Research On Distributed Photovoltaic Power Plants Based On Microgrid

With the increasingly serious energy shortage,new energy power generation has got more and more favor of people.The distributed photovoltaic power station based on micro-grid contains photovoltaic power generation and energy storage unit,which can efficiently solve the problem of voltage fluctuation due to the randomness of photovoltaic power generation and improve the grid-connected power quality.In this paper,the security and stability of the distributed photovoltaic power station system based on micro-grid is studied at both the unit-level and the system-level.First,the operational principle of photovoltaic module is analyzed.The input and output characteristics of the photovoltaic cells are verified by Simulink simulation.The maximum power point tracking control strategy is introduced.The photovoltaic controller platform based on the Boost conversion circuit is set up.The performance of the control strategy is validated by simulation and experiments.The simulative and experimental results show that the control strategy can realize the function of photovoltaic controller quickly and effectively.Second,the battery charging and discharging control system is studied.The DC/DC bidirectional converter is adopted as the battery charging and discharging controller which mathematical model is established.In order to realize the energy bi-directional transformation of the battery and maintain the DC bus voltage stability,the voltage/current double loop control strategy is employed.The simulation and experiment verifies that the converter can reduce the DC bus voltage fluctuations by storing surplus power to battery in the case that the photovoltaic power output is redundant.And when the photovoltaic energy is shortage,the battery is controlled to discharge for maintaining the power balance.Based on the control strategies mentioned above,the stability of the DC bus voltage and the reliability of the system operation are improved.Third,it would cause the grid impedance to change when the distributed photovoltaic power generation system is connected into the grid,which could affect the operation stability of the grid-connected inverter seriously.In order to improve the ability of the inverter to deal with the variation of grid impedance,a two-degree-freedom control strategy with grid impedance detection is proposed in this thesis.A model of grid-connected inverter with grid impedance is built to explore the influence on the stability of the grid system which is caused by the impedance mismatch due to the grid impedance changes.A two-degree-freedom control strategy adjusting tracking and anti-interference control separately is proposed,both thetracking controller and anti-interference controller are designed,the uncharacteristic harmonic injection method is adopted to detect the grid impedance in real time.The complete two-degree-freedom control scheme with grid impedance detection is given,its tracking controller and active disturbances rejection controller are designed.The Matlab/Simulink simulation and experiment platform are built.The results show that the proposed two-degree-freedom control strategy has the ability of rapid current tracking and grid impedance disturbance resistance under different grid impedance conditions,which can enhance the stability of the grid-connected inverter effectively.Finally,the 1.5kW distributed photovoltaic power station test platform is established through combining the photovoltaic module,the energy storage unit with the grid inverter.In view of the operation mode of off-grid or grid-connected,the coordinated control strategy of the system is studied.In the off-grid mode,the voltage and frequency of the system are adjusted through the inverter.Photovoltaic power generation units work in maximum power point tracking mode or the limit of power mode according to the power change.The balance of power compensation is finished with battery charging and discharging.In the grid-connected mode,the voltage and frequency of the system are maintained by the grid,the photovoltaic power generation units maintain maximum power output,while the DC/DC bidirectional converter maintains the stability of the DC bus voltage and brings down the photovoltaic power fluctuations.