| Adequate energy supply is an indispensable condition for the development of human production,accompanied by environmental degradation and energy crisis caused by the rapid consumption of non-renewable energy.Distributed power generation gradually replaces the non-renewable energy generation mode because of its sustainable development,which conforms to the trend of energy planning in the world in the new era.Multi-inverter parallel grid-connected structure has the advantages of strong capacity redundancy and low input cost,and has been gradually put into use in distributed power stations.However,there is strong coupling between multi-inverter and power network impedance,which may lead to coupling resonance of the system,endanger the safety of equipment and reduce the reliability of power supply system.In order to improve the stability of the system,the resonant suppression strategy which combines the series virtual impedance of filter inductor and active damping of capacitor current is adopted in this paper to reshape the impedance network of the system and to actively dampen the high frequency resonance in the system.In this paper,based on the research background of multi-inverter parallel grid-connected system with independent parallel structure,the equivalent model of multiple three-phase grid-connected inverters in parallel is established.Through the analysis of the impedance network of the system,the resonance mechanism and vibration characteristics of the system under different operating conditions are different,and the frequency domain analysis and simulation are carried out to verify the correctness of the theory.For the single inverter grid-connected system,the capacitive current feedback active damping method is adopted in this paper to avoid the additional power loss while damping the LCL resonance spike in the system.For the multi-inverter parallel system,the high frequency resonance in the system is compensated when the resonance occurs,and the harmonic components of the inverter output and the harmonic components at the parallel nodes of the inverter are superimposed to counteract the harmonic components of the inverter output.The compensation control of high frequency resonance harmonic is realized.In order to overcome the shortage of harmonic compensation control,the virtual inverter inductor series resistance is introduced to realize the high frequency impedance remolding of the system.The resonant network is destroyed by active damping,and the resonance suppression is realized.In this paper,the above control schemes are modeled and simulated respectively.The simulation results show that the active damping of capacitor current feedback has a good effect on the resonant suppression of a single inverter under digital control.The harmonic compensation control based on virtual impedance has a certain damping effect on the two high frequency resonance peaks of the system,but its parameter tuning is limited in flexibility and has limitations.The resonance suppression control based on virtual impedance can achieve the desired control effect in synchronous and non-synchronous operation of multi-inverter,and the high-frequency resonance of the system is effectively suppressed.Finally,two parallel grid-connected three-phase inverters are built.The experimental results show that the single inverter resonant suppression strategy and the multi-inverter resonant suppression strategy can effectively suppress the resonant spikes.The virtual impedance resonance suppression strategy proposed in this paper has the advantages of LCL capacitor parallel resistance and inductor series resistance in passive damping scheme,and avoids the defect of inductor series resistance to achieve an ideal resonance suppression effect.The stability of the system is improved. |
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