A tuned filter for connection to a busbar in a high voltage electric power system. The filter incorporates a variable inductor comprising a main winding, auxiliary windings and switching circuitry associated with each auxiliary winding. The auxiliary windings are arranged to act in inductive series with respect to the main winding, and the switching circuitry is used to selectively connect the auxiliary windings in electrical series with the main winding, so allowing the inductance of the inductor to be varied. To achieve enhanced variability of the inductance, each auxiliary winding has a number of turns substantially equal to x2(nnull1) where n is a positive integer corresponding to the position of each auxiliary winding in the inductive series of auxiliary windings and x is also a positive integer. The switching circuitry is preferably constituted such that each auxiliary winding is selectively connectable to the main winding in any one of three connection modes, comprising a first connection mode in which the auxiliary winding assists the inductance of the main winding, a second connection mode in which the auxiliary winding opposes the inductance of the main winding, an a third connection mode in which the auxiliary winding is bypassed with respect to the main winding.