P S Amaro

Impact of corrosive sulfur in transformer insulation paper

In recent years a significant volume of research has been undertaken in order to understand the latest failures in oil insulated power apparatus due to deposition of copper sulfide on the conductors and in the insulation paper. Dibenzyl Disulfide (DBDS) has been found to be the leading corrosive sulfur compound in the insulation oil. As there are currently no nondestructive techniques available to analyze the degree of damage from sulfur corrosion, electrical plant operators are faced with a choice of either replacing an asset which might still have several years of operational life or keeping a transformer operational with an increased risk of failure. This paper investigates permittivity and loss factor curves of transformer oil impregnated insulation paper, at different corrosive sulfur degradation stages. Three types of transformer conductors are investigated; new, artificially aged and service aged. To increase the understanding of the electrical behavior of Cu2S this paper demonstrates how the electrical properties change with moisture content. Dielectric spectroscopy is used to demonstrate some of the electrical changes across a wide frequency range, from 1mHz to 1MHz.

X-ray fluorescence as a condition monitoring tool for copper and corrosive sulphur species in insulating oil

Currently there are no standard techniques to detect copper sulphide deposits in transformers to act as an early warning system. This paper develops and tests a procedure using X-Ray Fluorescence (XRF) spectroscopy to tackle this problem and to monitor dissolved copper in oil, which can have a major impact on the dielectric properties of insulating oil. Initially calibration curves for sulphur and copper demonstrate the XRFs limits of quantitation. Subsequently, a laboratory scale sample preparation procedure is developed to demonstrate that corrosive sulphur depletion trends may indirectly monitor copper sulphide formation. To confirm depleting sulphur species are corroding copper samples are analysed with Energy Dispersive XRay spectroscopy. Finally XRF demonstrates that corrosive oil has a low impact on the long term concentration of copper in oil. An XRF condition monitoring technique feasibility study is conducted and recommendations are given in how to implement it in the field.

Silver corrosion in transformers

The impact of corrosive sulfur on high value assets such as transformers is widely reported and can be considered a global problem. A great deal of research and development is focused on the remediation of corrosive transformers and the mitigation of future corrosion. Regeneration of aged and or corrosive oil by filtration through bauxite clays is a common method for removal of polar molecules and many sulfurous compounds such as dibenzyl-disulfide (DBDS). This process is largely successful, although over the last several years, increased corrosion and even total failure of transformers has been linked with the reclamation process. In some cases, the corrosion associated with a recent reclamation is selective for silver rather than copper. By thoroughly understanding the chemistry occurring during aged oil reclamation, it will be possible to take appropriate measures to limit the formation of corrosive species during future reclamation and regeneration processes. There is a requirement to minimize the amount of oil wastage during reclamation for both environmental and commercial reasons. It is also important that any changes made to the reclamation process do not have a detrimental effect on the health and life-expectancy of the transformer. This paper describes studies carried out to track the total sulfur content of oil samples taken from two different locations (transformer main tank and reclamation rig buffer tank), at multiple time intervals during an oil reclamation procedure. By tracking the total sulfur content, it is possible to determine when and where maximum sulfur accumulation occurs, possibly indicating the best point at which a portion of oil should be disposed. Once a fuller understanding of the sulfur content is obtained, GC-MS will be used to gain a detailed understanding of the types of sulfurous species present in the oil.

Tracking copper sulfide formation in corrosive transformer oil

Corrosive sulfur in the last decade has been recognized as a major risk to high voltage transformers. An initial study undertaken by CIGRE on copper sulfide in transformer insulation investigated possible sources, i.e. Dibenzyl Disulfide (DBDS), it also looked at possible mitigating techniques, i.e. passivation, environmental influences, i.e. temperature, and improvement to oil corrosion standards, i.e. EN 62535. Although there was significant volume of research undertaken the high voltage industry still felt there is a lack of understanding in the process of copper sulfide formation and long-term effects of mitigation techniques. Hence a new CIGRE transformer working group, A2.40, was created. The identification of techniques with the ability to track the mechanism of copper sulfide (Cu2S) formation is fundamental to the current research objectives. The technique suggested in this paper involves the use of X-ray fluorescence (XRF), which is used for elemental analysis, to track the quantity of sulfur in the oil. As the deposits of Cu2S form in the conductors and paper insulation the amount of sulfur in the oil decreases. By using a series of laboratory experiments is possible to investigate how the rate of formation of Cu2S changes with time, suggesting that there is an initial chemical reaction needed for the Cu2S deposits to form. Scanning Electron Microscopy with Energy-dispersive X-ray spectroscopy (SEM-EDX) has been used to investigate the surface of copper at several different stages of the copper sulfide formation. By manipulating variables, i.e. temperature, it is possible to map their influence and provide a more precise risk assessment of transformer with corrosive oil to electrical utility companies. The experimental results suggest the possibility of developing a method to indirectly measure Cu2S deposition on conductors and insulation paper by tracking sulfur concentration changes in the transformer oil.

An initial study into silver corrosion in transformers following oil reclamation

The problem of corrosive sulfur is extremely well documented in the electrical power industry and to some extent remains a global problem. The presence of corrosive sulfur in transformers is widely reported as a cause of sudden and unexpected failure. Regeneration of transformer oil using a reactivating sorbent in the form of bauxite clays has proven effective in removal of oxidation products in transformer oils. Unfortunately, in some cases, increased levels of silver corrosion have been observed shortly after oil reclamation. It is, therefore, important to understand fully the chemistry occurring both during the oil reclamation and during silver corrosion. Once a fuller understanding of the causes of increased oil corrosivity is obtained, effective precautions can be taken to limit the formation of corrosive species, whilst minimizing the volume of oil wastage during reclamation. In this paper, the problem of silver corrosion is discussed. Initial results from corrosion tests using transformer oils are described, with particular attention paid to the analytical methods used. Scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy (SEM-EDX) was found to be a good method of characterizing the topology and elemental composition of the corroded silver surface. In terms of the oil composition, x-ray fluorescence spectroscopy is a very effective method for monitoring total sulfur and copper content. It is anticipated that once corrosive species are identified from the buffer tank samples, it will be possible to target such species in future analyses, allowing the detection of corrosive species at very low concentrations.

Investigation of the electrical and chemical processes causing the failure event in a copper sulfide related transformer failure

Copper sulfide related failures of oil-filled plants have become more common around most parts of the world over the last couple of decades, which has led the industry to re-evaluate their asset risk analysis policy for mineral oil insulated power assets. Two main theories for the failure event suggested by the current state-of-the-art are thermal runaway and turn-to-turn disk electrical breakdown. This paper provides an over view of two possible failure scenarios, electrical breakdown and low degree of polymerization, and the likelihood of corrosive oil causing each scenario. Empirical DP studies have demonstrated that the corrosion process degrades the chemical cellulose chain bonds, where DP-life expectancy models demonstrated that the corrosion process reduces 33 % of the transformer life expectancy. The electrical breakdown strength experiments demonstrated that the CuxS deposits reduced the electrical breakdown strength of each Kraft paper layer. Finally the results are considered in the larger context of the transformer insulation life-expectancy and its probability of causing the failure event.