Introduction to HVDC transmission
At an early stage in power system development, the choice for AC systems was made. The transmission of electricity over large distances requires high voltage levels. Because ohmic losses are proportional to the square of the current, every doubling of the voltage reduces losses to one-quarter. AC became the standard because transformers can quite easily transform the voltage from lower to higher voltage levels and vice versa. Nowadays, power-electronic devices make it possible to convert AC to DC, DC to AC, and DC to DC with a high rate of efficiency, and the obstacle of altering the voltage level in DC systems has disappeared.
AC transmission systems have an important drawback: they require reactive power. We speak, in our daily practice, about voltages and currents when we analyze the power flow in the network. Current refers to the net flow of charge across any cross section of a conductor. The net movement of 1 C of charge through a cross section of a conductor in 1 s produces an electric current of 1 A. The potential difference or voltage between the terminals of a conductor creates an electric field that forces the charges to move. The moving charges, which we call current, create a magnetic field around the conductor. The interaction between the electric and magnetic field is described by the Maxwell equations and the energy is stored in the electromagnetic field. For 50 or 60 Hz phenomena, the power system is, so to speak, of “electrically small dimensions” compared to the wavelength of the voltage, and Kirchhoff’s laws may fruitfully be used to compute the voltages and currents. But we have to realize that the electric energy transported by underground cables and overhead lines is stored in the electromagnetic field around the conductors and that that field has to be continuously charged to accommodate the oscillation of voltage and current. Reactive power is the flow of energy that continuously charges the electromagnetic field. This charge energy is not wasted, because the energy is recovered as the fields discharge, but to charge and discharge the field, a current has to flow through the conductor, and this current gives ohmic losses and causes a voltage drop across the line. As the electromagnetic field increases with the length of the conductor, the required reactive power also grows until a point is reached that for cables the reactive current level reaches the value of the nominal rated current and energy transmission is not possible anymore.
Bringing DC transmission at higher voltages was done in Edison’s days through the series connection of generators, but it was a cumbersome solution that could not compete with AC.