Comparison between separated and not separated positive and negative sequence control in a high voltage direct current transmission system during unbalanced grid faults

Especially during unbalanced grid faults the behavior of voltage source converter based generation or transmission units is highly dependent on the implemented control of the converter. For the control strategy of the converter the two general alternatives are on the one hand a separated control of positive and negative sequence quantities and on the other hand a not separated control in which voltages and currents are not decomposed in the symmetrical components.Furthermore the quickness of the converter control allows the injection of reactive current during grid faults in order to stabilize the voltage locally. The possibilities and limitations of this so called dynamic voltage control depend highly on the implemented control strategy as well.In this publication the advantages and disadvantages of the two control strategies with separated and not separated positive and negative sequence are explained and demonstrated during an unbalanced grid fault. For this an AC-DC hybrid transmission test system in which the converter control of the high voltage direct current transmission system is equipped with both control strategies forms the basis for this comparison.