Sulphur in my Transformer- what now?

Background

Several hundred large power transformers and reactor failures that have occurred internationally over recent years, were attributed to the presence of Corrosive sulphur in the oil.   These failures mostly involved equipment operated at elevated average temperature due to ambient loading or design, but within accepted temperature limits.  They occur normally without warning in commonly used oil tests, including DGA. Many experts have pursued intensive investigations of the failure phenomena, associated detection, and remedies on an international scale and have gathered a fair amount of experience of different mitigation techniques.

Oils containing reactive sulphur species, identified in most cases as dibenzyl disulphide (DBDS), can react with copper to form copper sulphide on the surface of the conductors and on the paper insulation surfaces, even under the normal operating conditions of the transformer.

Influencing factors

A prerequisite for the formation of copper sulphide is the presence of corrosive or potentially corrosive sulphur species in the oil.  The corrosiveness of some oils may change during service. Furthermore, even though most failures in recent years, can be attributed to the presence of corrosive or potentially corrosive components in the new unused oil, the possibility that oil may become contaminated during handling or service, must not be ruled out as a potential source of corrosive sulphur.  It was observed in laboratory experiments that copper sulphide deposition on the insulating paper is promoted during oil ageing.

Temperature is obviously a strong influential factor, since most chemical reaction rates are temperature dependent.  This is supported by the fact that most failures, due to corrosive Sulphur, have occurred in transformers in countries with high ambient temperatures and/or transformers with high loads.  Oxygen content must be listed here (and is discussed in more detail in sections 2.1.1 and 2.3), since it has a strong influence.  Low oxygen content is worse than a higher one. 

IGS Solution: Corrosive Sulphur Fleet Management

IGS developed a separate module for the assessment of transformer fleet failure risk due to Sulphur Corrosion.  It is similar to the health and integrity program and can be done as one integrated exercise.    

Phase 1:  IGS uses a predefined set of criteria to assess the risk on each of the transformers in the fleet.  Note that this is much wider than simply assessing the DBDs levels in oil.  Transformers are ranked using our scoring system.  High risk transformers are indentified and discussed with the client. 

Phase 2:  Further detailed testing and assessment is carried out on the high risk transformers and the scores revised. 

Phase 3:  Corrective measures are then considered for each, or each type of high risk transformer.  This may range from only doing monitoring, passivate, on-line oil regeneration to full rewind.  Some of the options are listed below.  Carry out test and assessment verifying effectiveness of method.

Phase 4:  Implement corrective measure and review of its effectiveness. 

Testing and monitoring

Whist there are some test that indicate that copper sulphide formation has taken place, DGA (dissolved gas analysis) has not been found to be effective as a preventive tool to intercept failures due to copper sulphide formation.

It should again be emphasized that moderate (or even extensive) amount of copper sulphide formation may not necessarily lead to failure. In fact, inspection of failed units shows that in some areas there may be quite heavy sulphide contamination without any indications of faults.

Mitigation techniques

If the oil has been identified as corrosive, a risk assessment should be carried out. If the result of this assessment is that the equipment is regarded as at risk, several mitigation techniques can be applied.

Passivator

The most widely used so far is the addition of metal passivator. Both oil exchange and on-site oil treatment to remove corrosive components have also been applied. It is sometimes also possible to modify operating conditions in order to lower temperature and dielectric stresses. 

Removal of corrosive sulphur from oil in service

Available oil treatment techniques may be divided into two groups, techniques based on the removal of corrosive sulphur compounds, and others based on the conversion of sulphur compounds. Combinations of these techniques are also available. Different treatments with activated earths may include removal of the corrosive sulphur compounds by absorption, and/or conversion into non-reactive compounds (chemisorpton). The Liquid/liquid extraction process is based on the selective extraction of corrosive sulphur compounds using solvents.

Several techniques have been proposed, and in some cases are already being used for the removal of corrosive sulphur from oil. These include the use of continuous on-line treatment with sorbents [34, 35, 36] “selective de-polarization” (a combination of reagents and sorbents) [37] mobile on-line reclaiming, with reactivating sorbents [38, 39] treatment with KOH/PEG, similar to established PCB removal technology [40] liquid-liquid extraction Oil exchange.

Each of these mitigation measures has side effects, a risk of not being effective or causing failure of the transformer.  These risks are different for each case.  For example, passivation of a transformer, where the oil has degraded significantly would not be viable, and aggressive actions as oil regeneration or refilling could lead to failure of a transformer.  We take both technical viability / risk and best economic value is taken into consideration when making recommendations. 

Services we can provide

We can provide a comprehensive transformer health and life management or this sulphur corrosion risk management service individually or coupled. 

We can provide:

·        All of the sampling and laboratory analysis.

·        Information gathering capturing on database and make it available to customer.

·        Expert Engineering assessment of risks, potential technical solutions, economic best economic or value assessments, mitigation measure impact assessments, operational change assessment & implementation, passivation, Sulphur removal by regeneration and oil refilling. 

Why Use IGS Engineering Services

·        IGS expert consultants have 40 years’  experience in power electrical plant assessment & management.

·        Uses the latest technology available.

·        International ties – know and use world best practices.

·        Experienced at overall plant management.

·        Use expert data base for transformer assessment.

·        Assessment results stored on data base introducing cost savings on later reassessments.

·        Assessments and recommendations based on international standards (IEC/IEEE). 

·        Assessments based on scoring system assisting bench marking in the future.  (Improvement / deterioration assessments).

·        Expert now based in Industrial Jubail City – close to your works.

·        KSA owned company focused on Power Electrical Consulting and Engineering Services and Transformers.

·        IGS services highly regarded in KSA.