A unit of Japanese industrial giant Hitachi plans to bring production of a key piece of high-voltage electric grids back to the United States with a $457 million investment to expand its plant in South Boston, creating 825 jobs.

The Japanese firm’s Hitachi Energy unit‘s move comes in tandem with a groundbreaking effort by the state’s housing agency and Halifax County to build homes for some of the people taking those jobs.

The expanded plant will make large power transformers, the custom-built equipment, sometimes as large as a house, that steps down the high-voltage electricity that flows through an electric system’s biggest power lines — in Virginia, lines that handle as much as 500 kilovolts of electricity — to the lower levels that travel down neighborhood lines.

“What they make is

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