DC electrical grids never really caught on for widespread public utility use. Their limited distribution range (a result of their late 1800s technology) couldn’t compete with AC’s transmission abilities. But one DC grid, located in San Francisco, has been operating since 1879, three years before Thomas Edison opened his New York DC power plant. In an article posted on the IEEE Spectrum web site, a 250-volt DC system coexists with AC lines, flowing through underground and overhead cables across the city. Such are the needs of some of its customers that PG&E (Pacific Gas and Electric), the local utility, has found it necessary to keep the system running.

One of the primary uses for DC power is elevator motors, dating from before 1930, that are still in use. These “winding drum” motors, outlawed after 1940 for safety reasons, were grandfathered in. They persist due to the enormous cost of replacing them with new elevator systems.

Due to technical issues that have arisen over the decades, mostly due to the aging of the equipment, PG&E has divided the DC grid into very small “islanded” sections, each serving about 7 to 10 customers. Instead of sending DC current over a separate grid throughout the city, the company now rectifies AC power nearby before sending it on to its destination.

Perhaps SF’s DC grid will stay in operation long enough to see a transition to a modern Direct Current grid, such as those operating in many parts of Europe. Transmission issues have been solved, meaning high voltage DC (HVDC) can be used to more efficiently power homes and businesses.

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