Analysis Of Resonant Mechanism For Multi-parallel Inverters And Research On Method Of Cluster Suppression In Micro Grid

Grid-connected inverter,as power electronic interface between microgrid and distributed power generations such as solar PV,wind power and fuel cell,etc,bearing the function of bidirectional conversion of energy.In order to satisfy harmonic standards of grid current,the grid-connected inverters output current is generally filtered by adopting LCL filter units.But inherent resonant point exists in the LCL filter because of the low damping characteristic,which will cause internal resonance under the external harmonic excitation.Large-scale distributed generations are connected to the microgrid via grid-connected inverters.Due to the grid impedance coupling,the parallel resonance will be introduced into inverters.Besides,series resonance will also be introduced into multi-inverter system with the introduction of power grid harmonics.Due to the existence of inverter output voltage harmonics and grid harmonic,the resonance in inverter itself,inverter-inverter,and inverter-grid will be leaded.The grid-voltage harmonic amplification caused by resonance will aggravate the risk of global resonance which seriously affects safe and reliable operation of grid.In this paper,LCL-type three-phase grid-connected inverters were selected as the research object.Firstly,the mathematic model of single three-phase grid-connected inverter in dq coordinate system was established.And the Norton equivalent models were obtained by the equivalent transformation for cont rol block diagram.Then,the model was extended to multiple inverters in parallel,and the Norton equivalent circuit models for multi-inverters in parallel were deduced with consideration of line impedances.The resonant coupling characteristics of multi-parallel inverters,including self-resonance,parallel resonance and series resonance,were qualitatively and quantitatively analyzed when the line impedances were considered or not.In the process of establishing Norton equivalent model for multi-inverters in parallel,the resonant characteristics of the system were compared and analyzed,whether or not considering the line impedances and difference of the line impedances.A cluster active damping method was proposed to suppress the resonance of multi-inverters in parallel.The virtual resistor connected with PCC was constructed by adding voltage feedback at PCC.And the harmonic resonance of impedance network was suppressed,which was caused by the harmonic voltage of inverter and grid.The system damping was increased by the high-frequency grid-current feedback to improve system stability.In addition,no extra sensors were required,which not only reduced the system cost,but also simplified the hardware control structure of system.Through the mathematical modeling of the cluster active damping strategy,the suppression effect of different virtual resistors on the resonance of multi-inverters in parallel was analyzed,and then the optimal virtual resistor was selected.By the simulation in Matlab /Simulink,the cluster active damping method to restrain the parallel resonance for multiple inverters was validated.Finally,the 30 kW PV grid-connected inverter was designed,and a experimental platform for multi-inverters in parallel was set up.The simulation d evice of photovoltaic array in the experimental platform,the main circuit design of the three-phase grid-connected inverter,the hardware and the software design of the control system were introduced in detail.Based on the experimental platform,the experiments of three grid-connected inverters in parallel were performed.The experimental results revealed the resonance mechanism of multi-parallel inverters,and verified the effectiveness of the cluster active damping proposed in this paper.