On the Ac stability of high power factor three-phase rectifiers

This paper presents a comprehensive study of the ac stability of high power factor rectifiers, proposing a new stability criterion to assess this condition under constant power load dynamics. Specifically, based on the multivariable generalized Nyquist stability criterion developed by MacFarlane and Postlethwaite, this paper shows how the fundamental stability problem at such ac interface is indeed reduced to a single-input single-output (SISO) case. It is shown that the ac stability is fully determined by the SISO return-ratio of the d-d channel-or the power transfer channel of the desired d-q frame alignment; that is, the stability is determined by the d-d channel input and output impedances. Correspondingly, it is shown that the q-q channel trajectory on the complex plane is such that it cannot encircle the critical point (-1+j0) and therefore be the cause of instability. The ac stability at the input terminals of the rectifier can then be fully assessed using the standard SISO Nyquist stability theorem, in sheer duality of Middlebrook´s criterion for dc-dc converters. Simulation results are used to verify and appraise the implications of the proposed ac system stability criterion.