The core challenge in power transmission is how to reduce losses. According to the law of physics, the power loss in a conductor is proportional to the square of the current (P_loss = I²R). To reduce losses, you can either lower the resistance (by using thicker conductors, which is extremely costly) or reduce the current. Since current and voltage are inversely related (P = UI), raising the voltage significantly reduces the current at a given power level — this is the fundamental logic behind high-voltage transmission.

Here, a key difference between AC and DC emerges: AC voltage can be easily stepped up or down using transformers, whereas for a long time, DC could not be efficiently converted in the same way.

The electricity generated at power plants (typically around 20 kV) is sent through a step-up transformer to raise it to 110 kV, 220 kV, or even over 1,000 kV for ultra-high-voltage transmission. Over long distances, the current is reduced to extremely low levels, keeping transmission losses within acceptable limits. Upon reaching the end user, the voltage is progressively reduced through step-down transformers to 220V (for residential use) and 380V (for industrial use), providing safe and convenient power for everyday equipment.

The inherent drawback of DC lies in the complexity of voltage conversion. In the early days, there were no efficient DC transformers. Achieving high-voltage DC transmission required bulky mechanical devices or expensive electronic equipment for voltage regulation — not only costly but also far less reliable than the simple transformer. This seemingly simple "voltage conversion problem" ultimately determined AC's dominance in the power grid.

In essence, the power grid adopted AC because it perfectly met the core requirements of large-scale, long-distance, and low-cost electricity transmission.

However, with rapid socio-economic development and rising electricity demand — particularly the large-scale integration of renewable energy sources such as wind and solar — the demands on transmission technology have continued to grow. Today, ultra-high-voltage direct current (UHVDC) transmission, as an important branch of ultra-high-voltage transmission, is playing an increasingly vital role in long-distance, high-capacity power delivery.