Research On Prevention And Control Of Ultra-low Frequency Oscillations In Power Systems With Wind Power Participation In Frequency Regulation

The active frequency control process of power systems consists of the primary frequency control and the automatic generation control(AGC).The primary task is to ensure the stability during the frequency control process,and the oscillation problem is the most important in the study of small disturbance stability in power systems.In recent years,the phenomenon of ultra-low frequency oscillations(ULFOs),strongly associated with primary frequency regulation and AGC control,has been repeatedly observed in practical power systems.It is characterised by a homogeneous variation in the speed of all generators and an overall oscillation in the frequency of the entire network,rather than a relative movement between groups of machines.The ULFOs are more pronounced in grids with a high proportion of hydropower units due to the "water hammer effect" of the water diversion system.With the rapid development of new energy sources,the potential for wind power to participate in frequency control of the power system has been fully exploited.However,large-scale wind power access exacerbates the traditional ULFOs,seriously threatening the safe and stable operation of power systems.In order to ensure the frequency stability of high-penetration wind power connected to the grid,the grid guidelines issued by several countries,including China,clearly state that wind turbines should have similar inertial response and frequency regulation capabilities as conventional synchronous generators.While the involvement of wind power in frequency regulation leads to significant changes in the characteristics of traditional ULFOs and more complex system coupling,posing certain difficulties for analysis and prevention and control.Therefore,it is of great theoretical significance and practical application to study the prevention and control of ULFOs in power systems with wind power participation in frequency regulation.This paper focuses on the impact of wind power participation in primary frequency regulation and AGC frequency regulation on ULFOs in power systems.By constructing the frequency stability analysis model with wind power grid and the wind power frequency response control model,control methods such as damping torque method,description function method and root trajectory eigenvalue method are used to analyze the impact of wind power participation in primary frequency regulation and AGC frequency regulation on ULFOs in power systems.The multi-objective optimisation function is constructed with the analysis of system stability,and the system parameters involved in deep oscillations are optimized.The strategy for the prevention and control of ULFOs in power systems is proposed with wind power participation in frequency regulation.Specifically,the main research work of this paper is as follows:(1)The wind power frequency control strategies such as virtual inertia control,overspeed control and pitch angle control are modeled,and the system stability analysis model with wind power participation in primary frequency regulation is established based on the single machine single load model.The closed-loop stability criterion for power systems is analytically derivated under a simplified model and the influencing factors are analyzed.The mechanism of wind power participation in primary frequency regulation on the damping change of the prime mover system,as well as the oscillation frequency characteristics and damping characteristics of the ULFOs are analysed by the damping torque method.The key influencing factors of system frequency stability under different wind power frequency control strategies are investigated.(2)Based on the system stability analysis model and wind power frequency control strategy in the previous chapter,the primary frequency regulation deadband characteristics are further considered.The frequency stability analytical model with deadbands is constructed and equivalently simplified.The impact of deadbands on the frequency stability of hydropower and wind power is analysed by the descriptive function method.The critical oscillation amplitude of the disturbance triggering system frequency oscillation is calculated under virtual inertia control and paddle pitch angle control strategies,and the conditions for oscillation occurrence are given.The theoretical analysis is verified by different deadbands of hydropower and wind power governors.(3)Based on the findings of the previous two chapters on the effect of wind power participation in primary frequency regulation on ULFOs,the wind power participation in AGC frequency regulation is further considered.The AGC control model with wind power is constructed,and the nonlinear links are linearised based on the Pade approximation.The effect of different parameters or links on the frequency characteristics of the open-loop transfer function is analyzed via root trajectory eigenvalue method and its effect on AGC oscillations is further investigated.The multi-objective optimisation function is constructed and the system parameters that deeply affect the AGC model are optimized.The oscillation prevention and control strategy are proposed,so as to effectively suppress AGC oscillations and improve frequency control performance.