Method for zero-sequence damping and voltage balancing in a three-level converter with split dc-link capacitors and virtually grounded LCL filter

A method and an apparatus implementing the method for an arrangement comprising a three-phase, multi-level invertern (11), an output LCL-filter (12) connecting the inverter to a grid (13), and a virtual-ground connection between the LCL-filter and the neutral point of the DC-link (l,m). The method comprises determining a zero-sequence component of an LCL-filter inverter-side current (i), calculating a zero-sequence damping and balancing voltage term (e*) on the basis of the LCL-filter inverter-side current zero-sequence component and voltages over the two halves of the DC-link (v, v), and adding the zero-sequence damping and voltage balancing term to the output voltage reference (e*).