Grate the potatoes in a food processor, using the shredding disk. Add to a colander and rinse with cold water. Press down to remove as much liquid as possible, then transfer the potatoes to a dish towel and wrap, squeezing so every last drop of moisture is extracted. Add to a large mixing bowl with the onion powder, egg, flour, baking powder, and salt. In a large skillet in about ¼ inch of vegetable oil over medium-high heat, fry ¼-cup dollops of the potato mixture, flattening with a spoon, for about 4 minutes a side. Remove to a paper-towel-lined platter, then place in a baking dish, covered with foil, to keep warm.