Evaluation And Parameter Tuning Of Equivalent Inertia For Offshore Windfarms With VSC-HVDC Connection Systems

As offshore wind power development in China gradually shifts to offshore areas farther away from the shore,offshore windfarms with VSC-HVDC connection systems are widely adopted.Similar to other renewable energy systems,offshore windfarms with VSC-HVDC connection systems lack active support for the receiving AC grids.As the capacity of integration increases,the equivalent inertia of the power system will continue to decrease.When there is an active power imbalance on the receiving grid side,the synchronous generator rotor speed in the AC grid will rapidly change,leading to system frequency issues.Therefore,it is necessary to study the inertia characteristics of offshore windfarms with VSC-HVDC connection systems.This thesis establishes a multi-device coupled model for offshore windfarms with VSC-HVDC connection systems and evaluates their equivalent inertia.The key parameters influencing the equivalent inertia of the system are obtained.Based on this,the determination of control parameters for the system’s equivalent inertia is carried out,optimizing the system’s frequency response characteristics and ensuring system frequency stability.The specific research results are as follows:(1)Introduction of the working principle and basic control strategy of offshore windfarms with VSC-HVDC connection systems,including the receiving end converter control strategy,the sending end converter control strategy,and the two different control strategies of the direct-drive wind turbine converter.On this basis,the control strategy of each module of offshore windfarms with VSC-HVDC connection systems was introduced that adjusts the power instruction as an additional frequency regulation control,the validity of this control strategy was obtained through simulation.(2)Introduction of the modeling method of amplitude-phase motion equations,modelling the common parts of the amplitude-phase motion equation under dual-loop control,and based on this,the amplitude-phase motion equation of the receiving end converter of offshore windfarms with VSC-HVDC connection systems was developed.To solve the amplitude-phase motion equation of offshore windfarms with VSC-HVDC connection systems with additional frequency regulation control,the concepts of forward path model and feedback path method were used.(3)Analysis of the factors that have a significant impact on the inertia of offshore windfarms with VSC-HVDC connection systems.The analysis results show that the equivalent droop coefficient of VSC-HVDC transmission system has a significant impact on the system inertia.Based on the proposed common-mode frequency and differential-mode frequency concepts,an equivalent inertia function model was developed for multiple synchronous generators and converters scenarios.The accuracy of the model is verified through simulation.According to the established model,a parameter design process flowchart was developed for the equivalent droop coefficient of offshore windfarms with VSC-HVDC connection systems,providing the droop coefficient under equipment constraints and system frequency guideline constraints to achieve the frequency support capability that complies with the system frequency guidelines under normal equipment working conditions.The flowchart can also provide the equivalent droop coefficient value corresponding to the maximum system frequency support capability under extreme equipment operating conditions.