As we already know that the basic components of a transformer are its winding’s, core, yoke and limbs. In the case of a single phase shell type transformer there are three limbs of a core and core is like E shaped. Unlike the core type both the primary and the secondary winding’s are placed on the central limb of the core. Hence, we can understand that winding’s are mostly covered by the core. Shell type has a double magnetic circuit.
Shell Type Transformer Diagram:
Shell Type Transformer Construction:
The coils in the this type of transformer winding are multilayered which are disc or sandwich type coils. Coils are generally made up of the strip copper. Low voltage coils are generally near to the top and bottom of yoke. The high voltage winding sits in between two low voltage winding’s or we can say that high voltage coil is adjacent to the low voltage coils. We can understand it easily by looking at the diagram.
The core consists of the laminated sheets. The natural cooling phenomena does not exist in this type of transformer as the winding’s are surrounded by the core of the transformer. For the maintenance purposes if any winding need to be removed then the lamination’s should be removed first.
3 Phase Shell Type Transformer:
In case of 3 phase, the construction of 3 phase is somewhat similar to the single phase as shown in the figure.
Three single phase transformers are stacked together to form a three phase shell type transformer. The three phases have individual magnetic circuits which are parallel to each other and are independent of each other. Three winding’s on either side of the transformer can be connected both in star(Υ)or delta(Δ). These transformers are rarely used.
Advantages of shell type transformer are:
- Shell type transformers have a greater mechanical strength.
- It has a compact design.
- It has high short circuit withstand capability means during an external short circuit it can withstand high amount of mechanical stress due to its design of greater mechanical strength.
- The control of the leakage flux is very good as the coil is surrounded by the core.
- It has an efficient cooling as oil can travel easily through the vertical coils and cools the transformer.
- The wire size in each coil of transformer can be selected flexibly.
- It has a flexible design as it can meet most requirements easily. Impedance can be easily adjusted between winding’s which ensure smooth operation with other transformers in parallel. It is also easier to design this type of transformer with tap changers.
- The power to weight ratio is better in this type of transformer.
- It has a high dielectric strength.
- It has a very reliable and robust type of design.
The following are the disadvantages of shell type transformer:
- It is more difficult to assemble
- Electrical clearance is more difficult.
- The repairing and the maintenance process is hard.
- There is no natural cooling in this transformer.
Following are the application of shell type transformer:
- For low voltage services, the shell type transformers are mostly preferred.
- These are also used for low voltage applications like in power electronics and electronic circuits.