This article provides a detailed analysis of the power electronics solutions enabling bipolar dc grids. The bipolar dc grid concept has proven to be more efficient, flexible, and higher in quality than the conventional unipolar one. However, despite its many features, these systems still have to overcome their issues with asymmetrical loading to avoid voltage imbalances, besides meeting regulatory and safety requirements that are still under development. Advances in power electronics and the large-scale deployment of dc consumer appliances have put this growing architecture in the spotlight, as it has drawn the attention of different research groups recently. The following provides an insightful discussion regarding the topologies that enable these architectures and their regulatory requirements, besides their features and level of development. In addition, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities. @en

Electrification has been a key component of technological progress and economic development since the industrial revolution. It has improved living conditions, spurred innovation, and increased efficiency across all sectors of our economy and all aspects of our lives. During the coming decades, electrification is expected to reach further and deeper into the transportation, building, and industry sectors, mainly motivated by the energy transition to a zero-carbonemission-based economy to mitigate climate change.@en

The Modular Multilevel Series Parallel Converter (MMSPC) not only allows dynamic interconnection among the modules in series connection, like the traditional Modular Multilevel Converter (MMC), but also in parallel connection. This extra switching state among the modules allows new features and operation modes. The parallel connection between modules enabled by the MMSPC, is not only limited to those modules belonging to the same arm, but can also be extended to the parallel connection of modules from different arms or even from different phases. All these different interconnections brings operational benefits as well as some constraints. In this paper, the power topology, operation and comparison of different MMSPC configurations are presented. Specifically, this paper describes in detail the traditional MMC (without parallel interconnection), the MMSPC with parallel connection of modules of the same arm only, the MMSPC with the option of paralleling connection among the phases of the system, the MMSPC with parallel connection between arms of the same phase and finally, the MMSPC with full-parallel capability.@en

The MMC converter is the key topology for HVDC applications. In these systems, a major complexity is the number of modules and components required per phase to achieve nominal voltage and current levels for applications in transmission system. In this paper, the topology and control of a two-phase MMC system for HVDC applications is presented. The main idea is to eliminate a complete phase of the classic MMC converter for reduce the number of modules and the complexity of the system, together with the use of MMSPC-type modules, in order to achieve internal voltage balance without the need of extra sensors and control loops.@en

Modular multilevel series-parallel converters (MM-SPC) have become a suitable solution to provide higher operating voltages, fault tolerance operation, and reliability at a reduced cost, due to their ability to achieve a simpler internal voltage balance. Therefore, MMSPC is a promising solution for applications where the nominal operating conditions are high voltage levels on the DC side and high current ratings on the AC side of the system. To allow this operation, it is necessary to design the system with parallelization in each phase. Therefore, this article proposes a design that incorporates parallelization in each phases and analyzes the internal cross-circulating current generated due to parallelization. This current creates an unbalance and internal losses in the converter. In order to compensate for the cross-circulating current, two cases are compared: the first one with decoupled inductors in their phases, and the second one with coupled inductors. This way, a comparison is made to determine which case provides greater compensation of the circulating current, allowing for the delivery of a high current at the converter's output with the least possible imbalance and internal losses.@en

The Hexagonal power converter has become a suitable solution to provide high-quality voltage waveforms while achieving decoupled control of three DC terminals as well. In this case, the internal voltage balance of the different storage units is the main concern for the correct operation of the Hexagonal converter. Moreover, the Modular multilevel series- parallel converters (MMSPC) have become an interesting solution to provide higher operating voltages, reliability at a reduced cost, due to their ability to achieve a simpler internal voltage balance. In this paper, a Hexagonal power converter based on MMSPC for decoupled DC terminals is presented. The proposed system allows to implement a simple and cost-effective way to achieve a decoupled control strategy in each output of the system, to control the corresponding voltage of the system, along with maintaining the internal voltage balance of the proposed topology.@en

Energy storage systems (ESSs) allow improving the stability and efficiency of the electrical grids with a high penetration of renewable energy sources. Moreover, the use of Hybrid ESSs (HESSs) enables storage solutions with both high-energy and high-power densities, by combining different storage technologies such as diverse battery chemistries, ultracapacitors, or hydrogen fuel cells to name a few. In this article, an HESS-based multioutput multilevel (MOM) converter is presented. The proposed topology enables decoupled control of each ac converter voltage output. The internal switching states further allow the use of different storage units and high-quality multilevel voltage in each ac output. The mathematical model of the proposed topology and the defined operation region of the system, besides a model-predictive control strategy, are developed. Finally, simulation and experimental results validate the performance of the proposed topology. @en

This article proposes a novel bipolar-type dc system suitable for both distribution and transmission systems based on modular multilevel series/parallel converters (MMSPCs). The system features decoupled operations of each pole of the bipolar system, being able to operate in both asymmetrical and regenerative modes. This enables two independent dc systems by using a single grid-tied converter. The MMSPC is based on a three-switch cell configuration and enables a simple balancing mechanism in combination with a wide range of output voltage frequencies. The simple balancing mechanism is the key to enable the dc operation and lead to simpler scalability for different voltage levels. Theoretical studies and experimental results are provided to verify and characterize the proposed system. @en

The increasing power levels handled by electric vehicle (EV) dc fast chargers will impose additional challenges to the switching devices in order to cope with the efficiency requirements. A cost-effective alternative to achieve highly efficient power conversion is through the partial-power conversion concept. This article validates the advantages of a step-down Type II partial-power converter (PPC), based on the phase-shifted full-bridge converter, for EV fast chargers. By exploiting the reduced voltage range of an EV battery pack along with the reduced power ratio for a Type II PPC, an extremely efficient charging process can be achieved. The concept is validated with the development of a 7-kW demonstrator, and hence, realistic efficiency measurements are obtained. Results indicate the effectiveness of charging a battery by merely handling 13.32% of the power provided to it, with a peak efficiency of 99.11%.@en

This paper presents a modular multilevel series/parallel converter (MMSPC) with intermodule switched-inductor power transfer. The switched-inductor voltage conversion feature allows controllable and efficient transfer of energy between modules with nonnegligible voltage difference, providing both step-down and step-up functionalities. Thus, this converter can accurately control and rapidly adjust the voltage of each module to generate an ac output voltage waveform with a controllable number of levels, increasing the quality of the output. Moreover, the intrinsic DC-DC conversion feature can generate a dc controllable output voltage and enable new applications. In this text, we specifically demonstrate how the flexibility of obtaining both ac and dc output with the same setup renders the topology promising for battery energy storage systems and dc microgrid applications. Experimental results validate the topology and concept of an MMSPC with intrinsic switched-inductor conversion.@en

During the last decades, AC systems dominated the power transmission and distribution applications almost exclusively. However, a recent convergence of needs originated in different sectors (renewable energy conversion, information technology and transportation) have accelerated the development of DC systems. Nowadays, DC systems are present at both transmission and distribution levels, offering high-performance solutions with enhanced efficiency and reliability, besides reducing the number of power conversion stages involved and uninterrupted power delivery. For LVDC active networks, two kinds of architectures are possible: unipolar and bipolar. Despite being a more sophisticated and technically complex solution, bipolar structure provide several advantages over conventional unipolar ones. Higher availability, efficiency and flexibility are just a few advantages featured by bipolar systems. This chapter presented a brief overview covering the different aspects of bipolar LVDC networks. Distribution converter topologies, balancing stages and also their control schemes are discussed in order to highlight the efforts being made in this growing architecture.@en

Hybrid Energy Storage Systems (HESSs) are based on different storage elements such as batteries or ultra capacitors (UC), aiming to implement a system with high energy and power density. These HESSs using multi-modular power converters to incorporate in each power electronics module a different storage element and then, these modules are interconnected to create the total system. On the other hand, the Modular Multilevel Series Parallel Converter (MMSPC) not only allows dynamic interconnection among the modules in series connection but also in parallel connection. This extra switching state among the modules allows new features and operation modes. In this paper, a HESS based on MMSPC topology is presented. The novel HESS-MMSPC topology is simulated with UC and batteries as storage units, and this proposed system is interconnected to an electrical grid. The control strategy and evaluation of the operation of the proposed system are presented.@en

This paper presents a new grid connected photovoltaic energy conversion system configuration for large scale power plants. The grid-tied converter is based on a modular multilevel converter using voltage source H-bridge cells. The proposed converter is capable of concentrating a multimegawatt PV plant with distributed string MPPT tracking capability, high power quality and increased efficiency compared to the classic two-level voltage source converters. The main challenge is to handle the inherent power unbalances which may occur, not only between the different cells of one phase of the converter, but also between the three phases. The control strategy to deal with these unbalances is analyzed in this paper. Simulation results for a downsized 7 level MMC composed of 18 H-bridge cells and PV strings are presented to validate the proposed topology and control method.@en

The use of conventional half-bridge submodules in the Modular Multilevel Converters brings important benefits such as the reduction in the power losses and semiconductor devices. However, the system with half-bridge submodule lacks the capability of blocking DC-faults currents. In this paper, a 5-level submodule with DC-fault blocking capability is presented. The proposed submodule allows to operate with different internal capacitances in order to reduce the cost of the system. Furthermore, a control and balance strategy for the novel 5-level submodule is proposed.@en