Research On Regulation Strategy Of ANPC Grid-Connected Inverter In Weak Current Network

With the increasing proportion of distributed energy in the power grid,and the double randomness of renewable energy power generation and power consumption load,the uncertainty of supply and demand sides of traditional power system is enhanced,and the traditional two-level grid-connected inverter is difficult to meet the requirements of renewable energy grid-connected power generation.In particular,the terminal power grid of renewable energy grid-connected system is weak,and the power flow distribution is more complicated after the distributed power supply is connected to the grid,which is easy to cause wide-band oscillation,so the stability of weak power grid-connected should be considered.Traditional three-level inverters,such as neutral-point clamp inverter and T-type inverter,have the disadvantages of unbalanced switching loss and high voltage stress on DC side.The active neutral-point clamped(ANPC)three-level inverter and its modulation strategy proposed in this article have the characteristics of low harmonic content of output voltage and current,many zero-level states and balanced loss.The DC-side voltage-dividing capacitor and switch tube of the converter bear low voltage,which can be widely used in the field of medium voltage and high power electric energy conversion.In this article,the working principle of ANPC inverter is analyzed,and an improved synchronous modulation strategy is proposed to balance the switching loss.The rationality and superiority of the proposed strategy are verified by analyzing the switching loss and power quality.In order to study the stability of grid-connected system,the impedance model of grid-connected system is established by harmonic linearization sequential impedance modeling method.The model comprehensively considers the characteristics of phase-locked loop,voltage loop and current loop,and analyzes the range of each control link in the impedance characteristics of grid-connected system in frequency domain,and enhances the stability of the system by exploring the coupling mechanism of bandwidth.In this article,Nyquist steady-state criterion is used to further verify the influence of control links and grid impedance on the stability of grid-connected system.According to the complementary advantages of voltage source and current source control modes,a strategy of switching the two control modes based on the harmonic distortion degree of grid-connected current is proposed.The proposed method can enhance the stability of grid-connected system and ensure good power quality in the process of grid impedance change.Finally,the hardware experimental platform and hardware-in-the-loop simulation platform of three-level ANPC inverter are built,and the proposed modulation strategy and dual-mode switching strategy based on adaptive power grid impedance are verified experimentally.The experimental results show that the proposed modulation strategy makes the switching loss of the system more balanced,and the conduction loss in zero-level state is reduced by half.The proposed control strategy keeps the grid-connected system stable in the process of wide-range changes of power grid impedance.