Passivity enhancement by series LC filtered active damper with zero current reference

Grid connected converters with the LCL filters suffer from the instability problems when they are connected to grid with non-negligible impedance. The active damper is an effective tool for improving the stability of the grid connected converters against the various grid conditions. The operation principle of the active damper in previous study is mimicking a damping resistance at the resonance frequencies, which is an intuitive approach but requires a complicated current reference generation algorithm and a high-bandwidth current control loop. However, the stability of the grid connected converters can be improved by enhancing the passivity of the total admittance seen by the grid, which allows for a zero current reference and a much simpler current controller for the active damper. To show the performance of the active damper with zero reference, this paper first carries out the impedance based stability analysis of grid converters in the weak grid. Based on the impedance model of the series LC filtered active damper, the real part of its output admittance is investigated and shown to be able to enhance the passivity of the admittance of the converters seen by the grid. Finally, simulation and experimental results are provided to show the validity of the analysis.