An Ignored Culprit of Harmonic Oscillation in LCL -Type Grid-Connected Inverter: Resonant Pole Cancelation

Resonant pole cancelation is usually adopted in the current control or active damping to tackle the filter resonance in the LCL -type grid-connected inverter. However, its potential impact on the system stability has not been adequately assessed. This article provides a comprehensive investigation on three typical control schemes that utilize resonant pole cancelation. An important finding is drawn that the resonant pole cancelation violates the so-called internal stability, which can give rise to the undesired harmonic oscillation in grid current. From the physical insights, it is revealed that the resonant pole cancelation does not essentially eliminate the LCL resonance but merely “hide” it in the system, which can still be triggered and, thereby, turns into the culprit of the harmonic oscillation. The theoretical expectation is validated by the simulation and experimental results.