| In order to guarantee the sustainable development of energy, fossil energy is gradually replaced by renewable energy. When the clean energy such as solar and wind energy are inverted to AC and added to the gird, then the use of them can be maximized. However, the phase-locked technique is important for the grid-connected inverter to provide accurate control reference. Especially under the condition of non-ideal voltage, a phase-locked loop(PLL) with better performance becomes more important.Firstly, the historical background and the significance of phase-locked loop are discussed, the characteristics of several traditional phase-locked loop are briefly described, their deficiencies are pointed out. For the grid-connected inverter, the requirement for PLL under the condition of non-ideal voltage is put forward, and the research status at home and abroad is introduced.Secondly, the current main realization methods of the PLL are discussed, including PLL scheme based on synchronous rotating coordinate system and PLL scheme based on double second-order generalized integrator. By establishing the mathematical models of the PLL schemes, analyzing the steady-state and dynamic performance of the system, then the disadvantages of these schemes are pointed out.Thirdly, on the basis of previous analysis, the PLL scheme based on adaptive observer is introduced, and the principle of this PLL scheme is explained in detail, then the gain matrix of the scheme is improved, and a new PLL scheme based on adaptive observer of variable gain matrix is proposed. This kind of PLL is applied in the three-phase grid-connected inverter, and the adaptability of the PLL is verified.Fourthly, the MATLAB simulation results show that the PLL scheme based on the adaptive observer of variable gain matrix performs well under non-ideal voltage condition. The requirement for the PLL under the non-ideal voltage condition is satisfied well.Finally, the PLL scheme, grid-connected circuit and control system, are designed. TMS320F2812 is the main control chip of the system. The PLL experiment is conducted under the conditions of voltage mutation, frequency mutation and complex voltage cases. The experimental results verify the correctness and feasibility of the theoretical analysis and simulation results. Based on the grid-connected experimental platform, the effectiveness of the PLL scheme based on the adaptive observer of variable gain matrix is verified, and this PLL scheme can be applied to three-phase three-wire system. The experimental results are consistent with the theory analysis above and simulation results. |
