Will Edison’s direct current win the current wars?

Edison’s direct current method is making a comeback with the development of high-voltage direct current transmission (HVDC) technology, which is overcoming the limitations of alternating current to deliver electricity economically and efficiently.

 

At the end of the 19th century, when electricity was first invented, two of the most renowned geniuses of the time went head-to-head to set the standard for how electricity should be delivered: Edison, who advocated direct current (DC), in which the direction and magnitude of the current is constant, and Tesla, who advocated alternating current (AC), in which the current is constantly changing periodically. Tesla’s alternating current system won out because it required a higher voltage to reduce power losses in the process of delivering power, and it was easier to control the size of the voltage. Today, we still see alternating current used in transformers, electrical outlets, and more. But recently, after more than 130 years of use, there’s been a push to switch back to direct current. But why?
In order to avoid obstacles, humans have to intentionally bend or stretch their bodies. The constant movement of the body to avoid obstacles causes us to lose energy, and the same is true for electricity. Obstacles that prevent electricity from flowing in a circuit are called resistance. Electricity loses energy because of resistance, and reducing the amount of resistance is a major challenge in power transmission. The resistance that doesn’t appear when using direct current, but is caused by the nature of alternating current, where the direction of the current changes periodically, is called reactance. The power lost as current flows through the reactance is called reactive power. Reactive power is the surplus power that is contained in the current but cannot be used as energy. This is not a big problem when the transmission distance is short, but as the line is stretched, the resistance of the line itself increases, and the line and the ground surface interact with each other to change the reactance of the line. In this case, the overall reactance value becomes large, causing the reactive power to spike, and the transmission efficiency becomes very poor. This creates a paradoxical situation where the alternating current method actually increases the amount of power lost when it needs to be transported over long distances.
In addition to the amount of power that is lost in transmission, it is important to send large amounts of electricity economically. With alternating current, the magnitude and direction of the power changes over time, so the system must be designed to account for these changing values and to deliver power in all cases. Design and installation costs are higher for alternating current systems than for direct current systems, which consider only one case because the power does not change. In addition, there are peculiar resistances and reactances that appear only in alternating current systems, making them more variable and less stable than direct current systems. Low stability also limits transmission capacity, making them unsuitable for moving large amounts of power.
As technology evolved, the need for a power system that could send electricity farther and at higher volumes decreased, making Tesla’s alternating current system less attractive. Instead, high-voltage direct current (HVDC) technology revitalized Edison’s direct current system. HVDC is a method of transmitting high-voltage alternating current power generated at a power plant by converting it to direct current through a converter, and then converting it back to alternating current through a converter at the desired location for use. It is difficult to change the voltage of direct current itself, but it is possible to obtain a high voltage direct current by using a thyristor, a conversion device that converts alternating current into direct current, or by using a semiconductor device (IGBT device). Not only is the direct current method more stable because the direction of the current is constant, so there is no reactance, but it is also more efficient than the alternating current method because there is no reactive power.
HVDC technology can be utilized in various fields due to its many advantages. In Korea, HVDC has been transmitting electricity between Jeju Island and Jindo, and Haenam and Jeju Island through submarine cables since the late 1990s, and in Europe, it has gone a step further by connecting power grids between countries to supply and generate electricity on a continent-wide scale. It can also facilitate the supply of power from offshore wind farms, a fast-growing source of renewable energy.
Since the majority of the electricity supply system has been alternating current for more than 130 years, many power facilities are built on alternating current, and it would be impractical to convert them to direct current on the fly. There are also issues that need to be resolved before it can be commercialized on a broad scale, such as harmonics that occur when converting high-voltage alternating current to direct current. With continued research, DC power will be a key contributor to building the next generation of green, nation-to-nation power grids in the near future. In the War of Currents, Edison had to drink the bitter pill of defeat when the limitations and problems of direct current were pointed out. Now, more than 130 years later, Edison’s spectacular revenge is beginning to unfold, as advanced technology has revitalized direct current power delivery.