Research On Key Technology Of Current Source Photovoltaic Grid Connected Inverter

With the advancement of China’s industrialization,the contradiction between production life and energy shortage is becoming increasingly acute.Traditional fossil energy is not renewable and has serious environmental pollution.In China’s future energy structure,solar energy plays a really important role.As the interface for photovoltaic cells to the grid and the electricity loads,grid connected inverter for photovoltaic is an important part of photovoltaic power generation system.And research of the grid connected inverter for photovoltaic has important theoretical significance and application value.The current source photovoltaic grid-connected inverter has the boost capability,which can reduce the boost link in the voltage source inverter,as well as the number of photovoltaic modules in series,and increase the reliability of the system.At the same time,the current source photovoltaic grid-connected inverter is also more suitable for the use of photovoltaic modules from weak light to strong light.In this thesis,a three-phase current source photovoltaic grid-connected inverter is comsidered as the research object,and the key technologies for its application are studied.The mathematical model of the three-phase current source inverter is established in the three-phase static coordinate system,the two-phase stationary system and the two-phase synchronous rotary coordinate system.On this basis,in order to study low voltage ride through control strategy when asymmetric three-phase grid voltage saging occurs,the mathematical model of three phase current source inverter under the condition of unbalanced power grid is established according to the theory of instantaneous power.The current type inverter type three value logic switch state and current space vector modulation strategy are introduced in this thesis,and the AC CL filter and DC energy storage inductor for current source inverter are designed.Then the method of damping CL filters is studied.The maximum power point tracking control strategy and the three-phase current inverter closed-loop control strategy are studied,and the parameters of the inner loop controller are designed.The current source inverter modulation process need to set up drive signal overlap time to prevent high induced electromotive voltage on DC inductor caused by discontinuous DC current,which is harmful for power switch device.However,in the commutation process of the inverter,the overlap time will cause the dwell time loss of the vector,causing the deviation of the synthetic vector,and introducing the extra low harmonics in the output current of the inverter.Vector deviation is analyzed in a fundamental wave cycle.It is concluded that the deviation vector equivalent to a stack in the waveform of the output current.The quantitative relationship of harmonic content and overlap time is determined and put forward a overlap time compensation method,which can effectively compensate extra low harmonics introduced by the overlap time.Simulation and experiment verify the effectiveness of the proposed method.The three-phase current source grid-connected inverter is a kind of booster grid-connected inverter.In order to control the DC current effectively,the line-line AC voltage must be higher than the DC side voltage.However,when the voltage of the grid sags seriously,the line-line AC voltage may be lower than the DC voltage,causing the DC current to be out of control.In this thesis,the controllable steady-state workspace of DC current in the three-phase current source grid-connected inverter is derived,and a modified three-phase current source grid-connected inverter topology and corresponding low voltage ride though control strategy are proposed.The simulation and experiment of voltage symmetry saging and asymmetric saging of three-phase grid are carried out to verify the effectiveness of the low voltage ride though scheme.