Research On Coordinated Control Strategy Of MMC-HVDC Transmission System For Offshore Wind Farm

With the strategic deployment of "carbon peaking and carbon neutral",the proportion of offshore wind power in clean energy generation is increasing,and more and more offshore wind farms are connected to the power system through flexible DC transmission systems.However,in the actual application scenario,the system inertia reduction,and power surplus problems caused by the flexible DC transmission of offshore wind farms have attracted widespread attention from industry and academia,and become one of the key research problems for the adequate consumption of offshore wind power and the safe and stable operation of the system.Based on the above background,this paper starts from the construction of a dynamic equivalent model of offshore wind power flexible DC delivery system,the frequency coordination control strategy of offshore wind power flexible DC delivery system,and the surplus power coordination control strategy of offshore wind power flexible DC delivery system.In response to the problem of long simulation time and low simulation efficiency of largescale offshore wind power grid-connected model,an offshore wind farm equivalence model based on an adaptive R-K clustering algorithm is proposed.The mathematical models of offshore direct-drive wind turbines and flexible DC systems and their control strategies are analyzed,and a detailed model of the flexible DC transmission system for offshore wind power is constructed.The dynamic equivalence model of the flexible DC delivery system of offshore wind power is obtained by selecting the sub-cluster indicators that can characterize the dynamic response of the detailed model,sub-clustering the offshore wind turbines using the adaptive RK clustering algorithm,and aggregating the parameters using the capacity weighting method.The simulation results show that the proposed dynamic equivalence model based on the adaptive R-K clustering algorithm can meet the simulation accuracy while greatly reducing the simulation running time and improving the efficiency of simulation analysis.In response to the problem of reduced system inertia caused by the continuous delivery of large-scale offshore wind power through a flexible DC system,a coordinated virtual inertia optimization control of offshore wind turbines and flexible DC systems is proposed.Based on the virtual inertia control of the offshore wind turbine,a virtual inertia optimization factor is introduced to change the output power in response to frequency changes,while the energy stored in the DC-side capacitors of the flexible DC transmission system can compensate for the power imbalance caused by frequency changes,thereby responding to the frequency changes of the system and thus achieving the purpose of coordinated frequency regulation.Simulation results show that the proposed frequency coordinated control strategy can improve the inertia of the system and better meet the demand for inertia response.In response to the problem of high DC line voltage and power surplus in the flexible DC delivery system of offshore wind power when symmetrical faults occur in the AC grid at the receiving end,the proposed coordinated control strategy of surplus power in the hydrogen production system coordinated with the voltage reduction control is proposed.The dynamic voltage reduction control is used to limit the active output of the offshore wind farm,thus reducing the power difference between the two ends of the flexible and straight system in the event of a fault in the receiving grid,and increasing the electrolysis power of the hydrogen production system to achieve coordinated control of the voltage reduction control and the hydrogen production system.Simulation results show that the proposed coordinated power surplus control strategy can ensure fast voltage recovery on the DC side and convert the surplus power into higher quality hydrogen energy.In summary,this paper proposes an adaptive R-K clustering algorithm-based equivalence model for offshore wind farms,a frequency coordination control strategy,and a coordinated surplus power control strategy for coordinating the hydrogen production system with dynamic voltage reduction control,which improves the inertia level and the surplus power consumption capability of the flexible DC transmission system of offshore wind power.