Wideband Measurement of Grid Impedance Using Chirp Signals in Grid-Connected Inverters

Grid connected inverters can suffer harmonic instability when connected to the grid due the interaction between the converter output impedance and the grid impedance seen at the point of common coupling (PCC). To counteract this it could be useful if the inverter were able to measure the impedance at the PCC over a wide frequency range. This can be done by injecting a current disturbance signal to the grid and measuring the resulting voltage disturbance. This paper investigates the use of a chirp signal injection in combination with Welch's method for signal analysis, as a means to measure the grid impedance up to 2 kHz. The accuracy of the approach is first investigated using MATLAB/Simulink simulations for a single phase inverter. The results demonstrate a good ability to accurately determine grid impedance with relatively low amplitude chirp signal injection. Experimental results show that the amplitude of signal injection needs to be higher in order to improve accuracy. There is however a trade-off between accuracy of estimation and total harmonic distortion of the current.