The Research On Strategy Of Restraining Circulation Of Parallel Three-phase Inverters Considering Non-linear Load

With the aggravation of energy crisis,environmental pollution and climatic issues,the development of renewable energy and energy conservation with the theme of the low-carbon economy has become the mainstream for future development.The microgrid plays an important role in optimizing and improving energy efficiency,reducing the impact of the power system to environment,and coordinating the contradiction between the distributed generation and the power grid.Under the condition of multi inverter in microgrid,the parallel technology without interconnection is the most obvious in many parallel technologies.Accordingly,the circulating current problem has become a hot topic on the research of parallel inverters in microgrid.In this paper,the parallel system of combined three-phase inverter in islanding microgrid is studied.First,the cause of circulating current is analyzed.One is the harmonic circulation problem caused by the dead time and the non ideal characteristics of switching devices,which can cause output voltage deviation between parallel system;Another is that the equivalent output impedance of parallel system does not match with the output power of each inverter at different harmonic frequencies,which can cause circulating current between parallel inverters.Then,the inverter control model based on harmonic disturbance is established.and that,the gain of controller can be attenuated by the instantaneous voltage feedback;In order to solve the problem that the inverter output impedance does not match at each harmonic frequency under the condition of nonlinear load,virtual impedance loop is adopted.Hence,a quasi proportional multi-resonance control strategy based on virtual resistance is come on.It makes use of the high gain of the controller to realize the good voltage waveform quality,and also has a certain attenuation effect on the harmonic circulation of parallel system caused by the dead time and so on.In addition,the operating frequency of the system will change with the load variations,which makes the traditional Repetitive Control lose the high gain at each harmonic frequency.To solve this problem,the adaptive sampling frequency repetitive controller is adopted,this controller is applied to the voltage controller,to ensure the high gain of system at the each harmonic frequency,and can suppress the harmonic circulating current.Lastly,the simulation model and experimental platform of the combined three-phase inverter parallel system are built.The two control methods are tested by simulation and experiment.The result shows the feasibility and effectiveness of the proposed control plan.