| New requirements such as large-scale grid connection of renewable energy and regional grid connection have promoted the development of VSC-HVDC(Voltage Source Converter based High Voltage Transmission)technology based on voltage source converters.Flexible DC transmission technology has control Features of flexibility and high resource utilization.At present,most of the domestic flexible DC transmission projects that have been put into operation or are under construction are double-ended flexible power grids,which can only realize point-to-point power transmission.When one end of the converter station exits operation due to a failure,the entire system will be shut down.As new energy has the characteristics of high dispersion and strong randomness,the new energy grid-connected system needs to involve multiple converter stations to transmit power.At present,the multi-terminal flexible DC VSC-MTDC(Voltage source converter based multi-terminal DC,multi-terminal flexible DC)The exploration of power transmission technology mainly focuses on the coordinated control of multiple converter stations.The main purpose of this thesis is to analyze the characteristics of coordinated control strategies by studying the operating principles of multi-terminal flexible DC transmission systems,and propose a fuzzy PID control based on the existing coordinated control strategies.The combined and coordinated control strategy is optimized to optimize the flexible switching of the grid-connected control strategy of wind power via the multi-terminal flexible DC transmission system,and improve the dynamic and steady-state operation characteristics of the system.The research content of this thesis mainly includes the following aspects:(1)Comparative analysis of the advantages and application scope of multi-terminal flexible DC transmission technology compared with traditional DC transmission technology,the current development status,application fields and topological structure of multi-terminal flexible DC transmission technology at home and abroad.The basic control strategies such as master-slave control,voltage deviation control and droop control of the VSC-MTDC system are analyzed by drawing operating characteristic curves and theoretical calculations.Using PSCAD/EMTDC,a four-terminal flexible DC transmission system is built for simulation verification and analysis.The simulation verifies The master-slave and droop control strategies have the defects of poor steady-state running rigidity and slow dynamic response speed.Combining the advantages of the above two control methods and the principle of DC voltage deviation control,a combined coordinated control strategy is derived.The control uses the master-slave control strategy in the steady state of the converter station to improve the accuracy of the transmission power;in the transient state,the droop control is used to improve the transient response speed,and the DC voltage deviation control is combined to achieve DC voltage tracking.(2)According to the principle of voltage deviation control,the master-slave-droop control scheme is derived.After that,the fuzzy PID control is combined on the basis of the strategy to realize the system voltage tracking and control strategy switching.The fuzzy PID controller parameters are designed,the voltage fluctuation rate is set,the upper and lower limit operating voltages of the controller are calculated,and the membership function is introduced to obtain the fuzzy The control rule table and the dynamic adjustment of the fuzzy control output parameters k_p、k_i、k_d、k_u in the three-dimensional space,through the tracking of the system voltage and voltage change rate and fuzzy calculation of the output system action parameters,a combined coordinated control strategy based on fuzzy PID control is obtained.(3)Use the simulation software PSCAD/EMTDC to build a5-terminal wind power grid-connected system via flexible DC transmission,and perform transient steady-state simulation analysis of the combined coordinated control strategy based on fuzzy PID control under three operating conditions.The simulation verifies that it is based on The combination and coordinated control strategy of fuzzy PID has good voltage rigidity,adaptive switching and transient recovery capabilities,and has good stability and reliability. |
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