Transient Stability Analysis And Enhancement Control Strategy Of Grid-connected Inverter During Grid Faults

In recent years,renewable energy generation with grid-connected inverter(GCI)as grid interfaces for wind turbines(WTs)and photovoltaics(PV)has been rapidly developed.In order to ensure the stable operation of the power system,many countries have issued grid codes,which requiring the grid-connected power electronic equipment to have capabilities for low voltage ride-through(LVRT).Due to the centralized development and long-distance transmission of renewable energy generation,the interaction between the GCI’s output currents and the transmission line’s impedance becomes more and more intense,which would affect the voltage of point of common coupling(PCC).As a result,during the severe grid faults,the GCI would lose synchronization with the grid and could not successfully achieve LVRT.The disconnection of new energy power generation equipment would further aggravate the active power shortage of the grid,which would seriously threaten the safety of the power system.However,there is lack of detail research about the transient lose synchronization mechanism and improvement control strategies for the GCI.Consequently,this paper studies the transient synchronization characteristics and the instability mechanism for the GCI under the symmetrical short-circuit grid faults.On this basis,the improved transient stability control strategies of GCI are studied,thus significantly improve the operate ability of GCI during LVRT.Firstly,this paper introduces the basic operating principle and mathematical model of the grid-connected inverter system.The control strategies of GCI system are analyzed during LVRT and the normal grid period.In addition,A time-domain simulation model of the GCI is established in Matlab/Simulink,which verified the basic operating characteristics of the GCI.Secondly,the transient stability of the GCI is studied during LVRT.The mathematical model of GCI suitable for transient stability analysis is established.On this basis,the frequency synchronization equation of the GCI is derived,which reveals the mechanism of transient lose synchronization of the GCI during LVRT.Furthermore,based on the expression of the equivalent inertia,damping coefficient and damping ratio,the GCI’s transient response characteristics during the grid faults are studied.The research results show that the grid voltage sag would cause the damping ratio of the GCI to decrease,thus deteriorating the transient synchronization characteristics of the GCI,and even cause transient instability phenomenon of the GCI system.Furthermore,in order to improve the transient stability of the GCI system during LVRT,an enhanced control strategy of the GCI system based on an improved phase-locked loop(PLL)is proposed in this paper.The basic principles of the proposed improved control strategy are described in detail.What’s more,the different transient stability strategies of the GCI are compared,and the effectiveness of the proposed control strategy is verified by simulation.Finally,an experimental platform for the GCI system is built,and the GCI’s transient synchronization behavior are studied through experiments.The experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed improved transient stability control strategy of the GCI.The research work of this paper has laid a good foundation for improving the transient operation capability of the large-scale grid-connected renewable energy generation systems under grid short-circuit faults.