Evaluation of Emerging Modular Multilevel Converters for BESS Applications

The electric power grid is undergoing significant changes and updates nowadays, especially on a production and transmission level. Initially, the move towards a distributed generation in contrast to the existing centralized one implies a significant integration of renewable energy sources and electricity storage systems. In addition, environmental awareness and related concerns regarding pollutant emissions have given rise to a high interest in electrical mobility. Advanced power electronics interfacing systems are expected to play a key role in the development of such modern controllable and efficient large-scale grids and associated infrastructures. During the last decade, a global research and development interest has been stimulated in the field of modular multilevel conversion, due to the well-known offered advantages over conventional solutions in the medium- and high-voltage and power range. In the context of battery energy storage systems, the Modular Multilevel Converter (MMC) family exhibits an additional attractive feature, i.e., the capability of embedding such storage elements in a split manner, given the existence of several submodules operating at significantly lower voltages. This thesis deals with several technical challenges associated with Modular Multilevel Converters as well as their enhancement with battery energy storage elements. Initially, the accurate submodule capacitor voltage ripple estimation for a DC/AC MMC is derived analytically, avoiding any strong assumptions. This is beneficial for converter dimensioning purposes as well as for the implementation improvement of several control schemes, which have been proposed in the literature. The impact of unbalanced grid conditions on the operation and design of an MMC is then investigated, drawing important conclusions regarding the choice of line current control and required capacitive storage energy during grid faults. [...]

Fractional frequency transmission system (FFTS) is a promising solution to offshore wind power integration. The back-to-back modular multilevel converter (MMC) and modular multilevel matrix converter (M3C) are universally regarded as two promising ac/ac converter candidates of the next generation. This paper performs a detailed comparison between back-to-back MMC and M3C with special emphasis on their total capacitor and semiconductor ratings. The arm current and capacitor ripples are first analyzed, then comparison results of a 220 kV, 500 MW case is given. In terms of peak-current-based and RMS-current-based total semiconductor ratings the M3C is 42.56 % and 22.97 % larger respectively, while its total capacitor rating is merely 33.15% of the back-to-back MMC. The better candidate can be determined according to the quantitative comparison results and the latest prices of semiconductors and capacitors.

This paper presents a new converter topology for voltage source converter based high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three level structure and consists of series-connected IGBT bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor stacks. The IGBT bridge stacks are leveraged to generate the output the voltage waveforms and to commute the thyristors. The basic operation principle and the methods to commutate the thyristors are explained in detail. Due to the three level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. The simulation results proved the analysis and verified the feasibility and benefits of the proposed converter topology.

Modular Multilevel Converter (MMC) with integrated Battery Energy Storage System (BESS), where the battery strings are connected to each Submodule (SM) through DC-DC converters, can transfer energy among the DC link, the AC grid, and the batteries. And this flexible structure is suitable to be applied in the microgrid and the wind power generation system, in which the BESS is needed to smooth energy fluctuations. However, compared with the common MMC, the Battery Manage System (BMS) which is used in BESS to balance the State of Charge (SOC) among batteries makes the capacitor voltage ripple of each Submodule different and complicated. The capacitor selection needs to take this into consideration. Some simulation results under different initial SOC situations derived from a Simulation model are given to evaluate the influence. And some corresponding conclusions and suggestions are obtained based on these simulation results.

To implement the STATCOMs for transmission and distribution applications, high power converters are needed. A multilevel converter is a logical choice for such applications as it can meet the imposed line side current and voltage harmonic distortion standards without extending its device ratings. Several types of monolithic multilevel converters have been proposed based on different structures of a DC link voltage (or current) to generate staircase output voltage (or current) levels. The multilevel current source converters (CSC) are also a potential configuration for STATCOM. Multilevel CSCs using fully controllable switches have the advantages of low output harmonics and controllable active and reactive power. During the last decade, modular multilevel converters have shown a breakthrough and have made their way to commercial high power applications. Modularity, in general, refers to a technique to develop comparably large systems by combining smaller subsystems. For power converter topologies, this means, a cascaded connection of converter cells seems to be an interesting solution to reach high voltage and high quality waveforms with a minimum complexity. This chapter provides an overview on monolithic and modular multilevel converter topologies, targeting high-power applications and in particular STATCOM. Modular multilevel converters can be built either straightforward by applying the modular building block cells or by a combination of cells with monolithic multilevel topologies. The monolithic and modular CSCs can also be realized by employing the duality concept on each corresponding Voltage Source Converter (VSC). This chapter summarizes the most recent developments made for multilevel converter topologies, by covering new promising topologies and operational issues. In addition, emerging trends, challenges, and possible future directions of the multilevel converter technologies are outlined.

In this paper a generalized control approach for modular multilevel converters is applied to a converter connecting a three-phase and a five-phase system directly without any central DC link. This new 3 × 5 modular multilevel matrix converter offers an alternative to other multilevel converters applied to five-phase machines, but foremost acts as example to prove the generality of the control. The cascaded control structure consists of an inner current control and an outer branch energy balancing control. Both are systematically derived and verified by simulation.

Modular Multilevel Converters (MMC) have many advantages in comparison with conventional multilevel converters in application to high voltage system. However, the control system of MMC requires a large amount of I/O signal ports which cannot be generated by a single digital signal processor (DSP) with maximum I/Oport of 24. This paper fucuses on the experimental results of a MMC that uses a Field Programmable Gate Array (FPGA) in which embedge the Nearest Level Modulation (NLM) method and capacitor voltage balancing algorithm. The MMC consists of 12 Sub-Modules (SM) in each phase to generate a 13-level ac output voltage. Applying the NLM method and capacitor voltage balancing algorithm have significantly reduced the switching frequency and produced an ac output voltage with very low harmonic distortion. The experimental results have verified by a laboratory-scale prototype and compared with the simulation results.

The modular multilevel converter (MMC) is emerged as the most favourable multilevel converter topology for medium to high-voltage applications. Reliability is one of the major concerns in an MMC due to high fragile component count. Due to low-cost and high energy density, electrolytic capacitors (ECs) are usually preferred as floating capacitors in the MMC. However, the ECs are gradually degraded over the time due to inherent chemical processes, which results in a decrease in capacitance value. This results in an increase in voltage ripple across the SM capacitors and would distort the output waveforms. Moreover, the prolonged use of these aged capacitors could disrupt the MMC operation. Therefore, monitoring and failure detection of submodule (SM) capacitors in an MMC are pivotal to enhance the reliability. This paper presents an SM capacitance estimation strategy for an MMC using nearest level modulation (NLM). A modified voltage balancing control structure with NLM is presented to reduce the unbalance in the capacitor voltages. The proposed approach uses available hardware for converter control with easy implementation in the same controller. Extensive studies are conducted on a three-phase MMC in PLECS simulation platform. Furthermore, experimental results are presented to substantiate the effectiveness of the proposed approach.

This paper demonstrates the interoperability of an emerging alternate arm converter (AAC) with the state-of-the-art modular multilevel converter (MMC) in high-voltage direct current (HVDC) systems based on a hybrid VSC-HVDC system. The paper also showcases the parameter derivation of the hybrid HVDC system and its detailed control structure. The study provides preliminary steps towards detailed analysis of AAC interoperability in complex hybrid dc grid configurations. A detailed set of results based on the 800 MVA hybrid voltage source converter (VSC)-HVDC system showcases the interoperability performance of the AAC under different operating scenarios and verifies its associated control functions.

The direct chain-link modular multilevel dc converter has raised great interest recently in medium/high-voltage dc-dc conversion due to the high power device utilization and lower power losses than the front-to-front configurations of modular multilevel converter. This paper introduces a single-phase chain-link modular multilevel buck-boost converter for medium voltage dc applications and presents an analysis methodology on its internal circulating current frequency. Its connection and comparison with the classic dc-ac modular multilevel converter are given first, and its dc and ac components are analyzed in the respective equivalent circuits. Then, the derivation methodology for the proper circulating current frequency with lowest internal reactive power is provided, which could minimize the current stress and decrease the power losses. Also, this method can be directly applied in the derivative topologies and further configurations to satisfy various conversion requirements. The theoretical analysis is verified by a set of full-scaled simulations and further verified against experimental tests on a down-scaled prototype.

Hexverter is a recently proposed direct AC/AC modular multilevel converter (MMC). It features significant advantages for low-frequency drives compared to traditional MMC in back-to-back configuration. This paper presents a novel branch energy control method based on the injection of optimized circulating current and neutral-point voltage. It can effectively balance the energy stored in the six branches of the hexverter and simultaneously minimize the module-capacitor voltage fluctuation. The control performance is verified by simulation studies.

An isolated modular multilevel dc-dc converter (IMMDCC) consisting of two modular multilevel converters (MMCs) coupled via a medium-frequency transformer is competitive for medium- and high-voltage applications because of its high modularity and high-voltage handling capability. Employing the quasi square-wave modulation (QSM) scheme for MMCs of the converter can increase the dc-link voltage utilization ratio and realize soft-switching operation, compared with sinusoidal wave modulation methods. However, the accurate reactive power control at the ac side of the IMMDCC can hardly be achieved under the QSM mode. The ac-side reactive power and current stress increase considerably when the conversion ratio deviates from the transformer turn ratio. In order to address this issue, this paper presents an arm voltage decoupling control scheme for the IMMDCC by introducing a virtual buck/boost conversion stage between the dc-side voltage and the transformer voltage. The transformer primary and secondary voltages are kept matched to suppress the reactive power when the dc-side voltage varies. The theoretical analysis shows that the proposed control enhances the power transfer capability and reduces the ac-side current stress of the IMMDCC. In addition, submodule voltages are maintained even when the dc voltages exceed the designed value under the conventional control scheme, which implies that the voltage range of the converter is expanded. The simulation and experimental results confirm the effectiveness of the proposed modulation and control strategy.

This paper presents a multiobjective real-time controller for a modular multilevel converter capable of parallel module connectivity, the so-called modular multilevel series parallel converter (MMSPC). The MMSPC topology allows the batteries to be dynamically rewired in various series–parallel configurations, generating a wide range of output voltage levels. The novel control method parallelizes the modules to balance their voltages without the need for individual module voltage monitoring. Additionally, the controller optimizes across the large number of feasible system configurations to minimize switching and conduction losses. Finally, the controller efficiently encodes the system configuration with module interconnection states rather than the module switch states, which substantially simplifies control. Furthermore, this work experimentally validates the MMSPC topology and concept. In the prototype, the parallel mode reduced the system losses at 5 kW output power by 18% and 24% for load power factors of 1.0 and 0.8, respectively. Sensorless balancing via parallelization maintained well-matched module voltages (standard deviation = 0.045 V) over a 5-h battery discharge with highly variable load current. The reduced conduction losses and simple balancing capability of the MMSPC can enable new applications at medium and low voltages that benefit from its high-quality output, elimination of filtering magnetics, fast response, and modularity.

In this paper, a high gain modular multilevel dc-dc converter topology is proposed. As compared with the conventional modular multilevel step-up converters, the proposed converter offers an additional degree of control-freedom and higher steady-state voltage gain. The proposed converter operates in continuous conduction mode. A detailed small-signal modeling for the proposed converter based on state-space average modeling is presented. In addition, this study covers both the small-signal and steady-state analysis for the high gain modular multilevel dc-dc converter operating in continuous conduction mode. The performance of the proposed converter is compared with conventional modular multilevel dc-dc converter. To confirm this design, simulations have been carried out on a 1 kW, 48 V to 288 V converter system. Finally, the validity of the converter is verified by simulation results.

The Doubly Fed Induction Motor (DFIM) is essential in Wind Energy Conversion Systems (WECS) for efficient wind energy harvesting. Conventional power converters in WECS often result in high Total Harmonic Distortion (THD) in output voltage and current, affecting wind energy economy. This research proposes replacing conventional converters with Modular Multi-level Converters (MMC) due to their scalable topology, low harmonic distortion, high efficiency, and the elimination of DC link capacitors. The MMC is connected in a back-to-back configuration between the rotor of the DFIM and the grid to enable four-quadrant operation. Circulating current in the MMC is minimized using a Modified Deer Hunting Optimization (MDHO) based Fractional Order PID (FOPID) controller and a Mexican Axolotl Optimization (MAO) based FOPID. Capacitor voltage balancing is achieved using Level-Shifting PWM (LSPWM) and Three-Dimensional Space Vector PWM (3D-SVPWM) techniques with voltage sorting algorithms. The impact of these control and modulation techniques on the DFIM is analyzed in terms of reduced THD, and steady state and transient performance. Simulations are conducted in MATLAB/R2022a, and experimental validation is performed using a 7-level 0.5 kW MMC in a Hardware-In-Loop (HIL) setup.