Stability analysis of dc distribution power systems

Abstract

This thesis addresses on the stability analysis of DC distribution power systems, including the bifurcation analysis of the current-mode controlled DC cascaded systems, the modeling and the stability analysis of the DC distribution power systems under large disturbance, the modeling and the stability analysis of the photovoltaic-battery hybrid power systems. The DC distribution power system, consisting of a common bus connecting to multiple DC-DC converters, has been widely used in micro-grids, electric vehicles, commercial communications systems, computers, multi-electric aircrafts and other applications. The study in this thesis focuses on the stability analysis of such kind of the systems. We will first describe the basic principle of the small-signal modeling approach, and apply it to the open-loop buck converter, the voltage-mode controlled buck converter, the averaged current-mode controlled buck converter and two averaged current-mode controlled buck converters in cascaded connection. The discrete-time mapping model and the averaged model are given and the key procedures for obtaining sufficiently accurate models for the purpose of nonlinear analysis are illustrated. In order to analyze the stability of the system, the discrete-time model and the averaged model are analyzed. Specifically, the eigenvalues of the linearized system around the equilibrium point are calculated. The judging criteria for the stability of the DC distribution power system based on the root locus are also given. In the DC distribution power system, the load converters usually adopt closed-loop control to maintain their output voltage. The inputs of tightly-regulated load converters behave as constant power loads (CPLs) which present themselves as the negative impedances to the source converter and may destabilize the whole system. Much research effort has been devoted to identifying the stability criteria of interacting converters in a DC distribution power system. However, much of the work has focused on the voltage-mode controlled cascaded converters. In this thesis, a simple DC cascaded system consisting of two cascading averaged current-mode controlled DC-DC converters is considered. The stability of the system is investigated through the impedance-based criteria and the discrete-time mapping model. The effectiveness of the impedance-based criteria on the current-mode controlled system and the complex behavior caused by the slow-scale instability such as border collision are analyzed in depth. An effective method for ensuring stable design of current-mode controlled cascaded DC converter systems is discussed. The existing analysis methods for large disturbance such as phase-plane analysis and bifurcation analysis are too complex to be used in DC distribution systems containing a large number of subsystems. So, a representative DC distribution system consisting of one source converter which regulates the bus voltage and a number of load converters, each of which regulates its own output is considered in this thesis. A simple and effective model is proposed to analyze the transient stability of the system under large disturbance. The parameters used in this model can be easily obtained, and the results presented in design-oriented forms can facilitate the choice of parameters for ensuring stable operation. DC distribution systems are dynamical systems, and can be treated as voltage-source systems or current-source systems under different conditions. Taking the photovoltaic-battery hybrid power system as an example, the interfacing converter of the PV panels may operate under maximum power tracking control (MPPT) or output voltage control under different sunlight density situations, and the bidirectional converter, interfacing the battery storage, may operate in boost mode for discharging or buck mode for battery charging. It is important to assess the stability of the system under all operating modes. Furthermore, the converters connected to the photovoltaic panels and the bidirectional converter used in a photovoltaic-battery hybrid power systems usually contain input capacitors which have a profound effect on the systems' stability. Small-signal models of all the subsystems, including the interfacing converter and the bidirectional converter, are used to assess the overall system's stability under different operating modes. The effect of the input capacitor on the stability of the system will be studied in detail, which help derive useful design information regarding the choice of the parameters that would ensure the system's stable operation under all operating modes.