From AC to DC

The discovery of the phenomenon of electromagnetic induction by Michael Faraday in 1831 opened the door for large-scale generation of electricity. Ampère continued Faraday’s and Oersted’s experiments, and on the basis of his findings , Antoine-Hippolyte Pixii built in 1832 a machine consisting of a fixed horseshoe armature and a rotating horseshoe magnet.  The first machine produced AC: the rotating horseshoe magnet induced each half rotation a voltage of opposite polarity. Antonio Pacinotti build in 1859 a prototype of a DC machine that he called “macchinetta” . The idea was, however, to generate DC. Pixii added a commutator to his original design, enabling the machine to supply electricity flowing in one direction. The first DC generators had a rather low efficiency, but they became an object of study and research. Many researchers contributed to the further development of DC generators in the 1860s and 1870s – Zenobe Theophile Gramme, Charles Wheatstone, and Werner Siemens were the most prominent ones (Figure 1).

The rotor of a DC generator produces an AC voltage as it spins. The commutator, basically a rotating switch, reverses the machine’s output every half cycle. The commutator mechanically rectifies the current.

So far most of the electricity we use is generated in power plants, where three-phase synchronous generators convert the mechanical energy from the steam or gas turbines into electrical energy. Today’s power system is an AC power system, as it has been for more than hundred years. In the last decades the challenges for the utilities and the transmission system operators have dramatically changed. The increasing costs of land for new transmission lines and substations, environmental concerns, and the price of the primary energy are making energy managers and power engineers to take another look at the transmission system. Innovations in DC technology have allowed us to consider forgrid extensions a high-voltage DC (HVDC) connection as a serious alternative for AC transmission lines. To understand how HVDC works, it is necessary to understand the operation of the converter bridge.