  The paper addresses an improved inner current reference calculation to be
employed in the control of modular multilevel converters operating during
either balanced or unbalanced conditions. The suggested reference calculation
is derived based on the AC and DC additive and differential voltage components
applied to the upper and lower arms of the converter. In addition, the impacts
caused not only by the AC network's impedances but also by the MMC's arm
impedances are also considered during the derivation of the AC additive current
reference expressions. Another issue discussed in this article regards that
singular voltage conditions, where the positive-sequence component is equal to
the negative one, may occur not only in the AC network but also internally
(within the converter's applied voltages). Several different inner current
reference calculation methods are compared and their applicability during the
former fault conditions is analyzed. The paper presents a detailed formulation
of the inner current reference calculation and applies it to different
unbalanced AC grid faults where it is shown that the presented approach can be
potentially used to maintain the internal energy of the converter balanced
during normal and fault conditions.
