Current Control Reference Calculation Issues for the Operation of Renewable Source Grid Interface VSCs Under Unbalanced Voltage Sags

This paper analyzes the current reference calculation for the control of grid-connected voltage source converters meant to operate under voltage unbalanced sags produced by grid faults. The well-known reference calculation method that allows to control the active power ripple produced by the existence of negative-sequence components in the grid voltage is extensively analyzed. A type of voltage sag that produces unfeasible current reference values is identified and a possible workaround is proposed. Also, the need to compensate the power ripple produced by the filter inductance of the converter is demonstrated and an extension of the calculation method to compensate for this is introduced. The theoretical results are confirmed using simulation tools and experimental tests.