Stability Analysis Of Grid-connected Inverter Based On Impedance Model

The depletion of global fossil fuels accelerates the process of renewable energy.As the main utilization of renewable energy,distributed generation has been a growing share in the major power grid.Grid-connected inverter is an important power conversion device for distributed generation,which takes a higher participation in the major power grid.However,different from the traditional resistance and reductance,due to the nonlinearities the gird-connected inverter produces a series of harmonics,which affects the stability of the system operation.In addition,the external factors,such as grid impedance,affect the stability of grid-connected system in a long-distance transmission.In this thesis,three-phase voltage inverter is considered.Due to the effect of high resonance peak of the grid-connected filter on the stability of system,three types of filters are analyzed.The frequency characteristics and filter application ranges are given by the analysis of frequency characteristic.Four damping schemes,including the typical passive damping scheme and active damping acheme,are considered to restrain the high-order filter inherent resonance peak in this thesis.The performances of the damping schemes are evaluated based on both the resonance peak damping effect and the damping power loss,which providing a basis to select a suitable damping scheme.Based on an appropriate filter structure,the impedance inverter is established.In this thesis,the control schemes under different coordinate systems are compared,and the control strategy under ?? coordinate system is adopted to simplify the process of establishing the inverter impedance model.The unit power factor grid-connected control and active damping are realized by selecting a reasonable control strategy and control parameters.Furthermore,the nonlinearity of the inverter is taken into account in the impedance model to build a more accurate impedance model.In order to achieve the stability of a single inverter,the influence of active damping parameters and digital control delay are analyzed based on classical control theory.The optimal range of active damping parameters is obtained.based on the theory of impedance stability analysis,the relationship between the equivalent impedance of the grid and system stability is analyzed,and a stable range of grid impedance is obtained.Moreover,simulation results illustrate the effectiveness of the theoretical analysis results.Multiple inverters in parallel are modelled as an equivalent impedance to solve the stability of distributed power generation system with multiple grid-connectedinverters.The influences of inverter counts on the stabilty of the grid-connected system is considered based on the theory of impedance stability.The results are then verified by multi-inverter simulation model.Finally,the control algorithm is designed and the theory analyses are validated by experiments with the help of the experimental platform of three inverters.