On Stability Of DC Distributed Power System

Compared to the centralized power system(CPS), the DC distributed power system(DC DPS) has the advantages of flexile system configuration, high-efficiency energy conversion, and high-density power delivery capability, and it has been widely used in the applications such as space stations, aircraft, communication systems, industrial autonomous production lines, and defense electronic power systems. However, the interaction between individually designed power subsystems may cause instability of the DC DPS. This dissertation is dedicated to the stability analysis and solutions of the DC DPS.This dissertation is composed of three main parts.The first part is chapter 2. It is known to the people that a reliabile and stable converter is a necessary requirement of a stable DC DPS. And compared to the other converters, the converters, which operating at high voltage and high power application, are rarely studied. So Chapter 2 is dedicated to improve the performance of these converters. It presents an improved phase-shifted boost-derived full-bridge converter. By connecting a switched-capacitor snubber in parallel with the primary winding of the coupling transformer, all the main switches realize zero-current-switching(ZCS) and the switches in the snubber realize zero-voltage-switching(ZVS). In addition, the snubber capacitor voltage is adaptively controlled: it is charged to the minimum required energy to ensuring ZCS for the main switches. Thus, the circulating energy and the duty cycle loss are minimized. The operating principle and control method of the proposed converter are described, and verified by the experimental results from a 530 V input, 15 k V output, 5 k W prototype.The second part of this dissertation is chapter 3. Chapter 3 focuses on the general impedance criterion for the stability analysis of DC DPS. Besides the popular cascaded configuration, the DC DPS has various configurations, such as the DC DPS with energy storage and the DC DPS whose source converter employing droop control. Unfortunately, the stability of such systems can not be judged by the existing criteria easily and fesiblely. In order to solve this problem, chapter 3 classifies the converters in the DC DPS into two types, namely, bus voltage controlled converter(BVCC) and bus current controlled converter(BCCC) instead of the source and load converters. As a result, the DC DPS can be described with a general system whose bus voltage and bus current are controlled by the BVCCs and BCCCs, respectively. By adopting the two-port small signal model, the general impedance-based stability criterion for the DC DPS is derived. The effectiveness of the proposed impedance criterion is verified by the experimental results from two DC DPSs.The third part of this dissertation includes chapters 4 and 5 for proposing the solution for the stability problem of the DC DPS.Chaper 4 solves the system’s instability problem by regulating the output impedance of the source converter and introduces the concept of adaptive active capacitor converter(AACC). The AACC is connected in parallel with the cascaded system’s intermediate bus and only needs to detect the bus voltage without any change of the existing subsystems. Hence it can be designed as a standard module for DC DPS. The AACC serves as an equivalent bus capacitor to reduce the output impedance of the source converter, thus avoiding the intersection with the load converter’s input impedance, and as a result, the cascaded system becomes stable. The equivalent bus capacitor emulated by the AACC is adaptive according to the output power of the cascaded system, and thus the power loss of AACC is minimized and the dynamic response of the cascaded system is not deteriorated. Furthermore, no electrolytic capacitor is needed in AACC, so the cascaded system’s lifetime is prolonged. The operation principle, control and design consideration of AACC are discussed, and a 480 W cascaded system comprising two phase-shifted full-bridge converters has been built and evaluated. The experimental results verify the validity of the proposed AACC.Chaper 5 proposes a virtual-impedance-based control strategy to regulate the input impedance of the load converter. The proposed virtual impedance is in parallel with the input of the load converter. Via this virtual impedance, the magnitude and phase of the load converter’s input impedance can be improved to ensure the whole cascaded system stable. The expression of the required virtual impedance is derived, and the control method of this virtual impedance is also realized by an input-impedance-regulator, whose design principle has been discussed in details. Finally, a 100 W cascaded system, which consisting of LC filter and 48V-12 V buck converter, is fabricated and the experimental results are in agreement with the theoretical analysis.