SACRAMENTO, Calif. — A century ago, thousands of Californians flocked to opening day for the Vaca-Dixon substation to hear the world’s longest and highest-voltage power line hum with electricity for the first time. It was an engineering marvel, built by hundreds of men and their mules in just two years. Today the Spanish renaissance building, sandwiched between Vacaville and Dixon along Interstate 80, remains a critical piece of the grid, powering homes across the Bay Area. It is also a museum, housing artifacts from the early days of California’s energy sector. The bygone substation, owned by...