Research On Control Method Of VSC-HVDC System Converter Station Participating In Grid Frequency Regulation

VSC-HVDC technology is an important technical means of improving the ability of renewable energy integration.Compared with the traditional AC transmission mode,the VSC-HVDC system can effectively isolate the negative influence of the fluctuating renewable energy on the operation stability of the traditional AC grid.But on the other hand,the conventional outer loop constant power vector control strategy adopted by VSC-HVDC system makes the converter stations at the sending end and the receiving end behave as non-inertia power supply.It is impossible to realize the mutual support of the frequency between the sending end and the receiving end.At the same time,a large number of renewable energy plants using power electronic conversion unit reduces the inertia of the power system and further deteriorates the frequency stability of the systemt.This dissertation,based on the VSC-HVDC system,and the traditional control strategy,applies the virtual synchronous generator and the inertia emulation control strategy to improve the power system frequency support ability,so as to enhance the ability of integrating fluctuating renewable energy sources.This dissertation takes the two terminal VSC-HVDC system as the research object.The mathematical model of converter station is established,the theory of vector control is analyzed,and the model of VSC-HVDC system with synchronous machines at both ends to simulate the characteristics of power grid is built.In view of the problem that the system at both ends of the HVDC system is isolated by constant power operation mode,so that the sending end cannot provide frequency support to the receiving end system,this dissertation puts forward the specific control strategy of the receiving end converter station to realize the operation mode of the virtual synchronous generator.Based on the physical operation characteristics of the synchronous generator,the ontology model,the governor model and the excitation controller model of the virtual synchronous generator are designed.By deducing the small signal model of the control strategy of virtual synchronous generator,the specific design and calculation method of the moment of inertia and damping coefficient involved in the control strategy are obtained.By building the simulation model based on MATLAB/Simulink platform,the dissertation verifies that the proposed control strategy of virtual synchronous generator used in converter station has the control effect of enhancing the system inertia and damping.During the period of frequency support,the converter station has the dynamic characteristics similar to the synchronous machine and the sending end system has the dynamic characteristics of frequency support for the receiving end system.Furthermore,in view of the lack of operational inertia in the power system,this dissertation proposes a suitable inertia emulation control strategy for the VSC-HVDC system.By analyzing the mechanical inertia time constant and capacitance time constant,this dissertation studies the analogy between the mechanical energy storage of synchronous generator rotor and the capacitance energy storage of HVDC system.On the basis of considering the capacitance,the control strategy of inertia emulation is designed and the selection method of control parameters is proposed.Finally,the dissertation proposes a joint control strategy of the inertia emulation of the converter station at the sending end and the virtual synchronous generator of the converter station at the receiving end,which supports the system frequency together with the sending end and the receiving end.The simulation results show that the joint control strategy can increase the inertia of AC system and enhance the frequency support capability of the whole system.Through the work of this dissertation,the technical feasibility and specific implementation strategy of improving the dynamic performance of frequency support of VSC-HVDC system are studied.It expands the operation mode and technical utility of VSC-HVDC system and further enhances the ability of the power system integrating the fluctuating renewable energy sources through VSC-HVDC technology.