Showing papers on "Converters published in 2020"

Current Limiting Control With Enhanced Dynamics of Grid-Forming Converters During Fault Conditions

Abstract: With an increasing capacity in the converter-based generation to the modern power system, a growing demand for such systems to be more grid-friendly has emerged. Consequently, grid-forming converters have been proposed as a promising solution as they are compatible with the conventional synchronous-machine-based power system. However, most research focuses on the grid-forming control during normal operating conditions without considering the fundamental distinction between a grid-forming converter and a synchronous machine when considering its short-circuit capability. The current limitation of grid-forming converters during fault conditions is not well described in the available literature and present solutions often aim to switch the control structure to a grid-following structure during the fault. Yet, for a future converter-based power system with no or little integration of synchronous machines, the converters need to preserve their voltage-mode characteristics and be robust toward weak-grid conditions. To address this issue, this article discusses the fundamental issue of grid-forming converter control during grid fault conditions and proposes a fault-mode controller which keeps the voltage-mode characteristics of the grid-forming structure while simultaneously limiting the converter currents to an admissible value. The proposed method is evaluated in a detailed simulation model and verified through an experimental test setup.

Frequency Stability of Synchronous Machines and Grid-Forming Power Converters

Abstract: An inevitable consequence of the global power system transition toward nearly 100% renewable-based generation is the loss of conventional bulk generation by synchronous machines (SMs), their inertia, and accompanying frequency- and voltage-control mechanisms. This gradual transformation of the power system to a low-inertia system leads to critical challenges in maintaining system stability. Novel control techniques for converters, so-called grid-forming strategies, are expected to address these challenges and replicate functionalities that, so far, have been provided by SMs. This article presents a low-inertia case study that includes SMs and converters controlled under various grid-forming techniques. In this article, the positive impact of the grid-forming converters (GFCs) on the frequency stability of SMs is highlighted, a qualitative analysis that provides insights into the frequency stability of the system is presented, we explore the behavior of the grid-forming controls when imposing the converter dc and ac current limitations, the importance of the dc dynamics in grid-forming control design as well as the critical need for an effective ac current limitation scheme are reported, and finally, we analyze how and when the interaction between the fast GFC and the slow SM dynamics can contribute to the system instability.

Model Predictive Control for Dual-Active-Bridge Converters Supplying Pulsed Power Loads in Naval DC Micro-Grids

Abstract: Pulsed power loads (PPLs) are becoming prevalent in medium-voltage naval dc micro-grids. To alleviate their effects on the system, energy storages are commonly installed. For optimal performance, their interface converters need to have fast dynamics and excellent disturbance rejection capability. Moreover, these converters often need to have voltage transformation and galvanic isolation capability since common energy storage technologies such as batteries and supercaps are typically assembled with low-voltage strings. In order to address these issues, a moving discretized control set model predictive control (MDCS-MPC) is proposed in this paper and applied on a dual-active-bridge converter. Fixed switching frequency is maintained, enabling easy passive components design. The proposed MDCS-MPC has a reduced prediction horizon, which allows low computational burden. The operating principle of the MDCS-MPC is introduced in the development of a cost function, which provides stiff voltage regulation. Resonance damping and sampling noise resistance can also be achieved with the proposed cost function. An adaptive step is introduced to enable a fast transition. Assessments on the performance of the proposed MDCS-MPC are conducted. Comparisons with other control methods are also provided. Experimental validations on a 300 V/300 V 20-kHz 1-kW dual-active-bridge converter are carried out to verify the theoretical claims.

Frequency Stability of Synchronous Machines and Grid-Forming Power Converters

Abstract: An inevitable consequence of the global power system transition towards nearly 100% renewable-based generation is the loss of conventional bulk generation by synchronous machines, their inertia, and accompanying frequency and voltage control mechanisms. This gradual transformation of the power system to a low-inertia system leads to critical challenges in maintaining system stability. Novel control techniques for converters, so-called grid-forming strategies, are expected to address these challenges and replicate functionalities that so far have been provided by synchronous machines. This article presents a low-inertia case study that includes synchronous machines and converters controlled under various grid-forming techniques. In this work 1) the positive impact of the grid-forming converters on the frequency stability of synchronous machines is highlighted, 2) a qualitative analysis which provides insights into the frequency stability of the system is presented, 3) we explore the behavior of the grid-forming controls when imposing the converter dc and ac current limitations, 4) the importance of the dc dynamics in grid-forming control design as well as the critical need for an effective ac current limitation scheme are reported, and lastly 5) we analyze how and when the interaction between the fast grid-forming converter and the slow synchronous machine dynamics can contribute to the system instability

DC-Link Voltage-Balancing Strategy Based on Optimal Switching Sequence Model Predictive Control for Single-Phase H-NPC Converters

Abstract: In this article, a model predictive control (MPC) strategy based on the optimal switching sequence (OSS) concept for a single-phase grid-connected H-bridge neutral-point-clamped (H-NPC) power converter is presented. The proposed OSS-MPC algorithm considers both the grid current tracking error and the dc-link capacitor voltage balance. Special emphasis is placed on the power converter control region in order to design suitable switching sequence candidates for this multiobjective control problem. Additionally, based on an analysis of the weighting factor effect over closed-loop performance, it is possible to demonstrate that this controller parameter is relatively easy to adjust. In fact, the weighting factor only affects the peak current during transients, with no effect over the steady-state performance. As a result, the proposed OSS-MPC provides a fast closed-loop dynamic to the H-NPC converter, which operates with a fixed switching frequency at all times. This predictive control strategy is experimentally validated in a 3.5-kVA laboratory setup.

A Tutorial and Review Discussion of Modulation, Control and Tuning of High-Performance DC-DC Converters Based on Small-Signal and Large-Signal Approaches

Abstract: Many commercial controller implementations for dc-dc converters are based on pulse-width modulation (PWM) and small-signal analysis. Increasing switching frequencies, linked in part to wide bandgap devices, provide the opportunity to increase operating bandwidth and enhance performance. Fast processors and digital signal processing offer new computational techniques for power converter control. Conventional control techniques rarely make full use of operating capability. The objectives of this paper are to present an overview and link to literature on conventional modulation and control techniques for hard-switched dc-dc converters, identify performance limits associated with conventional small-signal-based design, discuss geometric control approaches, and compare strategies for control tuning. The discussion shows how current mode controls have alternative state feedback implementations, and describes unusual opportunities for large-signal control tuning. Considerations for minimum response time are described. Comparisons among tuning methods illustrate how geometric controls can achieve order of magnitude dynamic performance increases. The paper is intended as a baseline tutorial reference for future work on power converter control.

Generalized State Space Average Model for Multi-Phase Interleaved Buck, Boost and Buck-Boost DC-DC Converters: Transient, Steady-State and Switching Dynamics

Abstract: This paper presents a generalized state space average model (GSSAM) for multi-phase interleaved buck, boost and buck-boost converters. The GSSAM can model the switching behavior of the current and voltage waveforms, unlike the conventional average model which can model only the average value. The GSSAM is used for the converters with dominant oscillatory behavior such as resonant converters, high current ripple converters, and multi-converter systems. The maximum current and voltage through the system can be predicted by modeling the switching behavior of voltage and current. The GSSAM in the literature is introduced for single-phase converters only, and it is not introduced for multi-phase converters due to the high complexity associated with it. Hence, the GSSAM for multi-phase buck, boost and buck-boost converters are introduced in this paper and the proposed models can fit with converters of any number of phases. The number of operating phases in the multi-phase interleaved converters is proportional with the output power to achieve the maximum efficiency over the operating range. Therefore, the proposed GSSAMs can describe the operation at any number of operating phases with switching dynamics of phases. The proposed GSSAM is validated by comparing the transient and steady-state dynamics between the GSSAM and a switching model from PLECS.

Flexible Converters for Meshed HVDC Grids: From Flexible AC Transmission Systems (FACTS) to Flexible DC Grids

Abstract: Flexible Alternating Current Transmission Systems (FACTS) have achieved to enhance the flexibility of modern AC power systems, by providing fast, reliable and controllable solutions to steer the power flows and voltages in the network. The proliferation of High Voltage Direct Current (HVDC) transmission systems is leading to the opportunity of interconnecting several HVDC systems forming HVDC Supergrids. Such grids can eventually evolve to meshed systems which interconnect a number of different AC power systems and large scale offshore wind (or other renewable sources) power plants and clusters. While such heavily meshed systems can be considered futuristic and will not certainly happen in the near future, the sector is witnessing initial steps in this direction. In order to ensure the flexibility and controllability of meshed DC grids, the shunt connected AC-DC converters can be combined with additional simple and flexible DC-DC converters which can directly control current and power through the lines. The proposed DC-DC converters can provide a range of services to the HVDC grid, including power flow control capability, ancillary services for the HVDC grid or adjacent grids, stability improvement, oscillation damping, pole balancing and voltage control. The present paper presents relevant developments from industry and academia in the direction of the development of these converters, considering technical concepts, converter functionalities and possible integration with other existing systems. The paper explores a possible vision on the development of future meshed HVDC grids and discusses the role of the proposed converters in such grids.

Grid-Connected Wind-Solar Cogeneration Using Back-to-Back Voltage-Source Converters

Abstract: This paper introduces a new topology, yet simple and efficient, for a grid-connected wind-solar cogeneration system. A permanent magnet synchronous generator-based full-scale wind turbine is interconnected to the utility-grid via back-to-back voltage-source converters (VSCs). The dc-link capacitor has been utilized to directly interface a photovoltaic solar generator. No dc/dc conversion stages are required, and hence, the hybrid system is simple and efficient. Moreover, the proposed topology features an independent maximum power point tracking for both the wind and the solar generators to maximize the extraction of the renewable energy. The regulation of the VSCs is achieved via the vector control in the rotating reference frame. The detailed small-signal models for the system components are developed to characterize the overall stability. The influence of the utility-grid faults on the performance of the proposed system is also investigated. Nonlinear time-domain simulation results under different operating conditions are presented to validate the effectiveness of the proposed topology.

Contemporary trends in power electronics converters for charging solutions of electric vehicles

Abstract: Electrifying the transport sector requires new possibilities for power electronics converters to attain reliable and efficient charging solutions for electric vehicles (EVs). With the continuous development in power electronics converters, the desire to reduce gasoline consumption and to increase the battery capacity for more electric range is achievable for EVs in the near future. The main interface between the power network and EV battery system is a power electronics converter, therefore, there is a considerable need of new power converters with low cost and high reliability for the advance charging mechanism of EVs. The rapid growth in power converter topologies brings substantial opportunities in EV charging process. In view of this fact, this paper investigates the significant aspects, current progress, and challenges associated with several power converters to suggest further improvements in charging systems of EVs. In particular, an extensive analysis of front-end as well as back-end converter configurations is presented. Moreover, the comparative properties of resonant converter topologies along with other DCDC converters are discussed in detail. Additionally, isolated, and non-isolated topologies with soft switching techniques are classified and rigorously analyzed with a view to their respective issues and benefits. It is foreseen that this paper would be a valuable addition and a worthy source of information for researchers exploring the area of power converter topologies for charging solutions of EVs.

LLC Resonant Converter Topologies and Industrial Applications - A Review*

Abstract: Owing to the advantages of high efficiency, high energy density, electrical isolation, low electromagnetic interference (EMI) and harmonic pollution, magnetic integration, wide output ranges, low voltage stress, and high operation frequency, the LLC resonant converters are widely used in various sectors of the electronics-based industries. The history and development of the LLC resonant converters are presented, their advantages are analyzed, three of the most popular LLC resonant converter topologies with detailed assessments of their strengths and drawbacks are elaborated. Furthermore, an important piece of research on the industrial applications of the LLC resonant converters is conducted, mainly including electric vehicle (EV) charging, photovoltaic systems, and light emitting diode (LED) lighting drivers and liquid crystal display (LCD) TV power supplies. Finally, the future evolution of the LLC resonant converter technology is discussed.

Review of Architectures Based on Partial Power Processing for DC-DC Applications

Abstract: This paper presents a review of advanced architectures based on the partial power processing concept, whose main objective is to achieve a reduction of the power processed by the converter. If the power processed by the converter is decreased, the power losses generated by the power converter are reduced, obtaining lower sized converters and higher system efficiencies. Through the review 3 different partial power processing strategies are distinguished: Differential Power Converters, Partial Power Converters and Mixed strategies. Each strategy is subdivided into smaller groups that entail different architectures with their own advantages and disadvantages. Also, due to the lack of agreement that exists in the sources around the naming of the different architectures, this paper seeks to stablish a nomenclature that avoids confusion when indexing this type of architectures. Regarding Partial Power Converters an extensive application oriented description is also developed. Finally, the main conclusions obtained through the review are presented.

H ∞ -Control of Grid-Connected Converters: Design, Objectives and Decentralized Stability Certificates

Abstract: The modern power system features high penetration of power converters due to the development of renewables, HVDC, etc. Currently, the controller design and parameter tuning of power converters heavily rely on rich engineering experience and extrapolation from a single converter system, which may lead to inferior performance or even instabilities under variable grid conditions. In this paper, we propose an $\mathcal {H}_{\infty }$ -control design framework to provide a systematic way for the robust and optimal control design of power converters. We discuss how to choose weighting functions to achieve anticipated and robust performance with regards to multiple control objectives. Further, we show that by a proper choice of the weighting functions, the converter can be conveniently specified as grid-forming or grid-following in terms of small-signal dynamics. Moreover, this paper first proposes a decentralized stability criterion based on the small gain theorem, which enables us to guarantee the global small-signal stability of a multi-converter system through local control design of the power converters. We provide high-fidelity nonlinear simulations and hardware-in-the-loop (HIL) real-time simulations to illustrate the effectiveness of our method.

Novel Four-Port DC–DC Converter for Interfacing Solar PV–Fuel Cell Hybrid Sources With Low-Voltage Bipolar DC Microgrids

Abstract: Bipolarity in dc microgrids is desirable as it enhances the system reliability and efficiency. However, a bipolar dc microgrid (BDCMG) demands multiple conventional dc–dc converters to feed power to both the poles of the BDCMG. To handle this requirement and to maintain high efficiency, a new four-port, dual-input-dual-output dc–dc converter topology is proposed for interfacing the solar photovoltaic (PV) and fuel cell sources to a low-voltage BDCMG. The proposed topology is unidirectional, efficient, and compact. It has fewer circuit elements with only one inductor compared to the conventional nonisolated dc–dc converters. The proposed converter regulates one of the pole voltages of the dc bus and also ensures maximum power point tracking of the PV source. Furthermore, the converter can be operated as a single-input-dual-output converter. The control complexity of the proposed converter is low as it can be operated in various modes with only one set of controllers. To design the control system for the proposed converter, a small-signal model is derived for each operating mode. Loss modeling and efficiency analysis of the proposed converter are carried out, and its efficacy and performance are validated by detailed simulation and experimental results under various operating conditions.

Transient DC Bias Elimination of Dual-Active-Bridge DC–DC Converter With Improved Triple-Phase-Shift Control

Abstract: A transient dc-bias current due to the voltage-second imbalance of isolated bidirectional dual-active-bridge (DAB) converters for the disturbance in line or load may result in the transformer saturation and oscillations in both sides dc currents. This article focuses on the transient dc-bias current elimination by using an improved triple-phase-shift (ITPS) control for DAB converters. The inductor peak current stress optimization is adopted in the proposed ITPS to determine the steady-state phase-shift variables. Originated from the dc-bias current model of DAB converters with the TPS control, the transient phase-shift adjustment strategy can be determined, which has the ability to improve the inductor current changing slope and shorten the settling time. Both simulation and experiments for different conditions are provided to evaluate main dynamic indexes such as the transient period, dc-bias current, and inductor current stress for three different transition cases. The proposed ITPS is proved as a promising solution in eliminating the dc-bias current, minimizing the transient current stress.

Comprehensive Conception of High Step-Up DC–DC Converters With Coupled Inductor and Voltage Multipliers Techniques

Abstract: Due to the plethora of non-isolated high gain step-up dc-dc converters presented in the literature, it has become important to comprehensively review and classify them, as well as to derive methods to generalize the usage of the commonly employed techniques. Motivated by this need, this paper not only proposes a new family of high gain step up dc-dc converter, but a generalized methodology to derive them from any classical dc-dc topology by applying a coupled inductor and voltage multiplier cells. For illustrating the methodology, high gain dc-dc converters based on the basic topologies (Buck, Boost, and Buck-Boost) are developed and analyzed. These converters are compared in terms of voltage gain, coupled inductor size, voltage stresses, total device rating, switching frequency effect in power loss, and output power regulation. Moreover, in order to verify the proposal, two practical experimentations are accomplished. Firstly, a prototype able to operate as any of the three basic topologies with different gain cells is developed for comparing theoretical, simulated, and experimental static gain results. Secondly, well-designed prototypes concerning to the Buck-, Boost-, and Buck-Boost-based converters are assembled for efficiency evaluation.

An ISOP Hybrid DC Transformer Combining Multiple SRCs and DAB Converters to Interconnect MVDC and LVDC Distribution Networks

Abstract: A hybrid dc transformer (DCT) combining multiple series resonant converters (SRCs) and dual-active bridge (DAB) converters with input-series-output-parallel (ISOP) configuration is proposed in this article. The proposed DCT is suitable for connecting medium-voltage dc and low-voltage dc distribution networks, which combines SRCs as the majority modules for power transmission and DAB converters as the minority modules for terminal voltage regulation or power regulation. Benefiting from the hybrid circuit topology, the proposed DCT can achieve a higher efficiency than the ISOP DAB converters with a reduced complexity in the communication and control, while maintaining the controllability of the dc voltage/power. The operation principles of the proposed DCT are analyzed and key parameters design are presented. Then, a 1 MW, 10 kV/750 V hybrid DCT is simulated and a DCT prototype of 2 kW, 400/100 V is built to verify the theoretical analysis.

An Overview of High-Conversion High-Voltage DC–DC Converters for Electrified Aviation Power Distribution System

Abstract: This article presents the state-of-the-art review of high-conversion high-voltage (HCHV) dc–dc converters for a modern aerial vehicle’s power distribution system. Higher dc bus voltages have become a trend in recent aerial vehicle development because of the potential reduction in size and weight of the rest of the power system and an increase in power density. Some front-end dc energy sources, such as fuel cells, batteries, and supercapacitors, may level at a low voltage and require HCHV dc–dc converters to integrate with the high-voltage dc bus. On the other hand, high-conversion step-down converters are required between the dc bus and various low-voltage electronic loads. A detailed review of HCHV dc–dc converters for an aviation power distribution system is limited in the literature. This article presents two main architectures of such converters. Architecture-I employs individual two-port dc–dc converters to link each source to the dc bus, and Architecture-II uses a single multiport converter (MPC) to connect all the sources to the dc bus. Architecture-I categorizes the two-port dc–dc converter topologies into unidirectional and bidirectional converters, followed by further classifications based on isolation and control schemes. Multiport dc–dc converters for Architecture-II are categorized based on port numbers and then source connection methods. This review investigates multiple topologies within each category or classification, highlighting selected circuit diagrams and their features and shortcomings. This article presents several insightful comparisons, among various bidirectional converters for Architecture-I, and MPCs for Architecture-II, for a designer to choose a proper converter. In terms of converter characteristics, this article focuses on dc voltage gain, power density, efficiency, and reliability, as these qualities are of utmost importance in an aviation application.

A New Coupled Inductor Nonisolated High Step-Up Quasi Z-Source DC–DC Converter

Abstract: In this article, a new nonisolated high step-up quasi Z-source (QZS) dc–dc converter with coupled inductor techniques is presented, which is suitable for renewable applications. The main advantages of the proposed topology are continuous input current, common ground between load and input dc source, low normalized voltage stress on the semiconductors (switch/diodes), and low total voltage stress on devices compared to the conventional QZS converter. However, increasing the turns ratio value of the coupled inductor windings not only limit the duty cycle but also leads to decrease the voltage stress on switch/diodes and increase the output voltage level. The operating performance of the proposed converter, theoretical analysis and comparison between the proposed converter and other published works (i.e., QZS converters) are provided. Finally, one prototype at 500 W is built and tested, which shows experimental results and theoretical analysis verify each other well.

Deadbeat Predictive Current Control for Modular Multilevel Converters With Enhanced Steady-State Performance and Stability

Abstract: Model predictive control (MPC) methods are popularly employed in modular multilevel converters (MMCs) due to their fast dynamic response and multiobjective control capability. However, they present some inherent problems, such as computational complexity, variable switching frequency, poor steady-state performance, and tedious weighting factor selection. This article develops a deadbeat predictive current control method for MMCs. This method can realize the reference tracking of ac current and circulating current in one sampling period without error, and thus provide a fast dynamic response as conventional MPC methods. Besides, switching state or voltage level evaluation, cost function calculation as well as weighting factor selection are not required. Therefore, it has a very low calculation burden, which is independent of the number of submodules (SMs). Since a modulation stage is utilized, a fixed switching frequency and consequently a satisfactory steady-state performance are obtained. The effects of time delay, parameter mismatch, and SM capacitor voltage ripple on the control algorithm are discussed. Also, the corresponding improvement measures are provided to further enhance the steady-state performance and system stability of MMCs. The effectiveness and performance of the developed control algorithm are verified by experimental results.

Improved Bootstrap Methods for Powering Floating Gate Drivers of Flying Capacitor Multilevel Converters and Hybrid Switched-Capacitor Converters

Abstract: A major challenge in the implementation of flying capacitor multilevel (FCML) converters and hybrid switched-capacitor (SC) converters is providing gate drive power to the large number of floating switches. A common solution uses isolated dc/dc converters, which are bulky, expensive, and energy inefficient. To design more compact and efficient gate drive power supply circuits, five methods are presented and compared in this article: bootstrap at deadtime, cascaded bootstrap with low-dropout (LDO) regulator, double charge pump, gate-driven charge pump, and synchronous bootstrap. By leveraging the inherent properties of multilevel converters, these methods can overcome the limitation of conventional bootstrap method (diode forward voltage drop) and make it possible to transfer ground-referenced power to all of the floating switches for any FCML or hybrid SC converters. Compared with the typical isolated dc/dc solution, these methods have simple structure and operating principle and can be implemented with a small number of diodes, capacitors, and LDOs. Experimental results show that an example power supply circuit can cut the size of the power stage of a state of the art seven-level FCML converter by half at 1/6 of the cost.

Continuous Nonsingular Terminal Sliding Mode Control of DC–DC Boost Converters Subject to Time-Varying Disturbances

Abstract: DC-DC converters work as one of the crucial components in DC microgrid intergraded power systems. In this brief, the robust output voltage regulation problem of DC-DC boost converter system is addressed by using a continuous nonsingular terminal sliding mode control (CTSMC) technique based on finite-time disturbance observer. By integrating the disturbance estimations into the controller design, an improved sliding mode control (SMC) approach is developed to achieve better voltage tracking performance. The proposed control method admits the properties of fast transient responses, strong suppression ability against time-varying disturbances and small steady state oscillations of output voltage. Experimental results in the presence of both load variations and supplied voltage fluctuations are provided to validate the effectiveness of the proposed algorithm.

Switched Capacitor–Inductor Network Based Ultra-Gain DC–DC Converter Using Single Switch

Abstract: A switched capacitor–inductor network (SCLN)-based ultravoltage gain dc–dc converter using a single switch is presented. The SCLN converter can achieve ultra dc voltage gain with minimum number of devices when compared with other existing converters. Moreover, the voltage gain is significantly improved at entire duty ratios. In addition, due to switched approach of inductor and capacitor, the converter provides lesser ripple content in the output voltage and current. The operation of the converter in continuous conduction mode (CCM) and discontinuous conduction mode (DCM) is discussed. The parasitic elements are considered to estimate the dc voltage gain and the efficiency of the converter more precisely. The small-signal model of the converter is derived, and the pole-zero locations are investigated. Moreover, the comparative performances of the SCLN converter with existing converters are reported. A hardware prototype has been developed and tested. To prove the applications of the SCLN converter in solar PV system, the experimental results are observed by considering the output voltages of 650 V for three-phase and 325 V for single-phase applications.

Medium Voltage Large-Scale Grid-Connected Photovoltaic Systems Using Cascaded H-Bridge and Modular Multilevel Converters: A Review

Abstract: Medium-voltage (MV) multilevel converters are considered a promising solution for large scale photovoltaic (PV) systems to meet the rapid energy demand. This article focuses on reviewing the different structures and the technical challenges of modular multilevel topologies and their submodule circuit design for PV applications. The unique structure of the converter’s submodule provides modularity, independent control of maximum power point tracking (MPPT), galvanic isolation, etc. Different submodule circuits and MPPT methods to efficiently extract the PV power are reviewed. The integration of the multilevel converters to PV systems suffers unbalanced power generation during partial PV shading conditions. Several balancing strategies to solve this problem are presented and compared to give a better understanding of the balancing ranges and capabilities of each strategy. In addition, the paper discusses recent research advancements, and possible future directions of MV converters-based large-scale PV systems for grid integration.

Review on fault-diagnosis and fault-tolerance for DC–DC converters

Abstract: The DC-DC converters operate extensively in a variety of industrial applications such as electric vehicles, renewable energy systems, aerospace applications, consumer electronics (smartphones, laptops, etc.) and energy storage solutions. The reliability of the DC-DC converters is of a greater significance. The research endeavours made in enhancing the reliability of the DC-DC converters are still very limited and dispersed. Due to the rapid growth in semiconductor technology, the DC-DC converters have an endless number of topologies, with various operating principles and functionalities. Enhancement of the reliability of all types of DC-DC converters is still a challenging task. Power switches are the most fragile components in converter circuits, which inherently fall prey to the faults occurring in the system. Hence, there is always a requirement to take appropriate remedial measures to deal with all kinds of faults. Further, in order to detect the occurrence of fault a fast fault-diagnosis and fault-tolerant strategies in the DC-DC converters is mandatory and the same has to be embedded in the converter for safety purpose. In view of the importance of the same, the fault-diagnostic algorithms and fault-tolerant strategies developed in the literature, by various researchers are furnished in a single document after conducting an exhaustive review.

A Family of Interleaved High Step-Up Converters With Diode–Capacitor Technique

Abstract: This paper presents a family of multiphase interleaved high step-up dc-dc converters using the diode–capacitor technique, which is an extension of the previously reported current-fed Cockcroft–Walton (CW) multiplier. The multiphase configuration has the advantages of low input current ripple, high current-handling capability, and high step-up voltage gain. Also, the switches and diodes have low voltage stresses. Thus, low-voltage-rating semiconductor devices are allowed reducing the conduction loss. Moreover, automatic phase current balancing can be achieved due to the charge balance of the series capacitors. A three-phase high step-up converter in the family is analyzed and evaluated in detail. The simulation and experimental results are provided to verify the theoretical analysis.

Coat Circuits for DC–DC Converters to Improve Voltage Conversion Ratio

Abstract: The voltage conversion ratio of basic dc/dc converters is limited due to the influence of parasitic elements of circuits and components. In this article, a family of passive circuits termed as “coat circuits” are proposed for conventional dc/dc converters. Not only voltage step-up capacity can be improved but also the voltage stress of components can be decreased by adding coat circuits to basic dc/dc converters. Moreover, no additional active switches are needed in the coat circuits, that means the control and driver circuits of the converters are not changed. The Cuk converter with the coat circuit has been analyzed as an example, and a 200 W experimental prototype has been built to verify the validity of the theoretical analysis.

Series-Connected-Based Offshore Wind Farms With Full-Bridge Modular Multilevel Converter as Grid- and Generator-side Converters

Abstract: Series-connected-based offshore wind farms can eliminate bulky central offshore converters, transformers, and platforms. In a series-connected-based offshore wind farm, the dc voltage of the converters should be changed within a large range to control the dc-link voltage (grid side) and maximize wind power acquisition (generator side). In this paper, the full-bridge-based modular multilevel converter (FB-MMC) is proposed as grid- and generator-side converters. The dc voltage of the FB-MMC can vary in a large range because the full-bridge submodule (FBSM) is used. The control method of the FB-MMC in a series-connected-based offshore wind farm is different from those of traditional ones. The dc current control and capacitor voltage balance control are proposed in this paper, with which the FB-MMC can operate normally in the series-connected offshore wind farm. Moreover, the capacitor ripple voltage in the generator-side FB-MMC is studied. The result shows that the capacitor in the FBSM can be designed in rated power and frequency, and it will be feasible when the power and frequency decrease. Simulation and experiment results verify the feasibility of the proposed configuration and the corresponding control algorithm.

Unified Modular State-Space Modeling of Grid-Connected Voltage-Source Converters

Abstract: This article proposes a modular state-space modeling framework for grid-connected voltage-source converters, where the different control loops, including the ac current control, the phase-locked loop, the dc-link voltage control, and the ac voltage magnitude control, can be modeled separately as building blocks. Moreover, the mathematical relationship between state-space models in the rotating ( dq -) frame and the stationary ( αβ -) frame is explicitly established, and, thus, the modal analysis can be performed directly in the αβ -frame, which allows intuitive interpretation of voltage and current oscillation modes in the αβ -frame. Experimental tests of a 3-kW back-to-back converter system validate the effectiveness of the unified modular state-space modeling and analysis.

A Generalized Carrier-Overlapped PWM Method for Neutral-Point-Clamped Multilevel Converters

Abstract: The neutral-point voltage unbalance problem holds back the extensive application of neutral-point-clamped (NPC) multilevel converters with more than three voltage levels in industry. Traditional phase-disposition pulsewidth modulation (PDPWM) or nearest-three-vector (NTV) modulation cannot achieve the voltage balance over the full modulation indexes and load power factors when the voltage level is higher than three. To solve this problem, a novel generalized carrier-based PWM method, named carrier-overlapped PWM (COPWM), for NPC multilevel converters is proposed in this article, which is proven to satisfy the volt–second balance principle. With this modulation method, the average values of all the neutral-point currents are equal to zero in a fundamental period, and therefore, all the dc-link capacitor voltages can be naturally balanced in the ideal and steady-state conditions. In order to simplify its implementation, an equivalent multireference modulation method with only one triangular carrier is also derived. Simulation and experimental results on a five-level diode-clamped inverter are presented to verify the proposed modulation method.