Fault-current handling capability of the modular multilevel converters (MMCs) under dc-cable short-circuit conditions is a major concern for the MMC applications on the high-voltage direct-current (HVDC) transmission systems, where the MMCs based on half-bridge submodules (SMs) cannot block the fault currents to protect the converter devices. In this paper, a comprehensive review for the fault-ride-through capability of the HVDC transmission systems based on the MMCs adopting different SM schemes is presented, where the MMCs can block the fault currents and compensate the reactive currents to the electric grid during the dc faults. An analysis of the dc short-circuit faults in the MMC is introduced and then the operation principle of different SM circuits building the MMC for blocking the fault currents is highlighted. The fault-tolerant operation of these MMC schemes as static synchronous compensator to enhance the ac grid stability during the dc faults is also investigated. A comparison in terms of investment cost, loss, volume, and controllability for various MMC topologies is performed. Comprehensive simulation results for the most promising topologies of the MMC with a capability of fault-ride through under dc-fault conditions are finally presented.

An Overview of Modular Multilevel Converters in HVDC Transmission Systems With STATCOM Operation During Pole-to-Pole DC Short Circuits

This article proposes a hybrid LLC resonant converter with three modes of operation for on-board chargers of electric vehicles. The converter can operate as a full-bridge converter, dual-phase half-bridge LLC converter, or single-phase half-bridge LLC converter to improve efficiency at low-voltage and light-load conditions. Without using additional switches, the operation mode of the proposed converter is switched by different modulation schemes. Moreover, the resonant inductance for LLC operation can be reduced so as to be integrated into the transformers, resulting in an increase in the power density. During the operation of three modes, all the primary switches achieve zero-voltage switching, thus minimizing the switching losses. A 3.2-kW-rated prototype using GaN devices with a power density of 65 W/in3 has been designed and tested to verify the effectiveness of the converter. Experimental results have shown smooth and stable transitions among three modes. A flat efficiency curve with a peak efficiency of 98.5% at full load condition has been achieved.

High-Efficiency Hybrid LLC Resonant Converter for On-Board Chargers of Plug-In Electric Vehicles

Many different types of electric vehicle (EV) charging technologies are described in literature and implemented in practical applications. This paper presents an overview of the existing and proposed EV charging technologies in terms of converter topologies, power levels, power flow directions and charging control strategies. An overview of the main charging methods is presented as well, particularly the goal is to highlight an effective and fast charging technique for lithium ions batteries concerning prolonging cell cycle life and retaining high charging efficiency. Once presented the main important aspects of charging technologies and strategies, in the last part of this paper, through the use of genetic algorithm, the optimal size of the charging systems is estimated and, on the base of a sensitive analysis, the possible future trends in this field are finally valued.

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Electric Vehicles Charging Technology Review and Optimal Size Estimation

This article proposes a novel isolated three-port dc/dc converter based on a series resonant converter and a dual active bridge (DAB) converter for electric-vehicle charging stations with fast and slow charging functions. With this three-port structure, the proposed converter has fewer components, which results in lower system cost and volume compared with separate charger systems. A simple control method using phase-shift and frequency modulations was developed to control the output power of the fast and slow charging ports simultaneously. An optimal phase-shift angle was also derived to minimize the transformer current for when only the DAB converter is operated for slow charging. To verify the converter operation, a 5-kW SiC-based prototype with a power density of 2.74 kW/dm3 was built and tested with an input voltage of 600 V. A high-efficiency performance over a wide output voltage range has been achieved, and the peak efficiency is 98.2% at the rated conditions.

High-Efficiency SiC-Based Isolated Three-Port DC/DC Converters for Hybrid Charging Stations

In this paper, a novel topology of six-level inverters for a medium-voltage high-power applications is proposed, which consists of inner flying-capacitor inverter (FCI) units and outer two-level inverter units. The proposed topology can reduce the number of devices, components, and isolated sources compared with the existing ones in the case of the same number of output voltage steps, which result in a reduction of the size and weight of the inverters. Also, the total power loss of the proposed topology is lower than that of the diode-clamped inverter (DCI) and FCI. Besides, the estimated cost of the proposed topology is about 60.5% and 81.3% compared with the DCI and FCI, respectively. In addition, the modulation technique and the control strategy for the dc-link capacitor and flying-capacitor voltages are developed for the proposed topology. The effectiveness of the proposed six-level inverter for the medium-voltage applications has been verified by simulation results. Also, the feasibility of the proposed inverter has been proved by experimental results for the prototype at the laboratory.

A Novel Six-Level Inverter Topology for Medium-Voltage Applications

This paper proposes a single-phase onboard battery charger (OBC) for plug-in electric vehicles, where the low-voltage (LV) battery charging circuit is utilized for an active power decoupling function. The OBC is operated in three different modes by sharing the transformer, switches, and capacitors. For a grid-to-vehicle mode or a vehicle-to-grid mode, the LV battery charging circuit serves as an active filter to eliminate the low-frequency power ripple at the DC link. Thus, the small film capacitors can be employed instead of large capacitors at the DC link. For the third operating mode, where the LV battery is charged from the HV battery, the isolation is provided by the dual active bridge (DAB) DC-DC converter. Since some components in the proposed OBC are used in common, the size and cost of the OBC can be reduced significantly. The simulation and experimental results have verified the validity of the proposed system.

Onboard Battery Chargers for Plug-in Electric Vehicles With Dual Functional Circuit for Low-Voltage Battery Charging and Active Power Decoupling

This article proposes a simple and effective method to reduce the bulky capacitor in onboard battery charger (OBC) where low-voltage (LV) battery charging circuit is utilized as an active power decoupling (APD) circuit to filter out the second-order ripple power. When the OBC is connected to the grid to charge the high-voltage (HV) battery, the switches on the primary side of the LV charging circuit are operated as those of an ac–dc converter with APD function. Consequently, small dc-link capacitors can be used instead of large capacitor banks. In the proposed OBC, the switching devices are shared for the ac–dc converter, APD circuit, and primary-side of the LV charger. For a simultaneous charging of the LV and HV batteries, a dc–dc converter is added, which shares the same transformer core and secondary side with the LV charging circuit. So the functions of the OBC are maintained in the proposed topology whereas the volume and cost are decreased by 52.3% and 46.9% compared with the conventional nonisolated OBC, respectively. A 2-kW SiC-based prototype has been designed and tested to verify the validity of the proposed system. The peak efficiencies of the OBC and LV charger are 96.1% and 95.3%, respectively.

A Novel SiC-Based Multifunctional Onboard Battery Charger for Plug-In Electric Vehicles

In this paper, a single-phase multifunctional onboard battery charger for electric vehicles (EVs) is proposed, where the active power decoupling capability is provided by utilizing the low voltage (LV) battery charging circuit. For this, the buck converter for the LV battery charger is used as an active power decoupling (APD) circuit while the high voltage (HV) battery is connected to the grid, by which the inherent second-order ripple power component of the single-phase charger is filtered out. Hence, small film capacitors can replace the large electrolytic capacitors, leading to the reduction of cost and the volume of the charger for EV applications. The effectiveness of the proposed charger is verified by experimental results.

Single-phase multifunctional onboard battery chargers with active power decoupling capability

This Article Proposes a Novel single-phase bridgeles power factor correction (PFC) boost rectifier with a small dc-link capacitor that can reduce the second-order power ripple at the dc output side. An auxiliary circuit was added to decouple the active power at the dc link, which forces the voltage ripple to divert to the auxiliary capacitor. Therefore, the dc-link capacitance can be reduced significantly. As a result, the bulky electrolytic capacitor can be replaced with a small film capacitor. The experimental results verified the validation of the proposed method.

Reducing the dc-Link Capacitance: A Bridgeless PFC Boost Rectifier That Reduces the Second-Order Power Ripple at the dc Output

In this paper, a single-phase non-isolated onboard battery charger with small DC-link capacitors is proposed, where the low-voltage battery charger is utilized as an active power filter to reduce the inherent second-order ripple power when the high-voltage (HV) battery is charged from the grid. In this scheme, the large DC-link capacitor banks of the HV battery charger can be eliminated without additional switches, leading to the reduction of cost and volume of the onboard battery charger. Also, this structure of EV chargers has distinctive features of three different modes such as grid-to-vehicle (G2V), vehicle-to-grid (V2G), and HV-to-LV battery (H2L) modes. The simulation and experimental results have verified the effectiveness of the proposed topology.

Hoang Vu Nguyen

Papers

Onboard Battery Chargers for Plug-in Electric Vehicles With Dual Functional Circuit for Low-Voltage Battery Charging and Active Power Decoupling

A Novel SiC-Based Multifunctional Onboard Battery Charger for Plug-In Electric Vehicles

Single-phase multifunctional onboard battery chargers with active power decoupling capability

Reducing the dc-Link Capacitance: A Bridgeless PFC Boost Rectifier That Reduces the Second-Order Power Ripple at the dc Output

Advanced Single-Phase Onboard Chargers with Small DC-Link Capacitors