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Modern wind turbines with increasing power levels are continuously emerging. These power levels are reaching a point where excessively high currents are obtained if the traditional low-voltage wind-turbine structure is adopted. High currents lead to excessively high losses and the need for bulky and heavy conductors to carry them. A medium-voltage structure should be a more competitive alternative to be adopted in high-power wind turbines. In this article, a new converter solution with a modular multilevel structure suitable for driving modern/future high-power medium-voltage wind turbines is proposed. This converter topology has desirable features common to modular multilevel converters, such as the improved reliability at high voltage levels, and the possibility to synthesize high-power-quality staircase-shape voltages, which leads to low requirements for harmonic and ${dv}/{dt}$ filters. Furthermore, the proposed converter presents reduced capacitance and inductance requirements compared with other well-established modular multilevel converter topologies, which should result in a lighter and more compact solution. The proposed solution could potentially allow for the reduction of the overall costs of the supporting structure required to withstand the weight of the wind turbine. Simulation and experimental results are presented in this article to validate the proposed topology. [ABSTRACT FROM AUTHOR]

A new state-observer-based current balancing method for Modular Multilevel Converters with Interleaved half-bridge Sub-Modules (ISM-MMC) is presented in this article. The developed observer allows estimating currents through interleaved half-bridge legs in each submodule of ISM-MMC based only on arm current and submodule's capacitor voltage measurements. Then, the interleaved current balancing control uses the estimated currents to reduce the interleaved currents imbalance caused by upstream control actions. This technique minimizes the number of required current sensors in ISM-MMC, thereby reducing the converter's cost, weight, and volume. Capabilities of the proposed sensorless interleaved currents balancing control have been tested against standard parameter tolerances of the composing passive elements. In addition to that, a novel capacitor voltage balancing strategy for MMCs is developed. The new algorithm contains the main advantages of the classical sorting-based capacitor voltage balancing methods while providing an opportunity to decouple two balancing tasks of ISM-MMC, namely capacitor voltage and interleaved legs current balancing. The feasibility of the proposed methods is verified by extensive simulation and experimental tests on a laboratory prototype by the corresponding system response under the output characteristics variation and interleaved current control perturbation. [ABSTRACT FROM AUTHOR]

This article proposes a decoupled control scheme for the modular multilevel converter (MMC) to reduce the total harmonic distortion (THD) and eliminate one specific harmonic. First, to realize the decoupled control between the ac and dc paths of the MMC, an improved nearest level control (INLC) method is proposed. It applies the round function to generate the ac output voltage levels, instead of the arm output voltage levels. Thus, a staircase wave is generated by the INLC method. Then, by adding an additional pulse to each quarter of the staircase wave, one specific harmonic can be eliminated without optimizing each conduction angle. Furthermore, a decoupled circulating current fuzzy control method is proposed to suppress the second-order harmonic and balance the arm energy. By this decoupled control structure, the output voltage and the circulating current can be controlled independently. Taking the fifth-order harmonic as an example, simulation and experimental results validate that the proposed scheme can simultaneously realize THD reduction and one specific harmonic elimination in the MMC output voltage. [ABSTRACT FROM AUTHOR]

A new Modular Multilevel Converter with Interleaved half-bridge Sub-Modules (ISM-MMC) is proposed in this article. The ISM-MMC exhibits higher modularity and scalability in terms of current ratings with respect to a conventional MMC while preserving the typical voltage-level adaptiveness. The ISM-MMC brings the known advantages of classical MMC to low-voltage, high-current applications making it a novel candidate for the sector of ultrafast chargers for all types of electric vehicles (EVs). This advanced topology makes it possible to easily reach the charging power of the EV charging system up to 4.5 MW and beyond with a low-voltage supply. To operate the new converter, a hybrid modulation scheme that helps to exploit the advantages of the interleaving scheme is implemented and explained in this article. It has been verified that the typical MMC control methods are still applicable for ISM-MMC. A comparative study between classical MMC and ISM-MMC configurations in terms of output characteristics and efficiency is also given. Furthermore, it has been demonstrated that the number of ac voltage levels is synthetically multiplied by the number of interleaved half-bridge legs in submodules. Simulations, hardware in the loop, and experimental tests are carried out to demonstrate the feasibility of the proposed topology and the implemented modulation scheme. [ABSTRACT FROM AUTHOR]

With the high penetration of the power electronic loads in the grid, the stability of static synchronous compensator (STATCOM) devices is greatly challenged. However, the conventional control methods for the modular multilevel converter (MMC) based STATCOM only consider the stability with small signal disturbances. This article proposes a novel dual-layer back-stepping control (BSC) for the MMC-based STATCOM. In the first layer, the BSC aims to regulate the sum of the capacitor energy and the reactive output current. In the second layer, the BSC aims to control the circulating current. Therefore, the proposed method possesses a fast dynamic response and accurate tracking with the Lyapunov stability of the MMC-based STATCOM. Compared with the arm-control-based BSC for MMC-based inverters, the proposed method has a simplified structure and a reduced computation burden. Moreover, the proposed method realizes the independent control between the output current and the circulating current. The simulation and experimental results verify the effectiveness of the proposed method. In addition, its robustness toward different circuit parameters and the operation ability under unbalanced grid fault is also verified. [ABSTRACT FROM AUTHOR]

As the penetration of renewable energy generation increases, the importance of energy storage systems becomes evident since these systems can contribute for the preservation of the power system stability. Wind turbine owners can also benefit from having energy storage systems as they can increase their revenues. The fast growth of wind turbine power ratings will eventually lead to the requirement of higher voltage levels as well. Proper power electronic converters will be required to drive these systems. Converters with a modular multilevel structure are considered the state-of-the-art solution for high-power applications. These topologies allow for a flexible integration of energy storage systems in both centralized and decentralized ways. This article presents a new converter solution with a modular multilevel structure and decentralized energy storage integration suitable to drive high-power medium-voltage wind turbines. This converter presents important structural and control characteristics that allows for a straightforward integration of an energy storage system in such a way that the wind turbine driven by it can operate with high flexibility and in a dispatchable fashion, benefiting both the power system operator and the wind-power-plant owner. [ABSTRACT FROM AUTHOR]

In recent years, the combination of battery energy storage (ES) systems and static synchronous compensators (STATCOM) in a single device, known as ES-STATCOM, has been studied by both academia and industry. The modular multilevel converter is an interesting candidate for ES-STATCOM realization. The integration of batteries based on dc/dc converter presents advantages in terms of design flexibility. However, there is a gap in the literature in terms of design and cost optimization of ES-STATCOM systems. To fill this void, this article provides a methodology to support engineers to design and perform cost optimization of ES-STATCOM systems. Both capital expenditures and operational expenditures due to energy losses and battery replacement are taken into account. The methodology identifies what is the optimum submodule voltage reference, which minimizes the ES-STATCOM total cost for a given target of operating time. A lifetime model is employed to predict if battery replacement is required. This model computes the battery capacity fade caused by the operation mission profile considering both calendar and cycling aging mechanisms. The case study is based on a 25 MVA/75 MWh ES-STATCOM connected to a 34.5 kV grid, which provides ancillary services for a photovoltaic power plant. Different rated blocking voltages are assumed for the semiconductor devices. Moreover, different maximum and minimum allowed state-of-charge are adopted in the converter design. The proposed methodology can also be used to find the optimum combination of the battery and insulated gate bipolar transistor (IGBT) part-numbers based on a given initial set. [ABSTRACT FROM AUTHOR]

Among the various topologies proposed in this literature, the modular multilevel converter (MMC) is considered the next generation of converters for medium- and high-voltage applications. This paper presents a comparison between two variants of MMCs for STATCOM applications: The double-star chopper cell-MMC and the single-delta bridge cell-MMC. These converter topologies are compared and benchmarked during positive and negative sequence compensation. The comparisons are supported by analytical results regarding minimum effective dc-link voltage, number of cells, current rating, energy storage requirements, and operation during unbalanced conditions. The dynamic behavior, power losses, and cost of both solutions are evaluated through simulation model of a 15 mega volt-ampère (MVA) MMC STATCOM. [ABSTRACT FROM AUTHOR]

The modular multilevel converters (MMCs) have become an attractive topology in recent years. The MMC has been employed in several applications as HVdc, energy storage, renewable energy, electrical drives, and static synchronous compensators (STATCOMs). One advantage of MMC-based topologies is the inherited fault-tolerant operation associated with the high number of submodules (SMs). This work proposes the study of four redundancy strategies, which can be employed in modular multilevel converters: standard redundancy, redundancy strategy based on additional submodules (RAS), redundancy strategy based on additional submodules optimized (RASO), and redundancy strategy based on spare submodules. These strategies are compared through a case study of a 15 MVA MMC STATCOM. A new approach for the SM capacitance design is proposed, including the effect of the negative sequence current in the converter storage energy variations. The comparisons of the redundancy strategies are accomplished based on dynamic behavior, capacitor voltage balancing, control complexity, and power losses. Finally, the advantages and drawbacks of each redundancy strategy are presented. [ABSTRACT FROM PUBLISHER]

The double-star chopper cell modular multilevel converter (DSCC-MMC) has been employed in several applications as HVDC, energy storage, renewable energy, electrical drives and STATCOMs. Generally, the DSCC-MMC main circuit parameter design presented in literature considers balanced currents flowing through the converter. Nevertheless, in STATCOM application, the converter can compensate negative sequence components and unbalanced currents flow through the DSCC-MMC, resulting in different stresses in the converter phases. Therefore, this work presents a detailed design methodology of the DSCC-MMC main circuit parameters, considering both positive and negative sequence current compensations. The dc-link voltage, number of submodules, power semiconductor thermal stresses, submodule capacitance and arm inductances are designed. Expressions for the energy storage requirements are derived when negative sequence is compensated. A case study considering a 15-MVA STATCOM is presented, and simulation results validate the proposed design methodology. Finally, the converter power losses and thermal stresses in the power semiconductors are evaluated. [ABSTRACT FROM AUTHOR]

Many high-power converter systems are often connected to the medium-voltage network using a step-up transformer. In such systems, the converter-side windings of the transformer can be configured as an open end, and multilevel voltage waveforms can be achieved by feeding these open-end windings from both ends using the two-level dual converter. An $LCL$ filter with separate converter-side inductors for each of the converter is commonly used to attenuate the undesirable harmonic frequency components in the grid current. The magnetic integration of the converter-side inductors is presented in this paper, where the flux in the common part of the magnetic core is completely canceled out. As a result, the size of the magnetic component can be significantly reduced. A multiobjective design optimization is presented, where the energy loss and the volume are optimized. The optimization process takes into account the yearly load profile and the energy loss is minimized, rather than minimizing the losses at a specific operating point. The size reduction achieved by the proposed inductor is demonstrated through a comparative evaluation. Finally, the analysis is supported through simulations and experimental results. [ABSTRACT FROM AUTHOR]

A converter system for high-power applications, connected to a medium-voltage network using a step-up transformer, is presented in this paper. The converter-side winding of the transformer is configured as an open-end, and both the ends of the windings are fed from two different converter groups. Each converter group comprises of two parallel voltage-source converters (VSCs), whose carrier signals are interleaved to improve the harmonic quality of the resultant switched output voltage of that converter group. However, an additional inductive component is required to suppress the circulating current that flows between the parallel-interleaved VSCs. An integrated inductor is proposed, which suppresses the circulating current in both the converter groups. In addition, the functionality of the line filter inductor is also integrated. Flux in various parts of the integrated inductor is analyzed, and a design procedure is also described. The volume and the losses of the proposed solution are compared with that of the state-of-the-art solution. The control of the proposed converter system is also discussed. The analysis has been verified by the simulation and experimental results. [ABSTRACT FROM PUBLISHER]

Photovoltaic systems technological development is driven by the request for higher efficiency and safety. These concerns influence also the choice of the power converter stage. Several topologies have been proposed and many of them are available commercially. Among them, the neutral point clamped (NPC) and derived topologies offers high efficiency, low leakage current, and low EMI. However, one main disadvantage of the NPC inverter is given by an unequal distribution of the losses in the semiconductor devices, which leads to an unequal distribution of temperature that can affect lifetime. By using the active NPC (ANPC) topology, where the clamping diodes are replaced by bidirectional switches, the power losses distribution problem is alleviated. The modulation strategy is a key issue for losses distribution in this topology. In this paper, two known strategies are discussed and a new PWM strategy, namely the adjustable losses distribution is proposed for better losses distribution in the ANPC topology. Simulations and experimental results help in evaluating the modulation strategies. [ABSTRACT FROM AUTHOR]

There is a strong trend in the photovoltaic inverter technology to use transformerless topologies in order to acquire higher efficiencies combining with very low ground leakage current. In this paper, a new topology, based on the H-bridge with a new ac bypass circuit consisting of a diode rectifier and a switch with clamping to the dc midpoint, is proposed. The topology is simulated and experimentally validated, and a comparison with other existing topologies is performed. High conversion efficiency and low leakage current are demonstrated. [ABSTRACT FROM AUTHOR]

This paper presents a new solution for filtering current harmonics in three-phase four-wire networks. The original four-branch star (FBS) filter topology presented in this paper is characterized by a particular layout of single-phase inductances and capacitors, without using any transformer or special electromagnetic device. Via this layout, a power filter, with two different and simultaneous resonance frequencies and sequences, is achieved-one frequency for positive-/negative-sequence and another one for zero-sequence components. This filter topology can work either as a passive filter, when only passive components are employed, or as a hybrid filter, when its behavior is improved by integrating a power converter into the filter structure. The paper analyzes the proposed topology, and derives fundamental concepts about the control of the resulting hybrid power filter. From this analysis, a specific implementation of a three-phase four-wire hybrid power filter is presented as an illustrative application of the filtering topology. An extensive evaluation using simulation and experimental results from a DSP-based laboratory prototype is conducted in order to verify and validate the good performance achieved by the proposed FBS passive/hybrid power filter. [ABSTRACT FROM AUTHOR]

In recent years, the integration of the high-power static synchronous compensator (STATCOM) and energy storage in the same device has gained interest. Such a system is referred to as ES-STATCOM. Modular multilevel converter (MMC) topologies constitute a promising converter family for ES-STATCOM realization, providing a modular and scalable solution with a high efficiency that handles high-power and high-voltage ratings in grid applications. There is a gap in technical literature discussing the design and the comparison of MMC-based ES-STATCOMs while utilizing batteries to find the most suitable MMC topology for ES-STATCOMs. Therefore, this paper benchmarks MMC family members for ES-STATCOM realization. Both centralized and distributed energy storage approaches are investigated. The proposed design flowcharts can be employed for comparison and optimization purposes. In total, seven topologies are compared in terms of number of cells, required silicon area and total battery volume. Different semiconductor devices and battery types are analyzed. The result indicates that centralized energy storage systems are the most suitable due to their design flexibility, low volume and small silicon area. Moreover, the possibility of using over-modulation in MMC using bridge cells has an important role in the optimization of ES-STATCOM. The results for the adopted case study shows that the decentralized approach can lead to 55% higher silicon area and 30% higher volume than the centralized approach. The double-star bridge cell MMC with centralized energy storage is determined as the most suitable solution for ES-STATCOM systems. [ABSTRACT FROM AUTHOR]