1. A modular multilevel voltage source converter comprising:a number of phase legs, each of the phase legs comprising an upper and a lower phase arm comprising cells with energy storage elements; and
a control unit operative to:
model a circulating current in one of the phase legs using a first expression (?VPR1, ?Vcap-PR1), a second expression (?VR2N, ?Vcap-R2N) and at least one modulation signal (mR, mcm), where the first expression comprises the sum of cell voltages measured in the upper phase arm ?Vcap-PR1 and the second expression comprises the sum of cell voltages measured in the lower phase arm ?Vcap-R2N;
determine a circulating current control signal (ER1?ER2+2VCM) for the one of the phase legs through the circulating current modelling without measuring the circulating current in the one of the phase legs; and
combine the circulating current control signal as a feed forward signal with a modulation signal used for one phase arm of the phase arms thereby obtaining a modified modulation signal controlling the one phase arm of the phase arms to reduce or introduce a circulating current while a waveshape is formed, wherein the at least one modulation signal comprises the modulation signal used for the one phase arm of the phase arms.