HVDC Grid Post-DC Fault Recovery Enhancement

Fast and stable post-fault recovery is an essential part of a fault clearing strategy in HVDC grids. As the post-fault recovery may involve de-blocking converters, unwanted oscillations may be noticed in the voltages and currents. These oscillations occur at resonant frequencies, which stem from the interaction between the converter and the grid. Methods to identify HVDC grid resonances exist, but the focus is mainly on control interactions instead of during transient conditions such as post-fault recovery. This paper provides a method to analyze oscillations during post-fault recovery process, discusses their origins and proposes two methods to mitigate these. It is found that these oscillations are mainly due to incompatibility of the converter- and grid-side DC voltage at the moment of de-blocking, and the low damping characteristic of the grid at resonant frequencies. The first mitigation approach acts on the first aspect and avoids triggering of the oscillations by matching the voltage of the converter to that of the grid at the moment of de-blocking. The second mitigation approach acts on the second aspect by increasing damping at the DC-side through controls. Time domain simulations have shown that both methods effectively reduce the oscillations seen during post-fault system recovery.