Current Limitation and Power Resynchronization after Large Disturbances for HVDC : A Case Study

Sammanfattning

The ongoing transition to renewable energy sources and the increasing penetration of power electronic converters in the power grid leads to challenges related low inertia and poor grid stability. To alleviate these issues, Grid Forming (GFM) Control is becoming increasingly important. However, power electronic converters are inherently sensitive to over currents, normally only able to handle over currents 1-2 per units of its rated value which risks damaging the equipment in case of short circuits and other grid disturbances. This thesis project has tested current limiting strategies based on Current Control-Grid Forming (CC-GFM) and Voltage Control-Grid Forming (VC-GFM) that uses Current Saturation Algorithm (CSA) to ensure that the current is kept within acceptable limits. Moreover, the so called ”Virtual power” method has been tested in order to improve the post fault recovery. Once the control schemes had been designed, different fault cases were simulated in the program PSCAD. The results imply that both the CC-GFM and VC-GFM control schemes successfully manage to limit the current. Furthermore, the use of virtual power lead to a significant improvement in post fault resynchronization with the CC-GFM performing the best. With the promising results future studies and tests would be of interest.