Bifurcation analysis of three-phase grid-connected converters

Abstract

This thesis aims to identify and analyze the nonlinear behavior in the three-phase voltage source converter, which is widely used as an interface converter in a distributed hybrid power system (DHPS). In this system, the converter does not work as a standalone converter but operates as a subsystem that connects to a non-ideal power grid and interacts indirectly with other subsystems via a point of common coupling (PCC). It has been observed that specific bifurcation phenomena occur in this grid-connected system. Bifurcation analysis is carried out to identify these phenomena, and a design-oriented analysis is adopted to derive practical parameter boundaries that divide the various possible operating regimes. Specifically, a catastrophic bifurcation is identified for the three-phase voltage source converter. When this bifurcation occurs, the DC output of the converter will collapse, and the input line current will expand to a very high level, which is dangerous for the grid. A set of design-oriented parameter boundaries are given. Also, the cause of this special bifurcation is identified by inspecting the nonlinear operation of the converter circuit. This phenomenon is studied experimentally in this thesis. Furthermore, an irreversible bifurcation phenomenon is reported in a three-phase voltage source converter connected to a non-ideal power grid with an interacting load, which represents a practical form of system configuration. Due to the limited input active power given to the converter by the 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 "abnormally". A large-signal analysis is adopted to identify the physical origin of the phenomenon and to locate the boundary of the instability. The phenomenon is verified experimentally. Finally, a low-frequency Hopf-type instability phenomenon in the three-phase voltage source converter is identified when the converter is connected to a non-ideal power grid. An averaged model has been developed for the grid-connected converter system to predict the low-frequency instability. Additionally, as a result of the emergence of low-frequency oscillation, it is found that the stability boundary leading to a catastrophic bifurcation is significantly varied. The low-frequency instability and its effects on the stability margin are verified experimentally.