.

2.2 IEC 60422:2024 – Mineral Insulating Oils
Table 3 of IEC 60422:2024 outlines recommended limits for mineral insulating oils after filling new electrical equipment, prior to first energisation. It categorizes transformers by voltage class:

<72.5 kV, 72.5 to 170 kV, >170 kV

Recommended tests include:
- Appearance
- Breakdown voltage
- Water content
- Acidity
- Dielectric Dissipation Factor (DDF) at 90°C
- Interfacial Tension (IFT)
- Potentially corrosive sulphur and corrosive sulphur
- Dibenzyl disulfide (DBDS)
- Total PCB content

2.3 IEC 61203:2025 – Synthetic Esters
Table 3 of IEC 61203:2025 outlines recommended limits for synthetic ester insulating fluids after filling new electrical equipment, prior to first energisation. It categorizes transformers by voltage class:

<72.5 kV, 72.5 to 170 kV, >170 kV

Recommended tests include:
- Appearance
- Breakdown voltage
- Water content
- Acidity
- DDF at 90°C
- Fire point
- Particles
- Total PCB content

2.4 IEC 62975:2021 – Natural Esters
Table 3 of IEC 62975:2021 outlines recommended limits for natural ester insulating fluids after filling new electrical equipment, prior to first energisation. It categorizes transformers by voltage class:

<72.5 kV, 72.5 to 170 kV, >170 kV

Recommended tests include:
- Appearance
- Breakdown voltage
- Water content
- Acidity
- DDF at 90°C
- Density at 20°C
- Viscosity at 40°C
- Fire point and flash point
- Total gas content (for hermetically sealed transformers)
- Total PCB content

2.5 Additional Recommended Tests
While not listed in the IEC standards, it is advisable to include:
- 2-Furfural and Estimated Degree of Polymerisation (DP)
These tests provide insight into paper insulation ageing and transformer life expectancy.

2.6 Pre-Energisation Testing Strategy
To establish a reliable baseline, it is recommended to perform all tests listed in the relevant standards. However, customers may choose a subset based on their risk tolerance and operational requirements. At a minimum, VPS Power recommend the following should be conducted:
- Dissolved Gas Analysis (DGA)
- Fluid assessment (Visual Appearance, Acidity, Water content & Breakdown voltage)
- Furfural and Estimated Degree of Polymerisation (DP)

2.7 Post-Energisation Monitoring Schedule
After the initial sample is approved:
- 24 hours after energisation (no load): DGA and Fluid assessment
- 48 hours after load operation: DGA and Fluid assessment
- Quarterly during the first year: DGA and Fluid assessment
Customers may opt for additional interim samples between the 48-hour and first-quarter intervals.

2.8 Custom Limits
Some customers may define their own acceptance criteria, which may be more, or less, stringent than IEC recommendations. These should be documented and agreed upon prior to commissioning.

3. Conclusion
Routine transformer oil testing—both before and after energisation—is a cornerstone of effective asset management. It ensures operational reliability, supports compliance with industry standards, and fulfils insurance and warranty obligations. By adopting a proactive approach, utilities and industries can safeguard their investments and maintain uninterrupted service.

Dissolved Gas Analysis (DGA) is a cornerstone diagnostic tool that identifies fault gases generated by thermal and electrical stresses within the transformer. Key gases include hydrogen (H₂), methane (CH₄), ethylene (C₂H₄), ethane (C₂H₆), acetylene (C₂H₂), carbon monoxide (CO), and carbon dioxide (CO₂). The presence and ratios of these gases can indicate fault types such as partial discharge, overheating, or arcing. IEEE C57.104 and IEC 60599 provide interpretation guidelines.

Post-energisation DGA helps detect early operational faults such as gassing due to load-induced thermal stress. Comparing gas levels to pre-energisation baselines allows for trend analysis and early intervention.

Fluid assessment includes visual inspection for clarity and contamination, acidity measurement to detect oil oxidation, water content analysis (Karl Fischer method) to assess moisture ingress, and breakdown voltage testing to evaluate dielectric strength. Elevated acidity and moisture reduce insulation performance and accelerate ageing. Fluid assessment post-energisation confirms that oil properties remain within acceptable limits under operational conditions.

2-Furfural and Estimated Degree of Polymerisation (DP) are critical indicators of the ageing condition of cellulose-based insulation in transformers. Furfural is a by-product of paper degradation, and its concentration in oil correlates with the extent of insulation deterioration. The Degree of Polymerisation (DP) measures the average length of cellulose chains, with lower values indicating more severe ageing. A DP below 200 typically suggests end-of-life for insulation, while values above 800 indicate healthy paper condition. These metrics help asset managers estimate remaining transformer life and prioritize maintenance or replacement.

Furfural and DP testing should be conducted using High-Performance Liquid Chromatography (HPLC) to provide a baseline for insulation health before energisation.

Custom limits may be based on historical performance data, transformer design specifics, or criticality of the asset. For example, a utility operating transformers in high-humidity environments may adopt stricter water content thresholds. These limits should be documented in asset management plans and integrated into commissioning protocols.

Quarterly monitoring during the first year is essential to capture any latent issues that may arise from installation defects, material incompatibilities, or environmental factors. Trending data over time supports predictive maintenance strategies.

For further information and support regarding transformer oil testing, please contact: james.robinson@vpsveritas.com