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| Fig. 1: A general layout of electrical grids. (Source: Wikimedia Commons) |
Since its development, electricity has played an ever more important role in people's lives. By now, even electricity outage for just a few hours can cause major disruptions to many common things that we depend on. In 2021, worldwide electricity generation has reached more than twenty-eight thousand terawatt-hours. [1] While we like to enjoy the benefits of electricity, we certainly don't want to maintain a power generator at home, and that's where an electrical grid comes in.
For a significant portion of human history, dating back to as early as when mankind has just mastered the art of using fire, we have preferred sourcing energy from nearby. Back then, the primary energy came from wood, which was widely available in the nature. When electrical appliances were invented, many buildings also had their own central power generators in the basement, much like water heaters that we still have today. However, this is clearly not practical for smaller buildings and typical households, given the operating cost and the equipment size. This has led to centralized power plants being built outside of urban areas and electrical grids supplying electricity to all users.
In the past, most local areas tend to rely on their individual sources, so there was no need for large-scale power transmission infrastructure. However, fast forward to today, when considering renewable energy, often some areas are far more suitable than other areas (such as offshore wind energy). This creates a demand to transport a large amount of power over long distance. Furthermore, centralizing those power plants can also reduce cost.
Before diving into the details of electricity transmission, I will first give a short introduction on alternating current (AC) versus direct current (DC). As the name implies, AC power periodically reverses its direction and continuously adjusts its magnitude in a wavelike pattern typically at 50 or 60 Hz, hence alternating. On the other hand, DC power only flows in one direction with a constant magnitude. Most modern appliances and electronics (especially smaller ones) operate on DC power, including all batteries, so that's why one might see some electronics have very big power bricks - those are actually AC-DC converters. Power from wall outlets is actually AC, thus a conversion is needed before being able to use that power.
By this point, one has to ask this question: if DC power is what we need, why the electrical grid use AC power? This ties back to Joule's law, which states that
where Q is the amount of heat dissipated, I is the current, R is the resistance, and t is time. We can see that, in order to minimize transmission loss, the current needs to be minimized, which means that we want to use higher voltage. Currently, most transmission grids are operating between 69 and 765 kV, about 60-700 times higher than the household voltage in United States, and even power plants produce electricity at much lower voltages (5-34.5 kV). [2] At this voltage, it is clearly not safe for anyone to use, so there are multiple substations to step the voltage down for different end users. This is where AC shines: it's very straightforward to step the voltage up or down with transformers, and the transformers are very cheap compared to its DC counterparts. [3] Furthermore, since AC generators tend to be more efficient than DC generators, there is no conversion involved in the electrical grid.
DC transmission, on the other hand, requires a lot more at both ends of the transmission grid, since we need two extra steps: AC-DC conversion when the power goes from power plants to the transmission grid, and DC-AC conversion when the power exits the transmission grid into the distribution grid. These processes require rectifiers and inverters, respectively, and they are much more expensive than transformers. [3] However, DC systems tend to be more reliable and only have one phase instead of three, thus requiring fewer lines to achieve the same capacity as an AC system. [4,5] Furthermore, long distance AC lines typically require intermediate switching stations, increasing the overall line cost. [4] DC transmission is also more efficient than AC transmission with less power lost during transmission. Over 1000 km, using the same material, DC line loss is estimated to be 3.5%, where as AC line loss is estimated to be 6.7%. [5]
Because of the different needs in every system, there isn't a one-size-fit-all solution to electricity transmission, and equipment costs can also vary widely due to such differences. By no mean DC is superior than AC in terms of transmission. Because of its characteristics, there are a few applications particularly suitable for DC transmission:
Submarine cables. Because of how water interacts with magnetic fields generated by AC lines, the transmission loss of AC transmission under water is much more significant compared to over land. [4]
Long distance transmission with large capacity, since rectifiers and inverters costs do not scale with line distances. [4]
Connecting between two unsynchronized AC interconnections, since there is no oscillation concerns, larger capacity can be carried. [6]
While AC transmission has been the dominant force in terms of power transmission, with the advancement in rectifiers and inverters, the cost of DC transmission has gone down significantly and started to gain traction. While the US and Europe already have much of the electrical infrastructure set up a long time ago, China has been building out many HVDC projects, which suit them particularly well since Chinese population density is very uneven: about 95% of the population lives on the east half of the country, but most natural resources, especially wind and solar, are most abundant on the west half. With its unified grid, HVDC transmission can help them carry electricity across the continent with high efficiency.
© Xuelin Yang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022.
[2] "United States Electricity Industry Primer," U.S. Department of Energy, DOE/OE-0017, July 2015.
[3] M. Rabinowitz, "Power Systems of the Future," IEEE 814649, IEEE Power Eng. Rev. 20, 5 (2000).
[4] M. P. Bahrman and B. K. Johnson, "The ABCs of HVDC transmission Technologies, IEEE 4126468, IEEE Power Energy Mag. 5, 32 (2007).
[5] M. Ardelean and P. Minnebo, "HVDC Submarine Power Cables in the World," European Commission Joint Research Center, EUR 27527 EN, 2015.
[6] J. Cohn, "When the Grid Was the Grid: The History of North America's Brief Coast-to-Coast Interconnected Machine," IEEE 8594689, Proc. IEEE, 107, 232 (2018).
