Noninvasive Online Parametric Identification of Three-Phase AC Power Impedances to Assess the Stability of Grid-Tied Power Electronic Inverters in LV Networks

This paper presents a noninvasive online parametric identification of three-phase ac power impedances to assess small-signal stability of grid-tied inverter systems by using well-known impedance-ratio-based stability criteria. The identification technique is integrated into the control of an existing grid-tied inverter for the estimation of wide bandwidth ac grid impedances, on top of its original power conversion function. This is accomplished in practice by injecting a short-time small-signal pseudorandom binary sequence (PRBS), a digital approximation of white noise which is wide bandwidth in nature, on the inverter control loop so that all frequencies of interest at the impedance measurement point can be excited at once. Then, digital processing is performed in the integrated control platform where the parametric ac grid impedance is extracted from the measurement of voltage and current over the length of PRBS injection. Moreover, a procedure on how to identify the output impedance of the inverter is deployed so that the parametric source and load impedances can be used to verify the system stability by means of the generalized Nyquist stability criterion. The technique is validated via hardware-in-the-loop real-time simulation. This paper focuses on the identification of balanced three-phase ac impedances in dq reference frame and a dq diagonal-dominant stability analysis which is typical of low-voltage distribution grids.