Research On Key Aspects In Large-Signal Stability Analysis For Current Mode Controlled DC-DC Cascaded System

With the development of electronic and renewable energy technique, DC distribution power system (DPS) becomes more and more attractive due to its unique advantages, and it has been widely used in the fields of electric vehicles, computer systems, medical electronic equipment, telecommunications industry, etc. The DC DPS has provided lots of benefits, but also negative factors. Increasingly larger size and complex structure not only highlights the stability issue, but also brings great difficulty for stability design. It is almost impossible to take the DC DPS as a whole for stability analysis. For this important issue, researches and analysis starts with cascaded system which is the most basic connection form of distributed architecture, and some useful theoretical achievements have been made based on small-signal analysis method. But the conclusions are only suitable to analyze the stability around equilibrium point, and will be invalid when large-signal disturbance occurs. Therefore, in order to analyze comprehensively, get in-depth understanding of system operation under large-signal disturbance, and improve system reliability, it is necessary to establish appropriate research methods based on nonlinear theory for large-signal stability analysis. This dissertation investigated and analyzed the large signal stability issue of DC-DC cascaded system in depth based on gyrator theory and relevant nonlinear analysis methods, which mainly contains the contents as below.Based on the comparative study of the existing large-signal model, this dissertation proposes a unified large signal model for current-mode controlled DC-DC converter based on the gyrator theory. The proposed model overcomes the shortcomings of existing large-signal models, such as complicated and difficult to be adopted for theoretical analysis. It is order-reduced, and the input-output expression is simple. Only single feedback loop is needed to achieve current mode control function, and it provides convenience for theoretical analysis. Because the gyrator theory is based on the energy conservation law, the proposed large-signal model is generalized and can be applied to all the basic topologies by modifying only one parameter in the model.In addition to reflect large-signal properties of original converter well, this model also includes all the small-signal information around the equilibrium point. Due to the characteristic of simplicity, generalization, structural uniformity, and completeness, this model provides foundation and possibility for theoretical analysis, and analytical large-signal stability criterion could be derived based on it.For the stability analysis of DC-DC cascaded system, this dissertation investigates the small-signal and large-signal stability of DC-DC cascaded system based on Lyapunov linearization method and mixed potential theory. To simplify the analysis, the proposed gyrator large-signal model is employed for simplifying the source converter, and the load converter is simplified as a constant power load (CPL). An analytical small-signal stability criterion is obtained based on this simplified system and Lyapunov linearization method. With the proposed criterion, the small-signal stability of system can be judged based on few parameters without complex input-output impedance calculation. Furthermore, the sufficient condition for stable operation of system under large-signal disturbance is proposed based on the mixed potential theory. It is also the analytical large-signal stability criterion for DC-DC cascaded system. Based on the same simplified system model, the proposed small-and large-signal criteria can be combined to derive a general stability criterion. When it holds, the system can work stable under both small-signal and large-signal disturbance, and the design process is simplified. Meanwhile, this dissertation also investigates the cascaded system in which the response speed of load converter is not so fast that it can not be simplified by CPL. Similar conclusion is derived. For convenience of application, the general stability criterion mentioned above can still be used to analyze the stability for this kind of system.Based on the proposed gyrator large-signal model and theoretical reseach results of large-signal stability analysis for cascaded system, chapter4of this dissertation studies the compensation methodology of DC bus voltage, and proposes a compensation control law for voltage bus conditioner (VBC). Effective compensation can be done based on the parameters of source and load converters. Meanwhile, the proposed control law is derived from the stability theorem of mixed potential theory, so the stability of cascaded system with VBC under large-signal disturbance is guaranteed theoretically. Besides, the proposed control law is universal, and the compensation result is satisfied.