Factors in Active Damping Design to Mitigate Grid Interactions in Three-Phase Grid-Connected Photovoltaic Inverters

Abstract

An LCL filter provides excellent mitigation capability of the switching frequency harmonics, and is, therefore, widely used in grid-connected inverter applications. The resonant behavior induced by the filter must be attenuated with passive or active damping methods in order to preserve the stability of the grid-connected converter. Active damping can be implemented with different control algorithms, and it is frequently used due to its relatively simple and low-cost implementation. However, active damping may easily impose stability problems if it is poorly designed.<br/><br/>This thesis presents a comprehensive small-signal model of a three-phase grid-connected photovoltaic inverter with LCL filter. The analysis is focused on a capacitor-currentfeedback (i.e., a multi-current feedback) active damping and its effects on the system dynamics. Furthermore, a single-current-feedback active damping technique, which is based on reduced number of measurements, is also studied. The main objective of this thesis is to present an accurate multi-variable small-signal model for assessing the control performance as well as the grid interaction sensitivity of grid-connected converters in the frequency domain.<br/><br/>The state-of-the-art literature studies regarding the active damping are mainly concentrated on stability evaluation of the output-current loop, and the effect on external characteristics such as susceptibility to background harmonics and impedance-based instability has been overlooked. As the active damping affects significantly the sensitivity to grid interactions, accurate predictions of the system transfer functions, e.g. the output impedance, must be utilized in order to assess the active-damping-induced properties. Moreover, the single-current-feedback active damping method lacks the aforementioned analysis in the literature and, therefore, the need for accurate full-order small-signal models is evident.<br/><br/>This thesis presents design criteria for the active damping in a wide range of operating conditions. Accordingly, peculiarities regarding the active damping are discussed for both multi and single-current-feedback active damping schemes. In addition, the parametric influence of the active damping on the output-impedance characteristics is explicitly analyzed. It is shown that the active damping design has a significant effect on the output impedance and, therefore, the impedance characteristics should be considered in the converter design for improved robustness against background harmonics and impedancebased interactions.