Research On Resonance Suppression Strategy Of Multi-Inverter In Parallel Based On Model Predictive Control

With the rapid development of new energy technologies,a series of distributed power generation such as wind power and photovoltaic power generation have become an important part of today’s power grid.Because the above-mentioned power generation methods need to use inverters to convert DC power to AC power for grid connection,and inverters It has non-linear characteristics,resulting in more harmonics in the output current.In order to suppress harmonics,filters are usually used on the inverter side.The LCL filter is widely used due to its small size,light weight and strong high frequency harmonic attenuation ability.However,because the LCL filter as a third-order system has low damping characteristics,it has inherent resonance itself,and it is easy to cause global resonance in the inverter parallel system.Therefore,how to suppress the resonance problem in the multi-inverter parallel system has become today’s A hot spot in the field of power electronics research.This article aims at the fact that when multiple inverters are connected in parallel and connected to the grid,there will be harmonics in the inverter output current,and the LCL filter has inherent resonance.When the inverters are operated in parallel,it may cause a global inverter parallel system.Resonance increases the output current THD at the grid end.If the resonance and harmonics of the parallel inverter system are not handled properly,it will not be able to meet the grid-connected inverter standards.In this paper,starting from the characteristics of the inverter itself,the component parameters of the inverter are designed.In the inverter system design,the parameter design of the AC side filter is very important.The replacement of L and LC type filters on the AC side of the inverter has become the trend of inverter grid connection.In the parameter design,first start with the aspects affected by each parameter in the LCL,and determine the parameters of the LCL according to the main constraints of the LCL filter design and the indicators of the selected inverter system.To study the resonance characteristics of multiple inverters in parallel systems,first establish a mathematical model of a single LCL grid-connected inverter,and combine the corresponding control block diagram to make the inverter Norton equivalent,and simplify the inverter to be equivalent.By controlling the current source in parallel with the output admittance,the equivalent Norton model of multiple inverters in parallel can be finally obtained.By drawing the amplitude-frequency curve of the open-loop transfer function of the system,the change trend of the resonance frequency with the increase of the number of inverters under different operating conditions is intuitively reflected,and the above-mentioned resonance mechanism is simulated and verified.It provides a theoretical basis and basis for the design of parallel resonance suppression control strategies for multiple inverters.Aiming at the resonance existing in the multi-inverter parallel system,the active damping is combined with the model predictive control strategy,and the design is carried out in the harmonic frequency domain and the low frequency domain respectively,which can suppress the resonance and increase the overall stability of the system;It can also improve the output current waveform and reduce the harmonic distortion rate.The principle of model predictive control is to predict the next moment of the output current,so that the current can track the change of the reference value in time.In each sampling period,8 voltage vectors are rolled into the designed value function.According to the value of the function determines the voltage vector that needs to be output at the next moment.The active damping scheme is to negatively feedback the high frequency components of the capacitor voltage,and its effect is equivalent to the parallel resistance of the capacitor in the harmonic frequency domain,thereby improving the damping of the LCL third-order system and suppressing the system resonance.Aiming at the goal of further reducing THD and suppressing resonance,the original algorithm is continuously optimized and improved to further reduce the grid-connected current THD and further strengthen the resonance suppression.Finally,simulation and experiment verify the dynamic and static performance and robustness of the above control strategy.