Passivity-Oriented Impedance Shaping for LCL-Filtered Grid-Connected Inverters

Abstract

The passivity theory provides an effective way to tackle the instability due to the inverter-grid interaction, which tells that the system stability can be guaranteed if the grid impedance and the inverter output impedance are both passive. Since the grid impedance is generally passive, the system stability can be immune to the grid impedance variation if the inverter output impedance is also passive. This paper develops a series and parallel impedance shaping method to ensure that the output impedance of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -filtered grid-connected inverter is dissipative up to the Nyquist frequency. Since the inverter output impedance can be decomposed into two passive elements and one active element, its non-dissipative property results from the latter, and thus the virtual series impedance shaping is introduced to cancel out the active element. Unfortunately, it poses impact on the low-frequency loop gain and system phase margin in the meantime. To alleviate this impact, a high-pass filter is inserted in the series impedance shaping function. Subsequently, a virtual parallel impedance is introduced to enhance the passivity robustness, such as against filter parameter variations. Moreover, the proposed method does not require any additional sensors. Experiments from a 6-kW prototype confirm the effectiveness of the proposed method.