Abstract
Three-phase voltage PWM rectifiers are widely used as DC voltage source to provide constant and regulated output voltage. However, if multiple rectifiers connected to power grid at the same point of common coupling (PCC), it may drive the converters to enter instability region of low-frequency oscillation. This paper studies a low-frequency oscillation phenomenon that is also a Hopf-type bifurcation in grid-connected converters. The instability phenomenon characterized by the oscillation of converters' output voltage, and large amount of harmonics appear on the line current. The input admittance model of converter and output impendence model of non-ideal grid have been established, and transfer matrix function between input current and input voltage of the system is derived. Nyquist stability criterion is adopted to judge the stability of the system. The phenomenon is verified by simulations in MATLAB.
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