Research On The Control Strategy Of Single-phase Photovoltaic Grid-connected Inverter In Weak Grid

With the shortage of traditional fossil energy resources and the environmental deterioration caused by large-scale use of them,new energy sources,such as solar energy,water energy,wind energy,and so on,have begun to receive more attention.Among them,the photovoltaic power generation has become the main form of solar energy utilization because of its convenience and integration.However,with the development of photovoltaic power generation technology and more grid-connected operation of photovoltaic power plants which are far away from the load center,the grid environment has become no longer ideal.In this weak grid,the grid impedance will increase the harmonic content of the grid-connected current and cause the generation of the peak resonance between the parameters,this will not only seriously affect the stability of the photovoltaic grid-connected system,but also cause sustained damage to the grid.Therefore,it is of great theoretical and practical value to study the control strategy of photovoltaic grid-connected inverter under the condition of weak grid,which can coordinate the operation between photovoltaic grid-connected system and grid.In this paper,the structure of photovoltaic grid-connected power generation system is modeled and analyzed firstly,photovoltaic cell and the maximum power point tracking that based on perturbation observation is studied emphatically,the operating principle of DC/AC inverter is analyzed and the parameter selection of the LCL filter is calculated.On the basis of this,the description of the weak grid is given,and the two forms of photovoltaic grid-connected in the weak grid are summarized,which is,single inverter and parallel inverter.In the single inverter that connected to grid,the grid impedance of the weak grid has some influence on the damping strategy of the inverter system.In this paper,the existence form of this kind of influence from the two damping strategies of passive damping and active damping is discussed,and the hybrid damping control strategy is adopted to avoid the negative influence of the grid impedance in this paper finally.At the same time,the proportional delay feedback integral control is also introduced in this paper.Based on the original proportional integral control,the PDFI adds a delay feedback circuit to achieve the control of the grid current without steady-state error.The results of the theoretical analysis and the simulation both show that the combination of these two control strategies has a great contribution to the control of the grid current and the steady operation of the system,and also has a good adaptability to the grid inductor.In addition,obvious peak resonance will be generated when parallel photovoltaic grid-connected inverters are connected to the weak grid.In this paper,the impact of the reference current of the grid-connected inverter,the grid voltage interference and the current disturbance between the photovoltaic inverters in parallel on the grid-connected inverters are analyzed,the grid impedance and the LCL filter of the photovoltaic inverter system are the key elements which lead to peak resonance is confirmed.On this foundatio,this paper presents the branch voltage and current double feedback suppression method,which effectively calms the peak resonance,weakens the harmonic content of the grid current of the photovoltaic inverter and the voltage that at the point of common coupling,improves the stability of the parallel operation of the photovoltaic grid-connected inverters in weak grid.At last,the simulation model is established to verify the reliability of this suppression method.