Harmonic cross-modulation is a term used to describe the variety of frequency interactions around the two converters that comprise and HVDC link and the effect on and interactions with the DC system and both AC systems. Because there are two converters involved, as well as two AC systems and any number of coupled frequencies, the number of interactions to be considered is infinite. Questions to be answered are : which interactions are the strongest, and whether they are likely to cause steady state or transient problems, and how to determine these interactions.