Transformers are an essential part of AC power systems. It was transformers that brought about the AC revolution across the world, thanks to their ability to efficiently convert power across voltage levels. Transformers are important because of losses. Electrical conductors aren’t perfect, which means that they have resistance. This resistance in the conductor leads to real power losses as the power is transported. In other words, the power you get into a cable isn’t the power you get out. For a three-phase power line, the losses are given by:
d = 3 L r I^2
Where:
d is the real power lost in the three-phase power line
L is the length of the circuit (length of one conductor)
I is the magnitude of the current flowing through the lines
r is the resistance per unit length of the line
The resistance per unit length of the conductor r is inversely related to how much metal is in the conductor. In other words, a larger conductor will have a lower resistance than a smaller conductor. Of course, more material means more cost, so larger conductors are more expensive than smaller ones, all else equal. Therefore, if we want to build the most economical system for transferring electrical power with low losses, we need larger conductors.
The amount of power flowing through the same three-phase lines described above is given by:
P = √(3) I V cos(θ)
Where:
P is the power flowing through the conductors
I is the magnitude of the current flowing through the lines
V is the voltage between the lines of the conductors
cos(θ) is the power factor of the circuit
What’s important from this relationship is that the power flowing through the lines is proportional to the current and voltage. In other words, we can transfer the same amount of power with a high current at a low voltage or a high voltage with a low current. Since the losses in a line are proportional to the current squared (as stated above), the preferred choice would be to transmit power at as high of a voltage as practical. This is why a transformer is so important. By increasing the voltage we can transfer power over long distances with negligible losses.
Why not just use high voltages for everything? Then you wouldn't need transformers.
High voltages aren’t without their problems. Voltage is a way of understanding the electric field in a circuit, a force field that acts on charges. Higher voltages mean that things need to be spaced out to make sure the electric field doesn’t get too strong. If the electric field becomes large enough, it can actually ionize materials and create arcing faults with dangerous consequences. To get around this problem, we space things out and/or use additional electrical insulation. Transmission lines have conductors spaced far apart to prevent these problems since there is plenty of room overhead. Within a residential or commercial space, devices that take up large spaces for high voltage would be incredibly impractical. Moreover, the savings in materials would be very low since conductor sizes for things like outlets or lights in your home are already very small.
What about DC?
Transformers are AC devices. They rely on Faraday's Law, a physical phenomenon by which an alternating magnetic field (as produced by an AC current) induces voltage. With a DC power source, the magnetic field produced is constant, and, as a result, no voltage can be induced into the secondary of the transformer. Nowadays, there's more to the story though. DC power systems can have their voltage altered just like AC systems using devices known as converters. Converters use semiconducting electronic devices like transistors to achieve this conversion. For a long time, electronic devices were too expensive and couldn't handle power at the levels required to make DC transmission viable. Now, that has all changed thanks to loads of innovation. DC transmission has its perks too, which is why we're starting to see more and more of it around the world.
In summary, we need transformers to get power from point A to point B on an AC system efficiently. Transformers make the power grid economical and are essential for AC power transmission and distribution.
High Voltage Transmission Lines with Conductors Spread Out to Prevent Ionization
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