High-voltage direct current power transmission

A DC link consists of a DC line connecting at least two AC networks via converter stations.

Today, electrical energy is produced, transported and distributed using alternating current. There are a few main reasons for this choice: simplicity of production (alternators are simpler and more reliable than direct-current generators), ease of changing voltage levels using transformers, and ease of cutting the current because it naturally cancels out twice per period.

However, the control of AC energy transfers in dense networks poses increasingly difficult problems:

• the distribution of energy flows in the various branches of mesh networks is governed by physical laws and cannot be easily controlled;

• reactive power must be compensated as closely as possible to its consumption in order to limit losses and voltage drops;

• frequency and phase settings of interconnected alternators must be coordinated.

Direct current poses other problems: its production requires the rectification of alternating current waves, and voltage change can only be achieved by means of complex devices. In both cases, expensive power electronics are required. The problem of interrupting direct current is technically solved, but at the cost of sophisticated and expensive processes.

However, there are situations in which direct current is more attractive than alternating current, or even mandatory.