| After large-scale photovoltaic power is integrated into the power grid,the frequency response ability of the power system is weakened.Virtual Synchronous Generator(VSG)technology has attracted much attention to simulate the inertia and primary frequency and voltage regulation characteristics of synchronous generators.The application of VSG technology in photovoltaic grid-connected system will improve the dynamic frequency response ability of the system.However,the traditional photovoltaic virtual synchronous Generator(PV-VSG)technology needs to rely on external energy storage systems.The realization of energy storage system will greatly increase the construction cost of photovoltaic grid-connected system.In view of this,this thesis takes the application of VSG technology in the photovoltaic grid-connected system as the research target,and takes the bipolar photovoltaic grid-connected system as the research object.Starting from the characteristics of the photovoltaic system(components),this thesis puts forward the PV-VSG control technology taking into account the active reserve,so as to realize the independent participation of the photovoltaic system in the grid frequency response without additional energy storage system.The main research contents are as follows:(1)The necessity of photovoltaic system participating in grid frequency response is analyzed.The advantages and disadvantages of traditional grid-connected inverter control method are briefly introduced.The research status of VSG is elaborated in detail.The existing control methods of PV-VSG are summarized.The research objectives of this thesis are determined by analyzing the shortcomings of traditional methods.(2)The basic principle and implementation process of VSG control algorithm are introduced.The physical essence of the moment of inertia is elaborated in detail.The corresponding simulation experiments are carried out to verify the inertia response and frequency modulation characteristics of VSG.Then,the principle and implementation process of two commonly used energy storage PV-VSG structures are compared.Aiming at the insufficient utilization of photovoltaic reserve power in the case of abandoning light and limiting electricity,a control method of deep utilization of photovoltaic reserve power is proposed.By using this part of reserve power,the start-up times of energy storage system can be effectively reduced,and the frequency modulation ability of the system can be increased.(3)Considering the poor adjustability of photovoltaic output power under the action of Maximum Power Point Tracking(MPPT),the problem that active reservecan not be realized without energy storage system,the control idea of photovoltaic power reserve control operation is put forward by analyzing the output characteristic curves of P-V and I-V of photovoltaic system,so as to realize active power self-reserved of photovoltaic system.Considering that the current maximum output power is unknown,a method of maximum photovoltaic power estimation based on Gauss-Seidel(G-S)iteration and Lambert W Function is proposed.Finally,by coupling the active reserve change of photovoltaic system with the frequency change of power grid,a control method of virtual governor of photovoltaic system is proposed.When the grid frequency changes,the photovoltaic system can adjust the active output according to its active reserve situation to participate in the frequency regulation of power grid.At the same time,the main factors affecting the reserve power capacity of photovoltaic system are analyzed in detail,and the applicability of the control method is analyzed.In order to verify the effectiveness of the proposed control method,a corresponding simulation model is established in Matlab/Simulink,and corresponding experiments are carried out under different working conditions.The experimental results show that the photovoltaic system can automatically adjust the output to participate in the system frequency regulation according to the system frequency change. |
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