sentences:  Dominant forms such as wind power and solar power are highly variable, and so the need for more sophisticated control systems became apparent, to facilitate the connection of sources to the otherwise highly controllable grid.Power from photovoltaic cells (and to a lesser extent wind turbines) has also, significantly, called into question the imperative for large, centralised power stations.The rapidly falling costs point to a major change from the centralised grid topology to one that is highly distributed, with power being both generated "and" consumed right at the limits of the grid.