This paper proposes a communication-free control strategy at the offshore wind farm (OWF) level to enhance onshore fault ride-through (FRT) grid code compliance of the voltage source converter (VSC)-based multi-terminal high voltage direct current (MT-HVDC) grid. In this proposal, the emerging virtual synchronous generator (VSG) concept is employed to equip the Type 4 wind turbine generator (WTG)s with inherent grid forming ability. Accordingly, it is proposed to switch the offshore HVDC converters control mode from grid forming to grid feeding during onshore FRT period to realize direct wind power in-feed reduction as a function of the severity of MT-HVDC grid's overvoltage. The related dynamics are mainly characterized by the high-speed current control loop, so improved OWF response is achieved during onshore FRT period as conventional voltage/frequency modulation strategies are not employed. New analysis/amendments are also proposed to study and improve the transient active power reduction sharing between the WTGs in first few power cycles under wind wake effect. Finally, with the objective of a smooth transfer of HVDC converters and WTGs in several proposed operation states, a set of state machines are proposed considering whole WTG's dynamics. Comprehensive time-domain simulations are performed with averaged electromagnetic transient models to demonstrate the improved onshore FRT behavior in terms of minimizing the electrical stress at both MT-HVDC grid and OWF levels. INDEX TERMS Fault-ride-through, multi-terminal HVDC grid, offshore wind farm, power reduction method, type 4 WTG, virtual synchronous generator.

This paper studies the effect of zinc oxide arrester (ZnO) and neutral earth resistance on controlling nonconventional oscillations of the unloaded power transformer. At first, ferroresonance overvoltage in the power system including ZnO is investigated. It is shown this nonlinear resistance can limit the ferroresonance oscillations but it cannot successfully control these phenomena. Because of the temperature dissipation of ZnO, it can withstand against overvoltage in a short period and after that ferroresonance causes ZnO failure. By applying neutral earth resistance to the system configuration, mitigating ferroresonance has been increased and chaotic overvoltage has been changed to the smoother behavior such as fundamental resonance and periodic oscillation. The simulation results show that connecting the neutral resistance exhibits a great mitigating effect on nonlinear overvoltage.