SAMPLING OF TRANSFORMERS [Monday, November 30, 2009]
SAMPLING OF TRANSFORMERS
It’s well known that regular oil analysis is extremely useful in monitoring the condition of transformer oils. The Oils are used to insulate all types of electrical transformers and other electrical distribution equipment.
The Analysis of transformer oils not only provides information about the oil itself, but also enables the detection of other potential problems, including contact arcing, ageing insulation paper and other latent faults and is an indispensable part of a cost efficient electrical maintenance programme.
Yearly routine oil analysis should be the cornerstone of any maintenance programme for all critically oil-filled electrical equipment, including transformers, circuit breakers and voltage regulators
Moisture content (PPM Test
One of the most important functions of transformer oil is to provide electrical insulation. Any increase in moisture content can reduce the insulating properties of the oil, which may result in dielectric breakdown. Water and oil due to their differing chemical properties are not mutually soluble. However up to a certain limit a small amount of water will dissolve in the oil. With fluctuating temperatures the dissolved water will become free as the transformer cools down, resulting in poor insulating power and oil degradation. Many transformers contain cellulose-based paper, which is used as insulation in the windings. Excessive moisture content can result in the breakdown of this paper insulation with a resultant loss in performance. The test performed is of extreme sensitivity and can detect water at levels down to a few part per million
Acid Test (Acid Test)
Transformer oils are oxidized under the influence of excessive temperature and oxygen, particularly in the presence of small metal particles which can act as catalyst. Oxidation products are usually acidic in nature and result in an increase in acid number. Further reaction of these acids with the bulk oil can result in sludge and varnish deposits. In the worst-case scenario, the oil canals can become blocked and the transformer is not cooled down adequately, which further exacerbates oil breakdown. Increase in the acidity has a damaging effect on the cellulose paper. Thus goes hand in hand with a decrease in dielectric strength and increased moisture content.
Dielectric Strength (KV-Test)
Dielectric strength of transformer oil is a measure of the oils ability to withstand electrical stress without failure. Transformer oils are designed to provide electrical insulation under high electrical potentials, any significant reduction in the dielectric strength will indicate that the oil is no longer able to perform this vital function. Some of the things that can cause a reduction in dielectric strength include contaminants such as water, sediment, conducting particles, oil degradation by-products and cellulose paper breakdown.
Dissolved Gas Analysis
(Gas Test) Dissolved gas-in-oil analysis has become a standard in the electrical maintenance industry throughout the world and is considered to be the most important oil test for transformer oil in electrical apparatus. As the insulating materials of a transformer breakdown from excessive thermal or electrical stress, gaseous by- products form. The by-products are characteristics of the type of incipient fault conditions, the materials involved and the severity of the condition. Dissolved gases are detectable in low concentrations (ppm level), which usually permits early intervention before failure of the electrical apparatus occurs, and allows for planned maintenance. Dissolved Gas Analysis is used not only as a diagnostic tool but also stem apparatus failure.
PCB Analysis (PCB Test)
PCB’s are a group of synthetic oil-liked chemicals of the organ chlorine family. Until their toxic nature was recognized and their use was banned in the early 1980’s, they were widely used as insulation in electrical equipment and particularly in transformers PCB’s are poisoness, which have been shown to cause damage to the reproductive, neurological and immune system of wildlife and humans, thus where explosions of transformers takes place it can cause costly decontamination work.
Oil samples can be taken at any time from most equipment without having to take it out of service, allowing a “window” inside the electrical apparatus that helps with diagnosing and troubleshooting potential problems.
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