Nonlinear Behavior And Analysis Of Three-Phase Grid-Connected Power Converters

This thesis aims to identify and analyze the nonlinear behavior of the three-phase boost rectifier, which is widely used as an interface converter in a distributed power system (DPS). In this system, the converter does not work as a standalone converter but oper-ates as a subsystem that connects to a non-ideal power grid and may interact indirectly with other subsystems via a point of common coupling (PCC). It has been observed that specific nonlinear phenomena occur in this grid-connected system. Bifurcation analysis has been carried out to identify these instability phenomena, and a design-oriented analysis is adopted to derive practical parameter boundaries that divide the various possible operating regimes and provide guidelines for achieving stable design.Specifically, an irreversible bifurcation phenomenon is reported in a three-phase boost rectifier connected to a non-ideal power grid with an interacting load. Such a system model represents a practical form of system configuration. Because of the limited input active power given to the converter by the non-ideal power grid, the DC voltage of the converter will drop when the converter fails to get the power it needs. The converter then sinks reactive power and operates in nearly zero power factor which be regarded as "abnormal operation". A large-signal analysis is applied to identify the physical origin of the phenomenon and to locate the boundaries of the stability regions. The phenomenon is also verified experimentally.Furthermore, a low-frequency Hopf-type instability phenomenon is identified in the three-phase boost rectifier when the converter is connected to a non-ideal power grid with significant impedance. An averaged model has been developed for the grid-connected converter system to predict the low-frequency instability. The low-frequency instability and its effects on the stability margin are verified experimentally.Finally, another low-frequency Hopf-type instability phenomenon is considered in two or more three-phase boost rectifiers coupled with the power grid having significant impedance at the same PCC. An impedance model and a general Nyquist criterion have been employed for such grid-connected systems to assess such typical instability. Design-oriented analysis has been presented to guide the design and to demonstrate how various parameters affect the system’s stability region. Also, this study has revealed that the stability of a grid-connected converter is no longer a standalone problem and the converter may be more prone to instability when connected to a power grid under certain conditions.