E+C August 2018

TRANSFORMERS + SUBSTATIONS

transformer oil throughout its operational life, es- pecially in humid atmospheric conditions. Howev- er, since paper has more affinity for water than oil, such moisture will primarily be stored in the paper rather than in the oil. During the manufacturing process, the core and coil assembly is heated for many days under vacuum to drive out any possible moisture that may be present. Hot oil is then admitted to the tank under vacu- um to ensure the removal of any gas or moisture remaining in the oil. The whole unit is further pro- cessed by circulating hot oil through a filter plant. At this stage the new transformer has a moisture content of <0.5% by weight in the paper and <5 ppm in the oil. During operation of the trans- former, thermal and chemical degradation of paper insulation also produces water as a by-product of chemical reactions and, in a severely deteriorated system, the moisture content of the paper may reach more than 4%. This process is accelerated by the heat produced when the transformer is on load. If the transformer spends much of its life at low load, its useful life will be greater than a generator transformer, which usually runs close to its nameplate rating. Measures to minimise oil degradation Oxidation is the major cause of oil degradation in free breathing transformer units. To prevent the in- gress of air into the transformer during the breath- ing process, nitrogen gas cushion transformers can be used, where dry nitrogen is used to fill up the tank space above the oil to minimise the expo- sure of oil to oxygen. Changing the silica gel in the breather before it is saturated with moisture can minimise the ingress of moisture via this route. Adding oxidation inhibitors such as DBPC to new oil effectively prevents the ageing of the oil. The ageing process of oil is accelerated by the presence of metals and adding amino group in- hibitors can eliminate this influence. Frequent fil- tration of the oil to remove excess moisture can slow down the degradation rate. Temperature is a dominant factor of oil degradation. The oil temperature can be limited to safe val- ues through the use of cooling tubes that aid in the heat convection and dissipation process. Cooling efficiency can further be improved by artificial cool- ing techniques such as forced fan cooling, forced oil cooling, or water cooling. Large transformers are often equipped with pumps in the oil line, and in the water line, to enhance cooling.

that the oil is replaced before it reaches 0.2 mg KOH/g as the formation of sludge is not irrevers- ible at this stage and it might be flushed from the unit. Acids cause the formation of sludge in oil. Sludge is a solid product of complex chemical composition and gets deposited throughout the transformer. Deposition of sludge can adversely affect the heat dissipation process by blocking oil circulation through the radiator pipes, leading to overheating and subsequent failure of the trans- former. Acid damages the structure of the insu- lation paper, with a subsequent loss of optimal transformer lifetime reached. Undesirable effects of moisture on oil All oil-paper transformers contain some water in their solid insulation. The paper in a transformer acts as the reservoir for moisture content through- out the insulating system. Water remains in a continuous dynamic state within the complex in- sulation system of a transformer. Depending on the operating temperature, water exchange takes place between the liquid and solid insulation. High levels of moisture in transformers can accelerate the ageing process of solid insulation, reducing the dielectric strength of the oil. Since at higher temperature the solubility of wa- ter in aged oil increases, even with an increased amount of water, the percentage saturation of the oil remains low and the dielectric breakdown volt- age of the insulation system remains high. Prob- lems arise in the oil when the transformer cools down again. It is possible for excessive moisture to remain in the oil because it migrates much more slowly back into the paper.This causes the oil to be- come supersaturated with water, leading to the for-

Corné Dames is the Sales and Service Manager at Transformer Tech Repair.

mation of free water in the transformer. The free wa- ter can settle where it will contribute to surface flash- over or could collect at the bottom of the cooler bank. Sudden operation of the oil pumps can direct this wa- ter onto windings, causing insulation failure and result- ing in major electrical break- down or short-circuit failure of the windings. In free-breathing trans- formers, a small quantity of water can enter the

16 Electricity + Control

AUGUST 2018