Three Phase Distribution Transformer Guide

Almost all industrial and commercial power distribution is three phase. A three phase distribution transformer is one unit with three windings on a common core, more compact, more efficient and cheaper per kVA than a bank of three single phase units.

This guide covers what to specify and the mistakes that cause problems later.

Almost all industrial and commercial power distribution is three phase. A three phase distribution transformer is one unit with three windings on a common core, more compact, more efficient and cheaper per kVA than a bank of three single phase units.

This guide covers what to specify and the mistakes that cause problems later.

Three phase power delivers more power for the same conductor material and gives rotating machines a naturally rotating magnetic field. That is why motors, compressors, chillers and lifts are three phase, and why distribution at meaningful capacity is three phase.

The vector group describes how the windings are connected and the resulting phase shift. Dyn11 means a delta primary, a star secondary with neutral brought out, and the secondary line voltage leading the primary by 30° (11 o’clock position).

A star secondary gives a neutral, so single phase loads connect between one phase and neutral. They should be spread evenly across the three phases to limit neutral current and voltage unbalance.

Two transformers can run in parallel only if they have the same vector group, the same voltage ratio, the same phase sequence and closely matched impedance. Ignoring this causes circulating currents that waste capacity and trip protection.

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