Secondary Frequency Support to Weak Grids Through Coordinating Control of Hybrid HVDC System

A recently proposed Hybrid-High Voltage DC (HVDC) system comprising a Capacitor Commutated Converter connected in series with a 2-stage Voltage Source Converter called 'Vernier' on the DC side and in parallel on the AC side is considered to transfer power from onshore wind farms (WFs) to the load centers. The flexible DC voltage polarity of the Vernier gives rise to circulating power. Although the circulating power leads to unwanted losses, it can be leveraged as an additional degree of freedom. This extra degree of freedom is used to provide secondary frequency control in the inverter-side grid by extracting power from the rectifier without any communication of signals while maintaining constant margin angle and firing angle at the respective ends. Moreover, to improve the rectifier-side grid frequency, the WFs provide primary frequency support while operating under deloaded condition. Analytical insight into this complex system is developed to damp frequency oscillations and increase utilization of Vernier capacity.