Research And Analysis On Stable Operation Control For PV Grid-Connected Inverter

In grid-connected photovoltaic(PV)power system,the grid-connected voltage source inverter(VSI)not only affects the system output power quality and operation efficiency,but also relates to the safe and stable operation of the power generation system as an interface connecting photovoltaic power sources and power systems.In this dissertation,some control strategies related to the grid-connected VSI stable operation are studied in detail,and the main work can be summarized as follows:First of all,the output characteristics of photovoltaic panels and the principles of MPPT algorithms are introduced,and the most commonly used one of them is introduced,that is,the fixed step disturbance observation method.The models of grid connected VSI is established,including the general model described by switch function,DQ model in synchronous rotating coordinate system and the improved linear model.According to the models and the ideas of feedback plus feedforward,the realization process of vector current control(VCC)is described in detail.The open-loop transfer function and closed-loop transfer function of the system are derived,and the influences of control parameters on the frequency characteristics and bandwidth of the system is analyzed.The effectiveness of VCC is verified by results of simulation,and the control performance is evaluated from the dynamic and static aspects such as overshoot and regulation time.Then,the essential differences between current control and power control are discussed,and how to define instantaneous power is described,and its calculation methods are discussed.The model of the VSI is established,and a nonlinear disturbance observer(NDO)based direct power control(DPC)scheme is proposed.The output power is observed in real time by using the NDO without additional sensors,and it is added to the reference of power closed-loop as power feed-forward,which facilitates the response performance to input disturbance power.For power inner loop,the control input is redefined to avoid the synchronous rotating coordinate transformation.Details on the design of the observer and the power inner loop are provided,meanwhile,the parameter tuning methods of inner and outer loop are given in this dissertation.the correctness and effectiveness of the proposed scheme are verified by simulations.Finally,Considering that the high impedance characteristic of the weak grid is easy to cause complex resonance problems,which leads to the distortion of the grid voltage waveform and affects the safe and stable operation of the PV generation systems,the stability of the grid connected photovoltaic inverter in weak grid is analyzed.This dissertation discusses the general interpretation of impedance analysis methods,and takes the inverter as an example to illustrate the applications of this methods in the electric circuit.The PV system is divided into several parts,and the influences of each part on the inverter output impedance is discussed.Then the output impedance model of the system is established.The frequency characteristics of the output impedance with phase-locked-loop(PLL),current and voltage control are given respectively,and the stability of the system is analyzed according to the results of frequency characteristics analysis and the Nyquist stability criterion.