The “Degradation of insulating papers” criticality is caused mainly by normal ageing mechanisms and by particular conditions of thermal, electrical and mechanical stress on insulating papers.

Causes in relation to life cycle phases

Causes of the “Degradation of insulating papers” criticality When it may occur (life cycle phases)
Lack of purchase requirements for insulating papers Requirements and purchase
Deficiency in quality control for individual batches or single insulating paper supplies (example initial DP before impregnation) Acceptance of insulating papers
Deficiency in analytical procedures for checking degradation of insulating papers Oil acceptance, factory test, installation and pre-energisation, operation, old age, post-mortem
Loss of protection gas and moisture accumulation on solid insulators Transport and installation of the transformer
Deficiencies in paper dehumidification treatments (e.g. vapour processes phase). A good insulating paper has a water impregnated value of between 0.5 and 1% by mass Construction, transport, installation and pre-energisation, operation, old age
Accumulation of air and moisture (for example, during oil change or other electromechanical maintenance) Factory test, installation and pre-energisation, operation, old age

The critical factors that characterise the ageing of insulating papers are temperature, water, oxygen, whether the system is closed or opened, thermal cycles and the relationship with the load profile of the transformer.Oil degradation processes are extremely complex, depend on many factors and call for mechanisms that are difficult to formalise quantitatively.Paper thermal degradation processes are the result of the interaction of the following 3 mechanisms:

1.      Hydrolysis. Through desorption of paper and the breakdown of cellulose chains, this phenomenon generates water as the main degradation product (in the presence of hot spots typically at a temperature of < 150 °C).

2.      Oxidation. This phenomenon typically occurs in the presence of hot spots, at temperatures between 150 °C and 300 °C, and is characterised by an increase in oxygen demand. The result is a decrease in the concentration of oxygen in the oil (for a silica gel respirator transformer it changes from 20000 ?l/l to values if 5000 0 10,000 ?l/l) which combines with the carbon originating from the degradation of cellulose forming CO2 and CO.This process simultaneously produces other by-products of cellulose, including furan compounds (in particular 2FAL), methanol and ethanol.

3.      Pyrolysis. This is a thermochemical decomposition mechanism typically occurring in hot spots at temperatures above 300 °C, such as to break the paper molecules with the formation of particles, water, methanol, ethanol and other characteristic gases.

The products of paper degradation are solid, liquid and volatile compounds (gas). These include those soluble in oil (furans, methanol, ethanol) and those insoluble in oil (particles, sludge) which amalgamate with the sludge resulting from the ageing of the oil. Oil and paper derived sludge add up to form the total sludge.

Schematic diagram showing the aging rate k, depending on the various degradation mechanisms (Figure 1 from IEC TR 62874: 2015)
Relationship between the mechanical properties of the insulating papers and the degree of polymerization of the papers (Figure 2 taken from IEC TR 62874: 2015)

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