Transformer Principles

A transformer changes voltage by electromagnetic induction. Alternating current in the primary winding produces an alternating flux in the core, which induces a voltage in the secondary winding. There is no electrical connection between the windings and no moving part.

Turns ratio

Voltage is proportional to the number of turns, and current is inversely proportional. A transformer that halves the voltage doubles the current, minus losses. Power in equals power out apart from losses, which is why transformers achieve very high efficiencies — well above 98 per cent for large units.

Losses

• No-load (iron) losses — hysteresis and eddy currents in the core. Present whenever the transformer is energised, regardless of load. This is why leaving lightly loaded transformers energised costs money continuously.
• Load (copper) losses — resistive heating in the windings, rising with the square of the load current.

Efficiency peaks where the two are equal, typically well below full load.

Impedance

Percentage impedance is the percentage of rated voltage needed on the primary to drive full load current in a short-circuited secondary. It matters for two reasons: it determines the fault level on the secondary side — a lower impedance transformer gives a higher downstream fault current — and it determines the voltage regulation, how far the output voltage falls as load is applied.

It also determines how load shares between transformers operating in parallel. Two transformers can only be paralleled if their impedances, voltage ratios, vector groups and phase sequence are compatible; otherwise circulating current flows between them and they overheat carrying no useful load.

Inrush

Energising a transformer draws a large magnetising inrush current, potentially several times full load current, decaying over a few cycles. Protection must ride through it, usually by restraining on the second harmonic content that distinguishes inrush from a fault.