Y. C. Fong

Topology, Modeling, and Design of Switched-Capacitor-Based Cell Balancing Systems and Their Balancing Exploration

A series of switched-capacitor (SC) cell balancing circuits is proposed for rechargeable energy storage devices like battery and supercapacitor strings in this paper. Taking a basic SC-based cell balancing unit as an equivalent resistor, the behavioral models of the proposed cell balancing circuits are developed to evaluate their balancing performance. Comparing with existing SC-based cell balancing circuits, the main advantage of the proposed circuits is that their balancing speed is independent of both of the number of battery cells and initial mismatch distribution of cell voltages. In order to improve the operation performance of SC-based cell balancing circuits in the respect of minimizing the equivalent resistance, optimizing methodologies of circuit parameters are introduced by referring the concepts of slow switching limit and fast switching limit as well as inductive switching limit of SC power converters. Simulation and experimental results are provided to verify the feasibility of the proposed cell balancing circuits.

An adaptive modulation scheme for fundamental frequency switched multilevel inverter with unbalanced and varying voltage sources

Stepped modulation operates at fundamental frequency allows the use of low speed switching devices and mitigates the switching loss and EMI concerns suffered by many other hard-switched inverters adopting carrier-based pulse-width modulation (PWM) such as space vector PWM and sinusoidal PWM. Half-height (HH) method is one of the simplest ways to compute switching angles of stepped sinusoidal output. Similar approaches have been utilized in digital-analog signal conversion and synthesis. However, there is limited research on HH-based modulation for power inverters. In this paper, a computational modulation scheme based on HH method was studied. The proposed modulation scheme is capable for real-time computation with most present microprocessors. This paper will demonstrate the implementation of the proposed modulation scheme at balanced voltage and the techniques of adaption to varying and unbalanced sources. Capability of having minimum total harmonic distortion (THD) under equal voltage sources of HH method was verified. The performance of the proposed modulation scheme was examined using numeric calculation and simulation with a 7-level cascade H-bridge inverter and the results are presented.

Centralized Regulation Scheme for a Parallel-Mode Switched-Capacitor Converter System With Simple Unit Commitment

Novel load regulation methods for switched-capacitor converter (SCC) system operating in parallel mode are proposed. SCCs are ready for integrated design which is ideal for mass production and also enables power conversion in compact footprint. At the same time, they are excellent candidates for parallel mode operation since the load current sharing among them is automatically allocated by the inherent droop characteristic, which is determined by the equivalent internal resistance. Instead of controlling the individual switching frequency of SCC units, this paper proposes a number of unit commitment schemes which regulate the output voltage by adjusting the number of activated SCC units. In addition to conventional discrete unit commitment, intermediate number of activated units is realized by employing pulse-dropping pulse width modulation. The proposed methods are verified by simulation and the experiment of a parallel SCC system consists of a number of double-mode zero-current switching SCC units.

Prediction of charging and discharging performances of supercapacitor modules

Due to higher specific power density and longer life time, super-capacitors (ultra-capacitors or electric double layer capacitors) are being considered in applications of energy storage systems. This paper presents the model of super-capacitor modules that consist of super-capacitors connected in series and in parallel. It provides the solution to predict the charging and discharging performances of supercapacitor modules. Thus, this paper provides a valuable approach for performance prediction of super-capacitor modules applied to energy storage systems.

Investigation of energy distribution and power split of hybrid energy storage systems in electric vehicles

In this paper, the model of the electric vehicles (EVs) with the hybrid energy storage system (HESS) consisting of battery and super-capacitor is proposed. Using the proposed model, the operation of the EV with HESS is simulated for various energy distributions of battery and super-capacitor (SC) energy storage system. The losses of the battery and super-capacitor, the travelled distance and the ratio of the SC power to the bus power are investigated and discussed. The study in this paper reveals the relationships between the losses of the battery and the super-capacitor, the energy distribution, the power allocation, the travelled distance and the load, and shows that the optimal ratio of the SC power to the bus power can be found to minimize the losses of the battery and the SC and maximize the travelled distance of the EV. Thus, this paper offers the valuable investigation for the optimal energy management and the maximum efficiency control of HESS.

An ESS charge balancing method based on current allocation with multi-source power converters for electric microcars

Batteries are one of the most costly components in electric vehicles (EVs) yet they also limit the performance and lifespan of the vehicles. Voltage imbalance between series connected battery cells worsens the limitations but is inevitable due to the manufacturing tolerance of capacity and deviations of environmental and batteries parameters. Industries have developed a wide range of solutions to keep cell voltage equalized including adding balancing circuits and specialized chargers to improve the battery capacity utilization and to extend the lifespan of whole battery pack. However, these methods are generally ineffective for the energy storage systems (ESSs) in EVs. Instead of developing circuitries for cell balancing, this study would like to investigate the effectiveness of battery charge balancing by load/charge allocation with multi-source power converters such as multilevel voltage source inverters (VSIs) that directly connect the batteries in ESSs as sources. A simulation model with an 11-level cascaded H-bridge inverter (CHBI) was employed to examine the proposed switching scheme. The simulation result is encouraging that voltage sources were balanced by utilizing a significant amount of the load/charging current as balancing current; the sources attained equal voltage within a short period in simulation environment.