What Is a Transformer, and Why Does It Only Work With AC?

Transformers were one of the late-1800s developments that made our alternating current (AC) power system possible, and you’ll still find them at nearly every stage of the grid today. A step-up transformer at a generating plant raises the voltage to send power onto high-voltage transmission lines. The same goes for large solar and wind farms. Substation transformers bring that voltage back down for distribution circuits. Pole-top and pad-mounted transformers then drop it to the voltages homes and businesses actually use.

The stakes can be high when one fails. As Tim Taylor, instructor of HeatSpring’s “Utility Distribution Systems” course, points out, a large generating plant that loses its step-up transformer has no way to sell its power onto the grid, and that outage can cost hundreds of thousands or even millions of dollars.

In this clip from the course, Tim covers the fundamentals. He explains what a transformer is, why it only works with AC, and how two coils that never touch can still pass power between them. He also walks through the relationship between voltage and current that applies to every transformer on the system, from large substation units down to the smallest pole-top transformers.

To build a working understanding of how power gets from the substation to the customer, enroll your team  in Utility Distribution Systems today!

Transcript below

So starting off with the conceptual diagram of a transformer, and the definition of the transformer: a device consisting of two or more coils of insulated conductor that transfers alternating current by electromagnetic induction from one coil to another to change the voltage or current value.

One of the key things here is alternating current. Transformers only work for alternating current. That’s because an alternating current produces a changing magnetic flux, and that changing magnetic flux, which is usually routed in the iron core of the transformer, is what enables a voltage to be produced in the other windings of the transformer. So it only works with alternating current and not direct current.

Looking at our diagram there, we see a primary winding and a secondary winding. Again, this is a very simplified type of diagram. In reality, transformers, particularly the large substation transformers, are much more complex than this, involving pretty elaborate cooling systems as well as some complex insulation systems.

When you’re moving from the primary winding over to the secondary winding, one of the keys is that these windings are electrically decoupled, meaning that they’re not connected together. It’s really only the magnetic flux that connects the two together.

And a very important relationship is shown in our last bullet there: the current change is inversely proportional to the voltage change. Let me explain that. If on the primary you have a high voltage, and the transformer is constructed so that on the secondary side you have a lower voltage, what happens is that the current on the secondary side in amps is going to be larger than the primary current in amps.

That’s an important relationship for everything from the substation transformers on down to our smallest pole-top distribution transformers. So you can see the equation there, that conceptually, we’ve got current change inversely proportional to the voltage change.