Research On Stability Control Strategy Of Power Converter System Under Non-Ideal Grid Condition

The promotion of distributed photovoltaic and wind power generation is an important means to achieve the goal of "carbon peaking and carbon neutrality".Since photovoltaic and wind power are often uncontrollable and intermittent,energy storage units with corresponding capacity should be configured for distributed photovoltaic and wind power generation.Energy storage units should be used to smooth the power fluctuations of distributed photovoltaic and wind power generation,so as to reduce the scheduling pressure of power system.Increase the penetration rate of renewable energy grids.Large-scale access of distributed renewable energy to the grid will lead to the weakening of grid support capacity,and it is easy to appear non-ideal grid conditions.Such conditions will affect the.performance of Power Converter(PCS),a key unit in the energy storage System.Therefore,in the case of non-ideal grid,The control strategy of energy storage converter is of great significance to improve the grid-connected performance of renewable energy.Taking LCL PCS as the research object,this paper proposes a composite control strategy based on active disturbance rejection current control and improved frequency lock loop with specified harmonic elimination to improve the stability and disturbance immunity of PCS in non-ideal power network conditions.The main research content of this paper includes the following four aspects:Ⅰ.Research on the Working Principle and Mathematical Model of PCSThe topological structure of PCS was determined,and the mathematical model of PCS in the three-phase static coordinate system was established.Through coordinate transformation,the mathematical model in the complex frequency domain in the dq coordinate system was obtained.PCS is a high-order,nonlinear and strongly coupled multi-variable system,which is difficult to be well controlled by traditional control strategies under the condition of non-ideal power grid.At the same time,the influence of internal and external disturbances on the performance of PCS system is determined,which provides a basis for the design of control strategy.Ⅱ.Research on PCS Control Strategy Based on Active Disturbance Rejection ControlTo solve the problem of system instability and performance degradation of traditional PCS control strategy under non-ideal network conditions,a PCS current loop control strategy based on active disturbance rejection control was proposed.The internal current loop coupling channel and the inherent resonance characteristics of the system are taken as internal disturbances,and the equivalent impedance and voltage fluctuation of the grid are taken as external disturbances.The extended state observer was designed to observe internal and external disturbances,and the state error feedback control rate was used to offset internal and external disturbances in the control loop,which improved the disturbance immunity performance of PCS system in non-ideal power grid conditions.Ⅲ.Research on Improving Strategy of Phase Locked Loop in Non-ideal Power GridSynchronous Reference Frame phase-locked Loop(SRF-PLL)is difficult to accurately obtain the voltage Phase and frequency information under non-ideal grid conditions.A second order generalized product decomposition coupled frequency locking ring with specific subharmonic cancellation function is proposed.The frequency lock ring can effectively eliminate the low frequency harmonics in the harmonic network through the pre-harmonic elimination module,and improve the accuracy of obtaining the frequency and phase information in the harmonic network.In order to eliminate the influence of coupling of positive and negative sequence components on output power,the positive and negative sequence components are controlled separately by separating them.At the same time,because the frequency of the adaptive filter is provided by the frequency locking ring,the frequency locking ring depends on the frequency feedback,which can realize the frequency self-adaptation to the input signal.Compared with the traditional phase locked loop,the frequency information of the power network is more stable than the phase information,so the frequency locked loop is more stable than the phase locked loop,and the frequency oscillation of the dynamic process is smaller and the smoothness is higher.Ⅳ Real-time Simulation Platform Verification Based on RT-LABThe construction and design of the simulation platform of hardware-in-theloop experiment based on RT-LAB are elaborated in detail.In the RT-LAB realtime simulation platform,the power parts of the non-ideal power grid and the energy storage converter were built,and the external actual controller and related signal conditioning circuit were designed,and a complete experimental platform of the energy storage converter hardware in the loop under the condition of nonideal power grid was built.The correctness and effectiveness of the proposed control strategy are verified by the platform.