Frequency Stability Control Of Offshore Wind Farm Connected By VSC-HVDC To Receiving-end Power Grid

Offshore wind farms in China are mainly located in the southeast coastal area.As the receiving-end power grid,it intakes a large amount of external power and has a heavy load,leading to low equivalent inertia.As VSC-HVDC can isolate the disturbance between offshore wind farm and AC system and realize independent control of active and reactive power output of offshore wind power system,offshore wind farm usually adopts VSC-HVDC to connect to the power grid.However,this further reduces the equivalent inertia of the receiving-end grid and brings great challenges to the system frequency stability.At the same time,offshore wind power and VSC-HVDC have flexible control mode and potential frequency regulation ability.Therefore,it is of great importance for the frequency stability of offshore wind power connected to the receiving-end power grid to study the frequency response characteristics between the receiving end power grid and offshore wind power,to analyze the maximum penetration level of receiving-end power grid for offshore wind power,and to optimize the method of offshore wind power in system frequency regulation.This paper analyzes the frequency response characteristics of offshore wind power connected to the receiving-end power grid,as well as the offshore wind capacity of receiving system,and proposes a coordinated frequency optimization control method for the offshore wind power system.The main work is as follows:(1)Firstly,the offshore wind turbine and VSC-HVDC are modeled separately,and on this basis,the operating characteristics of the offshore wind power system are analyzed;a comprehensive frequency response model of the receiving-end system suitable for offshore wind power integration is proposed.The influence of system equivalent parameters and wind power penetration rate on the frequency response of the system is qualitatively analyzed in this model,and thus the frequency response characteristics of the system after the offshore wind power is connected are revealed.These results are verified through simulation examples,and the influence of different types of disturbances on the frequency response of the receiving end power grid is discussed.(2)Secondly,aiming at small disturbances which endanger the steady-state frequency quality of the power system,risk indicators of the sensitive frequency bands of wind power fluctuations are proposed,so that the wind power bearing capacity of the receiving system can be analyzed.Aiming at large disturbances,the maximum penetration level of offshore wind power is analyzed with the threshold value of frequency protection action under the possible maximum power perturbation,moreover,a calculation method for maximum penetration level of offshore wind power that takes into account wind power fluctuations and frequency constraints is proposed.This paper also takes Jiangsu power grid as a simulation example to analyze and calculate the maximum penetration level of offshore wind power to ensure the frequency stability of offshore wind power connected to receiving-end system.(3)Finally,combining the potential frequency modulation capabilities of offshore wind farms and VSC-HVDC,an improved coordinated frequency control strategy is proposed for offshore wind power systems connected to the VSC-HVDC grid.Based on the virtual inertia control of offshore wind turbines and the VSC-HVDC system,additional power-voltage control is applied to the onshore converter station to ensure a certain inertia support while effectively improving the maximum frequency deviation of the system,and the main parameters of the DC side controller Carried out analysis and adjustment to ensure its control effect.We also use the established detailed control model of offshore wind power system and frequency response model of offshore wind power connected to receiving system in Jiangsu to simulate and verify its effectiveness.