Research On Power Modulation Strategy Of Wind Farms For Power System Stability Improvement

As the most mature technique of renewable energy generation,wind power generation is developing rapidly across the world.The power electronic converter based variable speed wind turbines(VSWTs)have a different dynamic behavior with synchronous generators.The integration of large-scale VSWT-based wind farms challenges the power system operation stability.On the other hand,the flexible power control ability of VSWT-based wind farms provides a new and effective measure for power system stability improvement.This dissertation focuses on the power modulation strategy design for VSWT-based wind farms to improve the power system stability,and a series of researches,including the VSWT power modulation ability analysis,power modulation strategy of VSWT-based wind farms for low-frequency oscillation damping and transient stability improvement as well as the integrations of different control strategies,are carried out.The main contents are as following:(1)Analyze the limitation factors on the power modulation ability of VSWTs,and quantify the power modulation ability of VSWT under different operating conditions.Based on the analysis,a modulation power dispatching strategy is proposed for VSWT-based wind farm,which can not only make full use of the power modulation ability of each VSWT,but also effectively avoid the overmodulation of VSWT.The simulation results verify the correctness of the evaluation method as well as the effectiveness of the power dispatching strategy.(2)As to the low-frequency oscillation,this dissertation proposes a wide-area adaptive reactive-power damping control strategy(WARDCS)for VSWT-based wind farms to damp the inter-area oscillation by adjusting the output reactive-power of wind farms.The proposed WARDCS identifies the system model using Kalman filter and chi-square test techniques,and chooses the suitable controller adaptively.The simulation results verify the effectiveness of WARDCS on damping the inter-area oscillation as well as its robustness to the operating condition variation of wind farms and power systems.(3)As to the transient stability,a transient stability control strategy(TSC)is proposed for VSWT-based wind farms based on the time-optimal control(TOC)and perturbation observer technique.The proposed TSC aims to utilize the active-power modulation abitliy of VSWTbased wind farms to improve the power system transient stability.Based on the extended equal area criterion theory(EEAC),the power system with wind farms integration is modeled as a cascade system,with an equivalent generator as the first subsystem and wind farms as the second subsystem.Based on the model,a TSC is designed for wind farms.Firstly,TOC is implemented to design the virtual control for an equivalent generator,which aims to improve the power system transient stability.Secondly,a control law is designed for wind farms based on perturbation observer technique to track the virtual control quickly.The simulation results verify the effectiveness of the proposed TSC on the transient stability improvement as well as its robustness to the operating condition variation of wind farms and power systems.(4)A synthetic control strategy(SCS)is proposed for VSWT-based wind farms to achieve the integration of transient stability control,oscillation damping control and frequency support control.The coordination rules between different controls are analyzed.Simulations are carried out on the modified IEEE 68-bus system,and the results verify the effectiveness of the proposed SCS.The research results of this dissertation can provide a good theoretical reference and practical guidance on the stability control design of power systems with large-scale VSWT-based wind farms integration.