Stability Analysis And Improvement Of Multiple Grid-connected Inverters In Weak Grid

Distributed generation plays an important role in modern life under an environment of fossil energy depletion. It has a particularly prominent performance in response to the country’s energy-saving and emission-reduction policies and the strategy of sustainable development of energy. Grid connected inverter acts as a bridge between the distributed power generation system and the grid, its performance counts much. As an output filter device, LCL filter has drawn experts’ notice for its more excellent filtering characteristics than L filter in high power system, and it tends to take place of L filter in application. However, LCL filter is a high order filter, and this may cause the system to resonate.In the distributed generation system, the grid environment is no longer ideal but one with background harmonics and large grid impedance called weak power grid. There will be a serious threat to the grid stability when multiple inverters are incorporated into the weak grid at the same time. Therefore, the stability analysis and improvement of multiple LCL type grid-connected inverters in weak grid have important significance to maintain the stability of the whole power system.Based on the open loop model built by scholars before, a closed loop model of multi-grid-connected inverters in weak power grid is established, considering the inverter closed-loop control method. And the influence of closed loop control on the stability of multi grid-connected inverters is analyzed in this paper. On the basis of the closed-loop model, a conclusion can be drawn: the zero-poles’ distribution of the system is affected by the impedance magnitude of the power network and the closed-loop control of the inverter. Thus, changing the zero-poles’ distribution can improve the stability. A regulator reconstructed control method is proposed to adjust the zero-poles’ distribution in this study, so as to improve the stability of the grid connected system.The effectiveness and practicality of the proposed control method are verified by MATLAB simulation and experiment. In the research of this topic, an experimental platform of two inverters with capacity of 5k W are designed and set up, and the control chip of the experimental platform is TMS320F28335. There is a three-phase voltage regulator and a group of magnetic inductors to imitate the grid voltage and its impedance. The experimental results show that the control strategy can effectively improve the stability of multi-grid-connected inverters’ system through the tests of different number of inverters which connect to different power grid environment.