A power apparatus applied in an SST structure includes a first AC-to-DC conversion unit, a first DC bus, an isolated transformer, a DC-to-AC conversion unit, a second AC-to-DC conversion unit, and a second DC bus. The first AC-to-DC conversion unit has a first three-level bridge arm and a second three-level bridge arm. The first DC bus provides a first DC voltage. The isolated transformer has a primary side and a secondary side. The DC-to-AC conversion unit has a third three-level bridge arm and a fourth three-level bridge arm. The second AC-to-DC conversion unit has a fifth three-level bridge arm and a sixth three-level bridge arm. The second DC bus provides a second DC voltage.