This paper was prepared as background for a Panel Discussion on Specification of Filters for High Voltage Direct Current Systems at the 1989 IEEE T&D Conference in New Orleans, on April 4, 1989. HVDC converters inject harmonic currents into the associated AC transmission lines. In the dc lines, they produce ripple voltage which in turn results in a flow of harmonic currents. Filters are customarily installed on both ac and dc side of converters. One of the most important uses of the filters is to control voice frequency interference (noise) in exposed telephone circuits .