Conventional HVDC converters always have a demand for reactive power. At normal operation, a converter consumes reactive power in an amount that corresponds to approximately 50 % of the transmitted active power. The least costly way to generate reactive power is in shunt connected capacitor banks. Some of these capacitor banks can then be combined with reactors and resistors to form filters providing low impedance paths for the harmonics in order to limit them from entering into the connected AC network.

A series resonance filter branch will give a very low impedance and thereby efficient filtering in a narrow frequency band around the tuning frequency. Such branches are therefore normally used for the largest harmonics, i.e. the 11th and 13th. But often it is advantageous to use a double-tuned filter for the 11th and 13th harmonics instead.

AC filter at Longquan, Three Gorges - Changzhou, China

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For the higher order harmonics, the current levels are lower, but these frequencies have the largest impact on telephone interference. Therefore they must also be attenuated, but the filter impedance can be larger than for the 11th and 13th harmonics. Thus, broadband filters, normally of high-pass type, are used to take care of all harmonics from the 23rd and upwards.

ABB has developed automatically tuned AC filters (ConTune). The ConTune filters, which replace the traditional series resonance filter branches, can be built to generate small quantities of reactive power but still provide good filtering.