Transformer

The modern grid spans continents, delivering power from generators hundreds of miles away to outlets in your home. This works because a device invented in the 1880s steps voltage up for efficient long-distance transmission and back down for safe local use, with losses of barely 1%. That device is the transformer, and it is the reason AC won the War of Currents.

A transformer transfers energy between two circuits through electromagnetic induction via a shared magnetic core. AC current in the primary coil creates an alternating magnetic field in the core; the changing field induces a voltage in the secondary coil. The voltage ratio equals the turns ratio: V_out / V_in = N_out / N_in. If the secondary has 10x more turns, voltage is stepped up 10x. Power is conserved (minus small losses), so current drops by the same factor.

Transformers only work with AC because they require a continuously changing magnetic field. This single requirement is why the entire grid runs on alternating current. A unit of electricity may pass through 4-5 transformers between generation and consumption, each 98-99.5% efficient.

Worked Example

A generator produces electricity at 20 kV. A step-up transformer with a 1:20 turns ratio prepares it for transmission.

• Calculate transmission voltage. 20 kV x 20 = 400 kV.
• The generator outputs 500 MW at 20 kV. Current at 20 kV: I = P/V = 500,000 kW / 20 kV = 25,000 A. Current at 400 kV: 500,000 / 400 = 1,250 A.

How do resistive losses (P = I²R) compare at these two voltages?

Losses scale with I². Ratio: (25,000)² / (1,250)² = 400. Stepping up voltage by 20x reduces resistive losses by a factor of 400. This is why transmission lines operate at the highest practical voltage.

Transformers are the physical enablers of the grid's architecture. Without them, power delivery would be limited to Edison's one-mile DC stations of the 1880s.

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