Two-stage Converter Research Articles

A Robust Control Approach for Frequency Support Capability of Grid-Tie Photovoltaic Systems

Abstract Distributed solar photovoltaic (PV) generation is growing rapidly around the world. However, unlike conventional synchronous generators, PV systems do not have any rotating masses to deliver inertia to support the grid frequency. The paper presents a detailed modeling of a new converter configuration and control scheme to enable PV systems to adjust the real power output and contribute to the grid frequency regulation. The proposed topology consists of a two-stage converter without an energy storage system. A DC–DC buck converter is used instead of a DC–DC boost converter, and this simplifies the control scheme which aims to keep the PV generator power in the right side of the P–V characteristic and can be varied in the range from near-zero to the maximum power. The proposed control scheme combines robust and nonlinear sliding mode theory with fuzzy logic. The PV system is connected to a low inertia microgrid and its ability to contribute to frequency regulation is assessed for different controls. The proposed converter and its control are validated experimentally on a 3-kW PV system using OPAL-RT real-time simulator and tested under varying temperature, solar irradiance, and partial shading conditions. The results show that with the proposed circuit, the operating point is always on the right side of the P–V characteristic irrespective of the operating mode. Furthermore, the proposed control scheme provides PV generators with a fast and effective inertial response to support the grid and enhance its stability during contingencies.

Secondary intramedullary nailing for open tibial shaft fractures. Comparison of a one-stage versus two-stage surgical procedure

IntroductionThe objective of this study was to compare the rates of radiological and clinical complications after sequential treatment of a tibial shaft fracture by external fixation followed by secondary nailing with an early-stage conversion (C1S) versus a two-stage late conversion (C2S). The hypothesis of this work was that treatment with C1S does not increase the risk of surgical site infections compared to C2S, and allows faster healing with a lower malunion rate. Material and methodsWe carried out a retrospective and comparative study based on the files of the traumatology department of the Édouard Herriot University Hospital in Lyon. We reviewed the records of patients who presented with a Gustilo grade 1, 2 or 3a open tibial fracture treated with external fixation with conversion by intramedullary nailing within 6 months from January 2010 to December 2020. We assessed the occurrence of Surgical Site Infections (SSI), consolidation time, time until resumption of weight bearing and the malunion rate. ResultsOf the 55 patients included, a C1S procedure was performed in 25 cases (45.5%) versus 30 cases (54.5%) for C2S. No significant difference was found concerning the occurrence of SSI after intramedullary nailing between the two groups (p=0.81). A significant difference was observed in terms of bone healing (p=0.036) and the malunion rate (0.0013) in favor of nailing in one stage. DiscussionThe strategy of converting an open leg fracture early, in one stage (C1S), after initial external fixator placement allows for faster healing and weight bearing, while ensuring a lower malunion rate compared to that of a late two-stage conversion. In the absence of a scar at the time of intramedullary nailing, C1S does not increase the risk of surgical site infections. While the choice of a late and sequential two-stage operation is likely related to the surgeon's apprehension around conversion of an open leg fracture by intramedullary nailing, this study could promote the use of C1S in a greater number of situations. Level of evidenceIV.

Partial-Power Post-Regulated LLC Resonant DC Transformer

This article presents a partial-power postregulated LLC resonant converter. In the proposed topology, the input power is first transferred to the secondary side of the transformer through the LLC-DC transformer converter to achieve galvanic isolation and voltage conversion functions. Then, most of the power is directly transmitted to the load side, and only a small portion of the power is converted to the output side by the partial-power processing converter. Therefore, unlike the two-stage solution, the converter proposed in this article achieves output voltage regulation while minimizing the efficiency loss caused by the two-stage conversion. The operating principle of the proposed converter is analyzed, and the system prototype is constructed. Experimental result validates the correctness of the analysis and proves the feasibility of the proposed converter.

A High-Power-Density High-Efficiency Soft-Switched Single-Phase Universal Input to 28-V Isolated AC–DC Converter Module Designed for Paralleled Operation

This article presents a high-power-density high-efficiency soft-switched single-phase universal input to 28-V isolated ac–dc converter module designed for parallel operation. The proposed two-stage converter utilizes a totem-pole bridgeless boost power factor correction (PFC) ac–dc converter stage followed by an isolated <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> resonant dc–dc converter stage. A comprehensive design and optimization methodology is presented that compares various designs and operating modes for the converter's power stages and selects the optimal design based on overall power density and efficiency. Additionally, a control scheme is presented for the PFC stage to minimize current distortion by accounting for various sensor, control, and gate-drive delays while ensuring zero-voltage switching (ZVS) of the transistors. Moreover, a new droop control strategy is presented, which utilizes the input current of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> dc–dc stage to ensure equal output current distribution between paralleled modules powering a common output load. To validate the proposed design and control strategy, two 1-kW universal input to 28-V isolated ac–dc converter modules are built and tested. Each prototype converter module achieves a high power density of 84 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and a peak efficiency of greater than 93% while ensuring a current distribution error of less than 0.3% when operating in parallel to deliver the maximum output power of 2kW.

Two-Stage Hybrid Isolated DC–DC Boost Converter for High Power and Wide Input Voltage Range Applications

A two-stage hybrid isolated dc–dc boost converter for high power and wide input voltage range applications is proposed. It can be used as a front-end dc–dc converter that can boost variable low voltage from a power source [battery (home/industrial inverter/industrial UPS application), fuel-cell or solar-PV] and interface it to a high-voltage dc-ink, which typically feeds an inverter. A detailed comparison among the different family of converter topologies for isolated dc–dc boost application is carried out that can prove to be useful for power electronics engineers to choose the correct topology for a given input voltage range. The quantitative comparison takes into account the power device utilization, transformer utilization, capacitance utilization, and inductive energy requirement. Based on the comparison, it is shown that with input voltage variation greater than ±20%, two-stage hybrid isolated dc–dc boost converter is superior compared to other family of converters. Design details for the two-stage converter are provided. The converter is developed and tested for a 5 KW inverter application with an input dc voltage range of 90–170 V and output ac voltage of 230 V. Theoretical analysis for loss calculations is also provided which closely matches the experimental results.

Impedance Modeling and Stability-Oriented Parameter Optimization of Isolated Dual Active Bridge-Based Two-Stage AC-DC-DC Converter

The isolated dual active bridge (DAB)-based two-stage AC-DC-DC converters have been widely applied in grid-connected power electronics systems. However, the impedance interactions between the DAB converter and the AC-DC rectifier can cause instability problems to the two-stage converter systems. The accurate terminal impedance characteristics of the DAB converters are still not clear due to the high frequency ac conversion stage. This makes it difficult to assess the impacts of the DAB circuit parameters on the stability of the two-stage converters. To address these issues, the impedance models of the DAB converters are originally derived in this paper. Based on the developed impedance models, the stability of the DAB-based two-stage AC-DC-DC converters is analyzed. The impacts of the DAB converter circuit parameters on the stability of the two-stage AC-DC-DC converters are comprehensively revealed by the Bode and Nyquist plots. The analysis results offer instructive implications to finetune the design rules of the DAB converters. The conclusions are validated by comprehensive simulation and experimental results.

Harmonic Analysis and Attenuation Strategy for a Two-Stage Matrix Converter Fed by Dual-Inverter Based on Pulse Barycenter Method

Since the DC-link of the dual-inverter two-stage matrix converter (TSMC) has no energy storage element, its grid-side current waveform is closely related to the load-side condition. The conventional modulation strategy of the dual-inverter TSMC is only the stack of the double inverters, without considering the cooperation between both modulation strategies. Thus, the fluctuations in the grid-side currents synthesized by both load-side currents will be obvious. To solve this problem, this paper analyzes the effect of the current pulse position on the grid-side current waveform quality, and relevant relationship between the pulse position and current harmonics is obtained according to Fourier transform. On this basis, the harmonic attenuation strategy for the dual-inverter TSMC is proposed. In the proposed strategy, the grid-side current harmonic is reduced based on the optimal duty-cycle combination, obtained from the harmonic optimization problem. The simulation and experimental results are provided to verify that the theoretical analysis of grid-side current harmonic content is consistent with the actual condition, and the proposed modulation strategy can effectively reduce the grid-side current harmonic.

Dual-Boost Inverter for PV Microinverter Application—An Assessment of Control Strategies

Photovoltaic (PV) microinverters have grown rapidly in the small-scale PV market, where typical two-stage converters are used to connect one PV module to the single-phase AC grid. This configuration achieves better performance in terms of energy yield compared with other PV configurations. However, the conversion efficiency of a two-stage system is the main drawback, especially when a high-voltage gain effort is required. In this context, single-stage microinverter topologies have been recently proposed since only one power conversion stage is required to extract the maximum power of the PV module and inject the AC power to the grid. This single-stage configuration allows considerable improvement of the overall efficiency of microinverters by reducing the number of elements in the system. However, the main challenge of these topologies is their control, since all variables of the converter are composed by the AC waveform with DC-bias. In this paper, four control strategies are analyzed for the mainstream single-stage topology, which is the dual-boost inverter (DBI). Classical linear control and three non-linear strategies, namely finite control set–model predictive control, flatness-based control, and sliding mode control, are detailed. The main contribution of this work is a complete comparison of the control strategies, to give insights into the most suitable control strategy for the DBI in PV microinverter application.

Estimation of the Output Characteristic of a Photovoltaic Generator under Power Curtailment and Considering Converter Losses

Power curtailment methods contribute to the frequency stability of power systems with a high share of photovoltaic generation. This paper focuses on an online strategy that estimates the output characteristic of a photovoltaic generator while operating in power curtailment mode by using voltage and current measurements and the nonlinear least-squares curve-fitting. In contrast to previously reported methods, this work introduces an improvement consisting of the estimation of the power losses of the electronic converter that connects the photovoltaic panel with the grid. Thus, the impact of a mismatch between the dc-side and the ac-side is reduced. The procedure is tested at different power levels and with fluctuations of irradiance and temperature. The results show that the converter efficiency, expressed as an exponential function with two quadratic equations, is associated with irradiance and temperature. It is also found that the two-stage converter efficiency is mainly affected by the irradiance level when the photovoltaic system operates on the left side of the power-voltage characteristic. In contrast, the temperature level influences the converter efficiency significantly when working on the right side of the maximum power point. The estimated efficiency curves can improve the accuracy of the power curtailment method and can be used for designing the two-stage converter. The effectiveness of the proposed power curtailment method has been tested in a two-stage photovoltaic system through MATLAB/Simulink.

Efficient production of isoquercitin, icariin and icariside II by a novel thermostable α-l-rhamnosidase PodoRha from Paenibacillus odorifer with high α-1, 6-/α-1, 2- glycoside specificity

Minor flavonoids, such as isoquercitin, icariin and icariside II, have desired bioactivity and bioavailability. To enhance the production efficiency on converting rutin and epimedin C into minor flavonoids, a novel GH family 78 α-l-rhamnosidase PodoRha from the Paenibacillus odorifer was cloned and heterologous over-expressed in E. coli BL21(DE3). PodoRha exhibited a high catalytic ability in cleaving outer α-1, 2-rhamnopyranosyl moieties of epimedin C and α-1, 6- rhamnopyranosyl of rutin. It displayed the highest activity at 45 °C and pH 6.5, and has great thermostability at 40 °C. Under the optimal transformation condition (40 °C, pH 6.5, 1 h incubation), 10 g/L of Rutin was 99.86% transformed into 6.95 g/L of isoquercitin by PodoRha with a crude productivity of 6.95 g·L-1·h-1; 1.964 g/L icariin was detected after the epimedin C of the total Epimedium flavonoids (10 g/L) were 99.98% converted by PodoRha. Moreover, by using a two-stage conversion system with a crude icariside II productivity of 2.081 g·L-1·h-1, 5 g/L epimedin C was 99.85% converted into 3.122 g/L of icariside II with 1 h incubation under pH 6.5 at 40 °C after addition of 5.5 U/mL PodoRha; then with 30 min incubation under pH 6.5 at 90 °C after addition of 0.8 U/mL IagBgl1.

A novel dual-leg DC-DC converter for wide range DC-AC conversion

This paper proposes DC–AC Dual-leg dual-stage Conversion (DDC) and DC–AC Direct single-stage conversions (DSC). Conventional energy conversion system has only two-stage conversion, so it has some drawbacks such as huge power loss, less conversion range and lower power rating. So direct conversion, dual-leg step-up and step-up conversions are the solutions to get wide voltage conversion efficiently. The proposed converter can perform the power conversion from battery DC supply into AC with 1:1 ratio, step-up AC, and step-down AC in both directions. Also, it can perform rectifier operation from grid AC supply into DC with 1:1 ratio, step-down DC, and step-up DC. Step-up, step-down and ideal operations are possible within a single circuit; its operation is similar to solid-state DC–AC/AC–DC transformer. The ideal operation, Step-down to Step-up conversion and Step-up to Step-down conversion are possible on both sides, so this converter can handle a wide range of voltage. Power distribution is achieved with voltage regulation between battery/DC-load and AC-load/grid using the proposed control strategy with proper modulation. A prototype model of a 2-kW power rating validates the advantages and feasibility of the proposed methodology.

Fault Tolerant Robust Passivity-Based Control Design for a Proton Exchange Membrane Fuel Cell Power Supply

Abstract The proton exchange membrane fuel cell (PEMFC) best exploitation needs to be based on reliability and fault tolerant control. One of the issues to fulfill this objective is ensuring high reliability indexes of the associated DC/DC converter, which is indispensable in practice. Three-stage interleaved boost converter is proposed in this context with a reliability-based reconfiguration control. The proposed control approach is passivity based, it is robust against high-temperature variations and load perturbations. However, this article proposes a novel method that adapts the reference current by a linear transformation. The fault tolerant control manages the open circuit failure to adapt the control algorithm to two-stage converter reconfigured structure. The matlab/simulink-based simulations show the effectiveness of the proposed control scheme as well as the reconfiguration method, which can give a technical improvement in the exploitation of PEMFC power supplies in the presence of faults and perturbations.

A Two-stage Single-phase Grid-connected Solar-PV System with Simplified Power Regulation

This study focuses on the design and development of a simplified active power regulation scheme for a two-stage single-phase grid-connected solar-PV (SPV) system with maximum power point (MPP) estimation. It aims to formulate and test an improvised new control scheme to estimate the real-time MPP of the PV panel and operate only at either the MPP or on the right-hand side (RHS) of the PV characteristics of the panel. A simple active power regulatory control scheme was formulated to provide frequency control services to a single-phase grid without using an energy storage device. The plant operator provides the reserve fraction as the input for the active power regulation controller. At any time, the reserve fraction is used to determine the magnitude of the reference power to be extracted from the PV panel for injection into the grid. A simple PI controller was used to track the calculated reference power. The different modes of operation of the regulatory scheme are presented in detail. All the above control schemes are integrated and implemented through appropriate switching of the DC-DC converter alone. The DC-AC converter maintains the DC link voltage and unity power factor at the single-phase grid terminals. The proposed control schemes were tested on a 250 Wp solar panel feeding power to a 230 V, 50 Hz single-phase grid through a two-stage converter. The entire scheme was modeled using the Matlab/Simulink platform, and the same was validated by hardware experimentation using Chroma Solar Simulator and NI myRIO controller under varied irradiation, temperature, and reserve fractions. The simulation and hardware results are compared and reported.

Model Predictive Control based Direct Matrix Converter fed Permanent Magnet Synchronous Machine drives for Traction and Electric Mobility Applications

There have been extensive research works going on electric mobility but most of these work and the existing electric mobility systems are battery-based DC systems. In some applications of electric mobility like traction and advanced technologies like electromagnetic induction charging, AC fed systems are employed due to the innate qualities of AC power transmission. Almost all the electric mobility systems we use are AC induction or permanent magnet machines. The conventional electric mobility systems including traction having AC as their energy source use two-stage conversion i.e. A fixed AC is converted to a fixed or variable DC link using a rectifier and finally, an inverter provides a variable AC in terms of frequency and magnitude according to the control algorithm. The two-stage conversion has its pros and cons but Matrix Converter (MC) will be a suitable and efficient alternative for AC fed AC motor drives. In the case of traction and other electric mobility applications, the load torque demand plays a significant role. The predictive control technique provides a suitable solution for these kinds of special drive applications due to their selective parameter control ability. Implementation of predictive control using a matrix converter is more effective than the conventional inverter fed drives, owing to the increased viability of matrix converter switching configurations. This paper discusses the mathematical implementation and comparison of Predictive Current Control (PCC) and Predictive Torque Control (PTC) with and without weighing factor for AC fed electric mobility applications. The efficacy of both the model predictive control techniques in concern of execution time, steady-state, transient, and dynamic conditions are analysed and validated along with the influence of diverse control variables in the cost function.

An Insight into the Second-Harmonic Current Reduction Control Strategies in Two-Stage Converters

Due to the components at twice the fundamental frequency of output voltage in the instantaneous output power of a two-stage single-phase inverter (TSI), the second harmonic current (SHC) is generated in the front-end dc-dc converter (FDC). To reduce the SHC, optimizing the control strategy of the FDC is an effective and costless approach. From the view of visual impedance, this paper conducts an intensive study on the SHC reduction strategies. Origin of the SHC is illustrated first. Then, the equivalent circuit models of the FDC under different control strategies are proposed to analyse the SHC propagation characteristic. The derived model can offer a better insight into how the inductor SHC is affected by the control parameters. According to the derived models, a synthesis of different control strategies is presented and the relevant parameters are listed for control design to achieve better suppression effect. The benefits and limitations of these control strategies are also discussed. Based on the proposed equivalent circuit models, several optimization methods are proposed to enhance the effect. A 1500 VA TSI prototype is built and simulated on MATLAB/Simulink, verifying the effectiveness of the proposed optimization methods. This paper is aimed to provide a guideline for the control design and control optimization of the TSIs.

Review on State-of-the-Art Unidirectional Non-Isolated Power Factor Correction Converters for Short-/Long-Distance Electric Vehicles

Electrification of the transportation sector has originated a worldwide demand towards green-based refueling infrastructure modernization. Global researches and efforts have been pondered to promote optimal Electric Vehicle (EV) charging stations. The EV power electronic systems can be classified into three main divisions: power charging station configuration (e.g., Level 1 (i.e., slow-speed charger), Level 2 (i.e., fast-speed charger), and Level 3 (i.e., ultra-fast speed charger)), the electric drive system, and the auxiliary EV loads. This paper emphasizes the recent development in Power Factor Correction (PFC) converters in the on-board charger system for short-distance EVs (e.g., e-bikes, e-trikes, e-rickshaw, and golf carts) and long-distance EVs (passenger e-cars, e-trucks, and e-buses). The EV battery voltage mainly ranges between 36 V and 900 V based on the EV application. The on-board battery charger consists of either a single-stage converter (a PFC converter that meets the demands of both the supply-side and the battery-side) or a two-stage converter (a PFC converter that meets the supply-side requirements and a DC-DC converter that meets the battery-side requirements). This paper focuses on the single-phase unidirectional non-isolated PFC converters for on-board battery chargers (i.e., Level 1 and Level 2 charging infrastructure). A comprehensive classification is provided for the PFC converters with two main categories: (1) the fundamental PFC topologies (i.e., Buck, Boost, Buck-Boost, SEPIC, Ćuk, and Zeta converters) and (2) the modified PFC topologies (i.e., improved power quality PFC converters derived from the fundamental topologies). This paper provides a review of up-to-date publications for PFC converters in short-/long-distance EV applications.

A wide band gap devices based switched-capacitor circuits for photovoltaic applications

A two-stage power converter for PV applications is developed in this paper. In the first stage, a multiple-input high step-up dc/dc converter is proposed. The second stage is the five-level dc/dc inverter with boosting functionality. The proposed two-stage converter harnesses the superior features of WBG devices to build a converter with high efficiency and high-power density. The high switching speed of WBG power devices allow both stages operating at high switching frequency, hence, large size film capacitors can be replaced by small size multilayer ceramic capacitors, furthermore, the high operating temperature of WBG devices will help to shrink the cooling size. In additions, a zero current-switching technique is implemented in order to reduce the switching loss and electromagnetic interferences. Prototype with 300 W is built and experimental results are presented with a peak efficiency of 98.3%.

Two-Stage Converter Standalone PV-Battery System Based on VSG Control

Standalone solar PV systems have emerged as potential alternatives to electricity problems in areas where a grid is unavailable. Obtaining full power from a photoelectric system, DC-DC inverter, DC-AC converter, and control system presents great difficulties when building these devices. A standalone two-stage approach is introduced in this work with a boost converter followed by an inverter and a battery with a bidirectional converter. In this paper, a novel virtual synchronous generator (VSG) controller is designed and implemented to adjust the inverter output. The VSG element and the maximum power point tracking (MPPT) used in this study serve the following purposes: to adjust the inverter output and to realize the maximum power of the PV scheme. The new control strategy design was evaluated and validated using extensive MATLAB simulations under different scenarios, including load variations. The system output was evaluated using extensive MATLAB simulations. A hands-on experiment was conducted for the VSG using console testing. Due to the lack of laboratory equipment, we could not experiment with the entire system.

Восстановительные сукцессии повреждений почвенного покрова при проведении рубок ухода в ельнике черничном северной тайги

Damage to the top soil layers during logging operations can be of various forms (compaction, mixing, pressing, etc.) and of a long-term character. The recovery of the morphological characters and the structure of the soil profile of the native soil can take decades or have irreversible effect. We have studied the soil damage during two-stage (1973 and 2002) conversion thinning operations in the spruce forest with blueberry cover in the northern taiga (tree-length log skidding, TDT-55 tractor). The types of damage that stay during the progressive succession for 50 years and have signs of disturbances due to the logging operations were identified. The mosaic structure of the mixed bedrocks still continues to exist. The forest litter pressed by the heavy machines is replaced by the newly formed one. And at the same time, the organic bedrock of the medium degree of decomposition, which is untypical for the native podzolic soil, is preserved. Mixing by tractor tracks and the formation of mixed bedrock is the most common disturbance of the upper bedrock during logging operations due to insufficient coverage of the skid roads by felling residues. The amount of such damage to the soil is 77 % and 79 % in the skid roads of 1973 and 2002, respectively. But the depth of damage is small, which is 10 cm on average, with fluctuations up to 22 cm. In nano- and micro-depressions, the processes of peat formation and gleying develop. The number of locations with the genesis of bog soils is gradually increasing. On the skid roads of 1973 the proportion of wetlands is 2,5 times higher than in the skid roads of 2002 (86,9 % and 37,3 %, respectively). The long-term soil disturbance in the structure of the soil profile makes it necessary to develop classification approaches to improve the analysis of anthropogenic disturbed soils in cutting areas. The classification units are suggested

La induced Si3 trimer monolayer on Si(111) surface: an ab initio study

The π-bond Seiwatz chain (SC) is one of the well-known reconstruction induced by alkaline or rare earth metals on Si(111) surface. Here we identify by ab initio calculations a new reconstruction of La induced quasi-two-dimensional Si3 trimer monolayer on Si(111)- surface. Its surface unit cell has one La atom and one Si3 trimer with the same La coverage (1/3 monolayer) as SC structure and the Si3 trimer satisfies the electron counting rule with a transfer of valence electrons from La atom, formally as , in correspondence to the milkstool model for Bi trimers on Si(111) surface. Band structure calculations show a semiconducting character with an indirect surface band gap of 0.76 eV. Moreover, a two-stage conversion process between the Si3 trimer and SC structure is verified by the climbing-image nudged elastic band method. These findings pave the way for further exploration of the new surface structure and its outstanding properties.