Showing papers on "Converters published in 2021"

Grid-Forming Converters: Control Approaches, Grid-Synchronization, and Future Trends—A Review

Abstract: In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for use in wider interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the expected behaviour of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e. angle stability, fault ride-through (FRT) capabilities, and transition from islanded to grid connected mode are discussed. Perspectives on challenges and future trends are lastly shared.

A Step-Up Multilevel Inverter Topology Using Novel Switched Capacitor Converters With Reduced Components

Abstract: In this article, basic cell (BC) of a novel swi-tched capacitor converter (SCC) has been proposed first. After that, the generalized structure of proposed SCC is developed. The developed SCC requires reduced number of switches, drivers, diodes, capacitors, and lower number of conducting switches in current flow paths and capacitor charging paths as compared to the other recently developed SCCs. A switched capacitor multilevel inverter (SCMLI) utilizing two numbers of generalized SCCs is developed next. Further, cascaded extension of proposed SCMLI is realized and analyzed for symmetric and asymmetric dc source configurations. A detail analysis of optimum selection of capacitance for switched capacitors of 13-level SCMLI is presented. An extensive comparison study shows that the proposed SCMLI requires lower number of components as compared to other SCMLIs. Further, the proposed structure has minimum cost function per level per boosting factor as compared to the other SCMLIs. Extensive experimental results considering fundamental switching frequency scheme are presented to validate the merits and effectiveness of the proposed structure.

Cyber Security in Control of Grid-Tied Power Electronic Converters—Challenges and Vulnerabilities

Abstract: Grid-tied power electronic converters are key enabling technologies for interfacing renewable energy sources, energy storage, electrical vehicles, microgrids, and high-voltage dc transmission lines with the electrical power grid. As the number of power converters in modern grids continually increases, their monitoring and coordinated control in a way to support the grid have become topics of increased practical and research interest. In connection with this, latest standards have also defined a mandatory set of control parameters for grid-tied converters, which should be adjustable by a remote entity that sends commands through a communication network. While such a remote control capability allows many new control functions in grid-tied converters, it also renders them vulnerable to cyber-attacks. The aim of this article is first to shed light on the portions of the power converter control systems that are vulnerable to cyber-attacks. Next, typical cyber-attacks are overviewed by considering different applications of the grid-tied converters. Further, the impact of different types of cyber-attacks on grid support functions is studied. Finally, this article is concluded with summary and recommendation for further research.

A Digital Twin Based Estimation Method for Health Indicators of DC–DC Converters

Abstract: This article proposes a health indicator estimation method based on the digital-twin concept aiming for condition monitoring of power electronic converters. The method is noninvasive, without additional hardware circuits, and calibration requirements. An application for a buck dc–dc converter is demonstrated with theoretical analyses, practical considerations, and experimental verifications. The digital replica of an experimental prototype is established, which includes the power stage, sampling circuit, and close-loop controller. Particle swarm optimization algorithm is applied to estimate the unknown circuit parameters of interest based on the incoming data from both the digital twin and the physical prototype. Cluster-data of the estimated health indicators under different testing conditions of the buck converter is analyzed and used for observing the degradation trends of key components, such as capacitor and MOSFET. The outcomes of this article serve as a key step for achieving noninvasive, cost-effective, and robust condition monitoring for power electronic converters.

Large-Signal Stability of Grid-Forming and Grid-Following Controls in Voltage Source Converter: A Comparative Study

Abstract: The grid-forming and grid-following controls are adopted in three-phase voltage source converters according to different grid conditions. However, the basic operation principle of the two kinds of control is different and will lead to the instability of the grid-connected system through various paths. In this article, the energy function model of the grid-forming and grid-following controlled converters are established and compared in detail. The influence of system parameters is studied, and the large-signal stability boundaries are derived through a general method. Moreover, compared to the grid-forming controlled converter, the grid-following converter will lose stability by exhibiting a varying damping coefficient transient. The equivalent damping coefficient can be used as a criterion of whether the grid-forming/following control is suitable for the weak/strong grid condition. The simulation and experiment results show the difference of the transient process and validate the control criterion for different grid strength under large-signal disturbance.

A review of multiple input DC-DC converter topologies linked with hybrid electric vehicles and renewable energy systems

Abstract: In this paper, the contemporary development in multiple input dc-dc converters are identified and examined. The quest to mitigate the difficulties associated with employing renewables in distribution systems and electric vehicles (EVs) has yielded many new converter topologies. These new topologies have easier control, lower parts count, are cheaper and are worthy alternatives to the typical series or parallel connection of converters. The converters are identified by three divisions that bother on the isolation between the respective ports. The electrically connected converters do not have isolation between the ports, and thus, a dc link connects the ports. Electromagnetically connected converters use a dc-link to connect input ports, but the input ports and output port are isolated. In magnetically connected converters, input ports are separated by multiple winding transformer, just as the output port is isolated from the input ports by the winding. The formation, structure, characteristics, operation, merits and demerits of the converters will be presented. Thereafter, comparisons will be done based on the distinct features of the converters. This review identifies that converter properties depend on the specific application requirement and thus, no converter fulfills all demands in the industry. Prospective future research trends are suggested. This work aims to update on research done during the time gap since the last comprehensive reviews.

A Novel Generalized Common-Ground Switched-Capacitor Multilevel Inverter Suitable for Transformerless Grid-Connected Applications

Abstract: Recent research on common-ground switched-capacitor transformerless (CGSC-TL) inverters shows some intriguing features, such as integrated voltage boosting ability, possible multilevel output voltage generation, and nullification of the leakage current issue. However, the number of output voltage levels and also the overall voltage boosting ratio of most of the existing CGSC-TL inverters are limited to five and two, respectively. This article presents a generalized circuit configuration of such converters capable of higher voltage gain and output voltage levels generation. A basic five-level (5L) CGSC-TL inverter is first proposed using eight power switches and two self-balanced dc-link capacitors. A generalized extension of the circuit for any output voltage levels and voltage gain is then presented while keeping all the traits of the proposed basic 5L-CGSC-TL inverter. The circuit descriptions, control strategy, design guidelines, comparative study, and the relevant simulation and experimental results for the proposed 5L-CGSC-TL inverters and its seven-level derived topology are presented to validate the effectiveness and feasibility of this proposal.

Model Predictive Control Using Artificial Neural Network for Power Converters

Abstract: There has been an increasing interest in using model predictive control (MPC) for power electronic applications. However, the exponential increase in computational complexity and demand of computing resources hinders the practical adoption of this highly promising control technique. In this paper, a new MPC approach using an artificial neural network (termed ANN-MPC) is proposed to overcome these barriers. The ANN-MPC approach can significantly reduce the computing need and allow the use of more accurate high-order system models due to the simple mathematical expression of ANN. This is particularly important for multi-level and multi-phase power systems as their number of switching states increases exponentially. Furthermore, the ANN-MPC approach can retain the robustness for system parameter uncertainties by flexibly setting the constraint conditions. The basic concept, ANN structure, off-line training method, and online operation of ANN-MPC are described in detail. The computing resource requirement of the ANN-MPC and conventional MPC are analyzed and compared. The ANN-MPC concept is validated by both simulation and experimental results on two kW-class flying capacitor multilevel converters. It is demonstrated that the FPGA-based ANN-MPC controller can significantly reduce the FPGA resource requirement while offering a control performance same as the conventional MPC.

A Review of Multilevel Converters With Parallel Connectivity

Abstract: Cascaded-bridge converters (CBCs) and modular multilevel converters (MMCs) enjoy growing popularity mostly due to modularity and scalability. Conventionally, their submodules allow only serial and bypass operation so that the use of low-voltage components for high-voltage output becomes possible. Dually, submodule parallelization adds the switched-capacitor behavior to CBCs/MMCs and has witnessed an upward trend in recent years. The salient advantages of parallel operation comprise sensorless voltage balancing, capacitance saving, current sharing, and system efficiency optimization. To capture the advancement in the field, this article reviews the state-of-the-art multilevel converters with parallel connectivity, covering various submodules, macrolevel circuit topologies, implementation challenges, and solutions, as well as control and optimization schemes. In particular, this article derives and classifies submodules as well as macro-level topologies according to basic H-bridge, asymmetrical half-bridge, and symmetrical half-bridge submodules. On top of that, this article introduces strategies for the simplification of submodules and the creation of novel topologies yet maintaining parallel connectivity. We highlight the role of graph theory in creating new analytic and synthetic methodologies for multilevel converters. In addition, this article discloses the relationship between multilevel converters with parallel connectivity and switched-capacitor converters.

Isolated and Non-Isolated DC-to-DC Converters for Medium Voltage DC Networks: A Review

Abstract: MVDC technology is a promising solution to avoid installation of new AC networks. MVDC can provide optimum integration of large-scale renewable energy sources, the interconnection of different voltage levels of DC and AC grids with the ancillary services. The development in MVDC depends significantly on the DC-DC converters. Such converters support the modern trends of utilising medium-frequency transformers in power networks. Research on isolated converters technology is in its infancy and limited by the conversion ratio and component ratings. Besides, there is no standards exist covering specific aspects of isolated converter product. Thus, a review of such converters is needed. This work presents, for the first time, a review of the DC-DC power converter families in MVDC grids including the leading families which are isolated and non-isolated converters, as well as other subfamilies comparing the specifications and characteristics. Also, the applications of these converters are provided by focusing on the essential requirements for each application.

Principle and Topology Derivation of Single-Inductor Multi-Input Multi-Output DC–DC Converters

Abstract: Single-inductor multi-input multi-output (SI-MIMO) dc–dc converters are attractive in the engineering applications due to the advantage of high power density and low cost. In order to explore as many as possible SI-MIMO topologies, this article proposes a simple and effective topology derivation principle that only requires three steps. First, three basic cells consisting of a single inductor and multiple sets of unidirectional switches as well as inputs/outputs are proposed. Second, integrate the mentioned three basic cells with the inductor branch of the typical single-input single-output converters. Finally, implement the topology simplification by removing unnecessary switches/diodes. Based on the proposed principle, a large number of SI-MIMO topologies are derived from buck, boost, buck–boost, and noninverting buck–boost converters in this article. With more topology choices having different performance characteristics, it is very beneficial for engineers to gain an optimized design that a preferred one can be selected out after comprehensive comparison. As an example, topology comparison and selection among a family of single-inductor single-input dual-output converters is also conducted in this article. Besides, performance analysis, design considerations, and simulation/experiment results of the selected optimum topology are demonstrated in detail to verify its advantages.

Improved Hybrid Switched Inductor/Switched Capacitor DC–DC Converters

Abstract: High voltage gain dc–dc converters are widely used in various applications like low-voltage sustainable sources. In traditional boost converters, the voltage gain is limited by high voltage stress, high current ripple, and low efficiency due to employing a high duty cycle ratio. In this article, we propose converters that are a combination of four substructures, namely active switched inductor, passive switched inductor, switched capacitor cell, as well as an auxiliary switch with nonisolated configuration. The proposed structures have high voltage gain compared to the conventional either switched inductor-based or switched capacitor-based ones in addition to a low duty cycle ratio. By adding an auxiliary switch, the efficiency is improved, particularly for high voltage gains. The principle of operation and steady-state analysis are discussed in detail. Also, simulation results from PSCAD/EMTDC software are validated by a prototype built for experimental examination. The results demonstrated that the voltage gain and efficiency could be improved by utilizing the auxiliary switch.

High Frequency Resonant Converters: An Overview on the Magnetic Design and Control Methods

Abstract: Resonant converters, especiallyLLC converters, are deployed in many applications thanks to their reliability and high efficiency. With the introduction of wide band gap (WBG) devices and the soft switching feature of the LLC converter, the switching frequency of the LLC can be pushed to the order of megahertz, to shrink the converter size and increase the power density. However, magnetic design and control challenges stand in the way of doing so. This article addresses the challenges of high-frequency magnetic design for the LLC converter, and presents an overview for the latest solutions on the magnetic design of the LLC transformer. The matrix transformer is suitable for an LLC converter where high output current at high frequency is required. Different integration methods for the matrix transformer are discussed and demonstrations of hardware prototypes are provided. The dynamic behavior of the LLC converter presents another significant challenge against deploying an LLC converter in applications where fast transient response is needed. This article discusses such challenges as well and provides an overview on the latest control techniques, including average current-mode control, charge control, and simplified optimal trajectory control, to improve the dynamic performance of an LLC converter. A 1 megahertz LLC converter with digital control is presented, using simplified optimal trajectory control with improved transient performance over a wide frequency range.

A Novel Nonlinear Deep Reinforcement Learning Controller for DC–DC Power Buck Converters

Abstract: The nonlinearities and unmodeled dynamics inevitably degrade the quality and reliability of power conversion, and as a result, pose big challenges on higher-performance voltage stabilization of dc–dc buck converters. The stability of such power electronic equipment is further threatened when feeding the nonideal constant power loads (CPLs) because of the induced negative impedance specifications. In response to these challenges, the advanced regulatory and technological mechanisms associated with the converters require to be developed to efficiently implement these interface systems in the microgrid configuration. This article addresses an intelligent proportional-integral based on sliding mode (SM) observer to mitigate the destructive impedance instabilities of nonideal CPLs with time-varying nature in the ultralocal model sense. In particular, in the current article, an auxiliary deep deterministic policy gradient (DDPG) controller is adaptively developed to decrease the observer estimation error and further ameliorate the dynamic characteristics of dc–dc buck converters. The design of the DDPG is realized in two parts: (i) an actor-network which generates the policy commands, while (ii) a critic-network evaluates the quality of the policy command generated by the actor. The suggested strategy establishes the DDPG-based control to handle for what the iPI-based SM observer is unable to compensate. In this application, the weight coefficients of the actor and critic networks are trained based on the reward feedback of the voltage error, by using the gradient descent scheme. Finally, to investigate the merits and implementation feasibility of the suggested method, some experimental results on a laboratory prototype of the dc–dc buck converter, which feeds a time-varying CPL, are presented.

Advanced Control Methods for Power Converters in DG Systems and Microgrids

Abstract: In recent years, different advanced control methods have been successfully proposed as alternatives to conventional cascaded linear controllers for power converters in distributed generation systems and microgrids. The prime movers of this research are strong capabilities of advanced controllers to improve the dynamic performance and robustness of power electronic converters in these applications. This article first introduces the key roles and functionalities of voltage source converters in distributed generation systems and microgrids. Then, it describes how these functionalities are traditionally achieved by using linear controllers, and addresses their fundamental dynamic performance limitations. Afterward, the most prominent advanced control methods are overviewed. In this context, the implementation principles, advantages, and disadvantages of prominent model- and data-based advanced control methods are critically discussed and experimentally compared. This article ends with a discussion about promising research directions in the area of advanced control for power electronic converters.

Three Levels Are Not Enough: Scaling Laws for Multilevel Converters in AC/DC Applications

Abstract: Single-phase inverters and rectifiers in 230 V $_{\text{rms}}$ applications, with a dc-side voltage of 400 V, achieve ultrahigh efficiency with a simple two-level topology. These single-phase designs typically utilize a line-frequency unfolder stage, which has very low losses and essentially doubles the peak-to-peak voltage that can be generated on the ac side for a given dc-link voltage. For certain applications, however, such as higher power grid-connected photovoltaic inverters, electric vehicle chargers, and machine drives, three-phase converters are needed. Because of the three-phase characteristic of the system, unfolders cannot be similarly used, leading to a higher minimum dc-link voltage of the three-phase line-to-line voltage amplitude, which is typically set to 800 V for 230 V $_{\text{rms}}$ phase voltage systems. Previous demonstrations indicate that significantly more levels—and the associated higher cost and complexity—are required for ultrahigh-efficiency three-phase converters relative to their single-phase counterparts. In this article, we seek to determine the fundamental reason for the performance difference between three-phase 800 V dc-link converters and single-phase 400 V converters. First, we build a 2.2 kW dc/ac hardware demonstrator to confirm the necessity of higher complexity converters, showing a simultaneous reduction in efficiency and power density between a two-level 400 V benchmark (99.2% peak efficiency at 18.0 kW/L) and a three-level 800 V inverter phase-leg (98.8%, 9.1 kW/L). With the motivation confirmed, we derive general scaling laws for bridge-leg losses across the number of levels and dc-link voltage, finding the efficiency-optimal chip area and the minimum semiconductor losses. With commercially available Si or GaN power semiconductors, the scaling laws indicate that six or more levels would be required for an 800 V three-phase ac/dc converter to meet or exceed the bridge-leg efficiency of a two-level 400 V GaN benchmark for a fixed output filter. With a complete Pareto optimization, we find that at least seven levels are necessary to recover the efficiency of the two-level 400 V benchmark, and we validate this theory with a seven-level 800 V 2.2 kW hardware prototype with a power density of 15.8 kW/L and a peak efficiency of 99.03%. Finally, two practical solutions that make use of the benefits of unfolder bridges familiar in single-phase systems are identified for three-phase systems.

Sliding Mode Control: Overview of Its Applications in Power Converters

Abstract: In this article, we present an overview of sliding mode control (SMC) and its applications in power converters. Owing to the distinguished features such as fast dynamic response, robustness, order reduction, and implementation simplicity, SMC is widely studied in power converters. First, we briefly review SMC design. Then, we compare the classical current control and SMC methods for a grid-connected inverter application. Next, we summarize the advantages and disadvantages of SMC. Existing SMC applications in various power converters are presented and a block diagram for each converter topology is given. Finally, we discuss future challenges in SMC.

Grid Impedance Estimation Through Grid-Forming Power Converters

Abstract: In more-electronics power systems, grid-forming power converters, which operate as ac voltage sources, regulate the grid frequency and voltages in replacement of synchronous generators. Notably, grid impedances greatly influence the small signal and voltage stability of grid-forming converters. As such, prior knowledge of grid impedances can be very helpful for controller design. However, grid impedance estimation schemes are normally designed for current-controlled grid-following converters. Moreover, they are either very complicated or only yield grid inductances in a generally intrusive way. To fill this research gap, an impedance estimation method well suited to grid-forming converters is proposed. The method consists of four operating modes, which work well in voltage and power control cases. In the voltage control case, the voltage amplitude perturbation or phase angle information is exploited. Subsequently, the grid inductance and resistance are derived from power measurement. Alternatively, the active or reactive power information serves to estimate the grid impedance in the power control case. The proposed method features an easy implementation without any harmonic distortion, safety concern, or dependence on control parameters. Moreover, the method operates nonintrusively in most scenarios. Furthermore, a novel Kalman filtering scheme is proposed to provide added incentives. Finally, simulation and experimental results validate the effectiveness and simplicity of the proposed method.

Large-Signal Stability Improvement of DC-DC Converters in DC Microgrid

Abstract: In DC microgrids, constant power loads (CPLs) reduce the effective damping of the DC-DC converter and may induce destabilizing effects into the DC-DC converter. To overcome such problems regarding CPL and ensure large-signal stability of DC-DC converters in DC microgrids, some feedforward terms are added to $V$ - $I$ droop-based dual-loop controller for a DC-DC converter based on the large-signal model. It is proven that the feedforward terms can not only improve the transient response but also guarantee the exponential stability of the closed-loop system in the whole operating range in regards to a large-signal manner, which is verified by using a singular perturbation model. Moreover, a disturbance observer is designed to estimate the output current, thereby enabling the removal of the current measurement sensor. The proposed technique can be easily plugged into a pre-defined $V$ - $I$ droop-based dual-loop controller without an additional sensor being required. Ultimately, both simulation and experimental tests verify the effectiveness of the proposed method.

Placing Grid-Forming Converters to Enhance Small Signal Stability of PLL-Integrated Power Systems

Abstract: The modern power grid features the high penetration of power converters, which widely employ a phase-locked loop (PLL) for grid synchronization. However, it has been pointed out that PLL can give rise to small-signal instabilities under weak grid conditions. This problem can be potentially resolved by operating the converters in grid-forming mode, namely, without using a PLL. Nonetheless, it has not been theoretically revealed how the placement of grid-forming converters enhances the small-signal stability of power systems integrated with large-scale PLL-based converters. This paper aims at filling this gap. Based on matrix perturbation theory, we explicitly demonstrate that the placement of grid-forming converters is equivalent to increasing the power grid strength and thus improving the small-signal stability of PLL-based converters. Furthermore, we investigate the optimal locations to place grid-forming converters by increasing the smallest eigenvalue of the weighted and Kron-reduced Laplacian matrix of the power network. The analysis in this paper is validated through high-fidelity simulation studies on a modified two-area test system and a modified 39-bus test system. This paper potentially lays the foundation for understanding the interaction between PLL-based (i.e., grid-following) converters and grid-forming converters, and coordinating their placements in future converter-dominated power systems.

A Review on Direct Power Control of Pulsewidth Modulation Converters

Abstract: Starting from the principle of instantaneous power theory, this article explores various direct power control (DPC) strategies for three-phase two-level pulsewidth modulation (PWM) converters. After summarizing the fundamental power formula of PWM rectifiers, this article studies the operating principle of the conventional table-based approach and its related improvements. It further looks into the advanced counterparts employing space vector modulation and different nonlinear control strategies. The emphasis is put on the prevailing predictive DPC. Besides, the voltage-sensorless and robust DPC methods based on the virtual flux concept and the state observer or estimator are investigated. Critical issues, including the sample delay, constant switching frequency, duty cycle optimization, objective function, and unbalanced operation are examined.

A Model-Data-Hybrid-Driven Diagnosis Method for Open-Switch Faults in Power Converters

Abstract: To combine the advantages of both model-driven and data-driven methods, this article proposes a model-data-hybrid-driven method to diagnose open-switch faults in power converters. This idea is based on the explicit analytical model of converters and the learning capability of the artificial neural network (ANN). The process of the method is divided into two parts: offline model analysis and learning, and online fault diagnosis. For both parts, model-driven and data-driven are combined. With the model information and data-based learning capability, a fast diagnosis for various operating conditions can be achieved without a high computation burden, tricky threshold selection, and complex rulemaking. This can greatly contribute to the practical application. The open-switch fault diagnosis in a two-level three-phase converter is studied for the method validation. For this converter, an ANN is trained with two input elements, seven output elements, and two neurons in the hidden layer. Experimental results are given to demonstrate good performance.

A Multiport Bidirectional DC–DC Converter for Hybrid Renewable Energy System Integration

Abstract: The bidirectional power flow in most of the existing four-port converters is achieved on the battery port located on the low voltage side, i.e., the battery is charged by the energy sources and discharged to the dc link on the high voltage side. The lack of the bidirectional power flow at the dc link prevents them from managing the power at the system level. In this article, a bidirectional four-port dc–dc converter is proposed for the integration of the hybrid renewable energy system to a dc microgrid. The proposed converter uses the least number of devices compared with the existing bidirectional multiport converters. The bidirectional battery and the dc-link ports make it a good candidate for the dc-microgrid application in which the system-level power management is desired. The working principle of the converter is first analyzed, and the power transferred by the transformer and the zero-voltage switching (ZVS) conditions are derived. Then, the converter is designed to meet the requirements of the power rating and soft switching. The proposed converter is used to interface a wind turbine, a photovoltaic panel, a battery bank, and the dc microgrid. The experiments are carried to testify the performance in both steady state and the transient. The steady-state waveforms have validated the power relationship as well as the critical ZVS conditions while maintaining relatively high efficiency. The transient testing results show that the converter can switch from one scenario to another under the different conditions. This article is accompanied by a video demonstrating the converter in the real-time operation.

Current-Limiting Droop Control Design and Stability Analysis for Paralleled Boost Converters in DC Microgrids

Abstract: In this brief, a novel current-limiting droop controller for paralleled dc–dc boost converters loaded by constant impedance Z, constant current I, or constant power P loads in a dc microgrid is proposed to guarantee closed-loop stability and power sharing. Using an improved version of the recently proposed nonlinear current-limiting controller, an inherent current-limiting property is guaranteed for each converter independently of the load type or magnitude variations. Then, sufficient conditions to ensure closed-loop stability for the entire dc microgrid system with a constant Z, I, or P load are analytically obtained. Hence, compared with existing droop control methodologies, the proposed controller ensures accurate power sharing, tight voltage regulation, and closed-loop stability with a current limitation when connected to Z, I, or P load, for multiple paralleled boost converters, which introduce nonlinear dynamics. To verify the effectiveness of the proposed controller and the stability analysis, simulation results for the three parallel operated dc–dc boost converters with Z, I, and P loads and experimental results for two parallel operated dc–dc boost converters with a P load are performed under several changes of the load power demand.

AC Grid Emulations for Advanced Testing of Grid-Connected Converters—An Overview

Abstract: High penetration of distributed generations and active loads have enabled the power electronics converter to become a vital component in the modern power grid system, and the broad employment of grid-connected converters at various power levels is making the grid impedance and characteristics complicated. Consequently, in order to validate more advanced features such as the reliability and stability performances of the grid-connected converters, it is becoming an emerging need to emulate the grid behaviors from more aspects. This article serves to foster and investigate the state-of-the-art techniques in the field of ac grid emulation from the perspective of multiple spatial-scales and multiple time-scales. Four major concepts used for grid emulation with featured principles, including concept I (analog simulation with under-scaled components), concept II (grid characteristics in the real-time simulator), concept III (grid characteristics in the converters structure), and concept IV (grid characteristics in the converters controller), are summarized, respectively, in this article. The practical implementation regarding to the circuit topology and the power supply for the grid emulation system are also discussed. Finally, the future trends and conclusions in the field of ac grid emulation are provided.

dq -Frame Impedance Modeling of Three-Phase Grid-Tied Voltage Source Converters Equipped With Advanced PLLs

Abstract: With the increased prevalence of power converters in power systems, especially three-phase voltage source converters (VSCs), the stability analysis of power electronics-based power systems has received much attention recently To this end, different impedance models, such as the dq -domain, sequence-domain, and phasor-domain impedance models among others, have been developed for three-phase VSCs in recent years A common trend in all these impedance models, which have no noticeable practical advantage compared to each other, is considering a standard synchronous reference frame phase-locked loop (SRF-PLL) for the synchronization of the VSC with the power grid The standard SRF-PLL, however, has a limited filtering ability and, therefore, may not be very practical in most applications To deal with this shortcoming of the SRF-PLL, a great number of advanced three-phase PLLs have been proposed in the literature These advanced PLLs may have different feedback/feedforward loops and filters in their structures, which make including their dynamics in the available impedance models complicated Bridging this gap in research is the objective of this article To this end, it is demonstrated that advanced three-phase PLLs have an alternative representation, which can be easily obtained and included in the available dq -frame impedance model Several case studies are presented to verify this idea

Review of High-Frequency High-Voltage-Conversion-Ratio DC–DC Converters

Abstract: The development of high-frequency power converters is continuously improving their power density, efficiency and fast dynamic response. Among them, high-voltage-conversion-ratio (HVCR) dc–dc converters are widely used in high gain, i.e., step-up/-down or bidirectional applications, such as power supply for data center, dc micro-grids, electrical vehicle charging systems, etc. In this article, the current main high-frequency HVCR dc–dc converters are classified into inductive-based and capacitive-based approaches, which can then be described further by four kinds in detail, namely transformer-based, coupled-inductor-based, and switched-capacitor-based, as well as combination of coupled inductor and switched capacitor. A comprehensive analysis and comparison is given, which can provide guidance for proper topology selection and further topology optimization.

Sliding Mode Control of Grid-Connected NPC Converters Via High-Gain Observer

Abstract: In this paper, a nonlinear high-gain observer (NHGO) based second-order sliding mode (SOSM) control strategy is proposed for the three-phase three-level neutral-point-clamped (NPC) converter. This controller applies the advanced SOSM algorithm both in the voltage regulation loop and in power tracking loop, which provides a fast dynamic for the dc-link voltage, and also assures a good steady state behavior for the NPC converter. Additionally, a NHGO technique is implemented in the voltage regulator combining with the SOSM algorithm. The conventional observer-based controllers suffer from the destructive effects of measurement noise, and it can only be addressed by diminishing the observer gain, which sacrifices the observer property. The NHGO technique adopts a time varying gain, that is, high gain in transient while low gain in steady state, which minimizes the adverse influence of measurement noise. The tuning method of the proposed NHGO-based SOSM controller is given to simplify the implementation process. Finally, the simulation and experimental results of the proposed control scheme for the NPC converter are given and compared with the conventional PI controller as well as the well-known linear extended state observer-based control method, which validates the feasibility and superiority of the proposed controller.

A Multifunctional Non-Isolated Dual Input-Dual Output Converter for Electric Vehicle Applications

Abstract: High voltage conversion dc/dc converters have perceived in various power electronics applications in recent times. In particular, the multi-port converter structures are the key solution in DC microgrid and electric vehicle applications. This paper focuses on a modified structure of non-isolated four-port (two input and two output ports) power electronic interfaces that can be utilized in electric vehicle (EV) applications. The main feature of this converter is its ability to accommodate energy resources with different voltage and current characteristics. The suggested topology can provide a buck and boost output simultaneously during its course of operation. The proposed four-port converter (FPC) is realized with reduced component count and simplified control strategy which makes the converter more reliable and cost-effective. Besides, this converter exhibits bidirectional power flow functionality making it suitable for charging the battery during regenerative braking of an electric vehicle. The steady-state and dynamic behavior of the converter are analyzed and a control scheme is presented to regulate the power flow between the diversified energy supplies. A small-signal model is extracted to design the proposed converter. The validity of the converter design and its performance behavior is verified using MATLAB simulation and experimental results under various operating states.