| With continuous increasing of the renewable energy penetration rate,the replacement of traditional thermal power units by renewable energy has been further deepened.However,most wind turbines are connected to the power grid through power electronic converters.Therefore,the wind turbine is decoupled from the grid frequency and does not have the frequency response capability as traditional synchronous unit.Especially in power grids with high wind power penetration,the replacement of traditional plants by a large amount of wind generation will reduce the total inertia of the grid,and the frequency regulation capability and frequency stability of power system will be further weakened.Notably,due to the fast and precise command tracking capabilities,energy storage units are suitable for frequency regulation services of power system.However,their expensive construction and maintenance costs have laid limitations on the possibility of participating in frequency regulations all by itself.Therefore,it is necessary to research how to combine wind generation and energy storage into a unified and coordinated system,and through flexible control of'the combined system to enhance the wind turbine's frequency regulation capability to actively participate in the frequency regulations of the power grid.Based on a comprehensive review of existing related research work from China and abroad,this thesis constructed a technical solution for the combined wind-storage system with multi-scenario adaptability and reliability to actively participate in the frequency regulation service of the power system.With the methods based on the flexible combination of fuzzy logic algorithms,model predictive control theory and frequency control theory.The combined wind-energy storage system is designed for solving the problem of reduced inertia and frequency reglation capability of high wind power penetration power systems caused by wind generation's replacement.The main research contributions in this thesis are as follows:1)On the level of DFIG's control method for inertial frequency response.Firstly,based on the vector control method,the mathematical model of DFIG and its rotor-side converter are derived and analyzed,respectively.Which provides a theoretical basis for further improvement on DFIG's control loop.Secondly,based on dynamic power tracking method,an inertial response control strategy for DFIG is proposed.By adding an additional response module to DFIG's active control loop,the MPPT curve can be revised dynamically according to the system frequency regulation requirements.Then,a dynamic response mechanism between the DFIG's rotor and the frequency fluctuations is constructed,enabling the wind turbine to use its rotor kinetic energy for fast frequency response.Finally,through simulation,the feasibility of the control strategy for improving the inertial response capability of DFIG is verified,which offers the theoretical foundation for the wind-energy storage frequency response research.2)On the level of short-term frequency response,a novel fuzzy theory-based coordinated control strategy is presented for improving the short-term frequency response capability of a DFIG-energy storage(ES)system.Firstly,to make the short-term frequency response capability of the DFIG-ES system adaptive for various wind speed conditions,based on the analysis of the DFIG operation characteristics under different wind speeds,four wind speed zones are classified,and accordingly different operation strategies are designed for each wind speed zone.Afterwards,two kinds of fuzzy logic controllers(FLCs)are designed and deployed in the DFIG-ES control system.Finally,case studies have been conducted to demonstrate the performance of the proposed control strategy via the comparison between different control strategies,and the simulation results illustrate that the coordinated control strategy can effectively improve the short-term frequency response capability of the DFIG-ES system under various wind speeds.3)On the level of long-term frequency response of the wind plant,a fuzzy-based coordinated operation and sizing strategy is proposed to realize the dynamic cooperation between the WTG and energy storage(ES)unit,and to improve the economic performance of the combined WTG-ES system for providing frequency reserve.Under the proposed control strategy,the WTG can operate at a closer rate to the maximum power point instead of maintaining a large power reserve level for frequency support.Firstly,through the proposed fuzzy controllers,the allocation factor of the frequency regulation demand can be determined,and WTG and ES can fulfill the regulation demand under the dynamic cooperation strategy.Afterwards,based on the proposed coordinated operation strategy,an optimization model is presented to optimally determine the de-loading level of the WTG in different time intervals and the rating power and energy capacity of the ES with the objective of minimum operation costs of the combined WTG-ES system.Finally,by using actual wind power generation and frequency data,case studies have been designed to validate the performance of the proposed control and sizing strategy.4)On the level of interconnected power system's AGC,in order to fully exploit the potential of wind turbines(WT)in the multi-area system for AGC support,based on the distributed model predictive control(DMPC)algorithm,a novel AGC model and control strategy for interconnected power grid with combined WT and energy storage(ES)system(CWES)is proposed.Firstly,the overall system is composed by several sub-areas,and local MPC controllers is designed for responding the AGC process in each sub-area by communicating the prediction and state measurement information with each other for better overall control performance.Then,the dynamic frequency response model of interconnected power system is introduced,in which the CWES model is integrated into the interconnected power system to take active participation in AGC as a part of the power system.Moreover,a fuzzy-based control strategy is designed for allocating the AGC regulation demand inside the CWES system dynamically.Finally,simulation and analysis results for a three-area interconnected power system with CWES system demonstrate apparent improvements on AGC performance,and the AGC potential of each subarea's wind plant are fully exploited by the designed CWES system. |
PDF Full Text Request