In a DC grid, contingencies such as converter outages give rise to a current imbalance that is reflected in the DC node voltages in the grid. This imbalance has to be accounted for by changing the currents flowing in and out of the DC system. These DC node voltages can be directly influenced by controlling the DC current of the HVDC converter at that node. Different control strategies can be applied to balance the currents in a DC grid after a contingency.In this paper, the different converter control strategies are introduced systematically, thereby aiming to provide a framework for classifying the different converter control strategies available in literature. It is discussed how all converter control strategies theoretically can be regarded as limiting cases of a voltage droop control. It is also explained how the different converter control concepts can be combined, leading to more advanced converter control schemes such as voltage margin control, dead-band droop control and undead-band droop control.Based on the introduced converter control strategies, different grid control strategies are introduced and classified. The application of the advanced converter control strategies results in advanced grid control strategies and the advantages of those are discussed.
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