Research On Resonance Mechanism And Suppression Methods For Inverter-Grid Interaction System

As renewable energy distributed power generation units continue to penetrate into the grid,the trend of power electronic power systems is accelerating.Grid-connected inverters are the main power electronic grid-connected devices,The grid-connected control methods are different,but they are divided into two categories:one is based on the current-controlled grid-connected strategy,which is connected by injecting current into the grid.The other type is a voltage-controlled grid-connected strate gy that outputs active/reactive power to the grid by controlling the phase and amplitude of the inverter's output voltage.With the grid-connected inverter based on these two control modes is connected to the grid,there may be potential oscillation and instability risks,Therefore,in order to cope with the problem of grid-connected adaptability under different control modes,this paper mainly carries out the following work:1.F or the current-controlled grid-connected inverter,the grid-side harmonic resonance mechanism between inverter and grid is explored from the perspective of impedance by establishing its output impedance model.Through the means of impedance reshaping,the basic forms of two kinds of impedance reshaping are derived,and a series of control loop topologies with the capability of impedance reshaping are found.In order to verify the proposed impedance reshaping method,this paper optimizes the design and verification of the proposed impedanee reshaping loop structure,and finds that the proposed control loop impedance reshaping can effectively suppress the influence of grid background harmonics to the grid-connected current of inverters.The control loop is simplified and easy to implement.Finally,the feasibility of the proposed control loop is verified by simulation and experiment.2.For the voltage-controlled grid-connected inverter,the virtual synchronous generator(VSG)is researched,and the mathematical model of the grid-connected inverter of VSG is established.Based on the analysis of VSG model,the low frequency oscillation mechanism of VSG-based inverter is described in detail.Combined with the analysis results of output power and frequency by external loop parameters perturbation,a parameter adaptive control strategy based on two-degree-of-freedom coordinated control is proposed.Simulation and experimental verification show that this strategy can effectively suppress fluctuations in output power and frequency and improve stability of system.3.In view of the inverter-grid interaction resonance mechanism and suppression strategy in the case of VSG cluster,this paper establishes an output impedance model based on VSG,and based on this,the output impedance model of VSG cluster is established and deeply explores the inverse network interaction based on VSG cluster.The system's resonant characteristics,meanwhile,consider the precise output impedance model that introduces the dead zone effect.Through the established VSG impedance model,it is found that the VSG-based grid-connected inverter exhibits low output impedance characteristics,and with the increase of the number of grid-connected stations,the dead-band effect and grid voltage background harmonics are applied to the VSG-type grid-connected inverter.The influence of grid current is large.Finally,the correctness of the impedance model of the inverse network interaction system based on VSG cluster is verified by simulation.4.Through the established VSG cluster impedance model,the sensitivity of the output filter parameters and controller parameters to the VSG impedance model is studied,and the grid-connected inverter based on the VSG control strategy and its clusters interact with the grid side harmonics is revealed.By comparing the current-controlled inverter cluster with the droop-controlled inverter cluster and grid interaction characteristics,it shows that the overall low-impedance spectrum characteristics of the VSG system are beneficial to weaken the interaction of the inverse-network,and the change of the number of parallels interacts with the resonance.The frequency interval has little influence,which in turn indicates the inverter-grid weak interaction characteristics of the VSG cluster grid-connected.Taking the LCL VSG grid-connected system as an example,it is found that the inner loop controller can effectively suppress the resonance peak.At the same time,a low-impedance characteristic of VSG is presented.A grid-connected control strategy based on multi-resonant virtual synchronous generator(VSG-MPR)control structure is proposed.This method can preserve the VSG advantage while effectively suppressing the harmonics of grid-connected current,wave.Finally,the correctness of the VSG cluster inverse-network interaction model and the comparative analysis results are verified by simulation and experiment.5.In order to verify the correctness of the proposed control strategy,support theoretical analysis,design and build a grid-connected inverter experimental platform based on TMS32 0F28335 controller,including hardware circuit and control algorithm.Experiments show that the designed grid-connected experimental platform can effectively verify the feasibility of the proposed control strategy and the correctness of theoretical analysis.