Grid impedance detection via excitation of LCL-filter resonance

Inverters adopted in distributed power generation, active filter and UPS are often connected to the grid through an LCL-filter. The impedance of the LCL-filter has a typical frequency spectrum with a resonance peak. Hence the LCL-filter has to be damped in order to avoid instability. However the resonance of the LCL-filter can be also excited in a controlled way in order to individuate the resonance frequency in the spectrum (using for example the FFT). This paper proposes to use a controlled excitation to measure the grid impedance, since this one influences also the resonance frequency. This paper addresses some possible limits, some solutions and some implementation issues (e.g. how to obtain a controlled resonance in the filter without damaging the system) in order to use the resonant peak for grid impedance detection. The analysis is validated both by simulations and experimental results.