This article presents the short circuit analysis of an offshore AC network which consists of wind power plants interconnected using HVAC cables. The power generated in the offshore AC network is transmitted to several onshore grids using VSC-HVDC system. The offshore AC network is formed by the VSC-HVDC systems using frequency and voltage droop control. A coordinated control scheme is proposed for wind turbines and offshore VSCs during short circuit conditions in the offshore grid to ensure fault ride through (FRT) without compromising the system stability. The theoretical analysis used for developing this control scheme allows to calculate the system limits taking into consideration the active and reactive power capability. In order to verify the proposed control scheme, three phase symmetric faults have been applied on a wind turbine busbar, HVAC busbar, and at the AC cable that interconnects the VSC-HVDC system. Additionally, a frequency coordination control scheme without communication between wind power generation and VSC-HVDC system has been proposed. The methodology and control system have been validated by performing a nonlinear simulation.
Many offshore wind power plants are being developed every year at North and Baltic Sea. From the prospective of environment and integrated European power, combined power transmission from several offshore wind power plants using VSC-HVDC transmission system to different onshore grids are suitable instead of connecting each wind power plants individually. Offshore AC hub is beneficial for the wind power plants that are far from shore but close to each other within the vicinity of 20 km. This paper presents a method of controlling reactive power flow in the offshore AC grid to minimize the power losses and voltage deviation. In the proposed scheme, offshore grid frequency and voltage are controlled through more than one converter. Using frequency and voltage droop schemes, the active and reactive power sharing is achieved among converters. Furthermore, the optimization algorithm is developed to acquire the set points for the wind power plants and VSC-HVDC droop gains for the optimum operation of the network.Postprint (author's final draft
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