In this blog post, we will look at the limitations of the AC method, which has dominated the power industry for 130 years, and find out why DC is gaining attention again.
At the end of the 19th century, when electricity was first invented, humankind was at a historic crossroads where it had to decide on a standard for electricity supply. It was clear that electricity would be the power source for industry and households, and the way in which this electricity would be supplied efficiently would determine the future electricity system. The two geniuses who clashed at this critical time were Edison, who advocated the direct current (DC) method, and Tesla, who advocated the alternating current (AC) method. Edison argued that direct current electricity, in which the direction and magnitude of the current are constant, should be used to supply power, while Tesla argued that alternating current electricity, in which the direction and magnitude of the current periodically change, should be used. The battle between the two was more than a simple technical debate; it was an important choice that would determine the way humanity would supply power and live. In the process, there was an in-depth discussion of the pros and cons of various power transmission methods.
Edison’s insistence on direct current was closely related to his invention, the incandescent light bulb. Incandescent bulbs required a stable voltage and a constant current flow, and direct current power was suitable for this. Edison advocated direct current, claiming that his invention and the direct current method were inseparable, but technically, direct current had the disadvantage of increased power loss when transmitted over long distances. Tesla, on the other hand, supported the alternating current method, which could solve the problem of efficiency in long-distance transmission. At the time, power loss was a major problem, and alternating current had a major advantage in reducing power loss in long-distance transmission because the voltage could be easily increased through transformers. This is why Tesla’s alternating current method eventually won, and today, alternating current is easily found being used through transformers and outlets.
However, recently, efforts are being made in various places to switch back to direct current from the alternating current, which has been the standard for power supply for over 130 years. What is the reason for this change?
Just as a person bends or stretches their body to avoid obstacles, electricity encounters resistance in a circuit and the flow of electricity changes. The obstacles that hinder the flow of electricity in a circuit are called resistors, and this resistance causes some of the energy of the electricity to be lost. Reducing this resistance in the process of transmitting power is an important task for improving efficiency. In the direct current (DC) system, such resistance is constantly present, but in the alternating current (AC) system, additional resistance is generated because the direction of the current periodically changes. This is called reactance, and the power loss that occurs as a result is called reactive power. Reactive power is surplus power that is included in the current but cannot be used as an energy source. When the transmission distance is short, this is not a big problem, but as the distance increases, the resistance and reactance of the line increase, and the reactive power also increases, resulting in a decrease in transmission efficiency. In other words, the AC method can be rather inefficient for long-distance transmission.
In addition to the amount of power loss during transmission, the way in which power is economically transmitted is also important. When using an AC system, the magnitude of the current and voltage constantly changes, so the design must take all fluctuations into account. On the other hand, the DC system reduces the complexity of the design and reduces the cost of equipment and installation because the current flows in a constant direction. Also, since the reactance that only appears in the AC system does not exist in the DC system, the DC system is relatively stable and suitable for large-capacity power transmission. From this point of view, the direct current method has the potential to deliver electricity more reliably and economically.
High-voltage direct current (HVDC) technology is emerging as a new technology with the development of technology. This method converts high-voltage alternating current generated at the power plant into direct current through a conversion device and transmits it, and converts it back to alternating current through a converter at the receiving site. It is difficult to convert the voltage of direct current itself, but it is now possible to generate high-voltage direct current through semiconductor devices such as **thyristors** and IGBT devices, which convert alternating current into direct current. The direct current method is not only stable because the current direction is constant and no reactance is generated, but also more efficient than the alternating current method because there is no reactive power.
HVDC technology, with its various advantages, is being used in various fields. In Korea, since the late 1990s, Jeju Island, Jindo, and Haenam have been connected to Jeju Island by undersea cables to transmit power in a direct current (DC) method, and in Europe, a continental-wide power supply system is being built by connecting national power grids. In addition, it is suitable for power transmission in offshore wind farms, a renewable energy source, enabling a stable power supply.
Of course, since an AC-based power grid has already been built over the past 130 years, it is difficult to switch to DC in a short period of time. There are also issues that need to be addressed for commercialization, such as the harmonic problem that occurs when converting high-voltage AC to DC. Nevertheless, if these problems are solved through continuous research and technological development, the direct current method will become the core technology of an eco-friendly and efficient power grid in the near future.
Although Edison lost the War of Currents 130 years ago due to the limitations of the direct current method, today’s advanced technology is bringing the direct current method back into the spotlight, marking the beginning of Edison’s revenge.
