Transmission, Distribution, Transformers — The Road from Power Plant to Wall Outlet

The Long Road Beyond the Outlet

Plug your phone charger into a wall outlet, and charging starts immediately. The power plant that generated that electricity is usually far from your home. Much of the electricity generated at China's Three Gorges Dam has to travel 1,000 kilometers east to Shanghai (DOI: 10.1002/9781118590423, book preface). Places with good wind and sunlight are also usually far from where many people live (DOI: 10.2139/ssrn.7009282, working paper).

Therefore, a long road lies between the power plant and the outlet. Long distances are hauled all at once on large roads, and as it gets close to the neighborhood, it is divided up finely into small roads. It resembles how a car that has run down a highway to a distant city exits at an interchange and enters a neighborhood alley. This article follows that road from the power plant to the outlet, and the example numbers and scenes are taken from South Korea.

There was one problem on this road that needed to be solved from the beginning. While passing through the wires, part of the energy carried by the electricity turns into heat and disappears. The way electricity travels a long distance starts from this problem.

Energy Turning into Heat While Passing Through Wires

When you plug in a charger, electricity flows along the wire. Current is the flow of electricity along a wire, measured in amperes by how much passes a single point in 1 second. For this flow to happen, there must be a difference between two points on the wire. Voltage is the difference that makes electricity flow between two points, measured in volts. South Korean household outlets are 220 volts.

Power is the amount of energy electricity carries per second, measured in watts. For a heat-generating appliance like a heater, power is voltage multiplied by current, so 1 ampere flowing from a 220-volt outlet is 220 watts.

While carrying electricity, a wire turns part of that energy into heat. This is because wires also slightly block the flow of current. Resistance is the degree to which an appliance or wire blocks the flow of current, measured in ohms. Here, the current flows all the way to the end of the wire unchanged, and what decreases is the energy the electricity was carrying.

Transmission loss is the share of energy that turns into heat while electricity passes through wires and fails to reach its destination. Even on small roads within a neighborhood, the same loss occurs, and a study was already published in 1989 that applied a method of reducing this loss simply by changing switches to fix the path electricity travels to several realistically modeled neighborhood power grids (DOI: 10.1109/61.25637).

Loss increases as the load carried by the wire grows. If multiple electric vehicles charge at any time in a neighborhood, losses increase, and wires can exceed their limits or voltage can fluctuate (DOI: 10.1109/tsg.2011.2159816). Distance also increases loss. Because longer wires mean greater resistance, making electricity close to where it is used can reduce loss (DOI: 10.1109/jestpe.2019.2959271), and that benefit is greatest when the size and location of generation facilities are chosen appropriately (DOI: 10.1049/iet-rpg.2015.0378).

A distant power plant means carrying that much more energy that turns into heat. This is why very high voltages are used over long distances.

Raising the Voltage Reduces Losses

Korea's highest-voltage transmission lines send electricity at 765 kilovolts. A kilovolt is 1,000 volts, making it 765,000 volts—more than 3,000 times higher than the 220 volts from a wall outlet.

In transmission lines, too, electric power is roughly the product of voltage and current, so sending the same amount of power at a higher voltage requires less current. Raising the voltage might seem like it would increase the current, but there are two different voltages involved here. One is the voltage at the sending end, and this is the one that gets raised in transmission. The other is the voltage dropped across the resistance of the wire itself, which is separate from the sending-end voltage and much smaller.

It is this second voltage that determines the loss converted into heat. Because wires produce heat just like the space heater we saw earlier, the power turned into heat in a wire is the product of the voltage dropped across the wire's resistance and the current. But this voltage grows and shrinks in proportion to the flowing current. Since the current is effectively multiplied twice, the amount converted into heat by the wire's resistance is proportional to the square of the current. When transmitting the same power through the same wire, raising the voltage by 10 times reduces the current to one-tenth and the losses to 100-plus-one. Sending the same power at a high voltage greatly reduces the loss converted to heat in the wire, which is why the voltage of electricity sent over long distances is raised (DOI: 10.1109/mpe.2013.2245587).

Transformers handle the job of raising and lowering voltage. A transformer is a device that raises or lowers the voltage of electricity. A transformer near the power plant greatly increases the voltage to send it to cities hundreds of kilometers away, and a transformer near the consumption site lowers it back down to a low voltage suited for appliances (DOI: 10.1109/mpe.2013.2245587). Transformers are easy to build and highly efficient to use (DOI: 10.1002/9781118590423, book preface).

Making the voltage very high creates new technical challenges to solve. What China studied as core technologies while developing ultra-high-voltage transmission include insulation to prevent electricity from flowing to unintended places, preparation for overvoltage and lightning, the electromagnetic environment around the lines, and equipment manufacturing (DOI: 10.1109/jproc.2009.2013613). High voltage reduces losses, but it brings these challenges along with it.

Highways, Interchanges, and Alleyways

You have probably seen a gray cylinder hanging on the utility pole in front of your house. That is also a transformer, and it is the final transformer electricity passes through before reaching an outlet. An electrical grid is a system that connects power plants and places that use electricity with wires to make them run as one, and the industry also calls it a power system. The electrical grid divides and uses high-voltage and low-voltage paths in separate layers.

The transmission network is a thick path that carries electricity in large amounts over long distances from power plants, and the distribution network is a thin path that chops up that electricity and delivers it down to neighborhoods and buildings. If the transmission network is a highway, the distribution network is a neighborhood alleyway. A substation is a facility that gathers multiple transformers in one place, and it is where two paths with different voltages meet. It corresponds to an interchange where you switch roads.

Let us trace the path that electricity travels in South Korea's power grid. Electricity coming out of a large power plant steps up to 345 kilovolts or 765 kilovolts at a transformer next to the power plant and gets onto the highway. Electricity that stepped up to 765 kilovolts first steps down to the 345-kilovolt path at a substation, then steps down again to the 154-kilovolt path at a substation close to the city, and then steps down again to 22.9 kilovolts at a neighborhood substation to enter the alleyway. Finally, a transformer on a utility pole or inside a building steps it down to 220 volts and sends it to the outlet. Every time the voltage drops a step, the path becomes thinner and the branches become more numerous. The 220 volts at your outlet is the final value after lowering the voltage that was raised next to the power plant across multiple stages.

Alleyways have long been designed as one-way roads. The distribution network was built on the assumption that the neighborhood substation is the sole source of electricity supply (DOI: 10.1109/tpwrd.2007.909115). When small power generation facilities like rooftop solar panels are attached inside the alleyway, voltage problems and protection device problems arise that did not happen in the old distribution network (DOI: 10.1109/tpwrd.2007.909115). As houses that generate electricity in the alleyways increase, designs built on the premise of one-way flow must be re-examined.

Why Alternating Current Became the Standard

On September 4, 1882, Thomas Edison operated a facility supplying electricity on Pearl Street in New York. The electricity sent by this facility was direct current. Direct current is electricity that flows in only one direction. A journal article contribution counts this facility as the first direct current electricity supply system (DOI: 10.1109/mpe.2013.2245587). Edison's direct current sent electricity only short distances at low voltages, and the usable incandescent lamp he released in 1879 drove this business (DOI: 10.1109/jproc.2017.2677558).

Alternating current is electricity where the direction of flow changes back and forth multiple times even within a second. Starting in 1886, the Ganz company in Budapest and the Westinghouse company in America began competing in business against Edison's direct current with alternating current systems that sent electricity long distances at high voltages (DOI: 10.1109/jproc.2017.2677558). This dispute grew from 1888 onward into a public fight pitting Edison's direct current on one side against the alternating current of George Westinghouse and Nikola Tesla on the other, which is called the "War of the Currents," and by around 1891 alternating current took hold as the primary method (DOI: 10.1109/mpe.2013.2245587).

Let us place the two systems side by side next to the previous calculation. When the voltage is low, a large current is needed to send the same electric power, and when a large current flows, that much more energy turns into heat in the wires. Edison's direct current was low voltage and short distance, while alternating current was high voltage and long distance.

The transformer is cited as the key reason alternating current won. With a transformer, the voltage could easily be raised to send electricity from a distant power plant to cities with little loss, and lowered again at the destination (DOI: 10.1109/mpe.2013.2245587). The alternating current side also had the advantage of the alternating current motor, which, although slightly lower in efficiency, was sturdy, required less maintenance, and could be made cheaply (DOI: 10.1002/9781118590423, book preface). Transformers work only with alternating current, whose flow continuously changes. This is why the ladder of voltage we followed in the previous section was built on top of alternating current.

The fight also played out on the stages of public opinion and politics. A historical writing revisiting this period notes that as alternating current competed, Edison reached into the "politician's toolbox" (DOI: 10.1109/jproc.2017.2677558). Management studies count this fight as one of the most fierce competitions in corporate history, analyzing it as a public relations battle where the two companies tried to alter impressions of each other and their products in front of the same audience (DOI: 10.1177/0001839218821439). In 1911, Westinghouse received the American Institute of Electrical Engineers' Edison Medal for 'developing the alternating current system for light and power' (DOI: 10.1109/jproc.2004.831201).

Since then, the voltage of alternating current transmission has grown immensely. One of the early commercial alternating current facilities, the facility in Redlands, California, USA, was 2.4 kilovolts, and China's first commercial long-distance ultra-high-voltage alternating current transmission line is 1,000 kilovolts, more than 400 times higher (DOI: 10.1109/mpe.2013.2245587). Direct current started being used again in power transmission over the past 50 years (DOI: 10.1109/mpe.2013.2245587).

Very Long Journeys Turn Back to Direct Current

Offshore wind farms are getting bigger and moving farther from the coast. That is why separate methods are being studied to bring that electricity back to land (DOI: 10.1186/s41601-022-00239-w).

High-voltage direct current transmission is a method where alternating current is converted into direct current, sent over long distances at a very high voltage, and then converted back into alternating current after arriving. Alongside the high-voltage alternating current transmission we followed earlier, this is a leading candidate for connecting large offshore wind farms to the land (DOI: 10.1186/s41601-022-00239-w). The voltage is stepped up using a transformer while still alternating current, and then a converter changes it into direct current.

When power lines become very long, alternating current becomes difficult to transmit because of the electrical properties of the lines themselves (DOI: 10.1002/9781118590423, book preface). This limitation arrives especially quickly in underground and underwater cables. In places that must travel long distances, such as underwater cables, high-voltage direct current becomes a low-loss and reliable choice (DOI: 10.17775/cseejpes.2015.00015). It is also an economical way to send a lot of power over a long distance (DOI: 10.1186/s41601-020-00173-9), and over long distances it takes up less land (DOI: 10.1002/9781118590423, book preface).

The price is paid at both ends. There must be converters that swap alternating current and direct current back and forth, and the overall efficiency and amount of power that can be sent depend heavily on these devices (DOI: 10.1109/eeeic.2016.7555677). Which option is better is determined by weighing distance, component costs, and the losses and failure risks of the converters (DOI: 10.1109/tec.2006.889624). Alternating current is still used for long journeys as well. China has developed ultra-high-voltage alternating current and ultra-high-voltage direct current side by side (DOI: 10.1109/jproc.2009.2013613).

To use the analogy of a highway, high-voltage direct current is a long-distance-only road that usually has entrance and exit points only at both ends. It proves its worth when sending a lot of power far away.

When the Road Jams — Congestion and Interconnection

Congestion is a state where you want to send electricity from here to there, but you cannot send it all because you hit the limit of what the wires on that path can handle. Here, the limit is the maximum amount of electricity that a single wire can safely carry. This limit is set because the more current flows, the more the wire heats up. It is just like a traffic jam on the way home. Like cars passing on a highway, someone else's electricity can also occupy the path. The author of the preface to a power cable book writes that in Belgium, sometimes over 90% of the electricity carried by the transmission grid is not used inside Belgium, but passes right across the country to connect neighboring countries on either side (DOI: 10.1002/9781118590423, book preface).

Congestion also depends on where you attach your equipment. Suppose we attach a 1 gigawatt—meaning 1 billion watt—solar complex and a data center of the exact same size to a power grid at the same time. After running 1,560 calculations while varying their placement in a test model mimicking the New England power grid in the U.S., there were only 14 arrangements where no wire exceeded its limit (arXiv:2509.01778, preprint, model calculation). If you picked the wrong spots, more than 220% of the limit's worth of electricity crowded onto a single wire, whereas if you picked well, even the busiest wire stopped at just 91.1% of its limit.

Interconnection is the act of attaching a new power plant or large electricity-using facility to the grid. The path it attaches to differs depending on its size. Ultra-large AI data centers consuming hundreds of megawatts—meaning hundreds of millions of watts—attach directly to the transmission grid (arXiv:2605.14105, preprint), while small or medium-sized data centers sometimes attach to the distribution grid (arXiv:2606.01407, preprint). As seen in the model calculation earlier, even with facilities of the same size, depending on which spot you attach them to, the road may jam or hold firm.

The leeway to accept more power with currently laid wires and transformers differs from spot to spot. This is because the layout of the power grid and how far apart things are electrically differ, and some spots hit their limits where wires handle current, while other spots hit limits where voltage strays outside the allowable range (arXiv:2606.01407, preprint).

Here, the highway analogy has limits. While human drivers choose their own roads on a car, electricity splits and flows across connected paths according to the laws of physics. Because of this, researchers even study methods to deliberately cut off certain lines to redirect the flow and relieve congestion. In one calculation mapping out a grid expansion plan for Denmark's 50% wind power target, it turned out that opening and closing lines this way allows better use of the grid and greater acceptance of wind power (DOI: 10.1109/tpwrs.2012.2224143, model calculation).

Frequent congestion is a signal that it is time to widen the road. There are ways to plan expansion by weighing the cost incurred by congestion against the cost of laying new lines (DOI: 10.1109/tpwrs.2004.831701). Laying new roads sometimes runs into issues outside of electricity. New transmission lines must resolve which land they will pass through, what impact they will have on the environment, and when and how permits will be acquired (DOI: 10.2139/ssrn.1762329, working paper). Methods of agreement are even studied separately to prevent environmental conflicts surrounding lines from spilling over into long lawsuits between stakeholders (DOI: 10.1111/nejo.12054).

When roads fall short, facilities looking to attach anew must wait in line. An interconnection queue is a line where applications submitted to connect newly built facilities to the power grid line up in order and wait for review. In the U.S., applications waiting for connection stand at 2,600 gigawatts, which is more than double the total capacity of all currently running power plants at 1,280 gigawatts. The time it takes for connection has increased by 70% over the past decade, and 80% of applications withdraw halfway through. Applications that turn out to require major transmission grid reinforcements also tend to withdraw. Researchers read the increased time and high withdrawal rate as a signal that the transmission grid is stretched thin, suggesting that connection systems should be reformed and tied more tightly to long-term transmission plans (DOI: 10.1016/j.joule.2024.11.008). Securing grid connection capacity for AI data centers can also take longer than building the physical building (arXiv:2608.29359, preprint).

The rules by which connection applications are reviewed vary greatly by region even within the U.S., and ways to reform them are under discussion (DOI: 10.2139/ssrn.6870718, working paper). The path from the power plant to the wall outlet is a path already laid, and for new power plants and new data centers to get onto that path, they must find spare capacity, widen the road, or wait their turn.

Where to Read Next

The article that picks up the story where this line stops is "Plugging It In Is Harder Than Building It — What an Interconnection Queue Actually Decides". It covers why that line grew so long and how people are trying to shorten it. The scale of the biggest new guest climbing onto this road continues in "How Much Electricity Does AI Use? — Why Every Source Gives a Different Number", and whether the users can shift their usage times to ease the road's burden follows in "Is a Data Center Really an Inflexible Load? — What Can Move and What Cannot". The task of balancing the electricity on the road at every single moment is handled by "The Few Seconds a Generator's Spinning Mass Gave Away for Free — What Gets Harder as Grid Inertia Falls".

On the other side of Electricity from Scratch, step-by-step pieces unpack the terms this article borrowed. These are the installments covering voltage, current, and power, watts and watt-hours, and how alternating current keeps its balance from moment to moment.

Source Papers

• "Extruded Cables For High-Voltage Direct-Current Transmission" (2013, book preface) — DOI: 10.1002/9781118590423 — Description by preface author (J. Fothergill): Three Gorges Dam electricity needs to be sent 1,000 km east to Shanghai; alternating current is easy to generate, raises and lowers voltage efficiently with transformers, and brings the benefits of robust, inexpensive AC induction motors; AC transmission has electrical (capacitive and inductive) line limits; in Belgium, sometimes over 90% of transmitted power is through-power; high-voltage direct current has lower losses and takes up less space over long distances. The same preface contains a historical error (attributing the 1882 Miesbach–Munich DC line to Edison), lowering its reliability.
• Variable renewable electricity is typically located far from load centres … (2026, working paper) — DOI: 10.2139/ssrn.7009282 — Variable renewable energy is typically located far from load centers, and curtailment increases rapidly unless transmission is expanded alongside it (UK wind correction model).
• "Reconfiguration of electric distribution networks for resistive line losses reduction" (1989) — DOI: 10.1109/61.25637 — Method for reducing line resistance losses through distribution network switch reconfiguration and actual distribution network application results.
• "Real-Time Coordination of Plug-In Electric Vehicle Charging in Smart Grids to Minimize Power Losses and Improve Voltage Profile" (2011) — DOI: 10.1109/tsg.2011.2159816 — Uncoordinated, random electric vehicle charging can cause increased distribution network losses, overload, and voltage fluctuations.
• "Grid-Forming Inverters: A Critical Asset for the Power Grid" (2019) — DOI: 10.1109/jestpe.2019.2959271 — Placing generation sources close to demand reduces physical and electrical distances, cutting transmission and distribution bottlenecks and losses.
• "Integration of renewable distributed generators into the distribution system: a review" (2016) — DOI: 10.1049/iet-rpg.2015.0378 — Review summarizing line loss reduction and other benefits of connecting distributed renewable generation to the distribution network.
• "Harmonizing AC and DC: A Hybrid AC/DC Future Grid Solution" (2013) — DOI: 10.1109/mpe.2013.2245587 — Pearl Street DC supply in 1882, the War of the Currents starting in 1888, and the AC dominance around 1891, with the key being the transformer that raises voltage to send it far with low losses and lowers it again. 2.4 kV Redlands to 1,000 kV in China. Conclusion that high-voltage direct current and other developments over the past 50 years mean it is time to rethink direct and alternating current.
• "High Voltage Direct Current Transmission in China: Developments, Current Status and Future Prospects" (2009) — DOI: 10.1109/jproc.2009.2013613 — Review of the development and core technologies of ultra-high-voltage AC and DC transmission in China (electromagnetic environment, overvoltage and insulation coordination, lightning performance, live-line work, equipment manufacturing).
• "Summary of Distributed Resources Impact on Power Delivery Systems" (2008) — DOI: 10.1109/tpwrd.2007.909115 — Traditional distribution networks are designed assuming the primary substation is the sole supply source, causing voltage and protection issues when distributed resources are connected.
• "A Contrarian History of Early Electric Power Distribution" (2017, historical contribution) — DOI: 10.1109/jproc.2017.2677558 — The 1879 incandescent lamp drove Edison's low-voltage, short-distance DC business, while Ganz and Westinghouse's high-voltage, long-distance AC competed from 1886 onwards, and Edison responded by reaching into "the politician's toolkit".
• "A Model of Competitive Impression Management: Edison versus Westinghouse in the War of the Currents" (2019) — DOI: 10.1177/0001839218821439 — Analysis of the War of the Currents as a competitive impression management campaign where rivals tried to alter the public's impression of each other and their products in front of a common audience (historical case study + grounded theory).
• "Electrical engineering hall of fame - George Westinghouse" (2004, profile) — DOI: 10.1109/jproc.2004.831201 — Westinghouse became the third recipient of the AIEE Edison Medal in 1911 for "The Development of the Alternating Current System for Light and Power".
• "A critical survey of technologies of large offshore wind farm integration: summary, advances, and perspectives" (2022) — DOI: 10.1186/s41601-022-00239-w — Review noting that large offshore wind farms are growing and moving farther from shore, with high-voltage AC, high-voltage DC, and low-frequency AC proposed as connection methods.
• "Review of high voltage direct current cables" (2015) — DOI: 10.17775/cseejpes.2015.00015 — High-voltage DC cables offer low losses and high reliability in submarine transmission and offshore renewable energy integration.
• "A comprehensive review of DC fault protection methods in HVDC transmission systems" (2021) — DOI: 10.1186/s41601-020-00173-9 — High-voltage direct current transmission is an economical choice for sending large amounts of power over long distances, with DC fault protection remaining a challenge.
• "A review of LCC-HVDC and VSC-HVDC technologies and applications" (2016) — DOI: 10.1109/eeeic.2016.7555677 — The efficiency and rated power of high-voltage direct current depend heavily on the conversion equipment that switches between AC and DC.
• "HVDC Connection of Offshore Wind Farms to the Transmission System" (2007) — DOI: 10.1109/tec.2006.889624 — Comparison of 150 kV and 400 kV AC versus high-voltage DC for offshore wind connections, factoring in capital and operating costs, losses, and supply disruptions, with sensitivity analysis on distance, component costs, converter reliability, and losses.
• "Grid Transmission Evaluation for Solar Deployment and Data Center Growth" (2025, preprint) — arXiv:2509.01778 — 1,560 calculations running 1 GW of solar and 1 GW of data centers across different locations on the IEEE 39-bus New England model: 14 congestion-free placements, up to 220% overload for bad placements, and up to 91.1% for good ones.
• "Hosting Capacity Assessment and Enhancement for Edge Data Centers in Active Distribution Networks" (2026, preprint) — arXiv:2606.01407 — A data center's hosting capacity in a distribution network varies greatly by location due to network topology and electrical distance, with current, voltage, or both setting the limit.
• "Line capacity expansion and transmission switching in power systems with large-scale wind power" (2012) — DOI: 10.1109/tpwrs.2012.2224143 — Reducing congestion effects stemming from Kirchhoff's voltage law through line switching allows better use of the grid and increased wind accommodation (Denmark 50% wind target).
• "Congestion-Driven Transmission Expansion in Competitive Power Markets" (2004) — DOI: 10.1109/tpwrs.2004.831701 — Transmission expansion planning method that uses congestion levels as a signal for expansion needs, balancing congestion costs against investment costs.
• "California's Renewable Energy Transmission Initiative as a Model for State Renewable Resource Development and Transmission Planning" (2011, working paper) — DOI: 10.2139/ssrn.1762329 — Examination of renewable energy transmission planning hurdles including transmission plans and costs, public and private land use, environmental impacts, line siting, permitting, and the timing of planning versus execution.
• "Conflict Assessment in Energy Infrastructure Siting: Prospects for Consensus Building in the Northern Pass Transmission Line Project" (2014) — DOI: 10.1111/nejo.12054 — Assessment of environmental conflicts in cross-border transmission line projects, presenting conditions for consensus-building to prevent prolonged litigation.
• "Grid connection barriers to renewable energy deployment in the United States" (2024) — DOI: 10.1016/j.joule.2024.11.008 — 2,600 GW of interconnection applications in the US (more than double the 1,280 GW of operating capacity), a 70% increase in interconnection wait times over 10 years, an 80% withdrawal rate, with applications requiring major grid reinforcements showing higher withdrawal rates. The authors interpret this as a signal of grid constraints and propose reforming the interconnection system.
• "Battery-Assisted Operation of Hyperscale AI Data Centers under Connect-and-Manage Interconnection Practices" (2026, preprint) — arXiv:2605.14105 — Hyperscale AI data centers with demands reaching hundreds of megawatts emerge as large loads connecting directly to the power grid, with methods to coordinate operation using batteries below real-time exchange limits (IEEE 39-bus case study).
• "Minimizing Grid Interconnection Capacity Requirements for AI Data Centers" (2026, preprint) — arXiv:2608.29359 — Securing grid interconnection capacity for AI data centers can be a bottleneck that takes longer than building the facility itself.
• "A Review of United States Energy-only Generator Interconnection Service Policy and Considerations for Reform" (2026, working paper) — DOI: 10.2139/ssrn.6870718 — US interconnection review rules and procedures vary widely by jurisdiction, summarizing the issues around reform proposals such as energy-only interconnection.