7.1.
CORE MATERIALS
Transformer cores are built from thin sheets of steel. These sheets are manufactured specifically for use in transformers.
Core steel has low carbon content < 0.1%. Increased carbon content has a detrimental influence on the hysteresis losses as well as the ageing properties.
Core steel is alloyed with silicon (Si). Silicon increases the specific electrical resistance, which again reduces the eddy current losses in the core. Increased silicon content makes the core steel brittle; therefore the content is kept below 3%.
Today, only grain-oriented steel is used. By cold rolling the steel sheets, the magnetic domains in the steel sheet will tend to be oriented in the rolling direction.
One gets a material with very good loss properties in the rolling direction, and correspondingly poor properties in the transversal direction. The rolling process requires special equipment with very high surface pressure.
The grain oriented core steel is available in several grades. The different properties are obtained by the raw material composition, the degree of cold rolling and different finishing treatments, for example laser treatment. Laser treatment is a mechanical treatment which divides the magnetic domains into smaller domains with lower losses as the result.
To minimise eddy current losses, the sheets must be insulated from each other. Earlier it was common to use varnish or paper. Today the core steel is delivered ready insulated from the manufacturer. The insulation is an inorganic material compatible with transformer oil and is corrosion and temperature resistant. The insulating coating is very thin < 4 µm. A thin coating means a good core fill factor.
The core is built up from many layers of core steel sheets. Suppose the core limb was built from a solid iron bolt, the core would represent a short-circuited winding around itself, and the transformer would not work. The mutually insulated sheets prevent such a short- circuit.
Eddy current losses in the core steel are proportional to the square of the thickness. Therefore the steel sheets have to be thin in order to reduce the no load losses. Typical thickness is from 0.23 mm to 0.30 mm.
7.2.
CONDUCTOR MATERIALS
From a technical and economical point of view there are two conductor materials that can be used, copper (Cu) or aluminium (Al).
The choice of conductor material depends mainly on price and availability. The supply and demand of copper and aluminium can vary quite significant on the world market, and thereby also price and availability of conductors from the manufacturers.
Some customers prescribe copper only, for different reasons.
The physical dimensions of a transformer with copper windings are normally smaller than that of a transformer with aluminium windings.
Conductor shapes are foil, round or rectangular wire.
The foil is not directly insulated. The round and rectangular wire is normally coated by varnish. The use of varnish instead of paper improves the winding space factor due to smaller thickness, and reduces the winding to liquid temperature gradient (winding to air temperature gradient for dry- type transformers).
Rectangular wire can also be insulated by paper for thermal class 105 or with aramid for higher temperature classes.
Cold working of the conductor increases the hardness and leads to increased the ability to withstand short-circuit forces.
7.3.
INSULATION MATERIALS
7.3.1. Solid insulation materials
7.3.1.1. General
A good insulation material must have the following properties: 1. High dielectric strength,
2. Good mechanical properties,
3. Long lifetime at operating temperature, 4. Easily workable.
Insulation material must withstand the operating temperatures that occur in the transformer during the lifetime of the transformer.
Insulation materials to be used in liquid-immersed transformers must be compatible to the liquid. 7.3.1.2. Cellulose materials
Mainly used in oil immersed transformers with thermal class 105.
Cellulose insulation is made of slow growing types of wood, having long fibres. Long fibres give long life-time, and high density gives high dielectric strength.
Cellulose products are compatible to mineral oil, and are easy to oil impregnate.
The impregnation is done under vacuum and elevated temperature, and the tiny cavities in the cellulose are filled with oil. Thereby the dielectric strength is further increased. In case the cavities were not filled with oil, these small air bubbles would cause partial discharges. Partial discharges may in the long term escalate to a dielectric break-down.
Contaminants represent weaknesses in the insulation that may lead to dielectric break-down. Cellulose insulation is specified in IEC 60554-3 for paper, and IEC 60641-3 for board. 7.3.1.3. Porcelain
Porcelain is mainly used for bushings in oil-immersed transformers. In some cases also used as supports or spacers in dry type transformers.
7.3.1.4. Solid synthetic insulation materials
These materials are mainly used in dry type transformers or reactors having higher thermal classes 130, 155, 180, 220. These materials are more expensive than cellulose insulation.
Enamels are used as conductor insulation, and normally double coated. Several qualities for different applications are available. Reference is made to IEC 60317
Epoxy resins used in combination with fillers, for example glass fibre and quartz powder is used for insulation barriers and complete vacuum cast windings.
Polyesters can be used as insulation barriers, spacers and duct sticks. Reference is made to IEC 60893-3 and IEC 61212-3
Aramid fibre are used to manufacture insulation paper or board sheets in different thicknesses. The material surface may be smooth or porous. The porous type can to a certain extent be oil impregnated. The material has very good thermal properties, thermal class 220.
7.3.2. Fluids
7.3.2.1. General
The fluid in a transformer has several functions; the two most important are certainly insulation and cooling. Another function is to carry information about the condition of the active part inside the transformer.
Several requirements have to be fulfilled;
Chemical Electrical
Oxidation stability Breakdown voltage AC
Oxidation inhibitor content Breakdown voltage, impulse
Corrosive sulphur Dissipation factor
Water content Streaming charging
Neutralization value
Physical Additional
Viscosity Low particle content
Appearance Compatibility with other transformer
materials
Density Gassing properties
Pour point Aromatic structure
Surface tension Poly-aromatic structure
Flash point Solubility properties
7.3.2.2. Mineral oil
Important properties of mineral oil are specified in IEC 60296. Flash point 145 °C, density 0,88 kg/dm3, relative permittivity 2,2.
Mineral oil is the most common liquid used. Mineral oil is normally the reference to which all other liquids are compared.
Mineral oil offers in most cases the best compromise between cost and technical properties, and compatibility with other transformer materials is also very good.
7.3.3. Other fluids
These fluids are reserved for special applications, and are typically 5-6 times more expensive than mineral oil.
The main motivation for using these fluids is improved fire safety and environmental impact.
Further these fluids are applicable for operation at elevated temperatures, however have limited capabilities in extremely cold climates.
7.3.3.1. Dimethyl Silicone
Important properties of silicone fluid are specified in IEC 60836 Flash point 310 °C, density 0,96 kg/dm3, relative permittivity 2,7.
Silicone fluid has slightly lower dielectric and cooling properties compared to mineral oil.
When it ignites it is self-quenching because it creates a layer of oxide. However, it is not self- quenching related to arcing and electrical failures.
7.3.3.2. Synthetic Ester
Important properties of synthetic ester are specified in IEC 61099 Flash point 275 °C, density 0,97 kg/dm3, relative permittivity 3,2. 7.3.3.3. Synthetic Hydrocarbon
Important properties of synthetic hydrocarbon are specified in IEC 60867 Flash point 230 °C, density 0,83 kg/dm3, relative permittivity 2,1.
7.3.3.4. Agricultural Ester
No applicable IEC specification.
Flash point 330 °C, density 0,91 kg/dm3, relative permittivity 3,2.
This is good compromise between fire safety and environmental friendliness.
BIOTEMP is an ABB developed and patented agricultural ester, based on sunflower oil, see also www.abb.com/ for additional information.