Research On Fault Ride Through Control Strategy For Offshore Hybrid Wind Farm Connected To Grid Via MMC-HVDC

Optimizing the energy structure and increasing the proportion of clean energy such as wind power in energy power generation is an important way to achieve carbon peak and carbon neutralization.Compared difference between onshore wind power with offshore wind power,offshore wind power has many advantages such as rich resources and stable wind power.Offshore wind power will play a significant role in future new power source generation.As the first choice of offshore wind power grid connection,VSC-HVDC transmission system must have a certain fault ride through capability to ensure that the offshore hybrid wind farm will not run off the grid in case of AC system failure.In this paper,the Grid-connected system of offshore hybrid wind farms via VSC-HVDC is taken as the research object,and the low voltage ride-through control of wind turbines,equivalent modeling of offshore hybrid wind farms and fault ride-through control of the Grid-connected wind farms via HVDC are studied.The detailed work follows:Aiming at the low voltage ride-through problem of variable-speed constant-frequency wind turbines during grid faults,a coordinated control strategy with additional protection circuits and reactive current compensation is proposed.First,the wind speed model,wind turbine model,doubly-fed wind turbine mathematical model and permanent magnet directdrive wind turbine mathematical model are established.Then,the reactive current compensation method is added to the original additional protection circuit for coordinated control to improve the low voltage ride-through capability of wind turbines.Finally,wind turbine simulation model with the proposed control strategy is established on the PSCAD platform.The results of the emulation indicate that the proposed low voltage ride-through control strategy can provide reactive power support to the grid while suppressing DC bus voltage fluctuations,thereby improving the low voltage ride-through capability of wind turbines.Aiming at the problem of too long time domain simulation time for the detailed model of hybrid wind farm,a clustering method of hybrid wind farm based on particle swarm optimization optimized K-Means algorithm is proposed.First,the wind speed,rotor speed and active power of the hybrid wind farm are selected as clustering indicators.Then,the particle swarm optimization algorithm is used to optimize the clustering center,and the equivalent model of the hybrid wind farm after the optimization of the clustering center parameters is established.Finally,the power output characteristics of hybrid wind farm detailed model,single-machine equivalent model and multi-machine equivalent model under different working conditions are compared and analyzed.The results of the emulation indicate that the hybrid wind farm equivalent model proposed has better consistency with the detailed model.Aiming at the problem of DC voltage exceeding the limit during the grid fault of the offshore hybrid wind farm through the flexible DC transmission grid-connected system,a fault ride-through control strategy that coordinated step-down control and super capacitor is proposed.First,the mathematical models of DC line,DC unloading circuit and super capacitor are established.Then,the step-down control and super capacitor are coordinated and controlled in stages.In the first stage,the AC voltage reference value of wind farm side is dynamically adjusted according to the DC voltage to reduce the output power of the wind farm.In the second stage,once the DC voltage exceeds the preset threshold,the super capacitor system will be activated to realize the coordinated control of step-down control and the super capacitor.The results of the emulation indicate that the proposed fault ride-through coordinated control strategy can well restrain the rise of DC voltage and accelerate the recovery of DC voltage.