Aging of power transformers insulation

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Long term improvements of all power transformers insulating system characteristics

Power transformers insulating system comprises of:

» liquid insulation (oil) and

» solid insulation (cellulose, press-board, wood)

Approximately 85% of all transformers breakdowns are related to the failures of insulating system.

Working life of power transformers is determined by the lifetime of its solid, i.e. cellulose insulation. As cellulose degradation is irreversible process, once cellulose depolimerization, measured as degree of polimerization- DP, reaches certain level, it is considered that power transformer has to be replaced.

Molecular structure of solid insulation is cellulose. The life of solid insulation depends mostly on the oil condition in which it is immersed. Our researches carried out on transformers that were in operation for over 10 years show that cellulose insulation between conductor and oil is in much worse condition closer to the oil. Aging products in the oil, which are not eliminated on time, eventually become main cause of insulating systems degradation.

Insulation degradation products are distributed in both liquid and solid insulation regardless of its origin due to diffusion and circulation of oil through cellulose.

Therefore it is necessary to remove as much as possible elements (pollutants) that degrade oil and cellulose, slow down processes that lead to it and eliminate oil and cellulose degradation products, because they also accelerate harmful processes.

Mechanism of insulating oil aging

Oxidation is reaction of unstable hydrocarbons from oil with oxygen and other catalysts such as moisture and metal particles, with the assistance of heat which significantly accelerate oxidation.

Process of insulating oil oxidation, i.e. aging begins under the influence of temperature, metals, water and oxygen, forming free radical molecules and peroxides.

A chain reaction starts, creating aldehydes, ketones, esters and organic acids, and finally forming acidic residues on the cellulose, windings and at the bottom of the transformer.

All insulating oils contain natural inhibitors (antioxidants), which among other things, prevent formation of peroxides. However, their concentration is not sufficient to prevent or slow down oxidation, so it is necessary to add artificial inhibitors, phenol or amine type, that block free radicals reactions.

Oil oxidation products are:

» Acids

›» Peroxides

» Resin (oil-soluble polar substances, which in the case of continuous oxidation turns into insoluble deposit- sludge, which is ultimate product of oil deterioration)

Degradation products are molecules of remarkably polar character and being such they can be factors of harmful currents in electrical and magnetic field in transformer insulating system.

Acids and peroxides created by oxidation of insulating oil deteriorate cellulose and accelerate its aging.

Acids can lead to creation of corrosion, attack the cellulose and in the later stages of oxidation can create acidic sludge which accumulates in the cellulose, side walls of the tank, in ventilating ducts etc. If the transformer is subject to overload and heating the more likely is sludge formation. Such created sludge will increase oil viscosity and reduce its cooling characteristics.

If Total Acid Number (TAN) exceeds 0.4 mgKOH/g oil starts to form sludge. Nowadays, it is considered that it is acceptable if TAN <0.1 mgKOH/g.

Mechanism of cellulose insulating aging

Cellulose consists of glycose molecules polymerised into chains of molecules. Solid insulation starts to degrade from the very beginning due to the presence of remained water from production, oxygen and temperature. Under the influence of high temperature and aging promoters, ie. water and acids, links connecting polymers broke irretrievably, reducing mechanical and dielectric properties.

During this process, molecules of H2O, CO2, CO, CH4 and furans compounds are created.

Water is created in significant amount but just like gases, CO2, CO, CH4, it can not be reliably taken into consideration when assessing the state of cellulose insulation because they are all created in the process of oil aging and under the environmental influence. Only furans compounds are exclusively products of cellulose insulation aging.

According to the law of the equilibrium of molecules between two media, all newly created compounds are distributed in both cellulose and oil.

Interaction of oil and cellulose aging products

Oil aging products accelerate the degradation of cellulose and vice versa. Under the influence of acid, water, peroxide-oxygen cellulose aging is significantly accelerated. In addition to temperature degradation, electrolytic degradation (acid and water) and oxidative degradation (oxygen peroxide) occurs as well. In the higher stages of degradation acids together with solid particles create a hard acidic residue at cellulose surface and inner sides of transformers. This, apart from chemical influence, worsen transformer cooling. Because polymerized cellulose molecules are broken, cellulose loses its mechanical and insulating properties, and because gas and water are created oil loses its insulating properties. With time, by spending inhibitors oxidation process is accelerated. Acidic residue which is chemically active is especially dangerous, prevents cooling and represent a weak insulating layer (partial discharge phenomena). All this dramatically accelerate aging and significantly increase the possibility of a definitive failure (insulation electrical breakdown).

Documentation \ Aging of power transformers insulation

MAIN INFLUENCES OVER CELLULOSE AGING

- Temperature

- Oxygene

- Water

- Acids

PAPER DEGRADATION:

Increase:

- Dissipation factor

- Mechanical strength

- Temperature

Decrease: