Impedance Modelling and Stability Criteria of LCL-type Inverters Operating at Higher Switching Frequencies Enabled by SiC MOSFETs

While the reduced switching losses of Silicon Carbide (SiC) MOSFETs has enabled higher switching frequencies, their impact on the stability of LCL-type converters has not been studied thoroughly. To investigate this, this paper presents the sequence impedance modeling and system stability analysis for an LCL-type grid-tied inverter with SiC MOSFETs, considering various switching frequencies and controller bandwidths. The analytical model and simulation model are built and compared in this paper. The discrete simulation model of the inverter is built by appropriately selecting the sampling frequency corresponding to different switching frequencies with proper LCL design. By comparing the Bode plots and applying stability criteria for different switching frequencies and controller bandwidth, their impact on system stability is analysed. The results show the reasonable controller bandwidth can reduce the impact of noise from Pulse Width Modulation (PWM) modulation. Besides, the lower switching frequency will potentially worsen the system stability for same controller bandwidth. The bandwidth of the phase-locked loop has a greater impact on system stability in a weak grid compared to the current controller bandwidth. However, the current control bandwidth may affect the damping of the converter impedance near the resonance frequency of the LCL filter.