Online Grid Impedance Measurement Using Discrete-Interval Binary Sequence Injection

Grid impedance affects the stability and control performance of grid-connected power electronic devices, such as inverters used to integrate wind and solar energy. Adaptive control of such inverters, to guarantee stability under different grid conditions, requires online measurement of the grid impedance performed in real time. Such online measurement can be performed by injecting a current perturbation from the inverter into the grid and by reading the grid voltage responses. To minimize the impact on the inverter operation, the injection must be kept as small as possible while producing enough voltage perturbation that can be reliably measured and processed to extract its various frequency components. This paper proposes the use of a discrete-interval binary sequence (DIBS) for this application to minimize the injection. The DIBS is a computer-optimized binary sequence, where the energy is maximized at specified harmonic frequencies based on the expected grid-impedance characteristics. Experimental results based on a three-phase grid-connected inverter are presented to demonstrate the effectiveness of the proposed methods.