Why the step down transformers are in delta star?

Answer 1

There is no such thing as a multi phase transformer. A transformer has one phase input and one phase output. There may be multiple windings on input and on output, but they are all in phase, insofar as the power factor allows it.

What we call a three phase transformer is really three Transformers operating together, one on each of the three phases. They do not share the phases. They do not share windings. They do not share cores. They do not share magnetic fields. They are independent. They only thing they might share is they same physical container.

Star-delta configuration for three transformers (emphasis on plural) is a common configuration, although star-star is supposed to be better from an eddy current perspective. It comes down to the objectives of the load, and whether or not there there is a neutral involved, and where the neutral is connected. You can still have a neutral in star-delta, but it won't be centered in the middle of the three phase power - it will be centered in the middle of one of the split phase single phases, in the style of 120/240 split phase like typical residential power. Again, it comes down to objectives.

Answer

Three-phase ('polyphase') transformers are widely-used in electricity transmission and distribution systems.

A three-phase transformer typically comprises a common, three-limb, silicon-steel core, around which are placed three pairs of primary- and secondary-windings, entirely enclosed within a sheet-steel tank containing a mineral oil for the purpose of insulation and cooling. For a delta/wye configuration, the primary winding is accessed through three ceramic bushings, while the secondary winding is accessed through four ceramic bushings.

Three, separate but identical, single-phase transformers can also be connected together to create a 'three-phase transformer bank'. Three-phase distribution transformer banks are more common in North America than in the UK, where three-phase transformers are the norm.

Most power transformers (i.e. transformers used in the transmission system) are three-phase transformers, although at really high MV.A levels, three-phase transformer banks are used because their very large physical size makes them easier to transport and install.

In the UK, distribution transformers normally use the delta/wye configuration, because the high-voltage primary is supplied from an 11-kV three-phase, three-wire, system, while the low-voltage secondary supplies a three-phase, four-wire, system with a line voltage of 400 V and a phase voltage of 230 V.

In North America, a three-phase distribution transformer's secondary configuration is normally delta-connected, with one phase centre-tapped in order to supply the standard, split-phase, 240/120 V low-voltage system.

Answer 2

because the phase voltage of the delta connection is more the the phase voltage is stare connection by root 3 so if we have 380 at primary u will get 220 at secondary ( i think )