The former one is built by suitably connecting three single phase transformers having same ratings and operating characteristics. In this case if the fault occurs in any one of the transformers, the system still retained at reduced capacity by other two transformers with open delta connection. Hence, continuity of the supply is maintained by this type of connection. These are used in mines because easier to transport individual single phase transformers.
Instead of using three single phase transformers, a three phase bank can be constructed with a single three phase transformer consisting of six windings on a common multi-legged core. Due to this single unit, weight as well as the cost is reduced as compared to three units of the same rating and also windings, the amount of iron in the core and insulation materials are saved. Space required to install a single unit is less compared with three unit bank. But the only disadvantage with single unit three phase transformer is if the fault occurs in any one of the phase, then entire unit must be removed from the service.
Advantages of Three Phase Transformers
- Being prewired and ready to install, these can be easier to install.
- To provide the same KVA, the core material required is very less compared to a bank of three single phase transformers.
- It is lighter and smaller.
- It requires less space to install.
- Higher efficiency
- Low cost compared with three units of single phase transformers.
- Transportation is easy and also transportation cost is less.
- Bus bar structure and switchgear installation for single three phase unit is simpler.
- Only three terminals are required to be brought out in case of a three phase transformer compared to six terminals from three single phase transformers.
Disadvantages of Three Phase Transformers
In case of fault or loss of one phase results to the complete unit shut down. This is because in three phase transformer, a common core is shared for all three units. If one unit is defective, the core of this defective unit would immediately saturate because the absence of an opposing magnetic field. This causes the greater escape of magnetic flux to the metal enclosures from the core. This further raises the heating of the metallic parts and in some cases this heat would enough to cause to fires. Therefore, a three phase transformer (or entire unit) must be shut down if any one phase is defective.
- Cost of repair is more for three phase transformer.
- To restore the service, spare unit cost is more compared with one single transformer spare unit.
- When these are self cooled, the capacity of the transformer is reduced.