Harmonic filters are necessary for the elimination of the harmonic waves and for the production of the reactive power at line commutated converter stations. At plants with six pulse line commutated converters, complex harmonic filters are necessary because there are odd numbered harmonics of the orders and produced on the AC side and even harmonics of order on the DC side. At 12 pulse converter stations, only harmonic voltages or currents of the order and (on the AC side) or (on the DC side) result. Filters are tuned to the expected harmonic frequencies and consist of series combinations of capacitors and inductors.