Effect of active damping on the output impedance of PV inverter

Three-phase photovoltaic inverters are usually equipped with an LCL-type output filter to reduce cost and size of the converter compared to a simple L-type output filter. However, an LCL-filter has an inherent resonance which has to be damped by a passive or active method to avoid instability. This paper presents an accurate small-signal model of the three-phase VSI-based photovoltaic inverter with LCL-type output filter using transfer matrices. Additionally, the effect of active damping on the output impedance is analyzed. The developed small-signal model has been verified by extracting frequency responses from a simulation model and a scaled-down prototype inverter. The model is shown to accurately predict the shape of inverter transfer functions such as the output impedance. Furthermore, it allows deterministic control design and accurate tuning of the active damper as well as performance evaluation of the control system.