Inverse Nyquist Stability Criterion for Grid-Tied Inverters

A new impedance-based stability criterion was proposed for a grid-tied inverter system based on a Norton equivalent circuit of the inverter [18]. As an extension of the work in [18], this paper shows that using a Thevenin representation of the inverter can lead to the same criterion in [18]. Further, this paper shows that the criterion proposed by Middlebrook can still be used for the inverter systems. The link between the criterion in [18] and the original criterion is the inverse Nyquist stability criterion. The criterion in [18] is easier to be used. Because the current feedback controller and the phase-locked loop of the inverter introduce poles at the origin and right-half plane to the output impedance of the inverter. These poles do not appear in the minor loop gain defined in [18] but in the minor loop gain defined by Middlebrook. Experimental systems are used to verify the proposed analysis.