The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

Due to increasing concerns about global warming, greenhouse gas emissions, and the depletion of fossil fuels, the electric vehicles (EVs) receive massive popularity due to their performances and efficiencies in recent decades. EVs have already been widely accepted in the automotive industries considering the most promising replacements in reducing CO2 emissions and global environmental issues. Lithium-ion batteries have attained huge attention in EVs application due to their lucrative features such as lightweight, fast charging, high energy density, low self-discharge and long lifespan. This paper comprehensively reviews the lithium-ion battery state of charge (SOC) estimation and its management system towards the sustainable future EV applications. The significance of battery management system (BMS) employing lithium-ion batteries is presented, which can guarantee a reliable and safe operation and assess the battery SOC. The review identifies that the SOC is a crucial parameter as it signifies the remaining available energy in a battery that provides an idea about charging/discharging strategies and protect the battery from overcharging/over discharging. It is also observed that the SOC of the existing lithium-ion batteries have a good contribution to run the EVs safely and efficiently with their charging/discharging capabilities. However, they still have some challenges due to their complex electro-chemical reactions, performance degradation and lack of accuracy towards the enhancement of battery performance and life. The classification of the estimation methodologies to estimate SOC focusing with the estimation model/algorithm, benefits, drawbacks and estimation error are extensively reviewed. The review highlights many factors and challenges with possible recommendations for the development of BMS and estimation of SOC in next-generation EV applications. All the highlighted insights of this review will widen the increasing efforts towards the development of the advanced SOC estimation method and energy management system of lithium-ion battery for the future high-tech EV applications.

A review of lithium-ion battery state of charge estimation and management system in electric vehicle applications: Challenges and recommendations

This paper investigates the characteristics of the active and reactive power sharing in a parallel inverters system under different system impedance conditions. The analyses conclude that the conventional droop method cannot achieve efficient power sharing for the case of a system with complex impedance condition. To achieve the proper power balance and minimize the circulating current in the different impedance situations, a novel droop controller that considers the impact of complex impedance is proposed in this paper. This controller can simplify the coupled active and reactive power relationships, which are caused by the complex impedance in the parallel system. In addition, a virtual complex impedance loop is included in the proposed controller to minimize the fundamental and harmonic circulating current that flows in the parallel system. Compared to the other methods, the proposed controller can achieve accurate power sharing, offers efficient dynamic performance, and is more adaptive to different line impedance situations. Simulation and experimental results are presented to prove the validity and the improvements achieved by the proposed controller.

Design and Analysis of the Droop Control Method for Parallel Inverters Considering the Impact of the Complex Impedance on the Power Sharing

With the rapidly evolving technology of the smart grid and electric vehicles (EVs), the battery has emerged as the most prominent energy storage device, attracting a significant amount of attention. The very recent discussions about the performance of lithium-ion (Li-ion) batteries in the Boeing 787 have confirmed so far that, while battery technology is growing very quickly, developing cells with higher power and energy densities, it is equally important to improve the performance of the battery management system (BMS) to make the battery a safe, reliable, and cost-efficient solution. The specific characteristics and needs of the smart grid and EVs, such as deep charge/discharge protection and accurate state-of-charge (SOC) and state-of-health (SOH) estimation, intensify the need for a more efficient BMS. The BMS should contain accurate algorithms to measure and estimate the functional status of the battery and, at the same time, be equipped with state-of-the-art mechanisms to protect the battery from hazardous and inefficient operating conditions.

Battery Management System: An Overview of Its Application in the Smart Grid and Electric Vehicles

A comprehensive review of lithium-ion batteries used in hybrid and electric vehicles at cold temperatures

In this paper, a hybrid switch with the combination of large-current silicon (Si) insulated gate bipolar transistor (IGBT) and small-current silicon carbide (SiC) metal-oxide semiconductor field-effect transistor ( mosfet ) is utilized in a three-phase T-type three-level grid inverter. The hybrid switch can optimize the cost of the system while achieving high efficiency. The relationship between efficiency improvement and cost of the hybrid switch is investigated. Several switching patterns, which can decrease the loss of the small-current SiC mosfet for the hybrid switch, are studied. Different switching patterns for the hybrid switch of the grid inverter are compared based on the inverter efficiency. Finally, a 20 kW three-phase T-type three-level grid inverter prototype with the hybrid switch of large-current Si IGBT and small-current SiC mosfet (1:2.4 SiC/Si current ratio) is built to verify the main results.

Efficiency Improvement of Grid Inverters With Hybrid Devices

Bidirectional DC/DC converter is an important part of the more electric aircraft (MEA). A high-performance bidirectional DC/DC converter with high efficiency and power density reduces the both volume and power loss of the converter. There are mainly two kinds of basic topologies applied to bidirectional DC/DC converter. One is a dual active bridge (DAB) and the other is symmetrical CLLC resonant topology. Both topologies can realise zero-voltage switch (ZVS). The DAB has been widely researched and applied to many fields including the MEA as DAB is easy to design and control. However the ZVS region of the DAB is limited. What is worse, the DAB always switches when the current is at the peak. Symmetrical CLLC resonant topology has also been widely researched in recent years. Compared with the DAB, the symmetrical CLLC resonant converter can reach ZVS in a wider region. In this study both the topologies are discussed and the design of the better one is demonstrated. A 3 kW symmetrical CLLC resonant converter is designed to verify the conclusion.

Design of high-performance bidirectional DC/DC converter applied for more electric aircraft

This paper proposes an optimized mathematical model of the voltage-source inverter parallel-operation system (VSIPS) and proposes an improved droop control method based on this model, which can realize accurate power sharing in VSIPS. First, this paper analyzes VSIPS as a multi-input and multioutput system and proposes a precise definition of circulating current. The circulating-current model, the steady-state model, and the small-signal model are proposed subsequently based on the optimum design of wire impedance, which constitute the optimized mathematical model of VSIPS in s -domain. The circulating-current phasor model and the circulating-current power phasor model (CCPPM) are also proposed. Second, the mathematical model of traditional droop control is built and analyzed based on CCPPM, which elaborates the tradeoff of droop control between the steady-state voltage bias and the load-sharing accuracy, and proves that the $V- Q$ droop control cannot realize accurate reactive-power sharing. Third, an improved droop control method ( $\omega - P_{{\text{cir}}}$ and $V- Q_{{\text{cir}}}$ control) is proposed, which can realize the accurate active- and reactive-power sharing and eliminate the steady-state voltage bias simultaneously. Finally, simulation and experimental results are presented, which validate the proposed mathematical model of VSIPS, the analysis of droop control, and the performance of the proposed method.

An Accurate Power-Sharing Control Method Based on Circulating-Current Power Phasor Model in Voltage-Source Inverter Parallel-Operation System

A micro-inverter could be directly integrated with a PV panel to generate grid-compatible AC power. Placing two flyback converters in parallel as the DC-DC stage and utilizing interleaving extends the power range of flyback micro-inverters. In this paper, open-loop interleaving technique for flyback micro-inverter operating at the boundary conduction mode (BCM) and discontinuous conduction mode (DCM) with a master-slave relationship are analyzed first. Then, a novel interleaving phase synchronization control method is proposed to achieve high power conversion efficiency and stable operation at the same time. In DCM operation region the open-loop phase regulation will be utilized while in BCM a closed-loop technique will dominate. A 200 W interleaved flyback micro-inverter prototype is built to verify the proposed interleaving synchronization control method. Simulation and Experimental results are provided to show that the proposed closed-loop control method could achieve stable interleaving phase synchronization while maintain good efficiency and low THD performance.

Design and Analysis of the synchronization control method for BCM/DCM current-mode flyback micro-inverter

With a high power level and a large transfer distance in a wireless charging system of electric vehicles, human exposure to electromagnetic filed becomes a major concern. In this paper, some key parameters of an aluminum plate and ring shield are studied with the aid of finite-element analysis. The shielding effectiveness and the power transfer efficiency are investigated simultaneously as one of the parameters changes. It is found that the shielding effect mainly depends on the size of the shield while increasing the distance between the shield and the ferrite layer attenuates efficiency drop due to the eddy-current loss in the shield. A 3.3kW wireless charging system with 20cm air gap was built to verify the simulation results.

Min Chen

Papers

Efficiency Improvement of Grid Inverters With Hybrid Devices

Design of high-performance bidirectional DC/DC converter applied for more electric aircraft

An Accurate Power-Sharing Control Method Based on Circulating-Current Power Phasor Model in Voltage-Source Inverter Parallel-Operation System

Design and Analysis of the synchronization control method for BCM/DCM current-mode flyback micro-inverter

A study on the shielding for wireless charging systems of electric vehicles