A filter apparatus includes at least one filter, each filter being tunable and including at least one capacitor arrangement. The capacitor arrangement includes a plurality of first capacitors, a plurality of second capacitors, and a plurality of switches. Each switch is switchable between a non-conducting mode and a conducting mode. The plurality of second capacitors and the plurality of switches are arranged to connect or disconnect the second capacitors on different potentials in order to tune the filter by adjusting the capacitance of the filter. An electrical power transmission or distribution system includes such a filter apparatus. A method is provided for filtering harmonics in an electrical power transmission or distribution system by means of such a filter apparatus.