IMAGINE what might have happened if, back in the 1880s, Thomas Edison had devoted his prodigious engineering talents to perfecting a direct-current transformer instead of wasting his energy disparaging the upstart alternating-current system from Europe that was being championed in America by George Westinghouse and his Serbian-American adviser, Nikola Tesla. Westinghouse, a hard-charging entrepreneur, and Tesla, an inventor with numerous patents on AC devices to his credit, were able to do so because they had something Edison did not—an efficient transformer to step the voltage up and down. Thanks to these transformers, AC could be distributed at high voltage and then tapped and stepped down to various voltages along the way, to satisfy the different needs of factories, offices and domestic customers. By contrast, Edison’s heavy-gauge copper cables delivered a fixed 110 volts throughout the system. Edison badly needed an equivalent transformer for DC. But instead of competing technologically, and winning, he turned to showmanship—and lost. It is true, and was the basis of Edison's showmanship, that low-frequency alternating current can be more hazardous than an equivalent direct current. By oscillating at a similar (ie, close enough) frequency to the human heart, a sufficiently strong alternating current can cause that organ to beat arhythmically and thereby induce ventricular fibrillation—a potentially deadly condition that needs to be corrected immediately. But all electricity is dangerous. So, selling the idea that DC is meaningfully safer than AC was always going to be a hard slog. Edison, nevertheless, organised publicity stunts galore involving animals (including, on one occasion, an elephant) being electrocuted by alternating current—as he sought to convince everyone who would listen that AC was too risky to have around the home. In a bid to highlight AC’s lethality, he even secretly financed the development of the electric chair. But in front of a crowd of reporters, the inaugural execution went horribly wrong, when a huge jolt failed to kill the condemned man. Repeated attempts had to be made to finish the writhing prisoner off. “They would have done better using an axe,” Westinghouse was quoted as saying. Thereafter, Edison’s smear campaign was doomed to end in jeers.

Edison's problem was that the best way to change the voltage of a DC supply with the technology available at the time was to use an electric motor coupled to a generator. Such motor-generator sets were expensive and needed careful maintenance. By contrast, the closed-core shunt-connection AC transformer, developed in the 1870s by Ganz Works in Hungary, was cheap, efficient, had no moving parts and required little attention. AC transformation has remained essentially the same ever since.



Edison chose 110 volts as his standard to meet the requirements of electric lighting—the dominant load of the day. To be brighter than gas lamps, the carbon-filament bulbs he had developed had to be driven at the maximum voltage they could stand. Experiment suggested 100 volts produced an acceptable trade off between brightness, reliability and safety. So, allowing for some voltage drop between the generating station and the customer’s premises meant starting with 110 volts. But because of the relatively low voltage and the drop in the line, a DC generator could reliably serve customers only within a radius of a mile or so. That meant sprinkling power stations throughout the neighbourhood, an expensive solution to the problem.



If electronic rectifiers and inverters had been around at the time, Edison would have been able to distribute his DC electricity at far higher voltages. His power stations could then have been bigger, more efficient and located further apart, each thus capable of serving a larger number of customers. Even rural areas could have been connected to the budding DC supply. The “war of the currents” Edison waged on Westinghouse and Tesla might then have had a different outcome, with DC the victor and AC remembered (if it were remembered at all) as an interesting but stillborn experiment.



That was not to be. The Ganz transformer allows AC to be sent over long distances at a high voltage before it is stepped down for retail sale. This high voltage requires a lower current for a given amount of power. Thermal losses caused by electrical resistance in a line are proportional to the square of the current, so the lower the current (and the higher the voltage) the more efficient the system. Modern AC grids operate at up to 765,000 volts.



Even so, DC distribution has, at least in principle, always had a lot going for it. Even now, at a sufficiently high voltage, it is cheaper than AC for transmitting large blocks of power over long distances. Not having to support three phases, as AC does, DC distribution requires fewer conductors. Meanwhile, the conductors themselves can be made thinner, because they do not suffer from the so-called “skin effect”—the tendency of an alternating current to flow mostly near the surface of a conductor, reducing its effective cross-sectional area and increasing its resistance in the process.



Direct current also uses transmission cables more efficiently. For instance, the power delivered by an AC line is defined by the root mean square (ie, 71%) of its peak voltage. A DC line, by contrast, can be made to operate continuously at its peak value. A high-voltage DC system can therefore carry 40% more power for a given current. Alternatively, it can use a thinner-gauge—and therefore cheaper—wire to carry the same current.

But it is when electricity has to be transported underground or underwater that DC truly reigns supreme. Unlike a cable hanging in the air, the live conductor in a buried or submerged cable has to be surrounded by a layer of insulation and then clad in a metal sheath. This makes it not only a means of transporting electricity, but also a huge coaxial capacitor. When an alternating current is applied to this capacitor, an additional current must flow continuously through the cable to keep the capacitor fully charged. The result is extra energy losses caused by the electrical and magnetic fields generated, as well as by the heat produced in the process. This capacitance effect limits the amount of power AC cables can carry, and the distance over which they can operate.



That is not the case with direct current. In a DC cable, the capacitance is charged only when the line is first switched on. Once it is in its steady-state condition, no additional current is required. This feature alone has made high-voltage DC the preferred way to link national grids separated by expanses of water, such as those of Britain and France. Offshore wind-farms, too, have benefited from the lower losses in submarine DC cables.



Finally, high-voltage DC provides a handy means for synchronising AC systems that operate on different frequencies. A number of countries (Japan, for instance) have one part of the national grid working at 50 hertz and another at 60 hertz. A high-voltage DC link between the two can prevent a sudden change in load (say, a catastrophic equipment failure) in one part affecting the other. Also, by being able to feed power between the two unsynchronised networks, it can help stabilise the overall system and avoid black outs.



So why, for all its advantages, is direct-current not used more widely? The transformer issue of Edison’s time is not DC’s problem today (subsequent development of rectifers and inverters using, first, mercury-arc valves and, later, thyristors provided a means for manipulating DC voltages). What has hobbled DC instead has been the inability to switch a high-voltage DC line fast enough when it has to be turned off in a hurry. Being able to do so is essential if a fault caused by a short-circuit is not to bring the entire grid to its knees. Because of a DC system’s much lower electrical impedance, faults can rip through it extremely quickly.



Breaking a high-voltage AC circuit is easy. The current alternates between positive and negative values—and therefore passes through zero twice every cycle. A rapid sequence of zero currents quickly snuffs out any arcing at the switch.



With a DC circuit, however, there are no natural zero currents to do the job. The current must therefore be forced down to zero by other means. This usually involves having the breaker generate an arc across the contacts with a voltage greater than the line voltage, and then arranging for the breaker to dissipate all the energy in the line until the arc is extinguished.



That takes time. Mechanical breakers can interrupt a high-voltage DC circuit in a few tens of milliseconds. Unfortunately, that is nowhere near fast enough. For a DC grid to function reliably, any fault has to be cleared within five milliseconds at most. Semiconductor-based breakers can switch fast enough to do the job, but they generate huge disruptive power losses in the process.



With no practical solution to the switching problem, the notion of a DC grid has remained for decades little more than a pipe-dream. At least, that was so until a couple of months ago when ABB, a power and automation group based in Switzerland, unveiled a breaker that was both fast and efficient enough to provide a high-voltage DC grid with proper protection.



The ABB breaker is a hybrid design that combines the speed of a semiconductor device with the efficiency of a mechanical switch. Like other hybrid breakers under development, the ABB device is capable of opening and closing in a few milliseconds. The company’s publicity notes that the switch can take the equivalent of a nuclear power station offline in a 30th of the time it takes to blink an eye.



Does this herald the dawn of a new age of DC? Not exactly. AC is so embedded as the world's electrical standard that converting to DC at a retail level is now unimaginable. In that sense, Westinghouse and Tesla remain the victors of the war of the currents. But from the point of view of power companies, the ABB breaker and its kin remove one of the biggest obstacles to the wholesale distribution of electricity by direct current. That could lead both to cheaper power, and to the burial of many of the pylon-borne power lines that disfigure so much of the rich world's countryside. In a deeper, sense, then, perhaps Edison will have the last laugh, after all.