Modular Converter Research Articles

Stabilization of voltages on capacitors of cells in modular multilevel inverters with space-vector PWM

Over the two past decades, a new type of semiconductor power converter has appeared – modular multilevel converter. This topology consists of several transistor-capacitive modules, allowing to realize multilevel rectifiers and invertors with various number of levels and phases. Currently, the modular multilevel converters are widely used in high-voltage direct-current power supply systems (HVDC), produced by Siemens and ABB companies, with a rated power 400-2000 MVA. Modular multi-level converters in comparison with other types of semiconductor converters have a number of significant advantages. The presence of a large number of levels (100 and higher) makes it possible to use relatively low-voltage switches and realize a low switching frequency, which leads to a high efficiency of the converter. The declared efficiency of the power transmission system of the HVDC power supply system with modular multi-level converters manufactured by Siemens is 99.5%.In addition, modular multi-level converters have increased reliability. This is due to the fact that modular converters can function even if they are output from individual transistor-capacitive modules. Constructive modularity and scalability of the converter is convenient in production. However, there is the problem of stabilizing the voltage on the capacitors of the cells. In the course of the research it was found that when using classical control algorithms for multi-level inverters (space-vector and sinusoidal pulse-width modulation) there is an imbalance of voltages on capacitors. Tension in one voice pair doubles. This leads to a significant deterioration in the shape of the output voltage. The article uses improved space-vector modulation algorithms for a modular multi-level converter, which provide the ability to stabilize the voltage in the converter cells, as well as the ability to control the amplitude and frequency of the output voltage. The Matlab / Simulink program has created a model of a modular five-level modular inverter. In the Matlab / Simulink program, a modular five-level inverter model was created. Studies have shown that the classical spatial vector modulation algorithms in modular multi-level inverters lead to an imbalance of tension on the capacitors and in modular multi-level inverters, and as a result, a voltage arises, which leads to a significant increase in the shape of the output voltage. The article uses an improved space-vector modulation algorithm for a modular multi-level converter, which allows stabilizing the voltage in cells. The simulation results confirm the proposed theoretical aspects.

Efficiency Optimization of Two Dual Active Bridge Converters Operating in Parallel

Growing interest in dc-type energy storage systems (ESS) and dc/dc converters has led to increased power ratings of such devices, which often requires parallel connection. The process of energy efficiency optimization of such modular systems is described in literature mainly in terms of the varying power demand, but the problem is more complex and not analyzed yet in relation to varying energy storage (ES) voltage level and bidirectional energy transfer at the same time. Therefore, this article considers the input parallel output parallel (IPOP) connection of two dual active bridge (DAB) modules as a representative modular converter, and to answer a question of: how to improve (maximize) efficiency of such a system, at different power demands and varying ESS voltage, only by a turn on/off operation of the module without any circuit and control algorithm changes. The proposed solution defines an operating area (enclosing different ES's voltage levels and loading power conditions) showing where one or two modules should operate to improve the overall efficiency. A curve fitting approach and polynomial approximation of the single module's input/output power curves are utilized for the proposed algorithm based on parametric functions. After simulation study, the method is validated with two 1.4 kW SiC-based DAB converters, operating at 100 kHz of switching frequency within a wide (0-270 V) ES's voltage range.

Failure‐mode analysis of modular multilevel capacitor‐clamped converter for electric vehicle application

The bi-directional buck/boost converter is being widely used as bi-directional DC/DC converter (BDC) in an electric vehicle (EV) traction system owing to its simple topological configuration. The major drawback of this configuration is in the event of failure of any of the power semiconductor devices and the power supply to the load will be interrupted. To overcome the above drawback, this work proposes a modular multilevel capacitor clamped converter (MMCCC) for EV application employing battery and ultracapacitor (UC) as energy storage sources. The battery-ultracapacitor (UC) combination provides better acceleration capability during high power demands and efficient energy recovery during regenerative braking. The design, control, and failure-mode analysis of MMCCC are discussed in detail to showcase the merits in terms of conversion gain, modularity, and reconfiguring capability under fault conditions. The high conversion gain owed to the modularity of the topology ascertains optimal stacking of UC thus reducing the weight and cost. Simulation is performed in MATLAB/SIMULINK environment, and hardware prototype is fabricated to validate the simulation results.

Graph-Theory-Based Modeling and Control for System-Level Optimization of Smart Transformers

Smart transformers offer the possibility to reduce the costs related to the integration of renewables and charging station into the grid and are gaining industrial interests. Grid system operators request modularity for easy maintenance and power scalability. Normally, the power is shared equally in the modular structure. However, differences in electrical and thermal parameters of the individual cells result in a nonoptimal efficiency and reliability characteristics with equal power sharing. In this article, a generalized approach for system-level modeling of modular converters with graph theory is presented. Graph theory simplifies the power flow management in modular architectures to a network flow problem, thereby facilitating the routing of power among the cells for optimizing reliability and efficiency. A power routing algorithm based on convex cost flow optimization is discussed, and case studies are presented to demonstrate the potential of the proposed method.

Handling Differential Mode Conducted EMC in Modular Converters

This article presents the analysis and management of differential mode conducted EMI in modular converters made from associations of standard conversion cells. Two main configurations, inputs in series—outputs in parallel and inputs in parallel—outputs in series are studied with the objective to define a generic EMI management technique, independent from the number of conversion standard cells implemented. First, analysis is carried out from a theoretical point of view based on simplified models. Second, filtering solutions are introduced and compared. Especially, centralized versus distributed filtering techniques are compared with the objective to find generic solutions. The results are compared and validated with practical characterizations.

Modular Two-Switch Flyback Converter and Analysis of Voltage-Balancing Mechanism for Input-Series and Output-Series Connection

The voltage-balance mechanism is a required ability to simplify the control scheme for modular input-series and output-series ( $\text{ISOS}$ ) connection, in spite of the fairly large number of studies, such mechanism has not been addressed clearly and satisfactory. This paper proposes a methodology, based on the converter output characteristics, to standardize the analysis of voltage-balance mechanism in steady state, which allows understanding the behavior of voltage sharing among the modules in steady state, in the presence of mismatched parameters for $\text{ISOS}$ connection of modular converters. In addition, the $\text{ISOS}$ connected modular two-switch flyback converter is presented. The intrinsic voltage-balance mechanism, even operating in continuous conduction mode, make this converter feasible, as an alternative to the predecessors modular converters. The operation and analysis of the proposed connection were corroborated, based on the experiment carried out on a laboratory prototype with three modules: ${\text{1}}, {\text{200 V}}_{\text{dc}}$ total input voltage, ${\text{1.5 kW}}$ rated power, and $\text{50 kHz}$ switching frequency. This paper is accompanied by a video demonstrating the operation, in steady state, of the laboratory prototype.

Challenges of Implementing Higher-Frequency Magnetics in Wide-Bandgap Converters [Women in Engineering

Designing system-level converters can be complicated. The interweaved intricacies of modular converters to achieve higher voltages while maintaining stable controls and magnetics design must be considered. While there are many difficulties to overcome, the magnetics tend to be the most uncomfortable for power electronics engineers to define in the converter design process due to the lack of direct transition from design and simulation to final hardware outcomes.

Research on an Asymmetric Fault Control Strategy for an AC/AC System Based on a Modular Multilevel Matrix Converter

This paper studies control strategies for an AC/AC system based on a modular multilevel matrix converter (M3C) when an asymmetric fault occurs in the secondary side ac system. Firstly, the operating principle of M3C is briefly introduced and verified by simulation. Then, based on its mathematical model by double αβ0 transformation, the decoupled control strategies for the primary side and secondary side systems are designed. In view of the asymmetric fault condition of the secondary side system, the positive sequence and negative sequence components of voltages and currents are separated and extracted, and then a proportional resonant controller (PR) is used to regulate the positive and negative sequence currents at the same time to realize decoupled current control in the αβ reference frames. The capacitor voltage balancing control, which consists of an inter-subconverter balancing control and an inner-subconverter balancing control, is realized by adjusting four circulating currents. Finally, the proposed control strategy is validated by simulation in the PSCAD/EMTDC software (Manitoba HVDC Research Center, Canada). The result shows that during the period of the BC-phase short-circuit fault occurring in the secondary side system, the whole system can still operate stably and transmit a certain amount of active power, according to their set values. Furthermore, the capacitor voltages are balanced, with a slight increase during the fault period. The simulation results verify the effectiveness of the proposed control strategy.

Improving Onboard Converter Reliability for More Electric Aircraft With Lifetime-Based Control

With the goal to decrease weight and increase overall efficiency, power electronics is proliferating in the aircraft applications. The conventional mechanical, pneumatic, and hydraulic systems are substituted with on-board electrical distribution systems. The new power electronics based distribution architectures should comply with the airworthiness standards by providing adequate redundancy to operate even under multiple failures. However, each redundant system adds to more weight and cost, and hence optimizing redundancy without compromising safety still remains a challenge. In this scenario, modular power converters offer a promising solution for obtaining reliable operation. Apart from ensuring availability and operation of the system, the modular converters enable controlling the stress for the individual converter cell and therefore their wear-out. This paper considers the failure mechanisms of the most sensitive devices in the system, the power semiconductors, and predicts their remaining useful lifetime (RUL). Based on this RUL, an optimization of the system loading is performed in order to delay the wear-out based failures in the system without reducing the mean lifetime of the building blocks. A study case is shown in order to demonstrate the lifetime extension of the system with the proposed algorithm.

A Reliable Medium-Voltage High-Power Conversion System for MWs Wind Turbines

With the increasing power ratings of large wind turbines, medium-voltage power conversion is drawing more and more attention because it generally offers higher power density and higher efficiency with reduced current levels. This paper evaluates the practical performance of a medium-voltage, fault-tolerant modular high-power conversion system based on cascaded modular converters. To avoid many large capacitor banks, the low-frequency voltage/power ripple in each cell has been attenuated through balancing the power flow between the H-bridge rectifier and the three-phase inverter by adding a frequency-adaptive quasi-resonant controller in the control loops. This can improve the system reliability and reduce the system cost and volume. In the experimental evaluation, an unexpected envelope oscillation issue was observed at the grid-side inverters. It was found that the above issue is caused by the discrepancy of the crystal oscillators on different digital signal processor boards. Then, a carrier-wave phase-locked loop has been proposed to eliminate the envelope oscillation issue. Experimental results of a downscaled three-phase five-level modular wind energy conversion system show that the dc-link voltage ripple can be eliminated without affecting the grid current quality.

Performance Analysis of Modular Forward Converter in Input Parallel Output Series (IPOS) Architecture Using PI and Fuzzy Logic Controller

This paper investigates the performance of modular dc–dc forward converter in Input Parallel Output Series (IPOS) configuration which is controlled by an effective controller. This IPOS topology facilitates the converter to have high output voltage gain. The role of controller is crucial in ensuring the power sharing between the converters. Fuzzy controller, known for its versatility, is employed in this research work. The fuzzy controller feeds appropriate pulse width modulated control signals for appropriate input and load regulation. The performance of the fuzzy controller is analyzed by comparing it with proportional integral controller and a advanced neural network controller. The proposed IPOS system has been analyzed using MATLAB/Simulink environment and hardware prototype. The simulation and hardware results reveal that the suggested topology with fuzzy controller is very effective compared to its counterparts.

A Self-Balanced Bidirectional Medium-/High-Voltage Hybrid Modular DC–DC Converter With Low-Voltage Common DC-Link and Sequential Charging/Discharging of Submodules Capacitors

This paper proposes a medium-/high-voltage high-power hybrid modular dc–dc converter to interconnect two different dc-voltage levels in medium-/high-voltage dc grids. The converter consists of half-bridge submodules (SMs) rated at a moderate voltage level, i.e., series connection of semiconductor devices is avoided while connecting two high dc-voltage levels. The proposed architecture provides self-balancing operation, i.e., unlike conventional modular converters, no need for capacitor voltage measurement, which enhances the system reliability and simplicity. Sequential charging/discharging of SMs capacitors is adopted in the proposed approach to ensure a self-balanced operation. The proposed architecture can provide a high conversion ratio as well as power transfer in both directions, i.e., bidirectional dc–dc converter. Detailed illustration of the proposed approach operational concept, design, and overall control system is presented. Simulation and experimental results are presented to show the viability of the proposed configuration and validate the claims.

A high voltage multi level arbitrary waveform generator for insulation testing

Due to the extensive applications of power electronic devices in power grids, many insulation components will be stressed by high voltage PWM, multilevel or even more complex waveforms. In this paper, a high voltage generator able to produce various waveforms is developed. The generator is based on cascade connected multilevel modular converter (MMC) topology, including 20 half bridge MMC cells with the capability of 21 output voltage levels. Half of the series-connected MMC cells are charged positively, while the other half are charged negatively. Each positive and negative half bridge MMC cell is connected alternately to optimize the insulation design. The power supply of the driving circuit for each MMC cell is provided by an individual fly-back converter fed by the cell capacitor to realize a compact design. On the basis of simulations, a multilevel arbitrary waveform generator prototype was constructed, with the maximum peak-to-peak voltage of ∼ 28 kV. A capacitor switching strategy is utilized to solve the voltage-imbalance problem resulted from the difference of the element parameters and other reasons. Finally, it was tested on a typical insulation with a simulated defect.

A novel energizing strategy for a grid-connected modular multilevel converter operating as static synchronous compensator

This paper proposes a novel energizing strategy for a three-phase modular multilevel converter connected to the electric network as static synchronous compensator. The energizing system is comprised of two thyristors and its controller. A comprehensive mathematical model is developed for each energizing stage. Based on the models, a control law is designed in order to compute the thyristors firing angle. This is performed in such a way that the maximum value of the energizing current drained from the grid is limited. First, during the positive half-cycle of the mains voltage, the current flows through two arms of the modular converter charging its DC capacitors. In the following half-cycles, the pre-charged capacitors transfer part of their stored energy to the other arms capacitors. The process is repeated until the total DC voltage of the modular converter reaches the grid voltage peak value. Digital simulation results are presented to validate the proposed methodology.

Current-Source Modular Medium-Voltage Grid-Connected System With High-Frequency Isolation for Photovoltaic Applications

Large-scale grid-connected photovoltaic energy generation systems are progressing remarkably benefiting from the latest developments in solid-state semiconductors technology. In such systems, the photovoltaic arrays can be connected directly to the medium-voltage grid without employing a bulky line-frequency transformer to step up the voltage. Nano-crystalline cores with a small size and a high permeability operating at medium or high frequency can be installed in the power conversion stage. Hence, the necessary isolation as well as voltage boosting features can be provided. However, only a few power converters allow this type of isolation. This paper proposes a new modular converter structure suitable for medium-voltage grid connected systems with high-frequency isolation. The output voltages of the series-connected modules are added in order to provide the necessary voltage boosting. Four different power converter topologies with small input capacitors can be used as submodules for the presented medium-voltage configuration having different advantages and drawbacks. These different topologies are analyzed in terms of power losses, footprint, and functionality. To validate the mathematical analysis and the computer simulations, a scaled-down 5 kVA three-phase, 1 kV prototype is built and tested with four modules for each phase.

A Modular Multilevel DC–DC Converter With a Compact Sub-Module Stack Suited to Low Step Ratios

The voltage ratings for dc transmission and distribution are not currently standardized because cable technology is advancing rapidly and therefore a low step-ratio dc-dc converter at multi-megawatt scale is a crucial technology for combining point-to-point dc links into multi-terminal networks. This paper presents a modular multilevel dc-dc converter configuration with only a single stack of half-bridge sub-modules (SM) and a resonant tank. The power processed through the stack is a small fraction of the throughput power if the step ratio is small (1 - v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> /v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> ). Detailed analysis shows the required semiconductor ratings are much smaller compared to the front-to-front MMC and modular converters derived from standard dc-dc topologies. Further, the requirement for SM capacitive energy storage is also significantly less than those converters. The theoretical analysis and the operating principles have been verified by full-scale simulation examples and experiments with a down-scaled prototype. The SMs are operated with an adjustable phase shift to create a high frequency excitation across the resonant tank for further volume reduction. The SM capacitors self-balance without the need for switching pattern adjustments and the resonant operation provides soft switching for all power switches to increase efficiency.

Novel Frequency-Doubling Modulation and Control Strategy for Three-Level Full Bridge based Power Electronic Traction Transformers

Power electronic traction transformers (PETTs) are widely investigated to substitute bulky line frequency transformers in railway traction system presently. This paper adopts the three-level topology for both grid-side cascaded converters and primary-sides of LLC resonant converters. The cascade number is reduced by half as well as isolated transformers compared to the traditional two-level configuration. Less cascade number of modular converters leaves relatively more space for electric insulation implementation, which is beneficial to PETTs in terms of the limited installation space in the train. A frequency-doubling modulation strategy for LLC resonant converters is proposed, which can enable the operating frequency of isolated transformers two times higher than the switching frequency to further reduce the transformer size without increasing the switching loss. Meanwhile the proposed modulation strategy can realize the capacitor voltage balance autonomously as well. Additionally, the control strategy of cascaded rectifiers and key parameter design guideline of LLC resonant converters are also introduced.

Multi‐hierarchy control strategy in abc coordinate for modular multilevel matrix converter in fractional frequency transmission system

This paper proposes a multi-hierarchy control strategy in abc coordinate for modular multilevel matrix converter (M3C) in fractional frequency transmission system (FFTS). The proposed control strategy avoids complicated coordinate transformations when the traditional control strategies based on coordinate transformation are adopted. Four hierarchies are designed to balance the capacitor voltages. In different hierarchies, the capacitor voltage ripples have different frequencies. According to this characteristic, different bandwidths of filters are adopted in different hierarchies, resulting in different control speeds among the four hierarchies. This will not only speed up the control but also assure the control accuracy. Simulation results verify the feasibility and effectiveness of the proposed strategy.

Power Converters Based Advanced Experimental Platform for Integrated Study of Power and Controls

With increasing interest in smart grid and renewable energy, significant investments have been allocated to promote related studies. Since there is a wide spectrum of topics to study, it is necessary to have an advanced experimental platform that can accommodate both system- and component-level studies, both hardware and algorithm designs, and both teaching and research. Unfortunately, such experimental platform is not commercially available. In this paper, an advanced power electronics based experimental platform is introduced. The system is consisted of one OPAL-RT real-time simulator, two one-bus microgrid testbeds, and two modular multilevel converters. The subsystems can form a multiple-bus microgrid testbed if connected through emulated power lines. The system can provide real-time simulation, controller hardware-in-the-loop (HIL) simulation, and power HIL simulation for power systems study. Various converter topologies can be configured with the modular converters for power electronics study. Both real-time simulator and DSP control boards can be used to implement advanced control algorithms. The designs and experiences shared in the paper will benefit many researchers that are in need of such system and promote power and energy related studies.

The Asymmetric Alternate Arm Converter: A Compact Voltage Source Converter Design for HVDC

The Modular Multilevel Converter (MMC) has become one of the most practical topologies for integration of renewable energy power plants into the grid, through submarine HVDC cables and HVDC lines. Nonetheless, the MMC requires significant large DC storage capacitors to mitigate for large power, low frequency pulsations due to the single-phase like AC to DC conversion process in their arms. This work addresses the recently introduced Asymmetric Alternate Arm Converter, a hybride MMC topology which uses modular converter arms only on the lower side of the main converter whereas the upper side uses only standard ON-OFF valves topology. This converter is similar to the Alternate Arm Converter (AAC) but dispenses of the modules on the upper side and of the director switch on the lower side. Like the AAC, the Asymmetric AAC offers a significant reduction in DC energy storage requirements and in the number of modules. In addition, the Asymmetric AAC offers a less restricted operation than the AAC, in terms of operating over a wide range of AC to DC voltage ratio, and the possibility of achieving both distortion-free AC current and ripple-free DC current. The principle of operation of the Asymmetric AAC is revisited here and a suitable control strategy to demonstrate its operation is developed. In addition, requirements for energy storage, voltage and current rating of converter modules and converter losses are examined by means of simplified analyses. Proposals are verified through simulation studies and experiments conducted on a low power prototype having three H-bridge modules per arm. Results confirm good operation of the proposed converter and control scheme.