Research On Virtual Synchronous Control Technology Of Grid-connected Inverter

Synchronous generators constitute the stable and easy to manage power system due to its two inherent characteristics of rotary inertia and rotary damping.However,with the rise of clean energy and micro grid in recent years,a large number of distributed power supplies with inverter as grid-connected interface are connected to the grid.With the increase of the permeability of the distributed power supply in the grid,the capacity of the traditional synchronous generator decreases year by year,and the inertia and damping characteristics in the power system are gradually replaced.In the foreseeable future,the mature and stable power system will be seriously threatened by low inertia and low damping.As a grid-connected inverter control mode which simulates the external characteristics of synchronous generator,virtual synchronous generator(VSG)becomes a new direction of grid-connected renewable energy.In this thesis,a virtual synchronous generator control scheme is designed based on the analysis of the inverter internal control structure and grid-connected principle,from the virtual rotor,virtual governor and virtual exciter three aspects of research,respectively.Through the comparison and analysis of virtual synchronous generator and droop control,the significance of its parameters in power system and the detailed principle which is better than droop control are obtained.In order to solve the problem that the practical renewable energy is connected to the network through the virtual synchronous generator,this paper then analyzes the difference between the inertia support of the virtual synchronous generator and the positioning of the primary frequency modulation from the perspective of energy supply and demand,and introduces the DC side structure of the virtual synchronous generator suitable for photovoltaic(PV)power supply.Then combined with the dynamic characteristics of photovoltaic power supply,a photovoltaic virtual synchronous generator(PV-VSG)control strategy is proposed.In the stability analysis of the original virtual synchronous generator and the photovoltaic virtual synchronous generator it is concluded that the added DC side additional control reduces the system stability and may cause serious power oscillation.For this reason,an adaptive control structure is added to improve the virtual rotor,and the inertia coefficient can be changed automatically according to the running state.The simulation verifies that the proposed control strategy can connect the photovoltaic power supply to the grid stably and friendly.Then the virtual synchronous generator experiment platform which can observe and control parameters online is built.The overall control scheme based on LabVIEW upper computer and DSP processor lower computer is adopted,and data acquisition and communication between upper and lower computer are realized by NI data acquisition card and high-speed serial port respectively.In this paper,the inverter control flow of the lower computer,the software design of the upper computer and the control scheme of the serial port are introduced respectively.In the experimental verification of the virtual synchronous generator control scheme,the experimental platform is verified to be intuitive and easy to use and the virtual synchronous generator control strategy is realized.