Grid impedance measurement for inverters with current controllers and virtual impedances

Modern inverter controllers such as the virtual synchronous machine (VSM) and other types of grid forming (GFM) inverters integrate an output current controller (OCC) and virtual admittances. The control structure is organized hierarchically; a higher level (slow) controller that generates reference voltages based on the control paradigm of the GFM, and the lower level (fast) OCC are combined to control the inverter. The voltage differences between these reference voltages and the grid voltages measured on the filter capacitors of the inverter are applied to the virtual admittance to obtain the reference currents that are employed by the OCC. The stability of such a combined control structure is known to depend strongly on the short circuit ratio (SCR) of the connecting line between the inverter and the grid. In particular, with lower SCRs, the region of stable operating points decreases. In order to be able to determine the strength of the connecting line of such an inverter and to adapt the tuning of the machine, this paper investigates a method for probing the grid impedance by injecting a small amplitude disturbance into the control loop of the GFM. Based on the measurement of the reaction of the grid voltages, it is possible to obtain an estimate of the grid resistance and inductance.