Research On Grid-Connected Control Strategy Of Energy Conversion System

Energy Conversion System(ECS)is an effective connection device between distributed power generation units and the power grid.Distributed power generation technology based on renewable energy is flourishing,playing a huge role in adapting to resource shortages and environmental challenges.ECS can directly convert the energy of power generation units into high-quality AC power and transmit it to the power grid,playing a significant role in the safe and stable operation of renewable energy generation units.ECS often uses pulse width modulation strategy in control to generate the main circuit driving signal,and the output voltage often contains rich switching harmonics.Therefore,it is necessary to select a suitable filter.There are a large number of background harmonics in the actual grid side voltage,which will affect the quality of ECS grid connected current;At the same time,due to the widespread distribution of grid connected equipment and the increase of its power level,the grid side impedance can affect the grid connection performance of ECS,and even cause its instability.After ECS is connected to the distribution network,it will also have an impact on the relay protection performance.Studying the grid connection control strategy and operation plan has important theoretical significance and practical value for improving the grid connection current quality,grid connection stability,and security of ECS.This article is based on a distributed power access architecture with a shared DC bus.Firstly,a two-level inverter bridge is selected as the main circuit topology of the ECS.In order to meet the harmonic limit requirements of the grid connected current,the LCL filter parameters are designed,and the system parameters of the ECS are determined.The mathematical model of ECS grid connection is established and the method of capacitor current proportional feedback is used to suppress the resonance spike of LCL filter.Through the analysis of system steady-state error,Loop gain and amplitude and phase margin,the limit values of capacitor current feedback coefficient and current regulator parameters are determined and the corresponding parameters of Current loop are designed.Then,the vector control strategy based on grid voltage orientation adopted by ECS was introduced,and the principle and algorithm implementation of the space vector pulse width modulation strategy used in its main circuit were introduced.In order to restrain the influence of grid voltage on grid connected current,the grid voltage feedforward strategy is added to the ECS Current loop control link,and the adaptability of this feedforward strategy to the changes of LCL filter parameters is analyzed.Subsequently,the sources and characteristics of harmonics at the grid connection point were analyzed,and a multi-resonance control strategy was introduced to address the issues caused by the full pass characteristics of the grid voltage feedforward strategy.This enabled the feedforward function to have frequency selection function and suppress specific harmonics from entering the control system.Analyze the system stability after introducing the multi-resonance control strategy,model the ECS output impedance and power grid impedance to obtain the system stability criteria,introduce the phase compensation strategy to improve the Phase margin of the system impedance ratio,and the above strategies constitute a composite control strategy.Subsequently,the impact of ECS integration into the distribution network on relay protection was analyzed,and the electrical characteristics of typical faults at the grid connection points were summarized.A fault detection scheme suitable for ECS integration into the distribution network was discussed and proposed.In response to the specific operating state of islands,unplanned island detection methods were studied,and an anti-island based ECS operation scheme was proposed.Finally,on the experimental platform,a control circuit with a DSP main control board as the core was adopted,and a software program was designed using the CCS 10.0platform.To verify the grid connection ability of ECS using composite control strategies,ECS grid connection experiments were conducted on the experimental platform.Build a simulation model to simulate non ideal operating conditions,and to verify the effectiveness of the composite control strategy under complex operating conditions,compare and analyze the grid connection experimental results before and after the introduction of each improved strategy.