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

The thermal management plays an important role in improving the reliability and lifetime of high power converters, especially for the high voltage direct current (HVdc) transmission system. In the voltage source converter based HVdc systems (VSC-HVdc), the hybrid modular multilevel converters (MMCs) are taken as the excellent candidates due to their dc short-circuit fault ride-through ability and less submodule (SM) devices. Furthermore, the voltage modulation index of hybrid MMCs can be promoted to achieve higher power rating and efficiency. However, in this article, it is revealed that thermal stress distribution inside the SMs becomes more unbalanced under a high voltage modulation index, which would lead to serious thermal fatigue. To overcome this problem, the active bypass with thyristor for half-bridge submodules (HBSMs) and symmetrical modulation for full-bridge submodules (FBSMs) based active thermal control strategies are employed. With the presented thermal control strategies, the junction temperature of the most stressed devices in both HBSMs and FBSMs is reduced enormously. Besides, the thermal control strategy does not bring negative influence on the steady-state output performance of hybrid MMC. Finally, the theoretical analysis and effectiveness of the proposed control methods are verified based on a laboratorial MMC prototype.

Active Thermal Control for Hybrid Modular Multilevel Converter Under Overmodulation Operation

A flyback converter (20) uses primary side sensing to sense the output voltage for regulating feedback. The flyback converter comprises a transformer with a primary winding (L1) and a secondary winding (L2), a first transistor (M1) coupled to the primary winding for conducting a current through the primary winding when the first transistor is on a second transistor (M2) for conducting a current through the secondary winding when the second transistor is on a regulator (14) coupled to the first transistor for controlling a duty cycle of the first transistor to regulate an output voltage of the converter, the regulator being configured to control the first transistor to have a minimum duty cycle, an output capacitor (C1) coupled to an output terminal of the converter, a synchronous rectifier controller (24) coupled to the second transistor for controlling the second transistor to be on or off, a comparator (42) having one input coupled to receive a voltage corresponding to the output voltage of the converter and having another input connected to a reference voltage representing a threshold voltage exceeding a regulated voltage of the converter, wherein triggering of the comparator signifies an over-voltage condition, an output of the comparator being coupled so as to control the synchronous rectifier controller to turn the second transistor on for an interval to conduct a reverse current through the secondary winding, upon an over-voltage condition being detected, to reduce the output voltage of the converter to mitigate the over-voltage condition, and a diode (D1) coupled to the primary winding to conduct a current through the primary winding after the interval without turning on the first transistor, such that power is transferred from a secondary side of the transformer to the power source while mitigating the over-voltage condition.

Flyback converter with primary side voltage sensing and overvoltage protection during low load operation

Single-phase zero-voltage-switching (ZVS) inverter with wide bandgap devices has higher efficiency and power density. However, the dc-side capacitor of the inverter will suffer double line frequency ripple and reduces the lifetime of the dc bus capacitor. Active power decoupling (APD) is an effective method to replace the short lifetime electrolytic capacitor with a smaller film capacitor. In order to help achieving the ZVS condition for ZVS inverter and the additional APD circuit, this article presents a ZVS topology for single-phase full-bridge inverter with APD circuit by adding an auxiliary branch between the dc source and the bridge legs. The auxiliary branch is composed of a clamping capacitor, an auxiliary switch, and a resonant inductor. A ZVS pulsewidth modulation scheme is proposed for a single-phase inverter with APD. The gate drive pulses are designed to realize the ZVS turn- on for the main switches of both APD circuit and single-phase full-bridge inverter. Circuit operation stages are analyzed and ZVS conditions are given. Finally, the proposed ZVS inverter with APD circuit is verified on a 1.5-kW inverter prototype.

Zero-Voltage-Switching Single-Phase Full-Bridge Inverter With Active Power Decoupling

Emerging ad-hoc wireless sensor nodes and other micro-scale applications demand long operational lives, small form factors, and total integration, which are next to impossible to fully achieve with conventional battery technologies. Efficient, power-moded, fully integrated systems inherently demand high peak-to-average power ratios (PAPRs), as in wireless sensor applications where telemetry is a power-consuming function with low duty-cycle operation. Lithium ion batteries (Li-Ion), while conforming to micro-scale dimensions and supplying moderate power densities, cannot store enough energy to sustain extended lifetimes, which is where fuel cells (FCs) excel. Although various control strategies for energy flow between batteries and FCs have been proposed in the past, none of them superimpose the severe constraints of a micro-scale system on the design, where volume, energy, and power are scarce and the performance of the MEMS FCs degrade with time. This paper presents a hybrid micro-scale MEMS FC-thin-film Li-Ion source and proposes a design methodology for the same wherein volume, energy, and power are optimized for peak-power and extended-lifetime performance. The FC is ultimately used to both charge and supply the load asynchronously, depending on the state of the load, while the Li ion mostly functions as a power cache. System simulations of a multi-sensor wireless system load show and validate how peak-power, average-power, duty-cycle, and frequency performance are achieved and how they relate to lifetime.

Design methodology of a hybrid micro-scale fuel cell-thin-film lithium ion source

Droop control method is widely used in wireless parallel inverters control strategy. In this application, inverters output active power and reactive power need to be calculated accurately for control of the angular frequency and amplitude of output voltage. Traditional calculation methods have a slow and oscillating transient response and could be easily impacted by disturbance and variation of load. To overcome these limitations, this paper suggests an more suitable method of power calculation to increase calculating speed and improve the dynamic response especially under nonlinear load conditions.

Min Chen

Papers

Active Thermal Control for Hybrid Modular Multilevel Converter Under Overmodulation Operation

Flyback converter with primary side voltage sensing and overvoltage protection during low load operation

Zero-Voltage-Switching Single-Phase Full-Bridge Inverter With Active Power Decoupling

Design methodology of a hybrid micro-scale fuel cell-thin-film lithium ion source

Power calculation method used in wireless parallel inverters under nonlinear load conditions