Transformer Life Extension

The Threat from Oxygen

Overview
The following notes are based upon a seminar presented by Lars Arvidsson¹ in the UK during March 2002 on the subject of Transformer Life Extension. They are written from the perspective of an asset manager and focus on the adverse effects of oxygen on the life of a transformer. It is Lars' view that oxygen has a far more detrimental effect on life expectancy than any other operational parameter.

It is important to be aware that these notes deal only with the effects of oxygen. Other important factors that affect the life of a transformer (such as temperature and moisture) will be added later as separate topics in this series.

Oxidation
The laws of chemistry state that oxygen has a strong affinity for electrons. An atom that has lost electrons to oxygen is said to be oxidised. In a transformer, oil and paper both degrade as a result of oxidation.

Oxidation of Oil
Transformer oil is produced to various specifications such as the paraffinic, naphthenic and ester types. Some oils possess a degree of natural protection against oxidation and are used without the addition of anti-oxidants. These are referred to as uninhibited oils. The more highly refined oils do not generally possess this natural protection and require anti-oxidants to be added. These are known as inhibited oils.

Ageing and oxidation are synonymous. The ageing of oil begins slowly as the anti-oxidants that are present work to neutralise the harmful peroxides and radicals as they are formed. However, with time the anti-oxidants decrease in quantity and the ageing process increases exponentially.

Ageing leads to the formation of acids, aldehydes, ketones, esters and eventually sludge (a mixture of long insoluble hydrocarbon molecules and particles). The process occurs in the presence of peroxides (unstable oxygen compounds) and free radicals and is accelerated by catalysts such as water and copper.

Oxidised oil presents a maintenance requirement that demands action. Left unattended, oxidised oil continues to deteriorate and transports contamination to the cellulose insulation within the transformer. Here the effects are much more serious. Whilst oil can be changed, cellulose cannot. If the oil is not maintained, sooner or later the condition of the cellulose will deteriorate to the point where it has to be accepted that the transformer has reached the end of its working life.

Oxidation of Cellulose (Paper)
Cellulose degrades (oxidises) much faster than oil because it contains oxygen within its molecular structure. The degradation process generates water, carbon dioxide and furfurals, and is accelerated by external sources of oxygen, high temperature and high levels of oil acidity. The water that is generated combines with moisture drawn in from the atmosphere to further accelerate the degradation process and set up a vicious cycle. The end result is broken molecular chains, a lower degree of polymerisation (DP) and loss of mechanical strength. In the absence of oxygen, decomposition occurs more slowly through the process of pyrolysis.

Action to Minimise Oxidation
Set out below are practical steps that can be taken to reduce the harmful effects of oxygen and extend the life of a transformer:

	Be aware that less oxygen means more life
	Use sealed systems where practicable
	Consider measures to prevent the ingress of oxygen (eg a bag or membrane in the conservator)
	Eliminate the frequent cycling of forced cooling systems to avoid pumping in large quantities of air
	Consider de-gassing treatments for the oil - either as a continuous process or as a maintenance exercise
	Measure anti-oxidant concentrations in oil and add new inhibitor to old oil where appropriate
	Measure the copper content in the oil (to reduce the catalytic acceleration of oxidation)
	Test oil for sulphur (sulphur behaves in a similar way to oxygen)
	Measure the peroxide number (PXN) of the oil

Conclusions and Observations
Oxygen has a seriously detrimental effect upon the life of a transformer. Oxidised oil can be maintained but once the effects have extended to the cellulose, the life of a transformer is at risk. Fortunately preventive measures are available that are cost effective and suitable for all asset management programmes.

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^{1 Lars Arvidsson MSc. Chemistry, is chief executive of Vasteras
PetroleumKemi AB, an independent chemical analysis and consultancy company based
in Sweden. His many published papers include Microbiological Activity within
Hydrocarbon Bulks and his pioneering work in the transformer field includes the
development of unique methods for assessing the condition of transformers. Email: lars.arvidsson@petroleumkemi.se}