Voltage Capability Research Articles

A Novel Multilevel Inverter With Self-Balancing Capability of Capacitors Voltage; Structure, Modulation, and Operation

This article presents a 17-level asymmetrical multilevel inverter (MLI) with a reduced number of components. This structure can also be used symmetrically to increase redundancy. It can be extended in a modular fashion to increase the number of output voltage levels. Moreover, the capacitor voltages are self-balanced and do not require a sensor to stabilize the voltage. Due to the application of a level-shifted modulation (LSM), most power switches are turned on and off at low and fundamental frequencies, which will significantly reduce losses. Other benefits of this topology are the inherent production of negative levels without H-bridge, reduced total standing voltage (TSV), and low total harmonic distortion (THD). Power loss analysis is also presented as well as calculation of capacitances. MATLAB simulation results and laboratory implementation are carried out to evaluate circuit performance in different operational conditions.

Study on Directivity of Underwater Circular Transducer Array

At present, in the detection and identification of underwater noise sources, one of the most commonly used transducer arrays is the circumferential transducer array. The circumferential transducer array is symmetrical in space and has high withstand voltage capability and small sound pressure fluctuation in the horizontal beam. Its directivity is an important index that affects the performance of the array. This paper focuses on the directivity maps of circular arrays with different sizes and numbers of elements. From the graph, it is concluded that the undulation of the directivity graph of the circular array is directly related to the array element spacing. The smaller the array element spacing is, the smaller the undulation is, and vice versa. The directivity formula and law can provide technical guidance for the design of a uniform circular transducer array.

A Serial Fault-Tolerant Topology Based on Sustainable Reconfiguration for Grid-Connected Inverter

Grid-connected inverters are widely used to integrate energy into the grid in renewable energy applications. However, the inverter usually has a high probability of failure due to a large number of semiconductor devices. In addition, especially in the field of marine renewable energy, the humid environment in offshore and coastal areas may make the inverter more prone to failure, which reduces system reliability. Therefore, a serial fault-tolerant topology based on a sustainable reconfiguration is proposed. The proposed topology can be reconfigured continuously by making full use of healthy devices, which can ensure the output voltage capability and improve the possibility of uninterrupted performance as much as possible. In addition, the cooperative modulation signal is selected for the recombined topology to output the desired voltage, which ensures the quality of the power generated by marine renewable energy. Principles of topology reconfiguration and modulation signal selection is described in detail. Simulation and experimental results verify the validation of the proposed method on the seven-level grid-connected inverter.

A double boost 9‐level switched capacitor‐based multilevel inverter for photovoltaic applications

SummaryA self‐balancing 9‐level inverter is proposed in this paper. To generate a 9‐level voltage with a gain of two requires a single DC source, eight power switches, three capacitors, and two diodes. Without additional circuitry, the voltages of the capacitors in the proposed topology can achieve inherent balance. The peak‐inverse‐voltage (PIV) of six power switches equals or less than the source voltage, and the other two are twice. As power switches are complementary, the proposed topology requires fewer gate drivers. Furthermore, the self‐balancing capability of capacitors' voltage helps in simple control complexity. The description of topology, mode of operation, modulation strategy, and design of the capacitor of proposed topology are presented. The advantages of the proposed inverter are examined by comparing it with switched capacitor multilevel inverters (MLIs). Finally, a 9‐level prototype is built to verify the theoretical analysis and the practical efficacy of the proposed inverter.

An SMPS-Based Lithium-Ion Battery Test System for Internal Resistance Measurement

A switching mode power supply (SMPS)-based lithium-ion (Li-ion) battery test system (BTS) is proposed to meet the increasing demands of tests in battery research. To improve the flexibility, while building the BTS with low cost and high reliability, an SMPS is employed as a charger/discharger, combined with a digital-to-analog hybrid control system to regulate its output voltage and current. Based on the BTS, two dc internal resistance (DCIR) measurement methods are designed, and a battery state classification method based on the DCIR cluster analysis with similarity as the judgment basis is proposed. An experimental prototype is developed to verify the feasibility of the proposed BTS and methods. The results show that the BTS possesses excellent voltage and current regulation capabilities for charging, discharging, and grid energy recovery. The internal parameter extraction method enables obtaining DCIR and other parameters with high accuracy. The online measurement method allows automatic acquisition of real-time DCIR and its variation trend without affecting the charging, which greatly improves the measurement efficiency. The proposed hierarchical clustering method based on DCIR effectively achieves the classification of battery states with excellent practicality and robustness.

Low-Cost and Compact Asymmetrical Unidirectional-Current Modular Multilevel Converters

Although the modular multilevel converters (MMCs) that contain submodules (SMs) with negative voltage capability have various advantages, they usually demand many more semiconductors than the conventional half-bridge SM-based MMC (HB-MMC). This study proposes a unidirectional-current clamp-double submodule (UC-CDSM) by combining two unidirectional-current full-bridge SMs (UC-FBSMs) using a shared switching device. The sharing design enables the UC-CDSM-based MMC (UC-CD-MMC) to have 25% fewer switching devices compared with the UC-FBSM-based MMC. The quantity of switching devices is rather similar to that in a conventional HB-MMC while the UC-CD-MMC still retains the advantages, such as low capacitor usage, dc fault clearing capability, and wide-range dc voltage adjustability. Moreover, a unidirectional-current hybrid MMC composed of UC-CDSMs and UC-FBSMs (UC-HYB-MMC) is presented to further enlarge the adjustable range of dc voltage. Detailed comparisons indicate that the UC-CD- and UC-HYB-MMCs can reduce the valve costs by 32% and 25%, respectively, and volumes by 39% and 34%, respectively, compared with HB-MMCs. Simulation and experimental results verify the steady-state and dc-fault clearance of the proposed topologies, and that the capacitor voltages in the UC-FBSMs and UC-CDSMs are well maintained and balanced in the UC-HYB-MMC.

Monolithic Bidirectional WBG Switches Rekindle Power Electronics Technology [Expert View

There are numerous mass volume power applications where it is necessary to control the flow of bidirectional power, including electric vehicles (vehicle to grid, vehicle to home, and vehicle to vehicle), distributed and grid-tie power systems using regenerated energy and/or energy storage components, and solid-state circuit breaker protection. Silicon carbide (SiC) and gallium nitride (GaN) based bidirectional power switches can enable these applications with their compelling advantages of high efficiency, high blocking voltage capability, and low system weight and volume. In particular, monolithic switches that allow for bidirectional symmetric conduction and voltage blocking with a chip area close to that of a similarly rated unidirectional switch are ideally suited to fuel a revolution in power electronics technology.

Simulation analysis on the capacity occupancy of reactive power equipment under DC bias of transformers

When the high voltage direct current (HVDC) system operates in ground-return-mode (GRM), ground return current with high amplitude up to several thousand amperes will penetrate into the earth. Such DC current will flow through AC lines with star-connecting and grounding at both ends, and intrude the neutral point of the transformers of the line in turn. Therefore, it will increase the reactive power consumption of the transformer and occupy the capacity of reactive power compensation equipment, as well as threaten the security and stability of the power grid. A simulation model of typical substation system operating under DC bias is established, based on which the quantitative analysis of the variation of the excitation current and the condition of reactive power consumption for transformers under different bias current is carried out. The analysis results show that the excitation current is seriously distorted and the reactive power consumed increases sharply under DC bias current with high amplitude, which occupies a large amount of reactive power compensation equipment capacity. Therefore, extremely adverse impact can be brought out on the reactive power balance and the voltage regulation of the power grid. In addition, the results of this paper are of significant reference value for researches on the reactive power characteristics of DC bias as well as the support capability of short-circuit voltage under DC bias.

Optimization of the proportion of the full-bridge submodules for the hybrid MMC under active fault-clearing mode

Hybrid modular multilevel converter (MMC) can actively clear the dc fault by outputting a negative dc voltage. The proportion of the full-bridge submodules (FBSMs) determines the fault clearing speed, and also has an important impact on the cost and loss of MMC. This paper analyzes the influence of the proportion of FBSMs of hybrid MMC on the negative dc voltage and ac-side voltage capability. An optimization method of the required proportion of the FBSMs of the hybrid MMC is proposed. During fault clearing process, according to the demand for the output reactive power, the required proportion of FBSMs can be reduced on the premise of maintaining the required negative dc voltage and the fault clearing speed. As a result, the volume, cost and loss of hybrid MMC can be reduced. The influence of the proportion reduction of the FBSMs on the PQ output range in the fault clearing stage is analyzed, which provides a basis for the selection of the proportion optimization degree of the FBSMs. Theoretical analysis and digital simulation results for a case study show the effectiveness of the proposed method.

A Si/SiC Hybrid Full-Bridge Submodule for Modular Multilevel Converter With its Control Scheme

Due to the high dc voltage utilization and capability of dc fault blocking, the full-bridge submodule-based modular multilevel converter (FBSM-MMC) becomes a prevalent topology for medium voltage high power applications. However, using the Si IGBT for each FBSM can induce high switching loss while the SiC MOSFET suffers from high cost because of limitations of fabrication technologies. To address this issue, a hybrid FBSM consisting of both SiC MOSFET and Si IGBT is studied in this article. In addition, an optimal control scheme is proposed, which adjusts the current conducting path of the FBSM and concentrates the high-frequency switching actions into the SiC module. The proposed control scheme only modifies the submodule (SM) level switching and has good compatibility with the existing MMC modulation schemes. Compared with the MMC solely formed by SiC MOSFET, the hybrid SM has similar switching loss and better performance in terms of conduction loss and cost. The experimental results prove the validity of the proposed topology and control scheme, and the further loss and cost analysis verifies that the Si/SiC hybrid FBSM can reduce the loss by 16.3% and save the device cost by 41.0% compared with the SiC FBSM.

An Ultra High Gain Converter for Driving HASEL Actuator Used in Soft Mobile Robots

Soft robots have the potential to fundamentally change interactions between robots and the surrounding environment, and between robots and animals, and robots and humans in ways that today's hard robots are incapable of doing. However, to realize this potential, soft robot actuators require extremely high voltage supplies of more than 4 kV. The electronics that can satisfy this need currently are either too large and bulky or unable to achieve the high power efficiency required for mobile systems. To meet this challenge, this paper conceptualizes, analyzes, designs, and validates a hardware prototype of an ultra-high gain (UHG) converter that can support extremely large conversion ratios up to ∼1000× to provide up to 5 kV output voltage from an input voltage of ∼5-10 V. This converter is demonstrated to be able to drive HASEL (Hydraulically Amplified Self-Healing Electrostatic) actuators, a promising candidate to realize future soft mobile robotic fishes, from an input voltage range of a 1-cell battery pack. The circuit topology employs a unique hybrid combination of a high-gain switched magnetic element (HGSME) and a diode and capacitor-based voltage multiplier rectifier (DCVMR) to enable compact magnetic elements, efficient soft-charging in all flying capacitors, and adjustable output voltage capability with simple duty-cycle modulation. Achieving an efficiency of 78.2% at 15 W output power, while providing 3.85 kV output from 8.5 V input, the proposed UGH converter proves to be a promising candidate for future untethered soft robots.

Internal Energy Based Grid-Forming Control for MMC-HVDC Systems With Wind Farm Integration

The virtual synchronous generator (VSG) control is regarded as an effective solution for operating converters in weak grid conditions due to its excellent grid support functions. However, successful implementation of the VSG control of an AC/DC converter relies on the existence of a steady DC voltage at the DC side, while this is not easy to achieve for the cascaded converter system of this work, i.e., the Modular Multilevel Converter-based High-Voltage Direct-Current (MMC-HVDC) transmission systems with offshore wind farm integration. To address this issue, a novel grid-forming control strategy with real-time inertia support and direct DC-link voltage regulation is proposed for the Receiving End Converter (REC) of the MMC-HVDC. The intrinsic power balancing regime of the internal energy stored in MMC's submodule (SM) capacitors is utilized for grid synchronization rather than emulating the swing equation as requested by the VSG control. For being more robust to sudden power variations, proper insertion strategies of the REC are developed to decouple the DC-link voltage of MMC-HVDC from the voltage of SM capacitors. Furthermore, in terms of the grid fault-ride-through issue of the REC, an associated FRT control strategy is proposed aiming for power angle stability and the stability of SM capacitor voltage. Finally, simulation results in PSCAD/EMTDC show that the proposed control can provide fast inertia support with satisfactory control of DC-link voltage and FRT capability, etc.

Coaxial Low-Pass Filter Design and Manufacture

In this study, a low-pass filter with coaxial transmission lines is designed and produced. The objective for designing a coaxial low-pass filter was to utilize both low loss transmission and high breakdown voltage capability for high power microwave applications to enable the suppression of the second harmonic of the 4.4-6 GHz C-Band ( C-Band was selected as a proof of concept ) signals with minimal insertion loss. Microwave and 3D electromagnetic simulators were used in the filter design. The design was started by establishing an 11 th order Chebyshev low-pass filter prototype giving 0.05 dB ripple. Based on this prototype, a filter circuit consisting of capacitors and inductors as lumped elements was designed. The low and high impedances to be selected for the stepped impedance method, which is the basis of the transition to coaxial transmission lines, were decided and the most suitable impedances for manufacturing was determined. The filter using these impedances and designed with stepped impedance lines was transformed into the filter in which coaxial transmission lines are used. Coaxial transmission lines in the low-pass filter circuit consist of conductors with an insulating material in between. While the insulating material is selected as air in the high impedance sections of the transmission lines, it is PTFE (Polytetrafluoroethylene) with an insulating constant of 2.1 in the low impedance sections. The designed filter was manufactured. The basic properties of the manufactured filter were characterized by using RF measuring devices and compared with the designed filter. The measurements results prove that the frequency responses of the designed filter and the produced filter are very close to each other. Namely, a low-pass filter with a cutoff frequency of 6 GHz, a sharp suppression of about 45 dB at the stopband of 9 GHz and a return loss of 20 dB was designed and produced by using coaxial transmission lines. The novelty in our work can be stated that the power handling capability is increased by two techniques. The first one is that the highest transmission line radius preventing the higher order mode excitations is used to decrease the conductor loss. Teflon is covered around the low impedance lines to enhance the dielectric breakdown capability. Teflon inclusion is also used to decrease the practical low impedance level that improves the filter characteristics.

Dynamic Performance Assessment of PMSG and DFIG-Based WECS with the Support of Manta Ray Foraging Optimizer Considering MPPT, Pitch Control, and FRT Capability Issues

Wind generators have attracted a lot of attention in the realm of renewable energy systems, but they are vulnerable to harsh environmental conditions and grid faults. The influence of the manta ray foraging optimizer (MRFO) on the dynamic performance of the two commonly used variable speed wind generators (VSWGs), called the permanent magnet synchronous generator (PMSG) and doubly-fed induction generator (DFIG), is investigated in this research article. The PMSG and DFIG were exposed to identical wind speed changes depending on their wind turbine characteristics, as well as a dangerous three-phase fault, to evaluate the durability of MRFO-based wind side controllers. To protect VSWGs from hazardous gusts and obtain the optimum power from incoming wind speeds, we utilized a pitch angle controller and optimal torque controller, respectively, in our study. During faults, the commonly utilized industrial approach (crowbar system) was exclusively employed to aid the studied VSWGs in achieving fault ride-through (FRT) capability and control of the DC link voltage. Furthermore, an MRFO-based PI controller was used to develop a crowbar system. The modeling of PMSG, DFIG, and MRFO was performed using the MATLAB/Simulink toolbox. We compared performances of PMSG and DFIG in reference tracking and resilience against changes in system parameters under regular and irregular circumstances. The effectiveness and reliability of the optimized controllers in mitigating the adverse impacts of faults and wind gusts were demonstrated by the simulation results. Without considering the exterior circuit of VSWGs or modifying the original architecture, MRFO-PI controllers in the presence of a crowbar system may help cost-effectively alleviate FRT concerns for both studied VSWGs.

Flexible Droop Coefficient-Based Inertia and Voltage Cascade Control for Isolated PV-Battery DC Microgrid

To realize the coordinated distribution of power in the multi-source system, maintain the charging balance among energy storage units, and improve the anti-interference capability of the bus voltage, a cascade control for an isolated PV-battery DC microgrid is proposed in this paper. First, to provide inertia for the microgrid so that it can cope with external disturbance, the parameters of the droop curve were adjusted by correlating the bus voltage variation rate, which improved the conventional droop control. Secondly, a nonsingular terminal sliding mode control was proposed to track voltage and current references and, thus, enhance the robustness of the DC microgrid. Moreover, the high-frequency chattering of the sliding mode control was inhibited with the exponential reaching law, and the Lyapunov function was utilized to verify the stability of the proposed nonsingular terminal sliding mode control method. Both the simulated and experimental results verified the correctness and effectiveness of the proposed method.

Common-Ground Grid-Connected Five-Level Transformerless Inverter With Integrated Dynamic Voltage Boosting Feature

A single-phase common-ground five-level (5L) inverter with a dynamic voltage conversion gain and capability of operating in a wide input voltage range and a single-stage energy conversion configuration is presented in this article. The proposed topology requires nine active power switches and is comprised of an integrated switched-boost (SB) module connected in series to a switched-flying-capacitor (SFC) cell. Two self-balanced capacitors with a single boost inductor in the integrated SB module are employed to generate a 5L output voltage waveform with a dynamic voltage conversion gain. The current stress profile of all the active and passive elements is kept within a permissible input current range. By adopting an extra diode-capacitor-inductor network into the integrated SB module and with the utilization of the same SFC cell, the proposed topology is extended to achieve a quadratic voltage conversion gain while retaining the quality of ac voltage waveform. Theoretical analysis, closed-loop control/modulation principles, design guidance, comparative study, and relevant experimental results obtained from a 1.5-kW laboratory-built prototype are presented to ascertain the operation and feasibility of the proposed system.

A Single Stage Doubly Grounded Transformerless Inverter Topology With Buck-Boost Voltage Capability for Grid Connected PV Systems

A novel single-stage doubly grounded transformerless inverter configuration which can buck-boost the output voltage is proposed in this paper. In this transformerless inverter configuration grid neutral is directly connected to PV panel negative rail which suppresses/eliminates the PV leakage currents. In addition, the tracking of maximum power point is possible for wide variation of weather conditions due to buck-boost voltage capability of inverter. Moreover, the proposed inverter topology has symmetrical operation in both half cycles hence injects negligible DC offset current in to the grid. Here the symmetrical operation of inverter is achieved due to one inductor is used for both half cycles of the grid. The proposed topology is realized using five switches, two of which work at 50 Hz and remaining three switches at high frequency which reduces overall semiconductor losses. Further, the proposed inverter topology is controlled through a newly developed modulation technique and possible working modes are analyzed and described thoroughly. For performance validation, the experimental analysis of grid connected proposed inverter is performed on a 1 kVA system and results show that proposed inverter feeds power in to the grid with lesser THD while achieving greater efficiency.

A 9- and 13-Level Switched-Capacitor-Based Multilevel Inverter With Enhanced Self-Balanced Capacitor Voltage Capability

Boost action is required for enhancing the output voltage of the sources such as photovoltaic (PV) sources, fuel cells, and battery storage devices, which eliminates the need for additional units. For this purpose, switched-capacitor (SC)-based multilevel inverters (SC-MLIs) are widely used. The proposed SC-based single-phase MLI is able to produce 13-level output ac voltage and furnishes voltage gains of 3 and 6. The same topology is also able to produce a single-phase nine-level ac output with a voltage gain of 4. The abovementioned voltage levels and voltage gains are achieved using the proposed topology just by modifying the switching strategy used for firing the switches and do not require any modification in the proposed SC-MLI structure. The proposed configuration of single-phase MLI requires less switch count to produce 9- and 13-level ac output voltages with the abovementioned voltage gains. The switches connected in the proposed configuration undergo less voltage stress compared to the MLIs suggested in the literature. The comparison of the proposed converter topology with the existing MLIs reported in the literature is included. The validation of the performance of the proposed inverter is carried out using experimental results captured on a low-power laboratory prototype.

Recent advances in developing organic positive electrode materials for rechargeable aluminum-ion batteries

Rechargeable aluminum-ion (Al-ion) batteries have been highlighted as a promising candidate for large-scale energy storage due to the abundant aluminum reserves, low cost, high intrinsic safety, and high theoretical energy density. However, the strong Coulombic interaction between the high charge density Al3+ and electrode host lattices leads to poor kinetics and insufficient cycle life in Al-ion batteries with inorganic electrodes. In contrast, organic electrode materials exhibit the advantages of designable molecular structure, flexible framework, coordinated energy storage chemistry, and resource sustainability. Nevertheless, organic materials encounter inherently high solubility, low active center utilization, and low electrical conductivity. Herein, the recent advances in developing organic positive electrode materials for Al-ion batteries is reviewed, and the charge storage mechanisms and electrochemical performances of different types of organic materials are systematically summarized. This review also discusses design strategies to improve specific capacity, operating voltage, cycle life, and rate capability of organic positive electrodes for Al-ion batteries. Finally, the challenges and research outlook toward high performance Al/organic batteries are presented.

Power FETs: Failure Analysis (FA) and Reliability Analysis (RA)

FET’s have become increasingly popular as possible replacements for microwave electron tubes and solid-state active diodes. Reliability is extremely important for these devices when used in communication systems, especially for space applications. The reliability of small, signal GaAs FET’s has been extensively investigated. As a result, the reliability of small-signal or low-noise devices is fairly well understood, and the essential problems are practically solved. However, the reliability of power devices is much more complicated because they are expected to operate in the vicinity of their maximum capability of voltage, current, or power dissipation, in the presence of large RF signals.