Impedance modeling and stability analysis of a three-phase three-level NPC inverter connected to the grid

The interaction between grid-connected inverters and the grid may cause stability issues, and compromise the reliable operation of the inverters. This study investigates the stability of a three-level neutral point clamped (NPC) inverter connected to the grid using impedance-based methods. Because the impedance model of a three-phase three-level NPC inverter has not yet been reported, this study fills the literature gap by analyzing the influence of three-level DC-side neutral point control on the impedance characteristics. By fully considering the DC bus dynamics and DC voltage control loop, and the current loop and phase-locked loop (PLL), the admittance model of a three-phase three-level NPC inverter is established and verified by simulation. Additionally, in the stability analysis of a three-level NPC inverter grid-connected system, the frequency coupling introduced by the PLL and DC bus dynamics is included with the help of an established admittance model. The stability of the grid-connected system under different grid short circuit ratios (SCR) and operating power levels is analyzed according to the Nyquist stability criterion. The experimental results revealed that the established impedance model of the three-phase three-level NPC inverter can properly represent the stability of this system.