Optimal State Estimation

Isolated dc–dc converters with galvanic isolation are commonly used in electric vehicle (EV) battery chargers. These converters interface between a dc voltage link, which is usually the output of a power factor correction stage, and an energy storage unit. CLLC and dual active bridge (DAB) dc–dc converters can achieve high power density, high-energy efficiency, wide gain range, galvanic isolation, and bidirectional power flow, and therefore, have potential applications as dc–dc converters for bidirectional EV charging systems. In this paper, full-bridge CLLC, half-bridge CLLC, full-bridge DAB, and half-bridge DAB dc–dc converters are evaluated and compared for their suitability for EV chargers. All the converters are designed with optimal soft-switching features. The operating principles, design methodologies, and design considerations are presented. Prototypes of the converters with power rating of 1 kW are designed and developed. The prototypes interface a 500 V dc link and a 200–420 V load, which is common for EV applications. The performances of the circuits are analyzed and a comprehensive comparison is conducted.

Comprehensive Analyses and Comparison of 1 kW Isolated DC–DC Converters for Bidirectional EV Charging Systems

High power density is strongly preferable for the on-board battery charger of Plug-in Hybrid Electric Vehicle (PHEV). Wide band gap devices, such as Gallium Nitride HEMTs are being explored to push to higher switching frequency and reduce passive component size. In this case, the bulk DC link capacitor of AC-DC Power Factor Correction (PFC) stage, which is usually necessary to store ripple power of two times the line frequency in a DC current charging system, becomes a major barrier on power density. If low frequency ripple is allowed in the battery, the DC link capacitance can be significantly reduced. This paper focuses on the operation of a battery charging system, which is comprised of one Full Bridge (FB) AC-DC stage and one Dual Active Bridge (DAB) DC-DC stage, with charging current containing low frequency ripple at two times line frequency, designated as sinusoidal charging. DAB operation under sinusoidal charging is investigated. Two types of control schemes are proposed and implemented in an experimental prototype. It is proved that closed loop current control is the better. Full system test including both FB AC-DC stage and DAB DC-DC stage verified the concept of sinusoidal charging, which may lead to potentially very high power density battery charger for PHEV.

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Dual active bridge based battery charger for plug-in hybrid electric vehicle with charging current containing low frequency ripple

https://cyberleninka.org/article/n/696659.pdf

Comparative assessment of maximum power point tracking procedures for photovoltaic systems

In this paper, a bidirectional battery charger is proposed for grid-to-vehicle, vehicle-to-grid, and vehicle-to-home operations of electric vehicles. The proposed charger adopts sinusoidal charging to eliminate the use of electrolytic capacitors. A nonregulating series resonant converter is employed for isolation, thereby minimizing the ratings of the components and achieving zero-current switching turn on and off of switches. Meanwhile, an ac–dc converter is employed to control each mode and their mode changes, making the whole control scheme simple and the mode changes seamless. A 3.3-kW prototype was built in the laboratory to validate the proposed concept. Our experimental results demonstrate that the proposed charger has high efficiency and can seamlessly switch between the three operation modes.

An Electrolytic Capacitorless Bidirectional EV Charger for V2G and V2H Applications

A bidirecitonal dual active converter with the power factor control capability is proposed for low-power on-board battery charger application. The source side half-bridge acts as a pulse-width modulation (PWM) converter that maintains the unity power factor. The battery side half-bridge converter acts as a dual active bridge (DAB) together shares the same DC link voltage with the PWM converter. Asymmetric duty cycle technique in which turn-on and -off time duration of primary switches in DAB converter are changed to control the charges of two capacitors is employed to eliminate imbalance voltage phenomenon. Thus, the input current can shape in phase with the voltage source and promising high efficiency of PWM converter. Simulation and experimental results are included to verify theoretical analysis.

A single-phase bidirectional dual active half-bridge converter

The second-order-generalized-integrator frequency-locked loop (SOGI-FLL) is typically employed to synchronize with the grid in the single-phase grid-connected converter. The SOGI creates two orthogonal signals from an input grid voltage, and estimates the magnitude while the frequency locked-loop identifies the grid frequency. If there is a DC component in the grid voltage due to the measurement process, it degrades the accuracy of SOGI-FLL estimates. The SOGI-FLL, therefore, cannot identify accurate information of the grid voltage. This paper introduces an improved control method for the SOGI-FLL when the grid contains a DC component. The value of the DC component voltage is estimated online and totally removed from the frequency control loop. The controller, thus, responses faster and more precisely detects both the magnitude and frequency of the input voltage. Simulation results are presented to verify the theoretical analyses.

Improving performance of single-phase SOGI-FLL under DC-offset voltage condition

The steady-state performance of low frequency AC (LFAC) systems for bulk power transmission is proposed and investigated in this paper. It is demonstrated that the LFAC is superior to the conventional 60 Hz AC system in terms of power transfer capability and voltage stability. An existing power system can increase its power transfer capability by up to more than eleven times if it operates at 5 Hz. In low operating frequency conditions, the transmission overhead line reactance is significantly lowered and thus results in less voltage drop along the line. If the system operating frequency is in the range of 1 to 10 Hz, the voltage profile along a transmission line is comparable to the surge impedance loading at the no-load condition or approaches that of the HVDC system at the full-load condition. More importantly, the V-P and Q-V characteristic curves show that the dependency of voltage on the real and reactive power flow is greatly reduced. In other words, the sensitivity of voltage on reactive power variations is diminished and results in a more stable voltage and a higher stability margin for the LFAC system compared to the conventional 60 Hz system.

Steady-State Analysis and Performance of Low Frequency AC Transmission Lines

This paper presents a thorough scope of a wide-used single-phase bidirectional battery charger which composes of a pulse-width modulation (PWM) converter in the front-end and a dual active bridge (DAB) converter for high power applications. The computer simulations of PWM converter and DAB converter are separately shown to support theoretical analyses. A prototype converter was built and tested at different operating conditions. Experiment results at 3.3 kW are given to verify theory and simulation results as well.

Study of single-phase bidirectional battery charger for high power application

With the advances in high power converter technologies, photovoltaic (PV) power has been integrated into the utility system in greater pace in recent years. This paper aims to provide a broad overview of the various grid-connected PV system configurations and their power converter technologies for applications in single- and three-phase utility power systems. Grid-connected PV systems presented include module-integrated, string, and centralized configurations. Single- and three-phase power converters discussed include full-bridge (with an additional switch in the DC link and back-to-back IGBT on the AC terminals), and neutral point clamped converters. The synchronization techniques for grid-connected PV applications are discussed in this paper. An improved method to detect the grid frequency and voltage magnitude for single-phase system is introduced. More importantly, harmonic distortions resulting from the operation of grid-connected photovoltaic systems are addressed and analyzed. Methods for reducing harmonic currents using three types of passive filters, L, LC, and LCL, are presented. The LCL-filter design guidelines for a specific PV system are presented in detail. The effectiveness of these filters is demonstrated on 3 kW and 50 kW photovoltaic systems.

Tuan Ngo

Papers

A single-phase bidirectional dual active half-bridge converter

Improving performance of single-phase SOGI-FLL under DC-offset voltage condition

Steady-State Analysis and Performance of Low Frequency AC Transmission Lines

Study of single-phase bidirectional battery charger for high power application

Grid-connected photovoltaic converters: Topology and grid interconnection