This blog post examines the limitations of the AC system that has dominated power for 130 years and explores why DC is gaining renewed attention.
At the end of the 19th century, when electricity was first invented, humanity stood at a historic crossroads, needing to establish a standard for power supply. It was clear electricity would become the power source for industry and homes, and the future power system would be determined by how this electricity was efficiently supplied. The two geniuses who clashed during this critical period were Edison, who advocated for direct current (DC), and Tesla, who championed alternating current (AC). Edison argued that power should be supplied via direct current (DC), where the direction and magnitude of the current remain constant, while Tesla insisted on alternating current (AC), where the direction and magnitude periodically change. Their rivalry transcended a mere technical debate; it was a crucial choice that would determine humanity’s power supply method and way of life. This process sparked in-depth discussions on the advantages and disadvantages of various power transmission methods.
Edison’s insistence on direct current was closely tied to his invention, the incandescent light bulb. Incandescent bulbs required a stable voltage and a constant current flow, making direct current power suitable. Edison championed direct current, emphasizing its inseparable relationship with his invention. However, technically, direct current suffered from significant power loss when transmitted over long distances. Tesla, on the other hand, supported the alternating current (AC) system, which could solve the efficiency problem in long-distance transmission. Power loss was a major issue at the time, and AC had a significant advantage in reducing losses over long distances because voltage could be easily stepped up using transformers. Consequently, Tesla’s AC system ultimately prevailed, and today, AC current is commonly used via transformers and outlets.
Recently, however, efforts to switch back to direct current (DC) are emerging in various places, challenging AC’s status as the standard power supply method for over 130 years. What could be driving this change?
Just as a person bends or stretches to avoid an obstacle, electricity also changes its flow when encountering resistance in a circuit. Resistance is the obstacle that hinders the flow of electricity in a circuit, and it causes some of the electrical energy to be lost. Reducing this resistance during power transmission is a critical challenge for improving efficiency. In direct current (DC) systems, this resistance remains constant. However, in alternating current (AC) systems, the direction of the current periodically changes, generating additional resistance. This is called reactance, and the resulting power loss is known as reactive power. Reactive power is included in the current but is surplus power that cannot be practically used as an energy source. While this is not a major issue for short transmission distances, as distances increase, the resistance and reactance of the lines grow, causing reactive power to rise and transmission efficiency to decrease. In other words, AC systems can become inefficient for long-distance transmission.
Furthermore, beyond the amount of power loss during transmission, the method of transmitting power economically is also crucial. When using AC, the magnitude of both current and voltage constantly fluctuates, necessitating design considerations for all possible variations. In contrast, DC systems feature current flowing in a constant direction, reducing design complexity and lowering equipment and installation costs. Furthermore, reactance—a phenomenon unique to AC—does not exist in DC systems, making DC relatively more stable and suitable for high-capacity transmission. From this perspective, DC systems hold the potential to deliver power more stably and economically.
High Voltage Direct Current (HVDC) technology is emerging as a new solution driven by technological advancements. This method converts the high-voltage AC power generated at power plants into DC using conversion equipment for transmission, then reconverts it back to AC at the receiving end using converters for use.
While converting the DC voltage itself is difficult, semiconductor devices like **thyristors** or IGBTs can now generate high-voltage DC. The DC system is stable because the current direction is constant, eliminating reactance. Furthermore, it has no reactive power, making it more efficient than AC systems.
HVDC technology, with these diverse advantages, is already being utilized in various fields. In South Korea, since the late 1990s, subsea cables have connected Jeju Island to Jindo and Haenam, enabling DC power transmission. In Europe, interconnecting national power grids is building a continent-wide power supply system. Furthermore, it is well-suited for transmitting power from offshore wind farms, a form of renewable energy, enabling stable power supply.
Of course, since power grids based on AC have already been established over the past 130 years, converting them to DC in the short term presents significant challenges. Furthermore, issues such as harmonic problems arising when converting high-voltage AC to DC must be resolved for commercialization. Nevertheless, if these problems are solved through continuous research and technological advancement, DC systems will establish themselves as a core technology for eco-friendly and efficient power grids in the near future.
Although Edison lost the War of Currents 130 years ago due to the limitations of DC systems, today, with advanced technology, DC power supply is being re-examined, effectively marking the beginning of Edison’s revenge.
