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IEC 62535: Test Method for Detection of Potentially Corrosive Sulphur in Insulating Oil
Wrapped Conductor Test Method for Identifying Corrosive Sulphur in Used and Unused Transformer Oils
IEC 62535:2008 addresses a critical problem in the power transformer industry: the detection of potentially corrosive sulphur in mineral insulating oils. In recent years, numerous transformer and reactor failures worldwide have been traced to copper sulphide (Cu2S) formation on cellulose insulation. These failures typically occur due to arcing between adjacent winding disks or conductors when conductive copper sulphide deposits bridge the insulation. This standard provides a reliable test method that overcomes the limitations of older tests such as ASTM D1275 Method A and DIN 51353, which were found unable to detect oils with potentially corrosive behaviour.
Copper sulphide formation on bare copper can create conductive particulates in the oil, which act as nuclei for electrical discharge and may lead to catastrophic transformer failure. The wrapped conductor test method described in IEC 62535 directly simulates the conditions inside a transformer winding.
The Wrapped Conductor Test Method
The principle is elegantly simple yet highly effective. A piece of flat copper conductor wrapped with Kraft paper is immersed in the oil sample and heated at 150 degree C for 72 hours in a sealed glass headspace vial. After the test, both the copper surface and the paper are examined for signs of copper sulphide formation. The method uses a wrapped conductor rather than a bare copper strip because this more accurately reproduces the conditions inside actual transformer windings, where copper sulphide migrates from the conductor onto the paper insulation.
Test Apparatus and Materials
| Component | Specification |
|---|---|
| Glass vial | Headspace type, ~20 ml capacity, 22.5-23 mm diameter |
| Vial seal | PTFE-faced silicone septum with aluminium crimp cap |
| Heating chamber | Oven maintaining 150 degree C +/- 2 degree C |
| Copper conductor | Flat Cu-ETP, ~7.5 mm x 1.5 mm, wrapped with 4 layers Kraft paper |
| Kraft paper | Density 0.70-0.85 g/cm3, thickness 0.060-0.100 mm |
| Oil volume | 15 ml per test |
| Heating duration | 72 hours +/- 30 min |
The paper adjacent to copper must be wound gap-to-gap with a specific tolerance. The PTFE face of the septum must seal correctly to the glass vial to prevent the hot oil from contacting the butyl rubber, which itself contains sulphur and would invalidate the test.
Inspection and Interpretation
After heating, both the copper and the paper are inspected under good lighting conditions. A positive result for copper is indicated by graphite grey, dark brown, or black discolouration. On the paper, copper sulphide deposition appears metallic — from clearly shiny to almost lustreless, often with a lead-like or tin-like appearance. The metallic surface may show blue and/or purple interference colours. Importantly, other discolourations from paper ageing and oil deterioration must not be mistaken for copper sulphide.
Engineering Design Insights for Transformer Oil Quality Control
The wrapped conductor method represents a significant advancement over traditional bare-copper strip tests. The key insight is that copper sulphide formation in transformers is not merely about corrosion of the conductor surface; the real danger is the migration of copper sulphide onto and through the paper insulation, creating conductive paths that short-circuit winding turns. The CIGRE working group A2.32 study that forms the basis of this standard demonstrated that many oils passing traditional tests would still cause copper sulphide problems under real operating conditions.
The test uses a paper-wrapped conductor rather than bare copper because this more closely replicates the winding environment. The paper serves as both a witness surface for sulphide migration and a more sensitive indicator of corrosive potential. Inter-laboratory tests showed 100% agreement on duplicate results.
Repeatability of this test method is excellent: inter-laboratory tests by CIGRE WG A2.32 showed 100% agreement for duplicate results. Reproducibility is also high, with only 2 out of 16 laboratories showing different results, and those were traced to differences in the oil batches delivered. The test must be performed in duplicate, and a blank test using white oil (sulphur content less than 5 mg/kg) must be carried out to ensure all materials are sulphur-free. The blank test also serves to detect any changes to the copper that might be caused by impurities in the test materials rather than by the oil sample itself, providing an important quality control check on the entire test procedure.
For the copper conductor material, strict specifications are defined: Cu-ETP according to EN 13601, with a copper content of at least 99.90% and oxygen content below 0.04% (w/w). The Kraft paper used for wrapping must meet IEC 60554-3-1 specifications for general purpose electrical paper, with a density of 0.70 to 0.85 g/cm3 and thickness of 0.060 to 0.100 mm. The paper must have an air permeability rating of M or H, conductivity below 4 mS/m, and zero free nitrogen content. These material specifications are critical because variations in copper or paper quality could affect the test results and lead to incorrect classification of the oil.
Practical Implementation and Reporting
The standard also provides an alternative copper strip method (Annex A) based on ASTM D1275 Method B but downsized to use only 15 ml of oil. For cases where visual inspection is inconclusive, the standard describes SEM-EDX analysis (Annex B) for definitive identification of copper sulphide on paper. The test report must include the testing laboratory identification, product type and identification, reference to the standard, the test result, and any deviations from the specified procedure. If both duplicate samples show positive results for copper or paper (or both), the oil is reported as potentially corrosive. If both show negative results, the oil is reported as non-corrosive. Different results between duplicates require repeating the test.
For utilities and transformer maintenance organizations, IEC 62535 provides an essential tool for screening incoming oil batches and monitoring in-service oil condition. Early detection of potentially corrosive sulphur can prevent costly transformer repairs and unplanned outages that may result from copper sulphide-related failures.
Frequently Asked Questions
Q1: Why is IEC 62535 preferred over older test methods like ASTM D1275?
ASTM D1275 Method A and DIN 51353 were found unable to detect oils that exhibit potentially corrosive behaviour under real transformer operating conditions. The wrapped conductor method in IEC 62535 more accurately simulates the winding environment and detects oils that form copper sulphide on paper insulation, which is the actual failure mechanism.
ASTM D1275 Method A and DIN 51353 were found unable to detect oils that exhibit potentially corrosive behaviour under real transformer operating conditions. The wrapped conductor method in IEC 62535 more accurately simulates the winding environment and detects oils that form copper sulphide on paper insulation, which is the actual failure mechanism.
Q2: What does a positive test result mean for transformer operation?
A positive result indicates the oil is potentially corrosive and may form copper sulphide deposits on winding insulation. Such oils should not be used in transformers, or existing oil should be treated or replaced. Continued use of corrosive oil can lead to copper sulphide bridging between winding turns, causing arcing and catastrophic failure.
A positive result indicates the oil is potentially corrosive and may form copper sulphide deposits on winding insulation. Such oils should not be used in transformers, or existing oil should be treated or replaced. Continued use of corrosive oil can lead to copper sulphide bridging between winding turns, causing arcing and catastrophic failure.
Q3: Can this test be used for both new and in-service transformer oils?
Yes, the standard explicitly states that the method is suitable for both used and unused mineral insulating oils. This makes it valuable for incoming quality control of new oil deliveries and for condition assessment of oil already in service.
Yes, the standard explicitly states that the method is suitable for both used and unused mineral insulating oils. This makes it valuable for incoming quality control of new oil deliveries and for condition assessment of oil already in service.
Q4: What should be done if visual inspection of the paper is inconclusive?
The standard recommends using SEM-EDX (Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy) to definitively identify copper sulphide deposits on the paper. X-ray diffraction or determination of copper and sulphur content of the paper may also provide guidance. The oil should only be reported as corrosive if the precipitate is positively identified as copper sulphide.
The standard recommends using SEM-EDX (Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy) to definitively identify copper sulphide deposits on the paper. X-ray diffraction or determination of copper and sulphur content of the paper may also provide guidance. The oil should only be reported as corrosive if the precipitate is positively identified as copper sulphide.
