Online measurements of high-band grid impedance using variable frequency carrier based PWM schemes

Grid-connected power electronic devices, such as voltage-source inverters (VSIs), are increasingly installed in the grid for the utilization of renewable energy sources. Under high penetration, the weak grid contains plentiful harmonics and com­plex grid impedance values, which have deep impacts on system stability as well as its control performance. Online impedance measurement is needed for impedance based adaptive control and analysis. This paper proposes a variable carrier frequency PWM (VCFPWM) based online grid impedance measurement technology. This method broadens the available working range in the high frequency band without increasing the low band perturbations. Through good analysis and design of the carrier frequency, the VCFPWM method injects sufficient harmonic energy at specified high frequency points and guarantees low THDs at the same time. The detailed design considerations of the carrier frequency are clarified and the characteristics of the VCFPWM based excitations are analyzed in this paper. Finally, the simulation and experimental results based on a three-phase grid-connected VSI are presented to verify the effectiveness of the proposed method.