Security Assessment And Control Strategy For Power System Interconnected With Wind Power

Wind power is evolving towards interconnection into the power system through tie lines.However,the frequent occurrence of various fault events(such as short-circuit fault,sudden load increase,generator fault removal,etc.)and the intermittent characteristics of wind energy may break the balance of the wind power interconnected power system and threaten the security and stable operation of the system.Therefore,it is a great challenge to study the security assessment and the control strategy to improve the security and stability of the wind power interconnected power system,to ensure a reliable and stable wind power interconnected power system.Till now,in the scenario of a large-scale grid connection of wind power,the wind power system is decoupled from the power system due to the adoption of converters.As a result,the equivalent rotational inertia of the wind power interconnected power system is reduced,that is,the ability to suppress frequency fluctuations is weakened.Therefore,ensuring frequency security and stability is the key to ensuring the security and stability of the wind power interconnected power system.After the system is disturbed by faults,according to the time sequence of the frequency response,the frequency response process can be divided into three stages: transient stage,dynamic stage and steady-state stage.The duration of the transient stage is short,and the security and stability of the wind power system will be affected if it cannot be intervened quickly under disturbances.In the dynamic stage,if there is no accurate frequency prediction value as a priori information,it is impossible to control the frequency security of the large-scale wind power interconnection system.Further,the frequency security assessment and regulation based on the transient stage and the dynamic stage can ensure the smooth transition of the system to the predetermined steady-state stage.In this dissertation,for the frequency challenge caused by the reduction of equivalent inertia caused by wind power grid connection and the fast response of wind power converters.We identify the issues associated with the frequency security and stability of wind power interconnected power systems,unravel the cause-effect relationships between measurable variables of power systems and system frequency security and stability,and seek out improving control solutions for the potential issues.The contributions are outlined as follows.(1)By analyzing the factors that wind power affects the transient frequency security of the power system,a security assessment method for transient frequency offset based on the hidden Markov model is proposed,which is combined with the characteristics of wind power replacement ratio and wind farm output power.Meanwhile,considering the frequency response mechanism of the system after being disturbed by a fault,a hidden Markov model with four states of normal frequency,fault,recovery and steady-state is constructed.The proposed method can realize the efficient and high-precision frequency offset security assessment of the transient stage of the wind power interconnection into the power system.In addition,the proposed method can deal with the problem of unbalanced sample data of the wind power interconnected power system.(2)A virtual synchronous generator control strategy based on model prediction is proposed.By combining the control principle of the virtual synchronous generator and considering both the quality of grid-connected current and the fluctuation amplitude of system frequency in the predictive control,this method can increase the equivalent inertia of the system and the frequency nadir.Therefore,the proposed method can optimize the frequency response curve of the transient stage and improve the frequency security and stability of the transient stage.(3)An integrated deep belief network is proposed to predict frequency quasi-steadystate value.By constructing the input feature set considering the influence of wind power and extracting the input feature set by stacked denoising autoencoder,the deep belief network can be trained separately and then the prediction result is obtained by integrating the result of two models.The high-precision prediction of the frequency quasi-steadystate value of the dynamic stage of the wind power interconnected power system is realized.(4)To restore the frequency of the system quickly and stably,a two-layer coordinated control strategy is proposed considering the distributed model predictive control between regions of the wind power interconnected power system and the fuzzy control within the wind farm.By constructing the model of the wind power interconnected power system and optimizing power distribution,the coordinated control realizes real-time control of the wind power interconnection into the power system and fast and stable frequency recovery.In addition,through the fuzzy control strategy in the wind farm,the secondary frequency drop is avoided by making use of the advantage that the energy storage can output a large amount of active power in a short time.Therefore,the frequency recovery capability of the dynamic stage of the wind power interconnection into the power system is improved.