A Coordinating Control for Hybrid HVdc Systems in Weak Grid

A coordinating control strategy is proposed for a hybrid high voltage dc (HVdc) system comprising a capacitor commutated converter connected in series with a two-stage voltage source converter called "Vernier." This approach ensures that 1) the tap changers and switched capacitor banks are eliminated by Vernier controls that maintain constant firing angle and margin angle at the rectifier and inverter, respectively, and provide volt-VAr control at the commutation bus, and 2) improves the power flow recovery and risk of commutation failure following sever faults. To that end, a detailed switched model of hybrid-HVdc is built in EMTDC/PSCAD platform. For the frequency-domain analysis, a new nonlinear state-space averaged model is also developed and benchmarked against the detailed model. The steady-state operating characteristics of the proposed hybrid HVdc system are established. Finally, the effectiveness of the proposed approach is demonstrated for weak ac systems interfacing the rectifier and inverter sides. Improvement in commutation failure and power flow recovery following severe disturbances are also shown through case studies.