Which of the following is a characteristic of AC transmission systems?

The characteristic of AC transmission systems that stands out is their requirement for reactive power to maintain voltage levels. In AC systems, as the current flows, it can cause the voltage to fluctuate. Reactive power—often produced by devices like capacitors or synchronous condensers—provides the necessary support to stabilize voltage and improve power factor. This is particularly crucial in long-distance AC transmission, where voltage drops and distortion can occur. Ensuring that sufficient reactive power is available helps in maintaining a stable and reliable AC transmission network.

This characteristic contrasts with the other options, which do not accurately describe the nature of AC transmission systems. For instance, AC systems are actually suitable for long distances, not limited to short ones. They also operate on alternating current, as opposed to high-voltage direct current, which is not a feature of AC systems. Additionally, AC transmission systems are increasingly integrated with renewable energy sources, such as wind and solar, allowing for a versatile and flexible energy grid.