Cascaded Converter Research Articles

DC‐link voltage stability analysis for three‐level boost + full‐bridge LLC cascaded converter using impedance modeling

AbstractThe cascade converter system has been widely concerned along with the medium power operating conditions, and it is crucial to address the intricate interplay among individual modules to ensure stability of both the source and load subsystems. This paper analyzes the DC‐link voltage stability based on impedance matching and proposes a normalized sensitivity calculation method based on the control strategy, which prevents the complex products and matrix calculations of traditional methods. The three‐level boost (TLB) output impedance model is derived based on state‐space averaging for open‐loop and considering the double‐loop, and the full‐bridge LLC (FBLLC) input impedance model is derived based on the fundamental equivalent circuit. Then the effect of the double‐loop PI parameters on the output impedance of the TLB is analyzed in detail based on the normalized sensitivity and verified by the Bode plots. The experimental results of the 30 kW prototype indicate that the control parameters were varied by a factor of 10 compared to the theoretical control group. The most significant alteration was the modification of the kp_i, resulting in an 8.9% increase in the DC‐link voltage ripple, while the ki_v was modified, resulting in a 0.53% increase, indicating that the effects of the PI parameter are consistent with the normalized sensitivity results.

On the Feasibility of an LCD-Based Real-Time Converter for Ionizing Radiation Imaging.

Here we present the cascade converter (CC), which provides real-time imaging of ionizing radiation (IoR) distribution. It was designed and manufactured with the simplest architecture, utilizing liquid crystal display (LCD) technology. Based on two merged substrates with transparent electrodes, armed with functional layers, with the cell filled with nematic liquid crystal, a display-like, IoR-stimulated CC was achieved. The CC comprises low-absorbing polymer substrates (made of polyethylene terephthalate-PET) armed with a transparent ITO electrode covered with a thin semipermeable membrane of polymer (biphenylperfluorocyclobutyl: BP-PFCB) doped with functional nanoparticles (NPs) of Lu2O3:Eu. This stack was covered with a photoconductive layer of α-Se and finally with a thin polyimide (PI) layer for liquid crystal alignment. The opposite substrate was made of LCD-type glass with ITO and polyimide aligning layers. Both substrates form a cell with a twisted structure of nematic liquid crystal (TN) driven with an effective electric field Eeff. An effective electric field driving TN structure is generated with a sum of (1) a bias voltage VBIAS applied to ITO transparent electrodes and (2) the photogenerated additional voltage VXray induced between ITO and α-Se layers with a NPs-doped BP-PFCB polymer layer in-between. The IoR (here, X-ray) conversion into real imaging of the IoR distribution was achieved in the following stages: (1) conversion of IoR distribution into non-ionizing red light emitted with functional NPs, (2) transformation of red light into an electric charge distributed in a layer of the photoconductive α-Se, which is what results in the generation of distributed voltage VXray, and (3) a voltage-mediated, distributed switching of the TN structure observed with the naked eye. The presented imaging device is characterized by a simple structure and a simple manufacturing process, with the potential for use as a portable element of IoR detection and as a dosimeter.

A Simplified single-phase neutral-clamped H-bridge cascade converter FCS-MPC control

A Simplified single-phase neutral-clamped H-bridge cascade converter FCS-MPC control

Clustered Voltage Control and Modulation Scheme With Switching Loss Reduction for High Power Hybrid Cascaded Converter

The hybrid cascade converters (HCC) is a potential solution to achieve harmonic current compensation in medium and high voltage applications due to the characteristics of having high frequency module. However, the high switching frequency inevitably leads to an increase of undesirable switching loss. In addition, since the reduction of power loss and the balance of clustered voltage are mutually interrelated, the existing modulation and control schemes cannot cope with them decently. To address this problem, the HCC is divided into a high-frequency module (HFM) and a low-frequency module (LFM), and a corresponding hybrid modulation scheme is proposed to reduce the power loss while maintaining a high harmonic compensation performance. Then, based on the analysis of the system voltage constraints, the dc-link voltage and threshold voltage under different parameters are redesigned. Furthermore, since the power flow between modules has been changed with the hybrid modulation scheme, the conventional clustered voltage control strategy is not capable of clustered control. Therefore, based on the re-modeling of the power flow, a negative-sequence voltage injection control strategy is proposed to achieve voltage balance. Finally, simulation and experimental results are demonstrated to verify the effectiveness of the proposed scheme.

Harris Hawks Optimization Algorithm for reducing THD using ZVT-ZCT-based QRCC: A comparative approach

This study proposes a novel solution to address harmonic issues in a Renewable Energy Resource System (RES) connected to the grid, using a Voltage Source Inverter (VSI) controlled by a QRCC based on Zero Voltage Transition and Zero Current Transition (ZVT-ZCT). We enhance system performance by integrating Harris Hawks Optimization (HHO) with the VSI. Key outcomes include evaluating the system's effectiveness in terms of voltage drop, current drop, real power, active power, and total harmonic distortion (THD). Notably, settling times for various converters are highlighted: 0.01 s for SEPIC, 0.008 s for CUK, 0.005 s for ZETA, and an impressive 0.0001 s for the Cascade converter. Under different operational conditions, open-loop operation yields a THD of 18.45%, reduced to 7.843% in closed-loop with PI controller. Optimization techniques further improve the system, achieving a low THD of 0.0549%. We emphasize the significance of MPPT-based INC-IR for cascade converters, resulting in a minimal switching loss of 0.38W, showcasing the system's efficiency in energy conversion.

Повышение адекватности моделей электромеханотронных систем

Development of electromechanotronic systems is connected with modeling and calculation of processes of various kinds. Electromagnetic, mechanical and thermal processes are analyzed, control, regulation, protection, monitoring and diagnostics problems are solved. Systems may contain a large number of power semiconductor elements. Short-term and multi-hour processes are calculated. Taking into account mutual connections of processes of various kinds in calculations of complex systems requires inadmissibly large expenditures of machine time. The problems are solved under more or less significant assumptions, which leads to a decrease in the adequacy of the results. The methods of systems calculation are proposed, which allow to reduce machine time consumption and to use more detailed description of installations and more accurate computer models. Possibilities of calculation of processes by «smooth components» of variables, and also application of dual models, in which speed and accuracy are combined, are considered. As an example, a system with an active cascade frequency converter containing 780 transistors and diodes, parasitic inductances and snubber circuits, and a vector-controlled induction motor with ladder circuits accounting for current displacement is analyzed. The model takes into account energy losses in the converter and in the motor, as well as the distribution of losses by components. It also takes into account emergency operation modes associated with disconnection of a part of low-voltage units of the cascade converter and maintenance of power quality at its input and output by symmetrization of the main current components and suppression of parasitic components.

ДВОНАПРАВЛЕНИЙ КАСКАДНИЙ ПЕРЕТВОРЮВАЧ ПОСТІЙНОЇ НАПРУГИ ДЛЯ ПОТУЖНОЇ СИСТЕМИ ЕНЕРГОНАКОПИЧЕННЯ В МЕРЕЖАХ ЕЛЕКТРОПОСТАЧАННЯ SMART GRID

Electromagnetic processes in a bidirectional cascade DC converter with the possibility of increasing and decreasing the output voltage levels in both directions of energy transfer for electric power storage systems are considered. Using the method of averaging in the state space, a mathematical model of the pulse converter was obtained. Analytical ex-pressions for calculating the ripple characteristics of a bidirectional cascade converter have been determined, with the help of which you can calculate the main parameters of the constant voltage converter, which provide permissible volt-age ripples on the DC buses and power choke current. To illustrate the developed calculation methodology, the ripple characteristics and parameters of a 30 kW converter for various load modes with 400V and 700V DC busbars for use in energy storage systems are determined. The bidirectional converter was calculated, analyzed and simulated in Math-cad/PSim packages at idle speed and at different load levels in order to confirm the effectiveness of the conducted re-search. Ref. 7, fig. 2. Keywords: method of averaging in state space, electric power storage systems, bidirectional constant voltage converter, non-isolated four-quadrant bidirectional converter.

New Modulation Method to Reduce Cell Capacitor Ripple Current in Modular Multilevel Cascade Converters and its Application to Active Power Filters

New Modulation Method to Reduce Cell Capacitor Ripple Current in Modular Multilevel Cascade Converters and its Application to Active Power Filters

An Input-Series-Output-Parallel Cascaded Converter System Applied to DC Microgrids

Direct current transformer (DCT) is a key piece of equipment in direct current (DC) microgrids, and the mainstream topologies mainly include LLC resonant converter (LLC) and dual active bridge (DAB). In this paper, a novel bi-directional buck/boost + CLLLC cascade topology is proposed for the input-series-output-parallel cascade converter system of a DC microgrid. To solve the problem that frequency variation causes the converter to deviate from the optimal operating point, resulting in low efficiency, and the inability to achieve a soft switching function. The CLLLC converter operates near the resonant frequency point as a DCT, only providing electrical isolation and voltage matching, while the buck/boost converter controls the output voltage and the voltage and current sharing of each module. Compared to other cascaded converter systems, the cascaded converter proposed in this paper has high efficiency, simplifies the parameter design, and is suitable for wide input and wide output operating conditions. The system adopts a three-loop control strategy, establishes the small-signal modeling of the system, and its stability is verified by theoretical analysis and simulation. The simulation and experimental results verify the correctness of the proposed cascaded converter based on buck/boost + CLLLC and the effectiveness of the control strategy.

Increasing PEM fuel cell performance via fuzzy-logic controlled cascaded DC-DC boost converter

The voltage level produced by Proton Exchange Membrane (PEM) fuel cell (FC), which is one of the clean energy sources, is low for many industrial applications. For this reason, for many industrial applications such as electric vehicles, it is necessary to use a boost type DC-DC power electronic circuit at the output of PEM FC. In this case, in addition to the PEM FC performance, the dynamic performance and efficiency of the DC-DC converter to be used affect the performance and efficiency of the total system. In this study, a single-switch cascaded DC-DC boost converter, which has both higher voltage gain and more efficiency compared to conventional cascade converter and other boost type converters, is proposed for PEM FC. Since the proposed single-switch cascaded DC-DC boost converter includes only one controlled-switch, it is efficient as well as smaller in size and less costly compared to the other types. In addition, the proposed single-switch cascaded DC-DC boost converter is tested with the traditional PID and fuzzy-logic control methods to observe the dynamic performance of PEM FC. When the fuzzy logic controller is compared with the PID controller, it is clearly seen that it gives 21% more successful results as per rise time, 71% settling time, 97% overshoot, and 26% RMSE (Root Mean Square Error) error rate. As a result, when the proposed single-switch cascaded DC-DC boost converter is regulated via fuzzy control method, a more efficient PEM FC system which has better dynamic performance can be designed.

Cascaded AC-DC Power Conversion Interface for Charging Battery

This paper develops a cascaded AC-DC power conversion interface (CADPCI) to convert AC power to charge the battery set. The proposed CADPCI is composed of a cascaded converter (CC) and a dual-input buck converter (DIBC). The CC is formed by connecting a full-bridge converter (FBC) and a bridgeless rectifier (BLR) in series. The CADPCI generates an 11-level input voltage and performs unity power factor correction. The switching loss is reduced because only the FBC with a lower DC port voltage is switched at a high frequency. The DIBC uses a buck converter and a selection switch set to generate a two-level DC voltage on the DC port of the BLR. By controlling the DC input voltage of the buck converter, the injected power of the BLR can match the input power of the utility. Therefore, the FBC does not require to handle the real power, saving an isolated converter for regulating the DC port voltage of the FBC, thus simplifying the power circuit of the CC. The buck converter also acts as a DC active filter to filter out low-frequency ripples of the charging current. A prototype is constructed to verify the performance of the proposed CADPCI.

Internal Energy Balance of a Modular Multilevel Cascade Converter Based on Chopper-Cells With Distributed Energy Resources for Grid-Connected Photovoltaic Systems

This article presents a methodology to control the internal energy balance of a modular multilevel cascade converter (MMCC) with distributed energy resources. The converter is characterized by a high voltage multi-terminal DC link connected to a three-phase MMCC with distributed photovoltaic arrays. Each modular arm structure is composed of bidirectional cascaded chopper cells with floating capacitors connected to the photovoltaic array through a DC/DC converter. Based on the power flow equations developed for the individual cells, a control methodology is applied to maintain the internal energy balance of the MMCC under power imbalance between the cells. The operation at different levels of irradiance is discussed to allow the operation with individual cells with zero energy generated from the photovoltaic array. Zero power cells are used to maintain three-phase output voltage and stable MMCC operation. Power balance between the arms is achieved through of the AC circulating currents in the MMCC arms without affecting the DC link and AC output currents. This converter topology and control methodology is suitable for hybrid renewable energy systems such as wind-solar power systems. Comprehensive experimental results are presented to validate operating principles and control methodology.

Research on High-angle operation Capability of Converter Valve Based on Thyristor Control Unit Optimization

When the converter valve operates at a high angle, the voltage stress borne by the internal electrical components increases, the loss of the damping circuit increases, and the junction temperature of the thyristor increases. For the influence of the harmonic component of valve voltage on thyristor energy extraction at a high angle, the thyristor control unit energy extraction circuit was optimized. Aiming at several typical high-angle operation conditions of ± 800kV hybrid cascade converter valve, the high-angle operation capability of the converter valve was studied and tested. Firstly, the electrical stress of the converter valve under different operating conditions of high angle was analyzed. Secondly, based on the engineering parameters of the Baihetan-Jiangsu UHVDC project, the operating parameters of the converter valve at high angles were calculated, and the most severe stress of the converter valve at high angles was extracted. Finally, based on the principle of the converter valve synthetic test circuit, the high-angle test parameters of the converter valve were calculated, and the high-angle operation test was carried out. The theoretical analysis and test results show that the electrical stress of the converter valve under high-angle working conditions is slightly higher than the rated operating condition, and the converter valve can withstand this stress, which can meet the requirements of long-term operation at a high angle.

Research on Control Strategy of High Voltage Cascaded Energy Storage Converters

High voltage cascaded energy storage power conversion system, as the fusion of the traditional cascade converter topology and the energy storage application, is an excellent technical route for large capacity high voltage energy storage system, but it also faces many new problems. How to use the control strategy to play better the advantages of high voltage cascaded energy storage has gotten more and more attention. This paper summarizes the research on power control, balance control, and fault-tolerant control of high voltage cascaded energy storage to provide a reference for related research and engineering application.

A Vector Inspection Technique for Active Distribution Networks Based on Improved Back-to-Back Converters

In this paper, an improved back-to-back converter is proposed, and the converter is used as a test power source for vector inspection of relay protection in an active distribution network, which effectively solves the problem that the output voltage and current of the test power source cannot be continuously and stably adjusted. Firstly, a three-phase back-to-back cascade converter is established to analyze the impedance characteristics of its DC terminal. Then a feedforward voltage is added to the inverter to improve the input impedance characteristics of the inverter. Secondly, the system stability and parameter stability of the improved back-to-back converter are analyzed. Finally, the improved converter is used as the test power source for vector inspection of relay protection in the active distribution network. The simulation results show that the stability of the improved back-to-back converter system is greatly improved. The experiment shows that the vector check technology based on an improved back-to-back converter can effectively check the vector of relay protection in an active distribution network and find various installation problems.

Boost Multilevel Cascade Inverter for Hydrogen Fuel Cell Light Railway Vehicles

This article presents a ground breaking traction drive for fuel cell-powered light rail vehicles based on a multilevel cascade converter with H-bridge cells. The converter provides dc–ac power conversion with unique buck–boost capability to compensate for the variation of fuel cell terminal voltage with the load power. In comparison to a conventional boost inverter, based on the cascade connection of a boost dc–dc converter and a voltage source inverter, the proposed converter has the significant benefits of much smaller inductor size, lower dc side current ripple, and multilevel voltage waveforms at the ac output. This article presents the converter modeling and design of the boost inductor and submodule capacitors, taking into account the full speed range of the motor. The concept has been fully validated on a bespoke experimental prototype driving an induction motor, showing the suitability of the proposed converter as a traction drive for hydrogen-powered rail vehicles.

A Unified Inner Current Control Strategy Based on the 2-DOF Theory for a Multifunctional Cascade Converter in an Integrated Microgrid System

Wider applications of integrated microgrids have been significantly restricted by converters with the sole function of power regulation. In microgrids with distributed generation and energy storage equipment, it is crucial for converters to be capable of reactive power compensation, harmonic suppression, voltage support, and other functions simultaneously. To achieve this, it is essential to modify the discrepancy between different control modes, both in control architectures and input signals. However, previous researches have focused on the stability and robustness of the system’s operation, rather than the instantaneous tracking performance of instructions during the mode switching. Instead, a unified control strategy based on the two-degree-of-freedom theory is conceived in this paper, to impose no reconfiguration of the inner current loop, so that the inherent stability can be guaranteed. In the proposed strategy, mode transition is replaced by the readjustment of referring instructions, and the complex tuning of filter parameters is abandoned. Thus, a desirable performance in a wide range of operating conditions for the microgrid system is provided and the effects of the disturbances associated with the mode transitions are eliminated. The simulations studied in MATLAB and experimental evaluations of the prototype both corroborate the simplicity and effectiveness.

Research on power quality comprehensive control device under the situation of medium voltage and high power based on H-bridge cascade converter

This paper proposes a power quality control device based on H-Bridge cascade converter, to solve the problems in current decentralised power quality control, such as huge investment in equipment, expensive operation and maintenance cost, hard to coordinate, and low global technical-economic ratio. It provides a novel for the comprehensive management of power quality in distribution networks. This paper has three parts. In the first part, the topology structure and working principle of the power quality comprehensive control device based on H-Bridge cascade converter are analysed, and its key parameters are analysed and designed. In the second part, corresponding coordinated control strategy is proposed according to the function of power quality comprehensive control device. Finally, a 10kV/1MVA medium-voltage and high-power power quality comprehensive control device is designed. The simulation through MATLAB Simulink proves that the parameter design is correct and the coordinated control strategy is effective.

Research on power quality comprehensive control device under the situation of medium voltage and high power based on H-bridge cascade converter

This paper proposes a power quality control device based on H-Bridge cascade converter, to solve the problems in current decentralised power quality control, such as huge investment in equipment, expensive operation and maintenance cost, hard to coordinate, and low global technical-economic ratio. It provides a novel for the comprehensive management of power quality in distribution networks. This paper has three parts. In the first part, the topology structure and working principle of the power quality comprehensive control device based on H-Bridge cascade converter are analysed, and its key parameters are analysed and designed. In the second part, corresponding coordinated control strategy is proposed according to the function of power quality comprehensive control device. Finally, a 10kV/1MVA medium-voltage and high-power power quality comprehensive control device is designed. The simulation through MATLAB Simulink proves that the parameter design is correct and the coordinated control strategy is effective.

Faults in Modular Multilevel Cascade Converters–Part II: Fault Tolerance, Fault Detection and Diagnosis, and System Reconfiguration

Modular Multilevel Cascade Converters (MMCCs) are considered a promising power electronics topology in industry. Their scalability allows to reach (ultra/very) high voltage levels with low harmonic content and high efficiency and makes MMCCs an ideal solution for high-power applications; such as electrical drives, solid-state transformers and high-voltage direct-current (HVDC) transmission systems. However, the high levels of thermal, electrical and mechanical stress on the power electronics devices and the large number of components (e.g. capacitors or semiconductors) make MMCCs prone to faults. Fault detection and diagnosis (FDD) in combination with fault isolation and system reconfiguration techniques, based on cell redundancy, can increase the reliability, availability and safety of MMCCs, which is crucial for their utilization in critical energy applications. This second part of the paper comprehensively surveys: (i) fault tolerance and fault detection & diagnosis (FDD; e.g. expert system, model- or hardware and data-based FDD methods) and (ii) system reconfiguration strategies (e.g. cold- or hot-redundant) for MMCCs. Finally, the state-of-the-art, challenges and future research trends and opportunities towards reliable MMCC-based systems are revealed.