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 invention relates to the field of power electronics, and aims to provide a modular multilevel circuit based on an active power filter. The circuit is characterized in that an active power filter circuit is arranged between an upper bridge arm and a lower bridge arm of each phase in a modular multilevel circuit, and the active power filter circuit comprises two switching power devices, two submodules, a capacitor and an inductor; the upper bridge arm and the lower bridge arm are connected in series through two sub-modules, and the two switching power devices are connected in series and thenconnected in parallel with the end portions of the sub-modules at the middle position after the sub-modules are connected in series; the capacitor and the inductor are connected in series and then connected in parallel to two ends of a switching power device connected with the lower bridge arm. The defect of the larger capacitance values of the sub-modules in the traditional modular multilevel topology is overcome, and on the basis of not changing the existing topology, each sub-module in the circuit can use a capacitor with a smaller capacitance value by adding an active power filter, thereby reducing the hardware cost and volume of the MMC topology; and moreover, the method has the advantages of simple implementation and low system loss.

Modular multilevel circuit based on active power filter

Techniques for multiple-phase, high-boost converters are provided. In an example, a multiple-phase switched-capacitor-inductor (MPSCI) boost converter can include a first phase circuit, a second phase circuit, and a capacitor. Each of the first phase circuit and the second phase circuit can include a first switch, an inductor having a first node coupled to a first supply rail, and a second switch configured to selectively couple a second node of the inductor to a second supply rail. The capacitor can be coupled between the second node of the inductor of the second phase circuit and the first switch of the second phase circuit.

Multiple-phase switched-capacitor-inductor boost converter techniques

The invention relates to the field of distributed photovoltaic power generation, and aims to provide a communication-free floating charge control method for a two-stage series photovoltaic power optimizer system The method comprises the following steps: a rear-stage direct-current converter controls a direct-current bus voltage at a set voltage level according to an output voltage of a battery, and transmits a charging condition of a battery system to a front stage through the change of the direct-current bus voltage; a photovoltaic power optimizer determines whether to track the maximum power point or not according to the change condition of the direct-current bus voltage; when the battery voltage reaches a preset floating charging voltage upper limit, tracking of the maximum power pointis stopped; and when the battery voltage drops to a preset floating charging voltage lower limit, tracking of the maximum power point is started, the photovoltaic power optimizer outputs power to a rear stage, and the battery voltage continues to rise A communication module does not need to be additionally arranged between the front stage and the rear stage, self-adaption to the floating charging process of the battery system can be achieved, the complexity of the charging system is reduced, and meanwhile the system cost is reduced

Communication-free floating charge control method for two-stage series photovoltaic power optimizer system

A analytical calculation method of DC fault current in high-voltage DC grid including current-limiting devices is proposed in this paper. Based on the simplified equivalent circuits of modular multilevel converts and different current-limiting devices, the approximate analytical solution of the DC fault current could be calculated. What’s more, an optimizing method of parameter in fault current limiter is proposed based on the proposed calculation method. By calculating the most suitable rated voltage of mental oxide arrestor in fault current limiter, the investment is effectively reduced and the current limiting effects are still ensured. Based on the proposed calculation method, the current limiting parameters could be reasonably selected. A ±500kV 4-terminal high-voltage DC grid is built in PSCAD/EMTDC. The feasibility and practicality of proposed calculation and optimizing methods are validated through simulation results.

A calculation method of analytical DC fault current in MMC-HVDC grid including current-limiting devices

For offshore wind power plant based on modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission technology, when a short-circuit fault occurs on the onshore AC network, the DC-link voltage will rise rapidly, which may even cause equipment damage. The existing scheme basically is to connect a DC chopper in parallel on the DC bus to absorb the excess energy, but the additional equipment will increase the construction costs and land use. For this reason, this paper proposes the MMC topology with integrated energy dissipation resistors, which is capable of passing through AC short-circuit faults. This paper studies the electrical parameter design method and the corresponding control strategy of the topology, and verifies the feasibility of the scheme through simulation. Compared with the existing DC chopper solution, this scheme is relatively simple in control, and can reduce the fluctuation of the DC-link voltage, construction costs and land use due to its integration with MMC.

Min Chen

Papers

Modular multilevel circuit based on active power filter

Multiple-phase switched-capacitor-inductor boost converter techniques

Communication-free floating charge control method for two-stage series photovoltaic power optimizer system

A calculation method of analytical DC fault current in MMC-HVDC grid including current-limiting devices

A MMC Topology with AC Short-Circuit Fault Ride-Through Capability and its Control Strategies