DC and AC in Ultra-High Voltage (UHV) Transmission

The greatest capability of UHVDC is its ability to transmit massive amounts of electricity directly from distant locations to a specific destination without interruption.

Its core advantages are most evident in long-distance, high-capacity transmission:

The longer the transmission distance, the more cost-effective it becomes: DC transmission lines have low costs, making them particularly suitable for tasks like transmitting hydropower from the Jinsha River and Yarlung Tsangpo River to the eastern coastal areas thousands of kilometers away.

More flexible connections: It acts like a universal adapter, connecting two AC power grids of different frequencies and synchronization without increasing the risk of short circuits. More precise control: Its fast adjustment speed acts like a "power emergency responder," providing rapid support during grid failures.

Currently, my country has built several world-class ultra-high-voltage direct current (UHVDC) projects. For example, the Kunming-Liuzhou-Longyan DC project, which just celebrated its fifth anniversary, was the first to adopt UHV flexible DC technology. It acts like a super voltage stabilizer for handling "unstable" clean energy sources (such as wind and solar power), having cumulatively transmitted over 113.5 billion kilowatt-hours of clean energy to the Greater Bay Area.

DC is "point-to-point," while AC is "point-to-multipoint." The core advantage of AC UHV lies in network construction and flexible allocation.

Strong networking capability: It can connect multiple regional power grids into a huge synchronous grid, much like a highway network.

More flexible allocation: This powerful network can achieve "excess power refunds and insufficient power subsidies." For example, in summer, when the southwest is abundant with water, excess hydropower can be sent to North China via the AC network; in winter, when the north has strong winds, it can conversely support the south.

It's the "deep-water port" of DC: This is the most crucial point. The massive amounts of electricity transmitted via DC must rely on a robust AC power grid for reception, distribution, and "digestion." If the receiving grid is too weak, it's like a giant ship sailing into a small fishing port-it simply can't dock, and the system is prone to instability.

Therefore, the core concept of my country's UHV development is "strong AC and strong DC," with both working in tandem.

Each performs its own function: For large, remote energy bases like Xinjiang and Tibet, DC efficiently transmits electricity; for load centers like North China, East China, and Central China, a robust receiving-end grid is built using AC, forming a massive synchronous network to ensure that electricity "can be sent in and received."

Mutual support: Without a robust AC grid, DC becomes a "rootless tree," unable to leverage its advantages in long-distance power transmission; conversely, without long-distance DC transmission, the AC grid cannot achieve optimal resource allocation over a wider area.

So, back to the question-having both DC and AC power isn't because the technical approach hasn't been decided;

it's precisely because we've found the optimal solution: let "direct flights" do what they do best, and let the "highway network" do what they do best.

The two will cooperate and work together to form an efficient, stable, and powerful modern power grid.