Impedance characteristics investigation and oscillation stability analysis for two-stage PV inverter under weak grid condition

In order to obtain impedance characteristics of the photovoltaic (PV) inverter and reveal potential stability issues of the PV inverter connected to a weak grid, a complete impedance model of the two-stage PV inverter is established in this paper. The model includes the PV arrays, front-end Boost converter, and rear-end inverter with output LCL filter. The impedance modeling of the PV inverter is derived at different interfaces of DC-link and point of common coupling (PCC), where the DC impedance characteristic at DC-link and d-q axis impedance feature at AC side are both investigated. It is worth noting that the DC output impedance of front-end circuit behaves as a first-order low-pass filter, and it is mainly influenced by the DC-link capacitor. Moreover, a significant feature is found that a reduced integral gain in the outer voltage loop may cause a negative resistance and capacitive impedance in d-axis impedance component. In addition, based on the impedance analysis method, the oscillation stability issues of the grid-connected PV inverter in case of a weak grid are evaluated by different divisions of source-load subsystems. It is demonstrated that the increase of the PLL bandwidth and the decrease of the integral gain of the outer voltage loop will both lead to the oscillation of the PV inverter. Moreover, the increase of the output capacity will deteriorate the stability margin, resulting in the high frequency oscillation of the system. Finally, the impedance model and the stability analysis are verified by the simulation results implemented in PSCAD/EMTDC platform.