A passive filter for connection between an AC source and a load, in either three-phases or in a single-phase arrangement. The filter includes, for each phase, a trap circuit having an inductor in series with a capacitor, the trap circuit having at least two terminals. A line reactor is connected between the AC source and the load, the line reactor having at least an input terminal, an output terminal and a tap terminal. A switch selectively connects at least one of the trap circuit terminals to a selected one of the line reactor terminals. The switch is capable of selecting which of the trap circuit terminals to connect to which of the line reactor terminals on the basis of a level of voltage distortion being experienced by the AC source, or on the basis of a calculated level of background voltage total harmonic distortion.