Determination of modular multilevel converters admittances and their impacts on HVDC power system stability

This paper deals with the determination of the complex admittances and trans-admittances of power converters in general and in particular of two modular multilevel converters (MMCs) connected by a high-voltage direct current (HVDC) link to two distinct AC systems. With the term trans-admittances we mean the cross admittance “seen” between the points of common coupling of the two MMCs in the two AC grids. These quantities show how the HVDC link couples the two seemingly separated AC systems. The accurate computation of admittances and trans-admittances requires careful consideration of several frequency harmonics and frequency up-conversion/down-conversion phenomena through the HVDC link. To determine them, we exploit the periodic small-signal (PAC) analysis that we briefly introduce from a theoretical perspective. Results show how AC and DC instabilities can be induced by MMCs and how they propagate through the HVDC link.