On Bifurcation Of Multi-operating-mode Stand-alone Renewable Power Generation Systems

Global environmental concerns and increased demand for energy consumption are opening up new opportunities for developing renewable power generation systems. In stand-alone renewable power generation systems,the renewable power conversion unit,such as photovoltaic panels and wind genarators may operate in a maximum power point tracking(MPPT) mode or off-MPPT mode, and at the same time, the energy storage unit may provide power to the load or store energy from the renewable power sources. The whole system is thus designed to operate with multiple structures and multiple operating modes. As a result, the dynamics behavior of such a system is rather complex and the closed-loop design has to take into consideration the different modes of operation. This thesis attempts to study the complex behavior of such kind of system, and to analyze how it affects the system’s stability. The main content can be divided into three sections.In the first section, we will describe the dynamic model by formulating the multi-operating-mode renewable power generation systems, and present a method for analyzing the stability of all operating modes and the boundary between two operating mode. Furthermore, we will give a systematic analysis procedure for analyzing the smooth and non-smooth bifurcations based on discrete-time model and averaged model, respectively. From an analytical viewpoint, we may look at such a conversion system as being comprised of a set of dynamical systems. When the conversion system operates as one dynamical system with a special structure, it stays in one operating mode and is a piecewise smooth system similar to conventional power electronic systems. The dynamic model of the renewable power generation system is thus comprised of several dynamic models of subsystems, which are piecewise smooth systems. In order to analyze the stability of period-1 solution of the renewable power generation system, the stability of the equilibrium of the discrete-time model or averaged model is analyzed instead, which is determined by calculating the eigenvalues of the linearized system around the equilibrium point. The system may also switch from one operating mode to another as system external parameters vary. To analyze how the system losses its stability through a non-smooth bifurcation when it crosses the mode boundary, we need to find the point at the boundary through which the trajectory crosses and focus on the neighborhood of the point, as well as derive the Jacobian around the point on each side of the boundary.In particular, the standalone photovoltaic-battery hybrid power system is taken as an example for detailed study in the second section(Chapter IV), which is based on the photovoltaic panels and the battery connected to the DC bus through Boost converter and bidirectional Buck/Boost converter, respectively. As the system has three normal operating modes, the dynamic model of each operating mode is described with mathmatic formulation firstly. Then, the control strategy is proposed to make the system switch its operating mode among three operating modes smoothly. Discrete-time models and the equilibrium are derived to analyze the stability of the period-1 solution; from the movement of the eigenvalues of the Jacobian, the bifurcation diagrams and stability boundaries are graphically presented to reveal how the external and internal parameters affect the system’s stability. From which, the worst condition for the control loops is found to design the regrulators. The analysis results can thus be used to avoid bifurcations and make the system more reliable. At last, the control strategy is verified by experiment. At the same time, smooth and non-smooth bifurcations that lead to slow-scale and fast-scale oscillations under some situations are also verified and shown by experiment.For further illustration, the dual-input buck converter(DIBC) serving two input renewable sources is taken as another example in the third section(Chapter V), which also has three normal operating modes and the dynamic model of each is described with mathmatic formulation. The operation principle is presented firstly. Then the control method is proposed to make the DIBC operate correctly and be sure that the system switches its operating mode among three operating modes smoothly. An averaged model and the equilibrium are derived to analyze the stability of the period-1 solution, and the movement of the eigenvalues of the Jacobian of averaged model presents the stability boundaries under different choices of parameter value. Furthermore, in order to design the regrulators, the worst condition for the control loop is found from stability boundaries. At last, the stedy operation, as well as smooth and non-smooth bifurcations that lead to slow-scale oscillations under some situations is also verified by circuit simulation.