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Three phase transformers are more economical for supplying large loads and large power distribution. Even though most of the utilization equipments are connected by the single phase transformers, these are not preferred for large power distribution in the aspect of economy. The three phase power is used in almost all fields of electrical power system such as power generation, transmission and distribution sectors, also all the industrial sectors are supplied or connected with three phase system. Therefore, to step-up (or increase) or step-down (or decrease) the voltages in the three phase systems, three phase transformers are used. As compared with the single phase transformer, there are numerous advantages with 3 phase transformer such as smaller and lighter to construct for the same power handling capacity, better operating characteristics, etc.

Three phase transformers are used to step-up or step-down the high voltages in various stages of power transmission system. The power generated at various generating stations is in three phase nature and the voltages are in the range of 13.2KV or 22KV. In order to reduce the power loss to the distribution end, the power is transmitted at somewhat higher voltages like 132 or 400KV. Hence, for transmission of the power at higher voltages, three phase step-up transformer is used to increase the voltage. Also at the end of the transmission or distribution, these high voltages are step-down to levels of 6600, 400, 230 volts, etc. For this, a three phase step down transformer is used.

A three phase transformer can be built in two ways; a bank of three single phase transformers or single unit of three phase transformer.

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.

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