Elemental sulphur in synthetic esters: What does research say?

Synthetic esters have been used as insulating fluids in transformers for many years. However, these fluids can also be affected by a substance that has so far been primarily associated with mineral insulating oils: elemental sulphur. Even low concentrations can, under certain conditions, react with metallic components inside the transformer and particularly affect silver-plated contacts. But how does elemental sulphur find its way into synthetic esters? How does its behaviour differ from that in mineral oil? How can it be detected and removed? And how are findings from current research being incorporated into international standards and test methods? 

We discuss these questions with Dr Jelena Lukić from the Nikola Tesla Institute of Electrical Engineering in Belgrade. She has many years of experience in insulating fluids, corrosive sulphur and power transformer diagnostics. She has convened two international CIGRE working groups – one addressing copper corrosion and an ongoing group focused on silver corrosion – and is actively involved in international standardisation work in this field. 

In this interview, she explains what research currently tells us about elemental sulphur in synthetic esters, how these findings have contributed to the development of the patented TeslaSorb^® technology, and what they mean for practical applications and international standardisation.  

Dr Lukić, elemental sulphur has been discussed in connection with mineral transformer oils for some time. When did research begin to show that it could also be an issue in synthetic esters?

Several years ago, we conducted the first testing of ester liquid samples, and further extensive research was initiated afterwards. 

What do we know today about how elemental sulphur finds its way into synthetic esters – and why can even very low concentrations become problematic?

Some of the root causes are known, coming from the liquid itself, but the overall view on all contamination routes and factors initiating silver corrosion is not yet known and is currently under investigation within CIGRE JWG D1/A2 84: Silver Corrosion in Power Transformers, which I am convening.

Low concentrations may be problematic due to the large volume/mass ratio of the insulating liquid to the affected silver contact surface in power transformers, providing enough elemental sulphur even in the low concentration range to deposit silver sulphide on a small affected silver-plated surface. 

Your research focuses particularly on the reaction between elemental sulphur and silver. What happens during this reaction inside a transformer, and why are silver-plated contacts particularly relevant?

Let me try to answer in a short, simple way. Silver-plated contacts are more endangered than copper conductors since silver is directly exposed to a faster, direct reaction with elemental sulphur, and risks of power transformer failure are higher due to the nature of OLTC dynamic operation and detachment of silver sulphide particles as a consequence of sliding contacts in normal service conditions.

This is leading to high interest from OEMs and PT owners in conducting Risk Assessment and getting involved with prioritised Maintenance activities, application of efficient mitigation solutions and remedial actions.

Nikola Tesla Institute is able to provide a comprehensive approach to solving this emerging problem in Power Grids, from Testing, Risk Assessment to Mitigation and Remedial Actions. 

You have compared the reaction of elemental sulphur with silver in synthetic esters and mineral oil. What were the most important findings from these studies?

The most important findings were related to the higher rate of the reaction of silver sulphide formation in synthetic ester liquid compared to mineral oils. 

Does this mean that synthetic esters are generally more susceptible to problems caused by elemental sulphur than mineral oils – or would that be too broad a conclusion?

We can say that if we compare the same compounds at the same concentration level, but not in general terms, because the problem of sulphur corrosion (both silver and copper) in mineral oils is still unfortunately very much present, but typically includes elemental sulphur and other species such as DBDS in much higher concentrations than those we find in synthetic ester liquids. 

How can elemental sulphur in synthetic esters be reliably detected? Are established test methods for corrosive sulphur sufficient?

Elemental sulphur is difficult to quantify in the low concentration range (sub-ppm level). Currently available methods are not sufficient. Even current standards for silver strip corrosion tests are not adequate.

Reliable elemental sulphur detection and quantification can be implemented with highly professional staff and a new generation of equipment, not necessarily very expensive equipment, but the application of methods needs to be supported by specialists trained to a high level.

At hand nowadays, a method performing the best of available ones regarding level of sensitivity is the ASTM D 1275-15.

The forthcoming years will be devoted to the introduction of new methods and guidelines.

How has the Nikola Tesla Institute translated its research into the patented TeslaSorb^® technology, and what have pilot projects and practical applications shown so far about removing elemental sulphur from synthetic esters?

Important lessons learned are related to a comprehensive approach in testing, applying testing of corrosive sulphur of insulating liquids for power transformers regardless of the type of insulating liquid.

On the basis of testing, R&D work shall provide Expert Diagnostics and Risk Assessment tools. Furthermore, there is an urgent need to develop and apply efficient technologies that can efficiently re-refine the liquid to mitigate the risks and extend the operating life of installed PTs, as well as to establish waste recycling at the end of PT life.

The Nikola Tesla Institute accredited laboratory and R&D team are able to implement a comprehensive approach to Diagnose, perform Risk Assessment and apply Long-term Mitigation solutions.

TeslaSorb patented technology was proven effective in several on-site treatments of corrosive synthetic ester liquid, to completely remove elemental sulphur and improve the properties of the treated synthetic ester to as-new quality in the course of technology validated with two OEMs and currently in the pipeline with another Big Utility and two OEMs.

Implementation and scaling of TeslaSorb technology on the market is targeted to be conducted as a firm partnership and collaboration with EOS as service provider.

Proven commercial records addressing the market need started with the first commercial track records of 50 tons of recycled synthetic ester, non-corrosive with quality as new liquid, delivered to an OEM in Europe.

The forthcoming years shall be devoted to the positioning of TeslaSorb technology on the market to bring solutions to power transformer renewal and life extension, as well as to the establishment of an effective recycling process for ester-based liquids and the introduction of governing standards.

Are there currently any changes to international standards regarding elemental sulphur? What new requirements can be expected from next year onwards – and how are you personally involved in developing these standards?

Available standardised testing methods are currently being introduced into specifications for new, unused synthetic ester liquids in IEC TC 10, while the guide for maintenance of synthetic ester insulating liquids in service is waiting for revision to address this emerging problem.

This revision should be based on findings and conclusions of the current CIGRE JWG D1/A2 84, which is working on the revision of silver corrosion testing methods to adapt them to be more sensitive and better reflect real conditions in power transformers.

Thank you very much for sharing your expertise and these valuable insights into your research, Dr Lukić!

EOS^® RePure: Recycling and reusing synthetic esters

Would you like to learn more about how used synthetic ester insulating fluids can be recycled and reused? Discover EOS^® RePure, a recycled synthetic ester insulating fluid, and find out how it contributes to a more circular approach to transformer insulating fluids. 

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• EOS^® RePure – A new lease of life for used synthetic esters

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