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

Due to current sensor errors, tolerance of power switching devices, and asymmetry of PWM gating driving pulses, grid-connected inverters without isolation transformer usually have certain amount of dc components injected to the ac grid. Many efforts, such as using a blocking capacitor, a current dc component feedback control, and a voltage dc component feedback control have been introduced to try to suppress the dc injection to the grid. This paper proposes a novel control strategy of suppressing dc current injection to the grid for a three-phase inverter by accurately sensing the dc component of line voltages of three-phase inverter and adding a dc component control loop. A dc suppression control scheme is presented, and the suppression performance of the proposed dc rejection method is evaluated and compared with the traditional method. Finally, the control scheme is verified on a 20-kW three-phase grid-connected inverter.

A Novel DC Current Injection Suppression Method for Three-Phase Grid-Connected Inverter Without the Isolation Transformer

This paper studies the impacts of grid voltage swell on the dc-link voltage for doubly fed induction generator (DFIG). A flexible algorithm of the dc-link voltage is proposed to meet the requirements of wide variation in grid voltage. The dc-link voltage reference is regulated with the variation of grid voltage. Thus, the dc-link voltage is maintained to a constant during normal operation but a relatively high value during grid voltage swell. The power losses are evaluated with the conventional control strategy and the proposed control strategy. The results show that the flexible control method will reduce power losses of the back-to-back converter. A laboratory platform is developed to verify the analysis and the control strategy.

Flexible control of DC-link voltage for doubly fed induction generator during grid voltage swell

The usage of too many sizeable capacitors significantly increases the volume and cost of the modular multilevel converter (MMC), and limits its spread in medium- and high-voltage applications. By adding a buck-type active power filter (APF) circuit into a particular MMC topology [of which each phase multiplexes one submodule (SM)], a modified MMC (APF-M3C) topology, which inherently has the ability to suppress capacitor voltage fluctuations and decrease the power losses compared with conventional MMC topology, is proposed in this article. Both the SM capacitor voltage ripples, which can be reduced up to 50% of original value and are related to the SM capacitance, and the arm currents, which are related to SM power losses, are calculated in detail. Its topology, and operating principle is presented in detail, and a novel modulation method for the APF-M3C topology are proposed. In addition, an improved control method for the voltage balancing is also recommended to further reduce system cost and power losses. Finally, the APF-M3C topology and traditional MMC simulation and experimental platforms were built. Simulations and experimental results verify the feasibility and effectiveness of the APF-M3C topology and control strategy by comparing with the conventional MMC.

Active Power Decoupling for a Modified Modular Multilevel Converter to Decrease Submodule Capacitor Voltage Ripples and Power Losses

This paper develops a systematic analytical method to calculate the harmonic components of uncontrolled single and three phase rectifiers. Based on frequency domain linearity (A.R. Wood and J. Arrillaga, 1995), the dc side voltage and current of discontinuous rectifiers are divided into on and off period segments, then through superposition, the analysis is greatly simplified. Similarly, the effects of the commutation and firing angle shift of continuous three phase rectifiers can be analyzed separately. Based on these analyses, simple expressions of the switching angle of discontinuous mode rectifier and the overlap period and firing angle shift of continuous three-phase rectifiers are obtained. All the calculations are purely algebraic, with high accuracy. The simulation results validated the proposed method.

Frequency-domain analysis of uncontrolled rectifiers

In this paper, we present a multiple-loop control scheme which is able to improve the transient response and power sharing accuracy of parallel-connected single-phase inverters with no control interconnections, and we also analysis the active and reactive circulating current of the parallel system model. A multiple-loop controller is developed, by adding four compensation loops to the traditional droop method. The output voltage and load current compensation loops are added in the scheme to improve the performance and robust of a single inverter. The instantaneous reference adjustment loop and variable output impedance adjustment loop are added to enhance dynamic performance of parallel inverters, as well as reduce the circulating current at the parallel inverter's startup and running. The proposed controller provides both steady-state objectives and a good transient performance. Two 1 kVA DSP-based single-phase UPS inverters are designed and implemented. Simulation and experimental results are all reported, confirming the validity of the proposed control technique.

Min Chen

Papers

A Novel DC Current Injection Suppression Method for Three-Phase Grid-Connected Inverter Without the Isolation Transformer

Flexible control of DC-link voltage for doubly fed induction generator during grid voltage swell

Active Power Decoupling for a Modified Modular Multilevel Converter to Decrease Submodule Capacitor Voltage Ripples and Power Losses

Frequency-domain analysis of uncontrolled rectifiers

An Improved Multiple-loop Controller for Parallel Operation of Single-phase Inverters with No Control Interconnections