Research On Low Frequency Oscillation And Its Control Strategy For Multi-machine System With Large-scale Photovoltaic Integration

As a static power generation unit,large scale photovoltaic access will have an impact on the damping characteristics of the system.Particularly,due to the lack of damping,low frequency oscillation occurs easily in link lines of large regional interconnected power grids.Therefore,this paper takes photovoltaic connected regional interconnection system as the research object,and analyzes in detail the influence of large-scale photovoltaic access on the low frequency oscillation characteristics of the system.Besides,the reactive power damping control strategy of photovoltaic directly participating in suppressing low frequency oscillation is studied.The main research contents are as follows:1)Based on the dynamic characteristics of photovoltaic power plants and synchronous generators,the mathematical models of single-stage photovoltaic power plants and synchronous generators are established.Based on this,the linearization model of multi-machine system before and after photovoltaic access is deduced in detail by using the linearization analysis method,and the influence of photovoltaic on the electromechanical oscillation of the system is preliminarily analyzed.2)Taking four-machine two-area system and 8-machine 36-bus system as examples,the effects of photovoltaic grid connection on local oscillation and inter-area oscillation of interconnected system are analyzed in detail by using the mode analysis method and TLS-ESPRIT method,aiming at the two research strategies of photovoltaic substitution synchronous units and photovoltaic direct access to power grid.The results show that under different research strategies,the grid-connected photovoltaic system will have varying degrees of impact on damping characteristics,and its impact is also related to grid-connected point and grid-connected capacity of photovoltaic.3)Based on the possible negative effects of photovoltaic access,the reactive power damping control strategy of photovoltaic system for low frequency oscillation suppression is studied.Firstly,the identification of reduced-order open-loop transfer function based on pseudo-random signal and ident toolbox of MATLAB is completed;Then,considering the observability and robustness of the signal,the wide-area feedback signal is selected based on the residue index and the residue ratio index.Then,The structure design of photovoltaic additional damping controller is completed with reference to PSS structure.And an optimal design method of photovoltaic damping controller based on artificial fish swarm algorithm is proposed.This method converts the controller parameter tuning problem considering multiple operating states into the objective function optimization problem based on the electromechanical oscillation eigenvalues of photovoltaic grid-connected system,which ensures the robustness of the controller.Finally,the simulation results based on the four-machine system show that the photovoltaic damping controller designed in this paper can quickly suppress the oscillation under different disturbances or operating modes.And compared with the controller based on single operating state,it has better control performance and robustness.