A method and cooling system that cools a power stack in a power conversion apparatus. The liquid cooling system includes a first cooling stage that includes first cooling components, wherein the first cooling components are connected to form parallel cooling branches; a mixing manifold configured to be fluidly connected to the parallel cooling branches so that cooling liquid streams from the parallel cooling branches are mixed in the mixing manifold; and a second cooling stage that includes second cooling components, and the second cooling stage is connected in series with the first cooling stage in terms of a cooling liquid that flows through the cooling system.