Engineering Expert Witness Blog

     Truth is the process of supplying our homes with power isn’t as straightforward as you might think, and the actual transmission of that power isn’t straightforward at all.  To begin with, the wires used in power lines are less than perfect conductors of electricity.  Along any given length of wire there are all sorts of imperfections in the metal, and these tend to resist the flow of electrical current.  These imperfections will always exist to some extent, even with the best manufacturing techniques and quality control, and the longer the power line, the more resistance the power flow will meet.  The result is loss of electrical power.  If there weren’t some kind of compensatory action at work to rectify this, your morning routine wouldn’t be nearly so smooth.

     To address the problem of power loss electric utilities use step-up transformers, similar to the one in Figure 1.  This enables voltage produced by the generator at the plant to be raised to a higher voltage, in turn enabling it to travel longer distances and remain effective.

Figure 1 – Electricity Leaving the Power Plant Goes Through a Step-Up Transformer

     For example, let’s say that an electric generator puts out 12,000 volts, and a step-up transformer raises that to 765,000 volts, enabling transmission to customers far away. If you will recall from last week’s blog, with electrical transformers, there is an inverse relationship between voltage and current.  So, when a step-up transformer increases input voltage, it actually results in a lowering of electrical current.  So how does this phenomenon aid in power transmission?  Simply put, when there is less current flowing through the wires, there is an accompanying reduction in power loss over the long length of the transmission line.

     Let’s take a look at what happens when the power reaches our homes.  Figure 2 shows a simplified distribution route from the power plant.

Figure 2 – A Step-Down Transformer is Used to Supply Electric Utility Customers

     First, the higher voltage originating from the step-up transformer at the power plant is decreased by the use of a step-down transformer located in a substation many miles away at the other end of the transmission line.  The use of this intermediary step-down transformer effectively lowers the voltage and at the same time raises the current at the other end of the line, the end where customers like you and I are waiting to use our hair dryers unimpeded.  The path that the power follows is somewhat circuitous, but well planned out, with numerous strategically positioned distribution lines acting as the final leg of delivery.  These distribution lines do what their name implies, they weave their way along streets and alleys, finally distributing electricity to customers.

     A step-down transformer located in a substation along the power transmission route allows this all to happen.  It can readily convert the 765,000 volts being sent by the power plant to the 25,000 volts needed to feed distribution power lines.  These, in turn, power individual homes, hospitals, etc.  Now you obviously can’t plug a television into a 25,000 volt wall outlet located in your house, so another step-down transformer is required to temper it into power that’s both usable and safe.  The one in our diagram is mounted on a nearby utility pole, and its job is to lower the 25,000 volts which it receives into a more manageable 240 and 120 volts, which is then fed into your home.

     That wraps up our series on electrical transformers.  Perhaps the next time you flip that switch in your home, whether it be on your hair dryer, TV, or what have you, you’ll pause for a moment to reflect on the long path it has followed to make your life just a little bit easier.

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