Depending upon the type of construction used, the transformers are classified into two categories viz
1. core type and
2. shell type.
Another recent development is the spiral core or wounds core type, the trade name being spiral core transformer.
Depending upon the type of service, in the field of power system, they are classified as
1. power transformers
2. distribution transformers.
Core Type Transformers:
In core type construction, as shown in fig., the coils are wound around the two limbs of a rectangular magnetic core.
Each limb carries one half of the primary winding and one half of the secondary winding so as to reduce the leakage reactance to the minimum possible.
The lv winding is wound on the inside nearer to the core while the hv winding is wound over the lv winding away from the core in order to reduce the amount of insulation materials required.
Small transformers may have cores of rectangular or square x-section with rectangular or circular coils but it is wasteful in case of large-capacity transformers.
In the case of large-sized transformers stepped cruciform core with circular cylindrical coils is employed. Such a core employs laminations of different sizes.
Though the cost of manufacturing of such a cruciform core is much greater, but the circular coils that are used are easier to wind and provide more mechanical strength, especially when short-circuit occurs.
Other advantages of using cruciform core are high space factor and reduced mean length of turns resulting in a reduced copper loss.
Shell Type Transformer :
In shell-type construction, as shown in fig., the coils are wound on the central limb of a three-limb core.
The entire flux passes through the central limb and divides into two parts going to side limbs, as shown in fig.
Consequently, the x-sectional area (and hence width) of the central limb is twice of that of each of the side limbs. Sandwich-type winding is used in such a construction.
If the core type and shell type transformers are compared, then we conclude that the core type transformer has a longer mean length of iron core and a shorter mean length of coil turn.
The core type transformer has smaller x-section of iron and, therefore, a greater number
of turns.
Core type construction provides more space for insulation making it best suited for EHV requirements.
In shell-type transformers advantage is gained through the core being used to protect the windings from
mechanical damage.
The shell type construction gives better support against electro-magnetic forces between current carrying conductors.
These forces are of considerable magnitude under short-circuit conditions. The shell type construction is commonly used for small transformers where a square or rectangular core Cross-section is suitable for economic consideration.
The shell type construction needs more specialized fabrication facilities than core type,while the latter offers an additional advantage of permitting visual inspection of coils in the case of a fault and ease of repair at the substation site.
For these reasons, the present practice is to use core type transformers in large high voltage installations.
Spiracore Transformer :
The fact that with flux in the direction of grain orientation of iron, permeability is increased while core losses are reduced, has led to the development of the wound core, wherein spirals of strip steel are wound through the window of a preformed coil.
The resultant structure is more rigid than cores made of stacked laminations, and because of the continuous magnetic path unobstructed by butt or lap joints encountered where individual laminations are used, the magnetic path reluctance is reduced. Such a construction is illustrated in fig.
Power Transformers:
The term is used to include all transformers of large size (250 kva and above) used in generating stations and substations for transforming the voltage at each end of a power transmission line.
They may be single-phase or 3-phase; 3-wire delta/delta or delta/ star connected, and of voltage rating 220/11 kV or in high voltage range.
They are put in operation during load hours and disconnected during light load hours i.e they are usually operated on approximately full load.
This is possible because they are arranged in banks and can be thrown in parallel with other units or disconnected at will. So power transformers are designed to have maximum efficiency at or near full load (i.e.with iron loss to full load copper loss ratio of 1:1).
Power transformers are designed to have considerably greater leakage reactance than is permissible in distribution transformers because in power transformers inherent voltage regulation is not as much important as the current limiting effect of the higher leakage reactance.
Power transformers usually make use of flux density of 1.5 to 1.7 T, have percentage impedance ranging from 6-18% and regulation of 6-10 percent.
They may be self oil-cooled, forced air-cooled or forced water-cooled.
Distribution Transformers:
Transformers of rating upto 200 kva, used to step down the distribution voltage to a standard service voltage are known as distribution transformers.
They are kept in operation all the 24 hours a day whether they are carrying any load or not. In such transformers iron loss occurs for all the time where the copper loss occurs only when they are loaded.
Therefore, distribution transformers should be designed with iron loss smaller in comparison to full-load copper loss (say with iron loss to full load copper loss ratio of 1 : 3).
In other words, they are designed for good 'all-day efficiency' and not for the highest efficiency on full load. They are of the self-cooling type and are almost invariably oil-immersed.
The distribution transformers are usually 3-phase, 4-wire 11 kv/400 V delta/star connected. Such transformers are designed to have an inherently good voltage regulation which is possible by arranging the coils in such a way as to have minimum leakage reactance.
Such transformers have a percentage impedance of 4-5 % and voltage regulation of 4-8%. They make use of cold-rolled steel with a flux density of 1.7 T.