[Power Industry Transformation]①The Farther Electricity Travels, the Cheaper DC Becomes... The Next Generation Is 'Voltage-Source HVDC'

AC System Won the First Current War
Long-Distance DC Transmission Now Possible with Power Semiconductor Advances
Power Losses Increase with Distance in AC Transmission
Expansion of HVDC Driven by Growth in Renewable Energy

Editor's NoteThe electric power industry is emerging as a driving force for the Korean economy, following automobiles and semiconductors. For a long time, electricity played only a supporting role in the industrial sector. However, with the explosive growth in power demand driven by artificial intelligence (AI) data centers, electric vehicles, and the spread of renewable energy, the electric power sector has become a key player in K-industry sought after even overseas. The four major domestic power equipment companies, including LS Electric, have secured orders worth 33 trillion won. The industry has established itself as a growth engine contributing to exports and job creation. The industry's focus is now on how to secure a virtuous cycle structure. Securing direct current (DC) technology for more efficient power transmission has become a core challenge. This article examines the challenges facing Korean companies as the power industry undergoes a major transformation.

Modern power systems have developed based on alternating current (AC). Generators that produce electricity by rotating turbines-such as thermal, hydro, nuclear, and wind power-generate AC electricity, in which polarity (+, -) periodically reverses. In contrast, solar power generates direct current (DC) electricity, where the direction of voltage and current remains constant.

In the so-called Current War of the late 19th century between Nikola Tesla and Thomas Edison, Tesla's AC system prevailed. AC made it easy to change voltage using transformers, allowing electricity to be transmitted over long distances through voltage step-up processes.

At that time, DC systems faced difficulties in voltage conversion, and it was cumbersome to convert the AC produced into DC. When long-distance transmission was not feasible, generators had to be installed everywhere, making AC more economical. Edison, on the other hand, advanced human civilization by inventing DC-based home appliances.

However, in the 21st century, power plants that generate DC electricity without rotating turbines-such as solar power, energy storage systems (ESS), and fuel cells-have emerged. With advances in power semiconductor technology, it has become easier to convert DC voltage, making high-voltage DC transmission possible. As DC-based power demand rapidly increases in data centers and electric vehicle charging stations, the need for DC transmission systems has grown.

In particular, as the share of renewables that generate electricity as DC has surged, high-voltage direct current (HVDC) transmission is expanding significantly. HVDC offers high transmission efficiency, making it suitable for renewable energy sources that must generate electricity far from demand centers.

HVDC requires conversion facilities at both ends to switch from AC to DC and back from DC to AC. Initial costs are inevitably high. However, because power losses are low, there is a tipping point beyond a certain distance where HVDC becomes less costly to invest in than high-voltage alternating current (HVAC) transmission.

With HVAC, power losses increase significantly as transmission distance grows. At the same voltage, HVAC incurs 25% higher transmission losses than HVDC. Due to the nature of AC, underground transmission lines have increased capacitance (the ability to store electricity), which greatly reduces their actual transmission capacity. To prevent this, additional compensation equipment must be installed, causing investment costs to soar when lines are buried underground.

Generally, for overhead transmission lines, HVDC becomes more cost-effective over several hundred kilometers, while for underground or submarine cables, it becomes cheaper over several tens of kilometers. Another advantage of HVDC is that, because it does not operate at a frequency, it is free from electromagnetic wave concerns.

HVDC systems are classified as current-source or voltage-source types, depending on their control method.

The current-source type uses thyristor semiconductors, while the voltage-source type uses insulated gate bipolar transistor (IGBT) semiconductors. Although the current-source HVDC technology is relatively mature, it has the drawback of being limited in bidirectional transmission.

The voltage-source HVDC allows bidirectional transmission and requires less installation space, but it is still in the technology formation stage and is expensive. It is expected that the use of voltage-source HVDC, which allows flexible control, will expand for applications such as offshore wind farm integration and cross-border transmission.