Impedance Modeling And Stability Analysis Of The Grid-connected Wind Turbine System Under Frequency-coupled Oscillations

In order to build a low-carbon energy system,the penetration of renewables keeps increasing,and the power system in China is gradullay evolving towards a power system with highpenetration renewables.The complex control effects of multi-time scale controllers in the renewable generators will be interwined with the power grid,which complicates the system characteristics and makes the system stability problems more severe.The oscillation accidents in the power system with high-penetration renewables not only have the time-varying oscillation frequency in the wide-band frequency range,but also show a new feature,which is the frequencycoupled oscillation phenomenon that multiple oscillation frequency points coexist and are coupled to each other.In the power system with high-penetration renewables,the wide-band oscillations will affect the operation of power generation equipments,and even induce chain reaction accidents,resulting in system collapse,large area blackout and other catastrophic consequences.Although some research achievements have been made in the stability analysis and stabilized control for the power system with high-penetration renewables,the existing impedance based stability analysis theory cannot reveal the mechanism of the frequency-coupled oscillation phenomenon.The traditional impedance model and stability criterion in the sequence domain ignore the relationship between the frequency-coupled oscillation phenomenon and the characteristics of renewable generators,which makes it unable to accurately predict the actual stable state of the power system,and can lead to the risk of mispredicting the system stability.Besides,the system stabilized control also lacks actual impedance models as theoretical basis.In oder to address above problems,this thesis focuses on the wide-band oscillation in the power system with high-penetration renewables and the frequency-coupled oscillation phenomenon reflected in stability accidents.Based on the theory of impedance stability analysis,this thesis studies the oscillation mechanism of the grid-connected wind turbine system,the impedance modeling,the system stability analysis,and the system stabilized control.The main work and innovation achievements of this thesis are as follows.1.The mechanism of the frequency coupling charactetistics in the wind power converters is clarified,and it is pointed out that the frequency coupling characteristics will lead to the frequency-coupled oscillation phenomenon in the wide-band oscillation accidents.On this basis,the sequence-impedance matrix based stability analysis method is proposed,which reveals the mechanism of frequency-coupled oscillation phenomenon,overcomes the problems of system stability analysis under this frequency-coupled oscillation phenomenon,and lays a theoretical foundation for the system stabilized control.Based on the developed frequency coupling analytical models,how the frequency coupling characteristics are influenced and how the frequency coupling characteristics shape the system impedances are analyzed,and it is proved that ignoring the frequency coupling characteristics or only considering part causes of the frequency coupling characteristics can lead to a misprediction of system stability.Besides,the system stabilized control by optimizing the key influencing factors of the frequency coupling characteristics is analyzed.2.The mechanism of dc dynamics in the wind turbine systems is clarified,and the modeling method of quantifying the dc dynamics in wind turbine systems is put forward.On this basis,it is pointed out that the dc dynamics are also the key influencing factors of the frequency coupling characteristics in wind turbine systems.Then,a novel impedance modeling method is proposed to establish the frequency coupling models of wind turbine systems based on the developed model of dc dynamics,which furtherly improves the grid-connection stability analysis theory under the frequency-coupled oscillation phenomenon.With the considerations of dc dynamics,the impedance model of wind turbine systems are developed,based on which the influence of dc dynamics on the system impedance characteristics and system stability are analyzed.The potential risk factors of causing weakly damped dc dynamics are pointed out,and in view of these risk factors,control methods are proposed to improve the adaptability of wind turbine systems to the weak power grid by optimizing the dc dynamics.3.In view of the oscillation problems between the wind turbine systems with traditional vector control strategy and the weak power grid,the method of improving the weak grid adaptability by replacing the vector control strategy with the virtual synchronous generator(VSG)control strategy in the wind turbine systems is discussed.Firstly,how the VSG control strategy influences the impedance characteristics and the grid-connected system stability of the wind power converters is analyzed,and a method is proposed to analyze the system impedance characteristics based on a space vector diagram.On this basis,the VSG controlled type-IV wind turbine system is taken as an example,and analysis points out that when the VSG controlled typeIV wind turbine system connects to the weak power grid,especially access to the weak power grid in parallel with the wind turbine system with conventional vector control strategy,the interconnected system will face the potential risk of instability.Then,control methos are proposed to improve the weak grid adaptability of wind turbine systems by optimizing the parameters of VSG control strategy and the dc dynamics.