Types of Windings Used in Core Type Transformers
The following mentioned windings are layered type and either rectangular or round conductor.
(a) Multi Layered Windings
(a) Single Helical Windings
(b) Double Helical Windings
(c) Disc- Helical Windings
Multi- Layer Helical Windings
Cross - Over Windings
Disc and Continuous Disk Windings
Cylindrical Windings - Cylindrical windings are windings with low voltage that use up to 6.6 kV for kVA up to 600-750 and have a current rating between 10 to 600A.
Multi Layered Cylindrical Windings Cylindical windings are mostly used in its multi- layered form. It is used with rectangular conductors in the two- layered type as it becomes very easy to secure the lead- out ends. The oil ducts separate the layers of the windings, this arrangement facilitated by the cooling process through oil circulation in the winding. In multi layered cylindrical windings, circular conductors are used; they are wound on vertical strips to improve the cooling conditions. The arrangement creates oil ducts which facilitate good cooling. These types of windings are used for high voltage ratings up to 33kV, 800 kVA and the current ratings up to 80 A. The maximum diameter that is used in this type of winding for a bare conductor is 4mm.
Helical Windings - Helical windings are used for low voltage and high capacity transformers, where the current is higher and at the same time the winding turns are lesser.The output of transformer in this case varies from 160 to 1000 kVA from 0.23 to 15 kV. For securing the adequate mechanical strength the cross- sectional area of the strip is made not less than 75 to 100mm square. The maximum number of strips that are used to make up the conductor in parallel is 16.
Multi-layer Helical Windings - The Multi- Layer Helical Winding is used commonly for high voltage ratings such as 110 kV and above. These windings include several cylindrical layers concentrically that are wound and connected in series. The outer layers of these windings are made shorter than the inner layers for distributing the capacitance uniformly. These windings basically improve the surge behavior of the transformers.
Crossover Windings - These windings are used for high voltage windings of the small transformers. The conductors of these windings are paper covered strips or round wires. The windings help reduce the voltage between adjacent layers as they are divided into a number of coils. The coils are separated axially by a distance of 0.5-1mm. The voltages between adjacent coils must not be more than 800 to 1000V. The inside end of the coil is connected with the output end of the adjacent one. The axial length of each coil in actuality is about 50 mm, while the spacing between the two coils is about 6 mm so that it can accommodate the blocks of insulating material. The coil width is 25 to 50 mm. Crossover winding has greater strength than cylindrical winding under normal conditions. Although, the crossover has lower impulse strength as compared to the cylindrical one. This type of winding also consumes a higher labor cost.
Disc and Continuous Disc Winding - The Disc and Continuous Disc Winding is basically used for high capacity transformers. These windings consist of numerous flat coils/ discs in a series or parallel formation. The coils of these windings are formed with the help of rectangular strips that are wound spirally from the centre outwards in a radial direction. The conductors of these windings can be a single strip or multiple strips in a parallel formation that are wound on the flat side. This formation of the conductors makes the construction for this type of windings extremely strong. The discs are separated from each other with the help of press- board sectors that are attached to vertical strips. The vertical and the horizontal spacers for free circulation of oil provide radial and axial ducts that come in contact with each turn. The area of the conductor ranges from 4 to 50 mm square and limits for current are 12 to 600 A. The minimum oil duct width is 6 mm for 35 kV. The advantage of these windings is that they provide greater mechanical axial strength and cheapness.