Modern society is effectively based on electricity - lights, clocks, computers, cars (even gas engines now have electronic ignition and control), ventilation systems, cooking appliances, cell phones, and countless other necessities depend directly on the availability of electrical power. And like the other necessities readily available in today's society, electricity has become accepted as a given and many individuals never have the interest or need to understand why or how electricity functions (the infrastructures of the water system and the internet come to mind in a similar light). The purpose of this article is to provide information for anyone interested in how electricity is generated, transmitted, and used to make our lifestyle possible. Many topics of interest, such as risk of electrocution and effects of high voltage, are also interspersed throughout the article for the avid reader.
With an understanding of current and voltage, it is necessary to distinguish between AC and DC, another two terms that are very common in electrical engineering. DC stands for 'direct current', and it is called direct because the electrons move one way in a DC circuit. A battery is a DC device, where the electrons flow from the negative (-) terminal to the positive (+) terminal. The electrons do not flow neatly in a line (like a marching army) but rather meander about randomly (like a group of unorganized protesters) with a mathematical sum showing a net movement in one direction. A more easily understood parallel to DC is wind - the air molecules travel from one place to another and the force of the wind comes from the molecules pushing an object in one direction.
AC stands for 'alternating current', because there is no positive or negative terminal - instead terminals alternate between positive and negative which causes electrons to vibrate back and forth but not move in any net direction. The electricity in standard wall outlets is alternating current at 120V, and one of the terminals switches between positive and negative about 120 times per second (but isn't it 60Hz? yes; the polarity switches twice per wave cycle). Continuing the air molecule analogy, AC is similar to sound - when you hear someone's voice it is very unlikely that the molecules they breathed out are anywhere near you (unless you are literally face-to-face), but you hear sound because molecules in air vibrate at a certain frequency, and transfer this vibrational energy to your ear without permanently moving from one area to another.
With the mentioned air analogy, in both cases energy tends to dissipate - it is difficult to hear someone's voice when they are far away, and the wind created by a fan has difficulty reaching the far corner of a room. Using pipes to transfer the air energy allows efficient energy transfer over long distances, and a similar goal is accomplished by using wires in the electrical system. However even with this understanding one may ask what is the purpose of having two different ways to transfer electrical power? This was a fact of contention between Tesla (AC proponent) and Edison (DC proponent) back in the days when the electrical infrastructure was being developed. The advantages of each system are given below.
The AC system has won out over DC for everyday use in buildings (to power lights, appliances, computers) primarily because of its higher efficiency in transferring power, and ease of converting from one voltage to another. Any type of turbine device at a power generation plant can be made to naturally produce AC, while producing DC would require rectification of the output. A solid-state device (transformer) can be used to efficiently change one AC voltage to another. The transformer is useful for consumers so different devices can be powered from the same power outlet if each device has its own transformer to meet its requirements, and it is useful for power companies because the generated electricity is low voltage while it is desirable to have high voltage for transmission over power lines. This type of voltage conversion is possible using purely DC but is more tedious and less efficient than using a transformer.
The DC system does have its merits, namely that DC current is naturally produced by chemical reactions (such as batteries), and lends itself more easily to digital logic operations. Thus computers and cell phones use DC supplied by the battery or by a rectifying transformer to perform calculations. Additionally, high-voltage DC can be used for power transfer and suffer less losses than a similar AC system, but because of the required equipment for voltage conversion this is generally not done unless there are substantial savings in reducing the power losses, as would happen with very long transmission distances (about 500+ miles) and undersea cables.
When an electric current travels through a wire, it creates some heat due to resistive losses, and this is wasted power for the consumer and the power company. Especially with long transmission distances, such as for cities far away from power generation facilities, these losses become significant. It turns out that having a higher voltage and lower current travel through the wire decreases resistive losses while transferring the same amount of power (power = voltage * current). Therefore in long transmission power lines, AC voltages of 200,000 to 500,000 volts are common, and these power lines are thick metal cables that are attached with insulator blocks to metal towers. (Wood poles and insulated cables are used for local power distribution at much lower voltages). As a general case, the longer the insulator used to attach the cable to a tower, the higher the voltage is carried in that cable.
Isn't it dangerous though to have such high voltage in a bare metal wire, out in the open, when all household electronics have insulated wires? This is a valid concern, and it is why the cables are so high up, so that it would be difficult for anyone/anything to touch them. Consider the size of the insulating structure attaching the cable to the tower, and imagine that amount of insulation around the entire length of the cable - it is completely impractical, and actually detrimental because high voltage tends to travel outside the wire; but more on that later. Then do the bare wires kill birds? No, because for electric energy to be dispersed there need to be two points of reference (consider the two prongs in an electric plug, and two poles of a battery). Along a single wire, there is no voltage difference, so a bird can sit on the wire without any harm. However if that bird was not content with just one wire and tried to straddle two it would get vaporized at the currents generally present in those wires. In that case, can I climb up the tower and get on a wire? That would be a bad idea, because the tower (and the ground) is electrically insulated from the power wire, so if you touch the wire while also in any way touching the ground or the tower, electricity will flow from the wire, through your body, to the ground. In essence, instead of getting a shock from touching two wires, you get a shock from touching one wire and the ground, because the voltage is high enough. This seems to be the biggest conceptual challenge for those who are learning about power transmission, that the ground is in fact a part of the electrical circuit, and can present an electric potential difference with regard to the wires. Tesla pushed this concept to its limit, hoping to transfer power worldwide without wires by using the entire planet as a resonant electronic system.
As implied above, the high voltage in power lines creates an electric field around the lines, which presents an electric potential with regard to the ground. That means that if you stand under a power line, there may well be a difference of a few volts (AC) between your body and the ground, since your body is closer to the cable than the ground is. This will be hard to measure using a traditional voltmeter since the current will be almost negligible, however the potential difference can be seen another way. If you get a long fluorescent tube, one that is about 4 feet in length, and stand with it under a high-voltage power line (your feet touching the ground, and holding the tube up pointing towards a power line), you will see a very faint glow from the tube. It is so faint that you would only see it at night and far from any residential lights, but it indicates the electric field surrounding the power lines, traveling through the fluorescent tube, through your body, and to the ground. The light given off can be used to measure relative strengths of electric fields, or effects of insulation (such as wearing shoes while holding the tube, or holding the tube along the middle vs on the end). If you try this experiment, make sure the weather is cooperative (no lightning, rain, wind).
Now that the idea of the electric field surrounding the wire is established, there is another reason for not having insulation on the power lines. Electricity at high voltages, especially AC, will tend to travel in the outer layer of the cable (this is called the 'skin effect'), and in fact in the space surrounding the wire. The electric field (at high voltages) travels through the air surrounding the wires, as well as through the wires! High voltage cables have twisted wires to reduce this effect, but it still occurs to some extent. If instead of air we had an insulator surrounding the wires, there would be large resistive losses through heating of the insulator, reducing the efficiency of power transfer while greatly increasing installation costs by requiring more materials for the wire, stronger towers, etc.
The power distribution system consists of a number of components required to transmit and convert the electricity. The most visible component is all the wiring, which actually transfers electricity. The most common way to transfer electricity today is the three-phase AC system (three sine waves, each lagging 120 degrees behind the next one) - that is why the big transmission towers have cables in multiples of three. One might be most aware of the positive and negative terminals on a battery (requiring two wires to power a device), however it is not always the case that two wires are necessary. In an alternating current, or high voltage direct current system, it is sufficient to have a single wire carrying an electric potential, in which case the ground (and anything touching the ground) acts as the second wire. This is an important point for anyone working on mains power systems, because it is indeed possible to receive a strong shock just from touching one wire, as long as one is also touching any object that is connected to the ground (including standing on the ground). Again for reasons of efficiency, high power alternating current systems have been designed to use three phases (or three wires), each of which is at a high potential with respect to ground. Note that the three prongs on a household plug (USA at least) are for a one-phase system (two flat prongs) with a protective grounding feature (the round prong) and not a three phase system. Many brushless motors require a three-phase system and have three wires (industrial machines have a four-prong plug, three prongs for the motor phases and one ground prong for the safety of users). How is three phase electricity converted to a one-phase form usable in a house? For this, just one of the three wires is connected to a transformer (along with ground), and thus a single phase of the three is selected to be sent along to a house. At the house the electricity indeed appears to be one phase. This arrangement may be visible on local power poles, where out of multiple electrical wires only one has a connection to a transformer on the pole. Only two wires on a tower may be used with a single-phase AC system where one of the wires is used instead of ground, or for a DC distribution system, but these are not likely to be seen often. Near homes, particularly in rural areas, you may see a large transformer on a pole from which only one wire leads to the house and provides electricity, this is a single-phase AC system with the ground used as a "second wire".
This brings us to the second important component, the transformer. As mentioned briefly above, it converts AC electricity from one voltage to another. Transformers can be seen nearly everywhere with digital electronics now (for instance a transformer is used to charge the laptop or cell phone, and it converts the 120V from the power outlet (in the US) to something like 5V or 19V that the digital device can use). Much bigger transformers than these household ones can be seen on wooden poles, particularly next to houses in rural areas. In neighborhoods where the houses are close together, the transformers usually supply many houses and are on the ground because they are large - these transformers usually have green rectangular covers and measure about 4 feet by 4 feet (1.5 x 1.5 meters). Still larger transformers can be found in substations - these serve entire cities or areas, and convert the 200,000 to 500,000V in the distribution power lines on steel towers to about 10,000V that is then sent to multiple 'wood pole' type systems. From there, the 'pole pig' transformers attached directly to the wood poles convert the electricity to the 240V sent to residential houses using single phase AC. Large users such as metro systems or factories get the high-voltage (10kV to 100kV) electricity directly, and are able to have access to a lot more power than the homeowner.
The next important component is the substation. As already mentioned, the substation converts high voltage electricity to a lower voltage for distribution. The local substation can be found by tracing wires from residential areas to their sources (wires from multiple distribution systems will converge on one substation) or alternatively going from the distribution cables to the next substation they reach. There are usually quite a few substations near/in large cities. The substations effectively cannot be enclosed due to the high voltage requiring a very large building to enclose it, thus substations will be easily visible once you find one. The substations do more than just transform the voltage, they also monitor the power load in the area they serve and are able to turn off the power in an emergency situation. They also may be able to provide power from more than one source. Another important function of substations is keeping the voltage in phase with the rest of the electric grid (and related, maintaining real vs imaginary power flow, but that is a bit outside the scope here) - since the AC voltage varies in time, it needs to vary equivalently everywhere the electricity is used. Catastrophic things could happen if a local network gets out of phase with the rest of the grid, since it will be trying to go against the power transfer already established by the power company generating the electricity, and that is like a car trying to stop a train.
That brings us to the final important component in the system, and that is the power plant. The power plant is what generates all the energy that the electric devices use, from the hair dryer to the computer to the cell phone charger. Most of the power plants in use now essentially use a steam engine connected to an electric generator (a spinning magnet that gets the electricity to start flowing through the wires). Wasn't a steam engine a thing of the past? For transportation it was, but for converting large amount of heat into usable power the concept of the steam engine is still the most profitable and well understood. What changes between different power plants is the source of heat to turn water into steam - in nuclear power plants the heat is provided by nuclear fission, in coal plants the heat comes from burning coal, and in geothermal plants the heat comes from hot areas of the earth about 1km below the crust. Hydroelectric plants do not use steam but rather the pressurized water due to the dam turns the electric generator directly, while wind generators simply use a propeller to turn the generator.
I will paraphrase a quote I heard by Richard Feynman (and paraphrase rather poorly, please do see the linked video) - somewhere in a dam there is a big wheel turning from water falling on it (hydroelectric plant), and that big wheel is connected by a bunch of copper wires to smaller wheels (fan, drill, etc) which then start spinning too. That interpretation emphasizes how amazing the idea of electricity really is. Something ('energy') that started out as coal burning, water going through a pressure gradient, or a nuclear reaction, can be used to generate light, play music, create heat, and countless other things. I am sure electricity will continue to amaze us and be an important part of our lives in the future, and hopefully this has been a worthwhile introduction to the topic.