Simulation And Small Signal Stability Analysis Of A Grid-connected Photovoltaic System

With the depletion of traditional energy resources and the deterioration of environment, solar energy is getting more and more attention for its abundant reserves and cleanliness. As an important form of solar power, photovoltaic power generation has become a main research direction for both domestic and foreign scholars. In this paper, a three-phase grid-connected photovoltaic (PV) system is studied. The power generation principles and the mathematical model of photovoltaic array are introduced. The simulation based on matlab is conducted and the output characteristics of the PV array are researched. Ac-cording to the output characteristics, maximum power point tracking methods and the grid-connected control strategy are developed. Finally, the influence of the grid-connected PV system on the small signal stability of the whole system is discussed, providing a reference for the actual project. The detailed researches are as follows:(1)In this paper, the power generation principles and the electrical characteristics of photovoltaic array are introduced. Then a simulation of photovoltaic array based on the mathematical model is conducted. The influence of solar irradiation level and temperature on the output characteristics and volt-ampere characteristics of photovoltaic arrays are analyzed. The relationship betweenλ(the ratio of the maximum power point voltage and the open circuit voltage)and the solar irradiation level as well as temperature is discussed, the conclusion is:λis little affected by temperature while it is affected more by solar irradiation level. If the solar irradiation level get stronger,λwill get bigger, however, the variation range is not big.(2)As solar irradiation level and temperature changes will affect the power output of PV array, it is necessary to take some measures to track the maximum power. Constant voltage tracking method, perturb & observe algorithm and incremental conductance method are three kinds of typical maximum power tracking methods. The principles of these methods are introduced in this paper. Then simulation models of constant voltage tracking method and perturb & observe algorithm based on MATLAB are established. Combined with the PV array model, the simulations of these two methods are conducted. The simulation results are compared and discussed. The results show that if the reference voltage is selected appropriately the constant voltage tracking method can always maintain the output voltage of PV array in the vicinity of the maxi-mum power point regardless of the solar irradiation level and temperature changes. For the perturb & observe algorithm, if the tracking step is short enough, the tracking accuracy will be ideal, but with more iteration. In order to track the output of the PV array quickly and accurately, these two methods are combined to locate the maximum power point: The constant voltage tracking method is used to provide an initial value. The perturb & observe algorithm is used to locate the maximum power point whose tracking step can be set relati(vel3y) shAo r tt.y pical mathematical model of three-phase grid-connected PV system consisting of a PV system, a distribution network and a resistance-inductance load, a rectifier load is described in this paper. For convenient analysis and control, the reference coordinate system is transformed from abc coordinate system to dq coordinate system that rotating synchronized with the former using Park's Transformation. The control strategy based on the dq coordinate system is proposed which includes phase-locked loop, voltage control scheme and current control scheme. Simulation for the control schemes are conducted and the results show that the strategies can meet the control requirements: The phase-locked loop can track the grid phase angle changes in time so that the dq coordinate system can keep pace with the abc coordinate system. The current controller can quickly detect the change of the reference value, and make timely and effective adjustments. The output of the voltage controller has slight fluctuation, however, it can become stable immediately.(4)Eigenvalue analysis of the linear state matrix to the whole system is performed and the results verify the small signal stability of the system because all eigenvalues of systems have negative real parts. Although there is a pair of eigenvalues whose oscillation frequency is in the range of low frequency oscillation, its oscillation modal still has good damping characteristics taking into account its relatively large damping ratio.(5)To determine the relationship between the state variables and the modal, the participation matrix of the system is also analyzed in this paper. It is shown that the influence of the state variables on the system's eigenvalues is different, however, there still exists some regularity: The modals of the PV system, distribution network and the load are mainly related to their own control methods and parameter settings, with little influence on each other. Though there is one state variable of the distribution network which has significant impact on the PV system modal, it will not make the system become unstable, for the real part of this eigenvalue is far away from the imaginary axis.(6)The changing trend of the eigenvalue real part of the system is also analyzed under different parameter settings including the normalized load distance from the PV system, length of distribution lines and ratio between X and R. It is shown that the grid-connected PV system dynamics are not influ-enced by those of the network.