Inherent damping of single-loop digitally controlled voltage source inverters with LCL filters

This paper studies the inherent damping of digitally controlled LCL-filtered voltage source inverters (VSIs) that adopt single-loop inverter-current feedback (ICF) or grid-current feedback (GCF), through investigating the virtual impedance introduced by each feedback control method. The virtual impedance consists of resistance and reactance which are found to be delay-dependent. The virtual resistor provides an inherent damping to the resonance of LCL filters when its resistance is positive, and thus making it is possible for the system to be stable. Subsequently, the damping ranges of time delay of the single-loop control systems are easily derived, within which a positive virtual resistance at the LCL resonance frequency is obtained. The time delay should fall into the damping ranges to ensure a possible stable operation. The study of the inherent damping of single-loop digitally controlled VSIs with LCL filters is validated and verified by simulated and experimental results.