Korea Electrotechnology Research Institute (KERI) said on Oct. 20 that it developed the country's first medium-voltage hybrid direct current (DC) circuit breaker, resolving a key challenge of the next-generation power transmission method, "multi-terminal direct current (MTDC)." This is expected to further enhance the stability and efficiency of Korea's power grid.
DC transmission has less power loss than alternating current (AC) and is easier to link with renewable energy such as solar and wind. But it has one big weakness: it is difficult to interrupt current when an accident occurs.
AC periodically changes the direction of current and has moments when it naturally becomes zero (current zero-crossing), making it easy to interrupt. In contrast, DC flows in only one direction, so when a fault occurs the current continues, requiring a device that forcibly brings the current to zero.
The 42 kV-class hybrid DC circuit breaker developed by KERI is a "composite device" that combines three existing technologies. A power semiconductor switch forcibly brings the fault current to zero to cut off the flow, a mechanical high-speed switch withstands the high voltage that follows and removes the electric arc, and an energy absorption device disperses the remaining energy to stabilize the system. Through this process, DC current can be interrupted quickly and safely.
Overseas companies also have similar hybrid breakers, but they excessively use power semiconductors, making the equipment expensive and reducing transmission efficiency. To improve this, KERI designed the mechanical switch to take over part of the semiconductor's role, lowering cost and increasing efficiency. It also produced two module types, 21 kV and 42 kV, allowing voltage to be expanded by stacking as needed.
Until now, Korea has operated a simple DC transmission network connecting only two points due to technological limitations. With this development, it is now possible to build a multi-terminal DC transmission network (MTDC) that connects multiple points, which is expected to greatly help prevent large-scale blackouts and stabilize the power grid.
Ahn Hyeon-mo, a senior researcher at KERI, said, "We have conducted long-term research to solve the issues of current interruption, insulation, and energy dissipation," and noted, "With this achievement, we can reduce reliance on foreign technology and achieve import substitution."
KERI has already completed prototype production and third-party testing and is promoting domestic and overseas technology transfers and exports. Going forward, it plans to localize high-voltage DC circuit breaker technology of 100 kV class and above and actively participate in the "West Coast energy expressway" and the "Korean Peninsula energy expressway" construction projects.