Research On VSC-HVDC Transmission Participating In Frequency Stability Control Of Self-provided Power Plant

With the continuous development of the power industry,more and more energy-intensive enterprises are building independent self-provider power plants to provide electricity for internal power consumption.Due to the short of circuit ratio and the moment of inertia of the self-provided power plant,when the system is connected to the impact load,the system will cause serious power imbalance,and the frequency of the generator set will fluctuate greatly,which seriously threatens the system frequency stability and equipment safety.The primary frequency modulation system of the generator set operates at this time.Since the primary frequency modulation function of the generator set has a maximum power limit,the primary frequency modulation will lose its effect when the load changes exceed the power modulation range.The Voltage Source Converter based High Voltage Direct Current(VSC-HVDC)can realize fast and independent adjustment of active power and reactive power,improve the frequency and voltage stability of the AC system,which can be used to solve the problem of frequency fluctuation in selfprovided power plant.The traditional VSC-HVDC system usually focuses on asynchronous isolation in design process,and delivers power according to a given power value during operation.When the AC power grid fails,it can prevent the expansion of the fault area,but the frequency support function of the two systems is also limited.In order to give full play to the rapid adjustment of active and reactive power of VSC-HVDC and enhance the frequency stability of the AC system,three additional frequency controllers based on the basic control of VSCHVDC are improved and designed in this paper,The main research contents are as follows:(1)The characteristics,application and research status of VSC-HVDC are introduced.By analyzing the characteristics of load frequency and generator frequency of the power supply system of the self-provided power plant,the causes of frequency fluctuation of the self-provided power plant are explored.After that,the two frequency modulation methods of primary frequency modulation and emergency power control of thermal power unit are introduced,and the application and deficiency of each method are analyzed.Finally,the primary frequency modulation system,steam turbine unit and electro-hydraulic servo system of thermal power unit are simplified and modeled,and the transfer function of thethermal power unit frequency modulation system is constructed,which lays a foundation for building a self-provided power plant simulation model.(2)The principle,structure and basic control strategy of the VSC-HVDC transmission system are introduced.In order to maintain the frequency stability of the self-provided power plant,a control method for adding additional frequency controller is proposed.Then,based on the vector control of the VSC-HVDC transmission system,the basic control strategy of the VSC-HVDC transmission connected to the power supply system of the self-supplied power plant is designed,and the proportional additional frequency controller,the proportional integral additional frequency controller and improved additional frequency controller are designed respectively on the basis of the fixed-powered power controller.Finally,the parameters of the three additional frequency controllers are designed by root trajectory analysis method,and the influence on system stability of parameters of each controller and the self-provided power plant frequency modulation system are analyzed.(3)Using PSCAD/EMTDC simulation software to build a model of selfsupplied power plant connected to the main network through the VSC-HVDC transmission system,and set up three different cases for simulation analysis.The results show that the three additional frequency controls can adjust the frequency of the self-supplied power supply system when the load of the self-supplied power plant suddenly increases or suddenly cuts off,reduce the impact of high capacity impact load access on VSC-HVDC transmission system and generator sets,and the improved proportional integral additional frequency controller has the best control effect.