The Severity-Lifespan Model (SL) provides a means of evaluating particular transformer risks to enable effective plans to be put in place for condition recovery. Present theory supports the notion that the transformer health is directly related to the condition of the insulation [Cigre WG12.18]. Insulation deterioration can be rapid due to high energy and mechanical forces or slow due to chemical degradation mechanisms. Ageing models relate thermal characteristics as the prime mechanism of insulation ageing yet a significant number of transformers that suddenly fail have not exhibited high elevated temperatures [McNutt1]. The SL Model was created with a specific focus on the holistic conditions that contributed to the transformer health.

The model was based on three levels of impacts. These are long term, medium term and short term impact and the details are captured in figure 1.

Long Term Impact

Long term ageing factors of figure 1 are those that are present in the transformer but do not significantly affect the transformer life immediately. These may however be catalysts and supporting mechanisms for other more severe degradation processes. These ageing mechanisms are usually contained and originate in the oil medium.

Oil Quality

Oil quality is a major aspect of transformer health [Lapworth1]. For oil to carry out the intended functions of dielectric and as a heat transfer medium it needs to be within specification. The following parameters are utilised for oil quality monitoring; dielectric strength, water, acidity and interfacial tension.

Dielectric Strength

All insulating material has some resistance to breakdown. This is measured as dielectric strength and in terms of transformer mineral oil; it is the ability of the oil to withstand electrical stress caused by high voltage gradients [Wang2]. This measure is applied predominantly to the oil giving an indication of its condition due to the dielectric strength being highly affected by contaminants such as water and free particles in the oil. The dielectric strength is measured in kV/2.5mm gap.

Water (H₂O)

Water is one of the major contaminants in a power transformer [Lundgaard1]. It is usually found in the paper insulation or dissolved and as free water in the oil [McNutt1]. Water is always present in an oil-immersed transformer and originates either from the atmosphere or internally from chemical reactions during the degradation process of oil and paper insulation [Garcia1]. Water content in the oil is dynamic as its solubility is highly temperature dependent and migrates between the oil and paper to reach an equilibrium state [Sheiretov1].

Water forms a significant catalyst in the breakdown process of the oil and paper insulation. When left uninfluenced, it creates a sustained cycle of cellulose breakdown and increase in water levels [McNutt1]. Water is measured in parts per million. 

Acids

The acidity of the oil originates from chemical reactions due to the degradation of the oil, which is usually as a result of oxidation of the oil and particles and this process is highly sensitive to temperature and levels of oxygen in the oil [Lundgaard1]. With the presence of water, the acidity can affect metal parts like the tank and supporting structures causing corrosion. If the acidity of the oil reaches high proportions, it results in the production of sludge [Gossling1]. Acidity is measured in mg.KOH/g oil.

Interfacial Tension

Interfacial Tension is the measure of tension at the interface between oil and water [Shaban1]. The measuring unit is dynes/cm with good oil having a reading between 40-50 dynes/cm. Both water and by-products of oil oxidation have the effect of reducing the interfacial tension measure [Eklund1].

Hydrolysis

Hydrolysis is the decomposition of a chemical compound by reaction with water. [Cigre WG12.18] highlighted that the energy of activation of pyrolysis is 1.4-2.0 times as great as the energy of activation of hydrolysis with hydrolysis being the dominant process for temperatures between 110-120 °C. The hydrolytic process requires an acid catalyst to provide a hydrogen atom to initiate the process [Emsley3].

Oxidation of Oil

Oxygen has a strong affinity for electrons and the process of bonding where oxygen consumes electrons from other atoms is oxidation [Sabau1]. This is evident in power transformers especially on the oil and paper insulating material. The oxidation of transformer oil produces acids, aldehydes, ketones, esters and sludge. A mixture of hydrocarbon molecules and particles form sludge [Sabau1]. 

Medium Term Impact

Medium term impact factors of figure 3-1 are composed of degradation processes, which have already been initiated and reaching levels that may reduce the transformer lifespan drastically. These consist of pyrolysis, oxidation of cellulose and the effects of corrosive sulphur. This section identifies methods of how these processes can be effectively measured.

Pyrolysis

Pyrolysis may be defined as the chemical decomposition of organic material when heated at high temperatures, usually greater than 300 °C and within 700 °C. One of the key aspects of pyrolysis is that it does not require oxygen or any other reagents [Demirbas1]. The process predominantly yields hydrogen and methane but for extreme cases yields carbon.

DiGiorgio identified relationships of typical fault gas distributions for pyrolysis in oil and of cellulose as highlighted in table 3.1 [DiGiorgio1].

Experiments conducted noted that at 700 °C the oil is completely decomposed to form methane, ethane and propane with no traces of acetylene [Buenomak1].

Oxidation of Cellulose

Due to cellulose having oxygen as one of its molecular components, it degrades at a faster rate than oil. The oxidation process produces water, carbon dioxide and furfurals [Liland1]. High temperatures and higher levels of oxygen and acidity significantly affect this process, which causes rapid breakdown of the glucose molecules into shorter chains decreasing the overall strength of the paper. Aging of the paper also increases the acidity of the oil [Lundgaard1].

Corrosive Sulphur

The formation of copper sulphides in transformers became a problem of note in the late 1990’s to present date. This was due to the changes in the methods of refining and oil purification with new suppliers in the transformer oil industry introducing new sources of crude oil having different profiles of naturally occurring sulphurous compounds [Sundin1].

The two mechanisms of copper sulphide are the corrosion of copper and the depositing of copper sulphide on conductors and paper insulation. The effect of this is either a reduction in the dielectric strength of the solid insulation or providing a conductive path [Lewand1]. Significant copper sulphide build-up results in the flaking of these conductive particles in the oil, which reduces the dielectric, and cause related failures [Griffin2].

The corrosive sulphur destructive process is highly temperature dependent. During conditions of low temperature, the process is one of either predominantly the formation of conductive sulphides, which then suddenly results in failure at elevated temperatures, or when the amount of copper sulphide is enough to cause conduction.

The Cigre WG A2-32 (Copper sulphide in transformer insulation) has identified the following key aspects of corrosive sulphur in power transformers [Cigre WGA2-32, Dahlund1]:

• Sulphur must be present in the transformer oil. This includes oil types such as naphthenic and paraffinic or intermediate oils.
•  Temperature plays an important role in the process but is not the sole factor for occurrence. Copper sulphide formation at temperatures from 80 °C to 150 °C was demonstrated in a laboratory.
• Most failures recorded to date have been found in GSU transformers, shunt reactors and HVDC transformers, which were attributed to the high loading tendency of the equipment.
• Failures recorded were also mainly found in closed units, which may indicate that the presence of oxygen slows down the copper sulphide production process.
• The DGA and the usual oil monitoring parameters like tan δ are not susceptible to the detection of copper sulphide production.
• It is generally found that the deposition of copper sulphide closely relates to the thermal profile in the transformer with typical location at the top of windings.

Short Term Impact

Short term impacts of figure 3-1 are conditions that when experienced will result in failure or initiation of failure of the transformer. These are further broken up into three categories which are controlled defective events, major component failure and defective events.

Short Term Impact are conditions within the transformer that are changing in the current environment with visible effects on the parts of the transformer. These are usually evident in the oil medium where measurements can be easily taken from oil samples. These parameters consist of the water content, dielectric strength, interfacial tension and top oil temperature of the transformer.

By combining these parameters, an effective indicator can be established on the immediate condition of the transformer insulation medium and control further deterioration to limit medium and long-term impacts.

Controlled Defective Events

These events are controllable by the engineers and consist of design, operating conditions and manufacturing of the transformer. Operating conditions cover events such as overloading beyond the capability or close to limits. Transformers in the modern era are pushed to limits especially when utilities are faced with under supply of power. Short time overloading can affect transformer life and the process may be irreversible. Transformer designs can also contribute to the lifespan of the transformer. The transformer must be effectively designed to handle a certain level of short circuits and over voltages. The design of the cooling of the active parts is also another important parameter to be considered. The manufacturing process is another risk area where poor workmanship of important components like the core and windings can result in premature failures.

Major Component Failure

Major component failures such as bushings and tap changers can be catastrophic to the life of the transformer. These events usually end the life of the transformer no matter how well the main transformer windings and core is designed and manufactured.

Defective Events

Defective events are system and environmental conditions that can severely affect the transformer. Defective system conditions consist of through faults, line over voltages and inrush currents. Environmental conditions consist of lightning and geomagnetic induced currents caused by solar storms. The nature of these events is such that they pose high levels of mechanical and thermal stress on the transformer electric and magnetic components and may be the one event that either causes failure or initiates failure mechanisms.

Partial Discharge and Corona

Partial discharge usually occurs within insulation systems that have voids, cracks, and along the boundary between different insulating materials or in bubbles within liquid dielectrics [McNutt2]. Discharges predominantly occur within localised areas with the discharges only partially bridging opposite electrodes [Mathes1].

Partial discharges within the winding paper insulating material are usually initiated within oil-filled voids within the dielectric. This causes the dielectric constant of the void to be significantly lower than the surrounding dielectric, which results in the electric field across the void being considerably higher than across an equivalent distance of dielectric. PD activity usually starts when the voltage stress across the void is increased above the corona 

inception voltage. The mechanism of deterioration caused by PD is slow and persistent until electrical breakdown occurs within the area [Mathes1].

Table 3.2 below summarises the chemical breakdown and fault gas distribution for typical corona in oil.

Arcing

An electric arc is formed from an electric breakdown of a gas which is able to sustain long enough to cause a current flow through the non-conductive medium [Perkins1]. In the case of a power transformer arcing can occur in the oil (non-conductive medium) between areas of high potential differences. An electric arc is associated with high temperatures that are capable of degrading or destroying the area of the medium around which is forms. The key difference between a partial discharge and an electric arc is that the electric arc results from a continuous discharge of electrons until permanent breakdown occurs [Perkins1]. Typical fault gas distribution for arcing is provided in table 3.3 below.

Relevance of Severity-Lifespan Model

The severity–lifespan model is significant in this study as it organises the transformer ageing risks into an integrated chart in relation to the impact and time. The key information from this model is the identification of the insulation degradation mechanisms and how these propagate over time to become catastrophic mechanisms to the life of the transformer. This model also provides a high-level summary of the degradation parameters that would be most effective in identifying the impact of the particular degradation processes.

References

1. [Cigre_WG12.18] Cigre Working Group 12.18, “Guidelines for Life Management Techniques for Power Transformers,” Cigre, 22 June 2002
2. [McNutt1] McNutt, W. J., “Insulation Thermal Life Considerations for Transformer Loading Guides,” IEEE Transactions on Power Delivery, Vol. 7, No. 1, pages 392-401, January 1992
3. [Lapworth1] Lapworth, J. A., Wilson, A., “The Asset Health Review for Managing Reliability Risks Associated with Ongoing Use of Ageing System Power Transformers,” IEEE International Conference on Condition Monitoring and Diagnosis, pages 605-608, Beijing, China, 21-24 April 2008

[Wang2]	Wang, M., Vandermaar, A. J., Srivastava, K. D., “Review of Condition Assessment of Power Transformers in Service,” IEEE Electrical Insulation Magazine, Vol. 18, No. 6, pages 12-25, November / December 2002
[Lundgaard1]	Lundgaard, L. E., Hansen, W., Linhjell, D., Painter, T. J., “Ageing of Oil-impregnated Paper in Power Transformers,” IEEE Transactions on Power Delivery, Vol. 19, No. 1, pages 230-239, January 2004
[Garcia1]	Garcia, D. F., Garcia, B., Burgos, J. C., “Modeling Power Transformer Field Drying Processes,” Drying Technology: An International Journal, Vol. 29, No. 8, pages 896-909, May 2011
[Sheiretov1]	Sheiretov, T., Zahn, M., “Dielectrometry Measurements of Moisture Dynamics in Oil-impregnated Pressboard,” IEEE Transactions on Dielectric and Electrical Insulation, Vol. 2, No. 3, pages 329-351, June 1995
[Gossling1]	Gossling, P. W. L., Welch, L. H., “The Stability of Oil in Transformers,” Proceedings of the IEE – Part II: Power Engineering, Vol. 99, No. 69, pages 231-238, June 1952
[Eklund1]	Eklund, M., “Mineral Insulating Oils; Functional Requirements, Specifications and Production,” IEEE International Symposium on Electrical Insulation, Conference Record of the 2006, pages 68-72, 2006
[Shaban1]	Shaban, K., El-Hag, A., Matveev, A. “Predicting Transformers Oil Parameters,” IEEE Electrical Insulation Conference, pages 196-199, Montreal, QC, Canada, 31 May- 3 June 2009
[Emsley3]	Emsley, A. M., Xiao, X., Heywood, R. J., Ali, M., “Degradation of Cellulosic Insulation in Power Transformers. Part 3: Effects of Oxygen and Water on Ageing in Oil. IEE Proceedings In Science, Measurement and Technology, Vol. 147, No. 3, pages 115-119, May 2000
[Demirbas1]	Demirbas, A., “Combustion Characteristics of Different Biomass Fuels,” Progress in energy and combustion science, Vol. 30, No. 2, pages 219-230, 2004
[Digiorgio1]	DiGiorgio, J. B., “Dissolved Gas Analysis of Mineral Oil Insulating Fluids,” DGA Expert System: A Leader in Quality, Value and Experience 1, Northern Technology and Testing, pages 1-17, http://www.nttworldwide.com/tech2102.htm, 2005
[Buenomak1]	http://www.buenomak.com.br, “Pyrolysis Add Electrical Arcing Investigation of Faults in Transformer Oil,” Milton Farber Space Sciences, Inc. 135 W. Maple Avenue Monrovia, California 91016, 14 July 2010