Studies On Participation Of Wind Power Inertia In Power System Frequency Control

China is the world’s largest producer and consumer of energy.In recent years,to meet the requirements of transformation of energy system and to achieve the energy development goal of"carbon peaking and carbon neutrality" proposed in 2020,wind power and other clean energy resources are developing rapidly in China and are gradually replacing the traditional fossil energy resources,playing an essential role in the energy system of China.A power system with high wind penetration level has been formed in some regions of China.However,since most wind turbines in the power system are connected to the grid through power electronic devices,they cannot provide the system with rotational inertia like traditional synchronous generators.In addition,to maximize the captured wind energy,wind turbines normally operate in "maximum power point tracking(MPPT)" mode,lacking reserves for system frequency regulation.Therefore,with large-scale wind power integrated in the power system and replacing the traditional power resources,the inertia of the system declines and the ability of the frequency regulation of the system is also weakened.In such circumstances,to maintain the system frequency stability,it is of great significance to study how wind power can participate in system frequency regulation.As shown in many recent studies,to get wind power capable of participating in system frequency regulation,the active power control strategy of the wind turbine can be modified to be able to increase the wind turbine’s active power output after the system experiences a large active power disturbance.In this way,the kinetic energy stored in the rotating mass of the wind turbine can be released to the power system after the disturbance and the system frequency response can be improved.This method is to participate the system frequency control by using the "inertia" of the wind power.In order to improve the system frequency response,and to guarantee the wind turbine’s and the wind farm’s stability and efficiency during the frequency control process,it is necessary to design a frequency control strategy for large-scale wind power integrated in the power system considering the influence of the control process on both the system frequency response and the dynamic change of the wind turbines.Based on the existing research,this dissertation firstly studies the frequency response characteristics of the power system with large-scale wind power integrated,and analyzes influences of different forms of frequency control of the wind power on the system frequency response characteristics.Then,this dissertation studies the system secondary frequency drop caused by the termination of the frequency control of the wind power,the over-deceleration of the rotor of wind turbines and the loss of captured wind energy during the frequency control of the wind power.To overcome the above issues,combining with theoretical analysis,mathematical derivation and advanced control methods such as model predictive control(MPC),this dissertation proposed a hierarchical,multi-objective frequency control framework by using "inertia" of the large-scale wind power integrated in the power system.Main contributions and innovations of this dissertation are described as follows:(1)Due to the limitation of releasable kinetic energy,the wind turbine has to terminate the frequency control and decrease its output power during the later stage of system frequency control,causing a secondary frequency drop.To solve this issue,based on the analysis of influence of termination time on the magnitude of secondary frequency drop,an analytical termination time for improving the secondary frequency drop is proposed.Firstly,this dissertation builds a mathematical model to formulate the change of the active power output of the wind turbine during frequency control.Secondly,based on the classical system frequency response model,the relationship between the secondary frequency drop and the termination time is derived,and the optimal termination time which can minimize the magnitude of secondary frequency drop is numerically calculated.Since the optimal termination time has to be obtained through numerical calculation,and it is related with operating conditions of the wind turbine,disturbance magnitude,etc.,it is not practical for application.Therefore,based on a simplified mathematical model of frequency control of wind power,the optimal termination time is derived and simplified as a fixed termination time which is only dependent on system frequency response characteristics.Finally,a method to obtain parameters of the system frequency response model and to determine the incremental power of the wind turbine is proposed to formulate the application strategy of the fixed termination time in a wind farm.The proposed fixed termination time utilizes the overshoot of the mechanical power of the synchronous generators during the frequency regulation,thus it has a clear physical explanation.Moreover,the application of the fixed termination time can realize the coordination between the wind turbine and synchronous generators in a power system during the frequency regulation.(2)To cope with the over-deceleration of the rotor speed of wind turbines in a wind farm and to decrease the wind energy loss of a wind farm due to the deviation from the maximum power points of the wind turbines during the frequency control of the wind power,an MPC-based centralized inertial control scheme for the wind farm is proposed.Firstly,to accurately predict the dynamic changes of the rotor speed,mechanical power of the wind turbines during the inertial control while avoiding the modelling errors caused by employing linearized model of the wind turbines,a predictive model of the wind farm during the inertial control process is built.Then,based on the predictive model of the wind farm,the MPC-based optimization problem is formulated with the objective of avoiding over-deceleration of the wind turbines’ rotor speed and decreasing the wind energy loss of the wind farm.Finally,the MPC-based centralized inertial control scheme for the wind farm is proposed.During the inertial control process,the MPC controller collects the real-time operating information of the wind turbines,calculates the MPC-based optimization problem,and then coordinates the active power outputs of the wind turbines in the wind farm.In this way,the over-deceleration of the rotor speed of the wind turbines can be avoided,and the loss of wind energy of the wind farm is significantly decreased.Accordingly,the proposed scheme improves the stability and efficiency of the wind farm and realizes the coordination of wind turbines in the wind farm(3)To get the inertial control applied in power systems with large-scale wind power in forms of multiple wind farm clusters,a hierarchical,multi-objective inertial control framework is proposed based on the above-mentioned studies.The proposed framework consists of wind farm cluster level control,wind farm level control and wind turbine level control.The wind farm cluster level control is designed based on the application strategy of fixed termination time.During wind farm cluster level control process,the linear programming optimization problems with the objective of improving system frequency response are solved to determine the demands for frequency regulation of each wind farm.Wind farm level control and wind turbine level control are designed based on the MPC-based centralized inertial control scheme for the wind farm.To avoid the decrease of the computational efficiency of the optimization problem caused by a larger number of wind turbines,the alternating direction method of multipliers(ADMM)algorithm is introduced to decompose the MPC-based optimization problem into sub-problems which can be solved in a distributed manner at wind turbine controllers.The proposed framework allocates different active power references for wind farms and wind turbines with different operating conditions.Accordingly,the coordination among wind farms and the coordination among wind turbines are both realized with the objective of improving system frequency response and improving wind farms’ stability and efficiency,respectively.