All-Port Unterminated Admittance Passivity for Robust Stability of AC-DC Interlinking Converters

This article presents the all-port unterminated admittance passivity criterion and establishes a termination independent robust stability-oriented controller design framework for ac-dc converters with $dq$ frame representation of ac ports. In addition to the passivity properties of the converter's dc and ac ports' unterminated self admittances, the coupling passivity properties are examined, which account for the possible destabilizing impact of port coupling. It is explained how, to ensure stability, the converter's control system should be designed so that all passivity properties are non-negative in the frequency range critical for stability. The proposed methodology is exemplified for a three-phase current-controlled voltage source inverter. As examples of stabilizing control strategies, multisampled pulse-width modulation and active damping impedance-emulation are shown to be capable of rendering all passivity properties non-negative at high frequencies. It is demonstrated that this prevents the converter's destabilizing interactions with the grid. Validations are performed both in frequency and time domains, using a laboratory prototype.