Research On Low-voltage Ride-through Technology Of Photovoltaic System Based On Virtual Synchronous Generator

With the increasing penetration rate of new energy sources,the lack of inertia and damping of power systems has attracted the attention of scholars.To this end,virtual synchronous generator that simulates synchronous motors to provide inertia and damping to the power system is proposed.When the PV installed capacity reaches a certain level,the instantaneous off-grid of PV under grid fault will have a great impact on the large grid.Therefore,Photovoltaic power generation needs to have low voltage ride through capability.In this context,this paper studies the low voltage ride through capability of a photovoltaic system based on virtual synchronous generator control.Firstly,the mathematical model of photovoltaic cell suitable for engineering was established,and the model was built by Matlab/Simulink.The analysis of its output characteristics verified the correctness of the model.Then,the mathematical model of the second-order virtual synchronous generator is established and a third-order small-signal model including power coupling is established by the characteristics of its power loop and transmission line transmission power.The root trajectory is used to analyze the influence of the main parameters on the system characteristics.It is concluded that the low-frequency oscillation occurs when the inertia coefficient and the active droop coefficient are improperly selected.The larger the inertia coefficient is,the smaller the system damping is.The larger the droop coefficient is,the larger the system damping is.The model of virtual synchronous machine is built in Matlab/Simulink,and the correctness of the conclusion is verified by simulation.In order to adapt to the process of low voltage ride through,it is necessary to improve the model of the existing virtual synchronous generator.Among them,it is necessary to block the reactive sagging link during the low voltage ride-through,and use the Notch filter to eliminate the double power frequency oscillation when the asymmetric fault occurs.Through analysis,the two targets of the inverter output current symmetry and active power without oscillation can not be realized at the same time.When the three-phase current outputted by the inverter is symmetrical for control purposes,the amplitude of the oscillation of the output power of the inverter and the reactive power are the same,the amplitude of the oscillation is related to the average value of the active and reactive power of the inverter output and the voltage imbalance.When the capacity of the inverter is large and the voltage imbalance of the grid-connected point is high,it is necessary to suppress the oscillation of the power frequency twice the power frequency.For this reason,an active and reactive power coordination control strategy is proposed to limit the maximum phase amplitude of the inverter output current,which can ensure that the steady state value of the current is within the limit range and the active power of the inverter output is not oscillating.The simulation model is built in Matlab/Simulink,and the correctness of the proposed control strategy is verified by simulation.Finally,Solutions is proposed based on two problems in the process of low voltage ride through.The two problems are that the Instantaneous current of the inverter is too high,and the photovoltaic output energy is too large when the grid voltage drops.The solution is as follows,1)Analyze the characteristic that the armature reaction of the synchronous generator has negative feedback to the short-circuit current,that is,the armature reaction has a restraining effect on the sudden change of the Instantaneous current,and the armature reaction is transplanted into the model of the virtual synchronous generator.And the principle of selection of the main parameters is given.2)Improved disturbance observation method makes the PV change from maximum power tracking to limited power tracking during low voltage crossing to balance the energy of the DC bus and keep the bus voltage constant.The simulation model is built in Matlab/Simulink.The simulation of the suppression of Instantaneous current by the armature reaction and the improvement of the DC bus power balance by the improved disturbance observation method are verified by simulation.