Harmonizing AC and DC: A Hybrid AC/DC Future Grid Solution

Abstract

It has been over 100 years since Thomas Edison built the first direct current (dc) electricity supply system on 4 September 1882, at Pearl Street in New York City. Many prominent events occurred in the electricity supply industry after that. The first one, ?the war of currents,? started in 1888. Thomas Edison and his dc distribution system were on one side, and George Westinghouse and Nikolai Tesla with the alternating current (ac) system were on other side. The war ?ended? in about 1891 when ac won as the dominant power supply medium. The key behind the ac win was the invention of the transformer that could easily step up medium voltage to high and extra-high voltage for long-distance power transfer from a remote ac generation station to load centers hundreds of kilometers away with lower transmission losses. Transformers can also step down high voltage back to low voltage at load stations to supply the low-voltage equipment. Since the end of the war, ac power systems have been developed and expanded at a tremendous speed from the initial small isolated networks, with each supplying only lighting and motor loads with a few hundreds of customers, to its current scale of super interconnected networks each supplying billions of customers over large geographic areas in one or several countries. The voltage levels and capacities of transmission networks have increased from the first commercialized three-phase ac system with only 2.4 kV, 250 kW in the town of Redlands, California, United States, to the first commercial long-distance, ultra-high-voltage, ac transmission line in China with 1,000 kV, 2,000 MW. Transmission distance has been increased from several miles to over thousands of kilometers (miles). With such major achievements, it is little wonder that the ac power system became the top engineering achievement of the 20th century. Does this mean that dc is gone? The answer is an unambiguous no. What has happened in the past 50 years, such as applications of advanced control technologies in conventional power system loads, the power electronics based high-voltage dc (HVdc) transmission, and the additional renewable power sources in low-voltage distribution system, calls for a rethink about dc and ac in electricity supply systems.