ChatGPT generated abstract representation of global energy transition from alternating to direct current
The Second War of Currents: How DC Is Quietly Taking Over
9 seconds ago
Michael Barnard
Tell Us What You're Thinking!
Support CleanTechnica's work through a Substack subscription or on Stripe.
Electricity has been shaped by two rival ideas that have taken turns defining our world. Alternating current, or AC, became the backbone of industrial civilization, while direct current, or DC, quietly survived in the background. The story of how AC first won, and how DC is now winning again, is a story about technology catching up with physics and economics catching up with technology. It is also about the shifting balance between generation, transmission, and consumption as we move into an era dominated by renewables and electronics.
Recently, someone asked why DC is winning again when AC had already won. I replied with a short explanation about transmission losses, breakers, and how our devices now all run on DC even though they receive AC from the wall. That answer made me realize how much the subject deserves a clear, accessible explanation. Most people know AC and DC from labels on chargers or power supplies, but the story behind them touches almost everything in the modern grid. This article grew out of that exchange—a chance to unpack why alternating current once dominated, and why direct current is quietly taking over again.
ChatGPT generated infographic comparing alternating current and direct current, showing AC’s oscillating waveform and DC’s steady one-way flow with everyday examples.
The first war of currents in the late 19th century was as much a public spectacle as it was a technical contest. Thomas Edison, whose business empire was built around direct current, saw Nikola Tesla’s alternating current as an existential threat. AC could travel farther and more efficiently, and that threatened to make Edison’s entire infrastructure obsolete. In response, Edison mounted a sensational campaign to discredit it, staging public demonstrations in which animals were electrocuted to show the supposed dangers of alternating current. He even encouraged the use of AC in the first electric chair execution, hoping the association with death would scare the public. Tesla, backed by industrialist George Westinghouse, took the opposite tack—lighting the 1893 Chicago World’s Fair with AC power and proving it could be controlled safely on a massive scale. The war was brutal and theatrical, but it ended decisively: AC’s technical advantages made it the foundation of the modern grid, even as Edison’s tactics became a cautionary tale of how business rivalry can distort science.
The United States settled on 60 reversals per second, or 60 hertz, while much of the world standardized at 50 hertz. The reason AC won that first war of currents was the transformer. A transformer can easily change voltage levels in AC circuits by using coils of wire wrapped around an iron core. That meant that electricity could be sent at high voltage, which reduces losses over distance, and then stepped down safely for homes and factories. Direct current did not have a practical way to do this at the time, so its reach was limited to city blocks.
For the first half of the 20th century, everything about electricity was shaped by that choice. AC motors were simple and reliable. Circuit breakers and switches were easy to design for alternating current. The losses from resistance were low enough at high voltage that the extra complexity of DC was unnecessary. Even household clocks used the regular rhythm of the grid’s frequency to keep time. That is why electric clocks on ovens and microwaves drifted if the grid frequency varied slightly from its nominal value. The drumbeat of AC quite literally set the tempo of daily life.
While AC conquered the visible world, DC retreated underground. It remained in niches that benefited from its steady flow. Telecommunication networks used it for reliability. Industrial processes that depended on electrochemistry, like aluminum smelting, required DC. Electric railways often preferred it for traction systems. Submarine cables also found DC more effective because AC’s oscillating magnetic field interacted with the water and the insulation of the cable, causing losses through capacitance. But these were exceptions, not the rule. The grid that powered the 20th century was almost entirely AC.
ChatGPT generated infographic illustrating how AC power from the grid is repeatedly converted to DC inside homes, powering laptops, phones, LEDs, and electric vehicles.
The balance began to shift quietly with the rise of electronics. Every computer, phone, television, and appliance that contains semiconductors runs internally on DC. The silicon chips that form the brains of modern devices require a stable voltage, not a swinging sine wave. Power supplies convert the incoming AC to DC, using rectifiers and capacitors to smooth the current. That is why every charger and every power brick on a laptop or phone is really a small converter. Inside homes, AC leaves every outlet, only to be immediately turned into DC. The world is still fed by AC, but most of what uses electricity now runs on DC.
The irony is that generation has also been moving back toward DC. Solar panels produce direct current, not alternating current, and they rely on inverters to transform it into the AC that the grid expects. Batteries are DC as well. The modern energy system has become a sequence of conversions: DC generation from panels or batteries turned into AC for transmission, then rectified back to DC for devices. Each conversion costs a small amount of energy, and the losses accumulate. The result is that DC, once seen as old-fashioned, has become the natural current of the digital and renewable age.
Long-distance DC transmission, known as high-voltage direct current or HVDC, was first developed to connect AC grids that operated at different frequencies. It allowed power to be sent from one unsynchronized grid to another without forcing them into the same rhythm. For decades, that was HVDC’s main role. It was also used for a few long-distance point-to-point links, like the Pacific DC Intertie between the Columbia River and Los Angeles. DC lines at the same voltage have lower resistive losses than AC lines, because there is no alternating field that constantly reverses direction. The savings become meaningful over hundreds or thousands of kilometers.
ChatGPT generated infographic comparing AC and DC circuit interruption, showing how AC current naturally falls to zero while DC maintains a continuous arc.
There was a problem, though. Breaking a DC circuit carrying thousands of volts is difficult. When a circuit is opened, the flow of current wants to continue, forming an electrical arc. In AC systems, the current naturally falls to zero 50 or 60 times per second, making it easier to interrupt. In DC systems, there is no zero point. Mechanical breakers were too slow, and early electronic breakers were fragile. That limited DC transmission to routes where the power flow could be controlled at the terminals, not along the line.
Before voltage-source converters arrived, HVDC systems relied on a different design called line-commutated converters, or LCCs. The first working LCC system appeared in 1954, built by ASEA (which later became part of ABB) to send power between the Swedish mainland and the island of Gotland. LCCs use a series of semiconductor devices called thyristors, which can conduct current once triggered but cannot turn off by themselves. To stop the current, they depend on the AC grid’s natural zero crossings—the moments when alternating current reverses direction and the voltage briefly reaches zero. That feature makes them reliable for moving bulk power between large, stable AC networks, but it also means they can only operate when a strong AC grid is present on at least one side. They cannot start on their own, reverse the direction of power easily, or support weak grids without causing instability.
ChatGPT generated infographic comparing Line-Commutated Converters and Voltage-Source Converters, showing how LCCs rely on AC zero crossings while VSCs use precise digital control.
By the 1990s, engineers were looking for something more flexible. In 1997, ABB developed voltage-source converters, or VSCs, a major step forward. Instead of thyristors, VSCs use modern semiconductor switches such as insulated-gate bipolar transistors (IGBTs). These can turn on and off electronically in microseconds, without waiting for the AC waveform. This lets VSCs generate their own AC waveform digitally from a DC source, adjusting voltage and frequency with great precision. They can also take AC from a weak grid and turn it into DC, or operate in both directions—sending power either way as conditions change.
Unlike LCCs, which draw reactive power from the grid, VSCs can produce or absorb reactive power as needed. That means they can stabilize grids, support islanded systems, and connect remote renewable generation like offshore wind farms that have no large rotating generators. The ability to start independently, control voltage and frequency, and operate in either direction made VSCs a breakthrough for HVDC.
The next major advance came in 2012, when ABB introduced the hybrid DC breaker, solving the long-standing challenge of interrupting DC at high voltage. Combining electronic speed with mechanical robustness, it allowed DC circuits to open safely in milliseconds. Together, LCCs, VSCs, and hybrid breakers mark the three generations of HVDC technology—each one expanding where and how DC transmission can be used, turning it from a specialized engineering solution into a core component of modern grid design.
ChatGPT generated infographic explaining ABB’s hybrid DC breaker, a DC-to-DC system that combines electronic speed and mechanical strength to safely interrupt high-voltage direct current.
Modern HVDC lines deliver 3.5% to 5% more of the electricity put into them at one end compared to AC lines of the same voltage and distance. The exact advantage depends on the design, but it is consistent and significant. For underwater cables, where AC’s alternating magnetic field interacts with the conductive seabed and water, the difference is even greater. That is why HVDC dominates offshore wind connections and interconnectors between islands and continents. For buried lines, DC avoids the extra resistance created by AC’s moving fields interacting with the soil. The break-even distance used to be about 800 km for overhead lines and 50 km for submarine cables, but those numbers are likely shorter today as equipment improves.
In densely populated urban areas, resistance to new overhead transmission lines has become a defining constraint on grid expansion. Communities object to the visual impact, land use, and perceived health risks of large pylons, especially as cities densify and real estate values climb. DC transmission offers a way around that barrier. Because direct current can be buried efficiently with minimal losses, it allows high-capacity lines to run invisibly beneath streets, rail corridors, or waterways, linking city centers directly to offshore wind farms or regional substations without altering the skyline. Unlike AC, which requires wide rights-of-way and complex electromagnetic balancing when buried, DC cables can be installed in smaller conduits and closer to existing infrastructure. This makes HVDC an enabler of urban electrification, allowing cities to expand renewable power delivery without public opposition to new towers. It turns underground space into a strategic asset, carrying the same energy quietly and efficiently where overhead lines would never be accepted.
The tradeoff is cost. The converter stations at each end of an HVDC line are complex and expensive, while the line itself is cheaper to build and loses less power. AC systems have the opposite profile. They require less investment at the terminals but more copper and aluminum in the lines. The decision between AC and DC for transmission is increasingly based on geography and use case. Over long distances, DC wins. For shorter or denser networks, AC still dominates.
That same logic explains why AC continues to rule distribution. Every substation, transformer, breaker, and piece of safety equipment on the grid is designed for alternating current. The infrastructure is vast and deeply standardized. Replacing it would be uneconomic. Even as DC makes sense at the edges of the system—between grids, under oceans, or inside electronic devices—it has not yet made sense in the middle. There are a few demonstration DC microgrids, mostly for data centers and research campuses, but they remain exceptions. The world still runs to the rhythm of AC.
The most realistic future is not a second war of currents but a synthesis. AC will continue to serve for wide-area distribution, where inertia and history matter. DC will expand where precision, efficiency, and distance are most valuable. Offshore wind farms already use DC backbones to collect and deliver power. Industrial systems, railways, and large battery storage sites often prefer DC internally. Buildings may one day have DC wiring alongside AC, feeding servers, lighting, and appliances without conversion losses. The direction of travel is clear, but it will take decades to unfold.
As the 21st century energy system matures, DC’s strengths align naturally with renewable generation and modern consumption. Solar farms, batteries, and electronics all speak DC. The grid that connects them still speaks AC, but the translators—converters, inverters, and power electronics—are getting smaller, cheaper, and more reliable. The future will not look like Edison’s short-lived DC networks or Tesla’s sprawling AC triumph. It will be a hybrid system, shaped by engineering and efficiency rather than ideology. The first war of currents was fought over who would control the flow of electricity. The second is being won quietly by the technologies that make both currents work together
cleantechnica.com |
