Research On The Control Strategies Of Grid-connected Photovoltaic System With Properties Of Virtual Synchronous Machine

The development of grid-connected photovoltaic technology can effectively ease the problems like environmental pollution and energy crisis caused by traditional power generation style.But the stability and security of grid faced more and more serious influence with the increasing penetration of photovoltaic power generation system.By simulating the mechanical properties and external characteristic of the synchronous generator,virtual synchronous generation(VSG)technology can effectively increase controllability of photovoltaic power generation,making it effectively participated in the grid's dispatching operation and the ease of load fluctuations.Previous research of virtual synchronous generation technology always use the ideal DC voltage source or optical storage systems to provide the power needed by the inverter,ignoring the DC/DC converter's control on the output power of photovoltaic cells.With the increasing penetration of photovoltaic generation,it's no need for the photovoltaic system working in fully station at any time.In order to guarantee the stability of the grid and balance between power supply and demand,photovoltaic generation can reduce the output power and keep some reserve capacity.A novel virtual synchronous generators control strategy which is suitable for photovoltaic system has been introduced.It achieved variable power point tracking(VPPT)of photovoltaic cells by the improvement of maximum power point tracking process of PV system,giving full use of the photovoltaic battery output power control function of the DC/DC converter under the premise of without additional energy storage devices.It makes accurate implementation of energy supply that virtual synchronous machine needed for imitating inertia and regulation.Combining with inverter control strategy,photovoltaic virtual synchronous generator co ntrol is realized.This paper has also presented a novel low voltage ride through(LVRT)control strategy of the two-stage three-phase photovoltaic system with variable power point tracking.With the new strategy,the PV array can generate as much active power as possible when a low voltage fault occurs at the point of common coupling according to the depth of the grid voltage dip,which can keep power balance between the two sides of the inverter to avoid the sudden increase of dc bus voltage and over current of inverter without changing the original configuration or adding any additional device,helping to realize low-voltage ride through.The correctness of the proposed control strategies have been verified on the MATLAB/Simulink platform.