How Does a Distribution Transformer Work?

A distribution transformer has no moving parts and no electronics. It works on one physical principle, electromagnetic induction, discovered by Faraday in 1831 and still doing the heavy lifting in every substation today.

Understanding the principle makes the rest of transformer engineering much easier to follow: why the rating is in kVA, why voltage drops under load, why losses matter and why cooling is part of the design.

A distribution transformer has no moving parts and no electronics. It works on one physical principle, electromagnetic induction, discovered by Faraday in 1831 and still doing the heavy lifting in every substation today.

Understanding the principle makes the rest of transformer engineering much easier to follow: why the rating is in kVA, why voltage drops under load, why losses matter and why cooling is part of the design.

Two coils of wire, the primary and the secondary, are wound around a common laminated steel core. When alternating current flows in the primary, it creates a constantly changing magnetic flux in the core. That changing flux cuts the secondary winding and induces a voltage in it.

Crucially, the two windings are electrically isolated. Power is transferred through the magnetic field, not through a direct electrical connection. That is what gives you voltage conversion plus galvanic isolation in one device.

The ratio of primary to secondary voltage equals the ratio of the number of turns on each winding: Vp / Vs = Np / Ns. An 11 kV / 415 V transformer therefore has about 26.5 times more turns on the primary than the secondary.

Because power is conserved (ignoring losses), current moves in the opposite direction: if voltage goes down by 26.5, current goes up by 26.5. This is why the low voltage side of a transformer has very large conductors and busbars.

Network voltage is not constant. Most distribution transformers include an off-circuit tap changer, commonly ±2 × 2.5% or ±5%, which changes the number of effective primary turns so the secondary voltage can be set correctly for the local supply.

Where voltage varies continuously during the day, an on-load tap changer (OLTC) can adjust without interrupting supply.

Two loss mechanisms dominate. No-load (iron) losses occur in the core whenever the transformer is energised, regardless of load. Load (copper) losses occur in the windings and rise with the square of the current.

Both appear as heat, which is why cooling, oil circulation and radiators, or air and fans, is an integral part of the design rather than an afterthought.

The windings have resistance and leakage reactance, together called impedance. As load current increases, the voltage lost across that impedance increases, so the secondary voltage falls. The percentage impedance quoted on the nameplate tells you how much: a 5% impedance transformer drops about 5% of secondary voltage at full load.

Not sure which rating fits your project? Send us your load list and we will size it for you, or email info@xsdfftransformer.com, call +86 158 6789 7761 or WhatsApp +852 5416 2620.