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

This paper presents the analysis and implementation of a PLL structure for single-phase grid-connected system. By comparing two different PLL structures based on SRF (synchronous reference frame) and p-q (instantaneous reactive power) theory respectively, a simplified control model of the PLL system is developed. This paper also analyzes the generation of the orthogonal voltage system based on SOGI which is essential in both of the PLL structures. Finally, simulation based on MATLAB and experimental results from a DSP-based system are all included to support the theoretical analysis.

Analysis and implementation of a PLL structure for single-phase grid-connected inverter system

Effects of lower order harmonic grid voltage on the stator current of the Doubly Fed Induction Generator (DFIG) are investigated. A stator harmonic current control method for DFIG wind power system is presented to suppress the effects of the lower order harmonic grid voltage on the stator current. The proposed control scheme is analyzed using transfer function. The performance of the proposed control method is verified by both simulation and the experiment on a DFIG wind power system prototype.

Stator harmonic current suppression for DFIG wind power system under distorted grid voltage

A hybrid AC-DC-AC topology for Uninterruptible Power Supply (UPS) is investigated, which is composed of a two level three-phase four-wire rectifier and a three level T-type inverter. It has distinct advantages for the applications of on line UPS. The grid side power quality can be easily satisfied with the front-end two level three-phase rectifier. Since output voltage's THD is challenge to UPS with sever nonlinear load, the T-type inverter is used to enhance the load power quality with higher conversion efficiency. The rectifier is transformed into step-up converter for the battery during the grid faults. The efficiency optimization of the proposed UPS with the hybrid AC-DC-AC topology is studied. The control strategies for the UPS are investigated. At last, the UPS prototype with 200kVA is designed and tested to validate the performance of the proposed UPS.

Design of hybrid AC-DC-AC topology for Uninterruptible Power Supply

A two phase ZVS full-bridge converter with a novel hybrid current doubler rectifier structure is proposed in this paper. With the proposed topology, the secondary windings of the transformers work in series when the input voltage is low but in parallel when the input voltage is high. The diode stress on the second winding is reduced and the interleaving operation reduces the current ripple in the output filter. It can be employed in low voltage high current ouput and high power applications. A prototype with 200–400V input and 50V/20A output is built up to verify the theoretical analysis.

A hybrid ZVS full-bridge converter with transformer winding series-parallel auto regulated current doubler rectifier

A novel ZVS dual bridge DC/DC converter is presented. The converter is composed of two dual-transistor-forward converter, coupled with a single high frequency transformer. Zero voltage switching is realized by introducing a proper leakage inductance to the secondary of the high frequency isolation transformer in cope with a designed PWM control. Operation principle and ZVS condition of the proposed converter are analyzed. Experimental results obtained from a 3.2 kW prototype are given. Extensions of the proposed converter topologies are presented.

Min Chen

Papers

Analysis and implementation of a PLL structure for single-phase grid-connected inverter system

Stator harmonic current suppression for DFIG wind power system under distorted grid voltage

Design of hybrid AC-DC-AC topology for Uninterruptible Power Supply

A hybrid ZVS full-bridge converter with transformer winding series-parallel auto regulated current doubler rectifier

Transformer secondary leakage inductance based ZVS dual bridge DC/DC converter