Research On Low Voltage Ride-through Characteristics For Photovoltaic Power System Based On Reachability Analysis Theory

In recent years,as a typical distributed generation in the field of new energy,the installed capacity of photovoltaic power system is increasing,and the impact of photovoltaic power system into the grid can not be ignored,so its ability to deal with when grid voltage drops.The traditional research improves the low voltage ride through(LVRT)capability of photovoltaic generation system by means of hardware,software,and the combination of hardware and software.And the existing power electronic equipment and control system can be used to test the low voltage ride through capability of photovoltaic power generation system for a set of finitely possible voltage fault waveforms,but to predict LVRT characteristics of the photovoltaic power generation system under all possible fault waveforms,the test system will be over designed and difficult to implement.It is very difficult to fully analyze low voltage ride through characteristics of photovoltaic power generation system using existing methods.This paper introduces the reachability analysis theory.As a predictive estimation method,the reachability analysis predicts not only the change of the system dynamic variables when the initial value of a certain point is given for a certain period of time,but also all possible situations of system variables under the influence of the input variable interval and disturbance variable interval,when a certain initial value interval is given for a given period of time.Therefore,this paper proposes a set class method based on the reachability analysis theory,aimed at finding out the set of all possible trajectories of photovoltaic power system state variables under a given collection of uncertain but bounded voltage disturbance,and test whether the voltage and current of the photovoltaic power system are within the tolerable range,so that the low voltage ride through characteristics of photovoltaic power system can be analyzed comprehensively.Firstly,the conventional dynamic model of photovoltaic power system has been established to understand the basic low voltage ride through characteristics of photovoltaic power generation,lay the foundation for the establishment of dynamic set model of photovoltaic power system,and provide the initial parameters for subsequent simulations.Then,the paper has described the reachability analysis theory,the reachable set representation method and the reachable set calculation principle.It is considering that the exact reachable set of nonlinear systems can not be solved,on the basis of solving the reachable set of linear system,the nonlinear system is linearized and the linearization error is taken as the uncertain input variable,this paper has summarized the solution principle of nonlinear system reachable set.Finally,the dynamic set model of photovoltaic power system has been established,and its reachability analysis procedure is summarized on the basis of the solution principle of the nonlinear system reachable set.The simulation results presented that all the possible trajectories of the system-related state variables are obtained when the grid voltage drops,and the low voltage ride through characteristics of photovoltaic power generation system can be analyzed according to the state variable trajectories.