A Novel Online Grid Impedance Measurement Method Based on Injecting Third Harmonic Voltage

Under grid-connected conditions, the adaptive control of the inverter can improve the stability of the system, as long as the grid impedance can be detected online quickly and accurately. The demerits of traditional active grid impedance measurement methods are complex data acquisition and processing, usually involving discrete Fourier transform (DFT), and the injected interference may deteriorate the power quality. In order to reduce the amount of calculation and minimize the impact on the operation of the inverter, this paper proposes to intermittently inject three cycles of the third harmonic signal into the inverter. Then the current and voltage variations at the point of common coupling (PCC) can be extracted through the Kalman filter, and the grid impedance can be estimated. The proposed method alleviates the total harmonic distortion produced by the continuous signal injection in the existing method, reduces the calculation amount and improves the detection speed at the same time. Simulation and experimental results show the effectiveness of this method.