In a large pot, heat the olive oil over medium-high heat. Add the onion, garlic, and ginger, and saute for 3-4 minutes, until tender. Stir in the red lentils, sweet potato, carrots, curry powder, and vegetable stock. Add 1 cup of water, and bring to a boil. Reduce the heat to low, cover, and simmer for 30 minutes, until the sweet potatoes and carrots are very tender.