An active filter (4) for compensation of variations in an apparatus' (3) current consumption from a power grid (1) as a cause of variations in the apparatus electrical load, through active frequency filtering and dynamic compensation of reactive energy/current. For each of the power grid's phases (a, b, c) the active filter includes a transistor bridge (Ta, Tb, Tc) connected between each phase and a DC-link (7). The active filter includes a filter circuit of LCL-type, arranged to reduce disturbances on the power grid caused by the active filter itself. The active filter further includes a control device (9) for calculating, with a given working frequency (f), points in time at which the transistor bridges are set to reverse the current direction through the DC-link, and the filter circuit is preferably adjusted to that working frequency.