C. D. Xu

Bio: C. D. Xu is an academic researcher from Hong Kong Polytechnic University. The author has contributed to research in topics: Converters & Buck–boost converter. The author has an hindex of 7, co-authored 27 publications receiving 258 citations.

A survey of distributed power system — AC versus DC distributed power system

Abstract: The history of the centralized and distributed power system is introduced far away from the power system being first built by Edison, which started the first confrontation between ac and dc power system. Although ac power system has dominated for a long century because of easy transmission and some other benefits, dc power system has still gave some good performances and attracted more attention during the past three decades. This paper will discuss the current developing stage of dc distributed power system by the following contents: the basic structure and characteristics; the benefits of the dc distribution system; the development of current techniques adopted in DPS and its analysis of protection and system interaction. In additional, the comparisons between dc and ac distributed power system are presented in detail, and finally, the challenge in future is also summarized.

A Novel Detection Method for Voltage Sags

Abstract: Determining the start and end of the voltage sag event is very important for sag analysis and mitigation. There are several detection methods for voltage sags in which sag voltages are usually expressed in the terms of RMS. The RMS method represents one cycle historical average value, not instantaneous value which may lead to long detection time when voltage sag has occurred. This paper will proposed a novel voltage sag detection method based on miss voltage technique. Proper dead-band and hysteresis are used in the method. The actual instantaneous voltage is compared with certain percentage of desired grid voltage and certain percentage of the amplitude of the grid voltage. Through instantaneous value comparison, low instantaneous value of the grid is shielded which overcome the mishandling turnover of voltage sags. The approach is fully described, and the results are compared with other methods for marking the beginning and end of sag, such as RMS value evaluation method and Peak-value method and simulation result provides that the method is efficient and fast and can be used to determine the initiation and recovery of voltage sags accompanied by missing voltage technique.

A Family of Dual-Phase-Combined Zero-Current Switching Switched-Capacitor Converters

Abstract: A family of dual-phase-combined resonant switched-capacitor (SC) converters with a similar structure is presented in this paper. Comparing with their conventional single-phase version, each of the proposed dual-phase converters is more than just a switched capacitor but also provides the service of two phases operated in the fully complementary manner. This design provides a more stable output voltage with low ripple and significantly reduces the quantity of switches. These converters therefore have the common features of smaller size, lower cost, and higher power density than the conventional multiphase SC converters. With the zero-current switching technique, the proposed converters also possess high-power conversion efficiency. The detailed analysis of the circuit operation and voltage ripples is presented. Experimental results are also provided to confirm the performance of the new family of converters.

Examination of bifurcation of the non-linear dynamics in buck-boost converters with input capacitor rectifier

Abstract: The exact formulation based on stroboscopic discrete maps is used to investigate the dynamical behaviours of the peak current-programmed buck-boost AC–DC converters using simple capacitor rectifier. The impact of the input capacitor on the dynamics of the converters is shown. Fast-scale instability and the relationship between the periodicity of the intermittent behaviours and input line cycle are reviewed under the condition of using input capacitor rectifier. Finally, a detailed investigation of input capacitor on the stable border in the buck-boost converters is studied and the Jacobian derivation has also been used to predict the border of the bifurcation. Experimental results are given to verify the analysis and simulation, which are very useful for the prediction of stable boundary and design of input filter in the power electronics systems.

Simulation of the integrated controller of the anti-lock braking system

Abstract: This paper presents simulation of integrated controller of vehicle anti-lock braking system, which consists of the traditional continuous PID and finite state machine theory. Based on the integrated controller, slip ratio and deceleration are selected as the main control parameters to optimize ABS. Vehicle speed, wheel speed, braking distance, pressure state and slip ratio are investigate to reveal the performance of anti-lock braking system. Single-wheel system is studied in this paper to prove that the controller can effectively reduce braking distance and time and also can improve the stability of anti-lock braking system, which can provide a guideline to the design of anti-lock braking system.

Grid-Interface Bidirectional Converter for Residential DC Distribution Systems—Part One: High-Density Two-Stage Topology

Abstract: With the emerging installations of multitype renewable energy sources and energy storage elements, the dc electronic distribution systems in residential buildings (dc nanogrid) are becoming an alternative future system solution, achieving a zero net-energy consumption and optimized power management. The concept of the energy control center (ECC), which interconnects the dc system to the traditional ac utility grid, is introduced, and the operation function of ECC converter suitable for dc nanogrid application is defined. This paper investigates a two-stage topology using a full bridge in series with a bidirectional synchronous rectifier dc–dc converter as a single-phase ECC for dc nanogrid, with a significant reduction of the dc-link capacitor value. The operation analysis and the design of passive components are provided. A bidirectional control system and the design process are also presented in terms of the system requirement and the small dc-link capacitor.

Intergrid: A Future Electronic Energy Network?

Abstract: Anticipated widespread usage of new power electronics technologies in electrical energy generation and consumption is expected to provide major efficiency improvements, while the deployment of smart grid technologies should improve the utilization and availability of electricity. This paper explores possible relationships between these two trends. Starting from an analysis of current and expected trends in the generation, transport, and consumption of electrical energy, this paper contemplates possible future ac and dc electronic power distribution system architectures, especially in the presence of renewable energy sources. The proposed nanogrid-microgrid-E-grid structure achieves hierarchical dynamic decoupling of generation, distribution, and consumption by using bidirectional electronic power converters as energy control centers. Several possible directions for modeling, analysis, and system-level design of such systems, including power flow control, protection, stability, and subsystem interactions, are briefly discussed.

Impedance-Based Local Stability Criterion for DC Distributed Power Systems

Abstract: This paper addresses the stability issue of dc distributed power systems (DPS). Impedance-based methods are effective for stability assessment of voltage-source systems and current-source systems. However, these methods may not be suitable for applications involving variation of practical parameters, loading conditions, system's structures, and operating modes. Thus, for systems that do not resemble simple voltage-source systems or current-source systems, stability assessment is much less readily performed. This paper proposes an impedance-based criterion for stability assessment of dc DPS. We first classify any converter in a dc DPS as either a bus voltage controlled converter (BVCC) or a bus current controlled converter (BCCC). As a result, a dc DPS can be represented in a general form regardless of its structure and operating mode. Then, the minor loop gain of the standard dc DPS is derived precisely using a two-port small signal model. Application of the Nyquist criterion on the derived minor loop gain gives the stability requirement for the dc DPS. This proposed criterion is applicable to dc DPSs, regardless of the control method and the connection configuration. Finally, a 480 W photovoltaic (PV) system with battery energy storage and a 200 W dc DPS, in which the source converter employs a droop control, are fabricated to validate the effectiveness of the proposed criterion.

A literature review of Microgrids: A functional layer based classification

Abstract: Operation of distributed energy resources and resilience related problems are becoming of most importance in the pursuit for a more sustainable electricity delivery. Microgrids (MGs) could contribute significantly to both issues and may play an important role in the new decentralized paradigm of power systems. This paper proposes a hierarchical organizational scheme of MGs with a clear distinction of the Microgrid, Nanogrid and Picogrid concepts, and addresses a detailed technical literature review to identify and classify MGs main features and design alternatives. Definitions; descriptions of MGs, Nanogrids, Picogrids; operation modes; business models and communication protocols are dispersed in the literature. In order to gather clearly all the existing information, this review has been organized according to four functional layers, inspired by the ones commonly used in Smart Grid architecture description. For each one and whenever possible, the paper classifies the outcomes of the review in a table kind of format that helps summarizing the different MGs׳ options. Regarding the first layer, the paper describes the different physical devices involved in MGs, such as generators, converters, electric vehicles (EV) and energy storage systems (DS) and the DC levels that are being discussed. For the second layer the paper addresses a review of the communication protocols currently used or proposed. Then, the intelligence layer is analyzed concerning any decision making issue related to operation and planning of the MGs as well as any other aspect that may be relevant for taking advantage of MGs potentiality. In the last layer, the paper reviews different business models adapted to a future where MGs will be deployed. Finally, the paper ends with an exhaustive review of the practical MG experiences in place worldwide, classified according to the issues presented previously, and a brief discussion of the key points to be addressed in the MGs deployment.