Adaptive virtual capacitor control for MTDC system with deloaded wind power plants

Coordinated droop control strategies can provide frequency support for AC area grids from wind farms integrated through multi-terminal high-voltage DC (MTDC) transmission. However, such a strategy inevitably causes DC voltage deviation for transmitting AC area frequency information, thereby deteriorating the stability and security of the MTDC system operation. This paper reports an improved adaptive virtual capacitor control strategy that can provide inertial support for the system. Instead of increasing the capacitance of the physical capacitor, a virtual capacitor is generated by utilizing the rest energy of the deloaded wind farm. Furthermore, an S-shape function is designed to adaptively adjust the capacitance of the virtual capacitor based on the operating points of the system, in order to provide a better inertial support than with the fixed virtual capacitor control strategy. The proposed strategy not only enhances the DC voltage nadir but also improves the frequency nadir of the AC area by releasing additional power from the deloaded wind farm., a MTDC system of a four-terminal voltage source converter with two wind farms is simulated using PSCAD/EMTDC. Case studies are conducted to compare and demonstrate the effectiveness of the proposed strategy under sudden load variations.