Showing papers on "Converters published in 2018"

Distributed Power System Virtual Inertia Implemented by Grid-Connected Power Converters

Abstract: Renewable energy sources (RESs), e.g., wind and solar photovoltaics, have been increasingly used to meet worldwide growing energy demands and reduce greenhouse gas emissions. However, RESs are normally coupled to the power grid through fast-response power converters without any inertia, leading to decreased power system inertia. As a result, the grid frequency may easily go beyond the acceptable range under severe frequency events, resulting in undesirable load-shedding, cascading failures, or even large-scale blackouts. To address the ever-decreasing inertia issue, this paper proposes the concept of distributed power system virtual inertia, which can be implemented by grid-connected power converters. Without modifications of system hardware, power system inertia can be emulated by the energy stored in the dc-link capacitors of grid-connected power converters. By regulating the dc-link voltages in proportional to the grid frequency, the dc-link capacitors are aggregated into an extremely large equivalent capacitor serving as an energy buffer for frequency support. Furthermore, the limitation of virtual inertia, together with its design parameters, is identified. Finally, the feasibility of the proposed concept is validated through simulation and experimental results, which indicate that 12.5% and 50% improvements of the frequency nadir and rate of change of frequency can be achieved.

Model Predictive Control of Power Converters for Robust and Fast Operation of AC Microgrids

Abstract: This paper proposes the application of a finite control set model predictive control (FCS-MPC) strategy in standalone ac microgrids (MGs). AC MGs are usually built from two or more voltage source converters (VSCs) which have the capability of regulating the voltage at the point of common coupling, while sharing the load power at the same time. Those functionalities are conventionally achieved by hierarchical linear control loops. However, they present severe limitations in terms of slow transient response and high sensitivity to parameter variations. This paper aims to mitigate these problems by first introducing an improvement of the FCS-MPC strategy for a single VSC that is based on explicit tracking of derivative of the voltage reference trajectory. Using only a single step prediction horizon, the proposed strategy exhibits very low computational expense, but provides steady-state performance comparable to carrier-based sinusoidal PWM, while its transient response and robustness to parameter variation is far superior to hierarchical linear control. These benefits are exploited in a general ac MG setting where a methodology for paralleling multiple FCS-MPC regulated VSCs is described. Such an MG is characterized by rapid transient response, inherent stability in all operating conditions, and fully decentralized operation of individual VSCs. These findings have been validated through comprehensive simulation and experimental verification.

Stability Analysis and Stabilization Methods of DC Microgrid With Multiple Parallel-Connected DC–DC Converters Loaded by CPLs

Abstract: Constant power loads may yield instability due to the well-known negative impedance characteristic. This paper analyzes the factors that cause instability of a dc microgrid with multiple dc–dc converters. Two stabilization methods are presented for two operation modes: 1) constant voltage source mode; and 2) droop mode, and sufficient conditions for the stability of the dc microgrid are obtained by identifying the eigenvalues of the Jacobian matrix. The key is to transform the eigenvalue problem to a quadratic eigenvalue problem. When applying the methods in practical engineering, the salient feature is that the stability parameter domains can be estimated by the available constraints, such as the values of capacities, inductances, maximum load power, and distances of the cables. Compared with some classical methods, the proposed methods have wider stability region. The simulation results based on MATLAB/simulink platform verify the feasibility of the methods.

Recent progress and development on power DC-DC converter topology, control, design and applications: A review

Abstract: Over the last few decennia, power DC/DC converters have been the subject of great interest due to its extensive increment of utilization in different applications. A thorough review on recent developed power DC/DC converters is presented in this paper. The study is focused on the topologies in different applications such as renewable energy, automobile, high-voltage and medium-voltage DC power systems, telecommunication, etc. In addition, an overview of the modulation techniques, the state-of-the-art of control strategies of well-established converters are discussed. Photovoltaic (PV) systems as the noticeable renewable energy resources generally suffer from poor conversion efficiency with instability and intermittent characteristics. Therefore, DC/DC converter with Maximum Power Point Tracking (MPPT) algorithm is essential to ensure maximum available power harnessed from the PV. Important features of DC/DC converters with MPPT are also figured with various performances. Furthermore, the design and optimization of different parameters are addressed systematically. Finally, the researcher’s future challenges and focusing trends are briefly described. For the next-generation converters design and applications, these are considered in details, and will provide useful framework and point of references.

Virtual Direct Power Control Scheme of Dual Active Bridge DC–DC Converters for Fast Dynamic Response

Abstract: One of the essential requirements for high-performance dual active bridge (DAB) dc–dc converters as the controlled dc voltage sources is to obtain the constant output voltage rapidly and accurately under all working conditions. In order to reach fast dynamic response, combing direct power control with feedforward control strategy, this paper proposes a virtual direct power control (VDPC) scheme with single-phase-shift control for DAB dc–dc converters to face with these following extreme conditions, such as start-up, load step-change, no-load, the input voltage fluctuation, and the desired output voltage step-change. The proposed VDPC scheme of DAB dc–dc converters can achieve no overshoot and fast transient response for the output voltage in load or input voltage disturbances and start-up stage. Dynamic response of the output voltage control has been also improved when the desired value steps up and down. Finally, four control schemes consisting of traditional voltage loop control, load current feed-forward control, model-based phase-shift control, and the proposed VDPC schemes are compared and tested in a scale-down DAB dc–dc converter experimental prototype. Experimental results verify the above excellent performance of the proposed VDPC scheme and the effectiveness of theoretical analysis.

Multilevel Power Converters

Abstract: This chapter covers the fundamental multilevel converter structures and modulation paradigms. The basic principles of the four main different multilevel converters have been discussed methodically: (1) cascaded H-bridges, (2) diode clamp, (3) flying capacitors, and (4) modular multilevel converter. Various modulation schemes for multilevel converters are also covered including fundamental frequency switching and harmonic elimination and several carrier-based and space vector-based pulse-width modulation (PWM) methods. The chapter also includes significant content on the operation and control of modular multilevel converter (MMCs) including techniques to balance capacitor voltage and reduce circulating current in converter arms.

Finite Control Set Model Predictive Control for Grid-Connected AC–DC Converters With LCL Filter

Abstract: This paper presents two new implementations of finite control set model predictive control (FCS-MPC) methods applied to ac–dc converters with an inductive.capacitive.inductive (LCL) filter. The LCL filter, despite its advantages, can cause a strong resonance in the grid current and also pose a substantially more complex control problem. The new solutions improve the FCS-MPC with an active damping algorithm by eliminating low-order grid current harmonics and decreasing sensitivity to grid voltage distortion. The new methods, i.e., PCi1i2uc and PCi1i2uc-2steps propose multivariable approaches using converter-side current, line-side current, and capacitor voltage. Another improvement involves extending the prediction horizon in PCi1 i2uc-2steps. Both methods have been tested and compared in steady and transient states as well as under grid voltage disturbances. The methods were also compared with a predictive control algorithm with active damping. The simulation results and experimental measurements validate the developed control schemes and show high quality of grid current (low THDi value), high dynamic performance, and immunity under distorted grid voltage conditions.

Control Design for Optimizing Efficiency in Inductive Power Transfer Systems

Abstract: Inductive power transfer (IPT) converters are resonant converters that attain optimal energy efficiencies for a certain load range. To achieve maximum efficiency, it is common to cascade the IPT converter with front-side and load-side dc/dc converters. The two dc/dc converters are normally controlled cooperatively for the requirements of output regulation and maximum efficiency tracking using a control technique based on perturbation and observation, which is inevitably slow in response. In this paper, a decoupled control technique is developed. The load-side dc/dc converter is solely responsible for output regulation, while the front-side converter is responsible for impedance-matching of the IPT converter by controlling its input-to-output voltage ratio. The controls are linear and therefore fast. DC and small-signal transfer functions are derived for designing the control parameters. The performances of fast regulation and high efficiency of the IPT converter system are verified using a prototype system.

Switched tank converters

Abstract: This paper presents a new class of Switched Tank Converters (abbreviated as STCs) for high efficiency high density non-isolated DC-DC application where large voltage step down (up) ratios are required. Distinguished from switched capacitor converters, the STCs uniquely employ LC resonant tanks to partially replace the flying capacitors for energy transfer. Full soft charging, soft switching and minimal device voltage stresses are achieved under all operating conditions. The STCs feature very high efficiency, density and robustness against component non-idealities over a wide range. Furthermore, thanks to the full resonant operation, multiple STCs can operate in parallel with inherent droop current sharing, offering the best scalability and control simplicity. These attributes of STC make it a disruptive and robust technology viable for industry's high volume adoption. A novel equivalent DCX building block principle is introduced to simplify the analysis of STC. A 98.92% efficiency STC product evaluation board (4-to-1, 650W) has been developed and demonstrated for the next-gen 48V bus conversion on data center server boards.

An Oscillatory Stability Criterion Based on the Unified $dq$ -Frame Impedance Network Model for Power Systems With High-Penetration Renewables

Abstract: Recent years have witnessed emerging oscillatory stability issues in power systems with high-penetration renewables. These issues are generally caused by the dynamic interaction between the AC/DC grid and power electronic converters used for the integration of renewables. Some previous studies have explored the issues with simplified system models containing only a few converters and idealized networks. However, the interactions between many converters and traditional generators/HVDCs have not been fully considered. To fill this gap, this paper first presents the concept of a unified dq frame impedance network model (INM), with which different converters as well as traditional generators/HVDCs can be incorporated to form an integrated s -domain model of a practical system. As the INM is aggregated into a lumped impedance matrix, a new criterion is then proposed to quantify the oscillatory stability, just by analyzing the frequency characteristics of the determinant of the matrix. Finally, the modeling method and the stability criterion are applied to a real-world system with a very high share of renewables and a realistic risk of oscillatory instability. Their effectiveness has been validated by both field measurements and electromagnetic transient simulations.

A Review on Grid-Connected Converter Control for Short-Circuit Power Provision Under Grid Unbalanced Faults

Abstract: As an increasing amount of converter-based generation on power electronics is connected to power systems, transmission system operators are revising the grid-connection requirements to streamline the connectivity of the devices to maintain security of supply. Converter-based generation can behave significantly different from the traditional alternators under grid faults. In order to evaluate the potential impact of the future converter-based power systems on protective relays, it is necessary to consider diverse current control strategies of voltage-source converters (VSCs) under unbalanced faults as the performance of converters primarily depends on their control objectives. In this paper, current control strategies of VSCs under unbalanced faults for short-circuit power provision are reviewed in two groups, namely, power-characteristic-oriented and voltage-support-oriented control strategy, respectively. As the fault current provided by converters should be restricted within secure operation limits considering semiconductor capabilities, converter current limit issue is also discussed.

Pulse Density Modulation for Maximum Efficiency Point Tracking of Wireless Power Transfer Systems

Abstract: Maximum efficiency point tracking (MEPT) control has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet the power demands with the highest efficiency against coupling and load variations. Conventional MEPT implementations use dc/dc converters on both transmitting and receiving sides to regulate the output voltage and maximize the system efficiency at the expense of increased overall complexity and power losses on the dc/dc converters. Other implementations use phase-shift control or on–off control of the transmitting side inverter and the receiving side active rectifier instead of dc/dc converters but cause new problems, e.g., hard switching, low average efficiency, and large dc voltage ripples. This paper proposes a pulse density modulation (PDM) based implementation for MEPT to eliminate all the mentioned disadvantages of existing implementations. Delta-sigma modulators are used as an example to realize the PDM. A dual-side soft switching technique is proposed for the PDM. The ripple factor of the output voltage with PDM is derived. A 50 W WPT system is built to validate the proposed method. The system efficiency is maintained higher than 70% for various load resistances when the power transfer distance is 0.5 m, which is 1.67 times the diameter of the coils.

Simplified Finite Control Set-Model Predictive Control for Matrix Converter-Fed PMSM Drives

Abstract: Finite control set-model predictive control (FCS-MPC) is emerging as an attractive alternative for power converters control. In comparison with the classical linear controllers, FCS-MPC needs a shorter control loop cycle time to achieve the same performance. But, the conventional FCS-MPC involves a large amount of calculation. The calculation efforts increase with the number of switching states of the power converter as well as the control objectives, which is a challenge for its application. This paper proposes an effective method to simplify the FCS-MPC and to reduce its calculation efforts for its application in matrix converter-fed permanent magnet synchronous motors. The experimental results which verify the good performance of the proposed method are presented.

Novel Nonlinear Droop Control Techniques to Overcome the Load Sharing and Voltage Regulation Issues in DC Microgrid

Abstract: In a dc microgrid, good load sharing and voltage regulation are desirable. These are affected by practical factors like sensor calibration errors and cable resistances. To enhance the load-sharing accuracy among the parallel-connected voltage-controlled sources and to improve the dc-bus voltage regulation, three novel nonlinear droop control techniques are proposed for the smart grid scenario. The proposed methods are completely decentralized methods and require only local information (output voltage and output current of the individual converter) for achieving aforementioned merits. Since no communication channel is required, it is easy to implement them. Furthermore, the absence of communication channel improves system reliability and offers plug-and-play features, as only local information is utilized. Also, failure of one converter does not affect the operation of other converters connected to the grid as no information is exchanged between the converters. Effect of sensor calibration errors and cable resistances is minimized by these techniques. Theoretical analysis and experimental results are presented to demonstrate the efficacy of the proposed control methods. Finally, a performance analysis of the three droop control techniques is presented along with their advantages over the conventional methods under different operating conditions.

Capacitor-Voltage Feedforward With Full Delay Compensation to Improve Weak Grids Adaptability of LCL-Filtered Grid-Connected Converters for Distributed Generation Systems

Abstract: LCL -filtered grid-connected converters are widely used for distributed generation systems. However, the current regulation of such converters is susceptible to weak grid conditions, e.g., grid impedance variation and background harmonics. Paralleling multiple harmonic compensators (HCs) is a commonly used method to suppress the current distortion caused by grid background harmonics, but the control bandwidth should be wide enough to ensure system stability. In order to enhance the adaptability of LCL -filtered grid-connected converters under weak grid operation, this paper proposes an improved capacitor-voltage-feedforward control with full delay compensation. When used with converter-side current feedback, the proposed control can keep system low-frequency characteristic independent of grid impedance and provide a high-harmonic rejection capability without using additional HCs. Moreover, it completely avoids the design constraints of an LCL filter, i.e., $\omega _{r} is required for single-loop converter-side current control. Therefore, a higher resonant frequency can be designed to achieve a wider control bandwidth and to lower the current distortion caused by the paralleled filter capacitor branch. Experimental results are finally presented to verify the proposed control, which are also in good agreement with theoretical analysis.

Dynamic Stabilization of DC Microgrids With Predictive Control of Point-of-Load Converters

Abstract: This paper investigates the possibility of deploying a finite control set model predictive control (FCS-MPC) algorithm for dynamic stabilization of a dc microgrid (MG) that supplies tightly regulated point-of-load (POL) converters. Within their control bandwidth, such converters behave as constant power loads (CPLs), where the MG sees them as impedances with a negative incremental resistance. Due to this characteristic, POL converters have a destabilizing impact that may cause large voltage oscillations or even a blackout of the whole MG. This paper proposes an active damping method realized by introducing a stabilization term in the cost function of the FCS-MPC algorithm that is used for regulation of the POL converter. This approach, on one hand, stabilizes a dc MG without implementing any additional active or passive components; thus, providing higher energy efficiency and better cost-effectiveness than methods that rely on such components. On the other hand, when compared to other approaches that focus on dc link stabilization via POL converter control, the proposed method has a significantly lower influence on the load voltage regulation performance. These findings are confirmed through comprehensive analytical investigation that shows how the proposed stabilization term affects the input impedance of the POL converter and the load voltage tracking performance. This is followed by experimental validation, where an FCS-MPC regulated uninterruptible power system inverter was used as a particular CPL example.

Mission Profile Based System-Level Reliability Analysis of DC/DC Converters for a Backup Power Application

Abstract: Reliability analysis is an important tool for assisting the design phase of a power electronic converter to fulfill its life-cycle specifications. Existing converter-level reliability analysis methods have two major limitations: 1) being based on constant failure rate models; and 2) lack of consideration of long-term operation conditions (i.e., mission profile). Although various studies have been presented on power electronic component-level lifetime prediction based on wear-out failure mechanisms and mission profile, it is still a challenge to apply the same method to the reliability analysis of converters with multiple components. Component lifetime prediction based on associated models provides only a $B_{X}$ lifetime information (i.e., the time when X % items fail), but the time-dependent reliability curve is still not available. In this paper, a converter-level reliability analysis approach is proposed based on time-dependent failure rate models and long-term mission profiles. Two different methods to obtain the component-level time-to-failure are illustrated by a case study of dc/dc converters for a 5 kW fuel cell-based backup power system. The reliability analysis of the converters with and without redundancy is also performed to assist the decision making in the design phase of the fuel cell power conditioning stage.

An Analytical Method to Evaluate and Design Hybrid Switched-Capacitor and Multilevel Converters

Abstract: This paper investigates the use of multilevel conversion in dc–dc applications that require a large voltage conversion ratio. A quantitative method that can serve as a guide to compare and design multilevel topologies for large conversion ratio applications is presented. The proposed method keeps the conduction loss and switching loss constant across the different converters and employs the passive component volume as the single performance metric. As examples, flying capacitor multilevel converters and hybrid switched-capacitor (SC) converters are compared to conventional two-level buck converters, and are shown analytically to have significantly reduced passive component size. Three converter prototypes are implemented, based on the presented methodology to experimentally validate the method as well as demonstrate the advantages of multilevel and hybrid SC converters.

Voltage-Lift Technique Based Nonisolated Boost DC–DC Converter: Analysis and Design

Abstract: In this paper, a new structure of nonisolated boost dc–dc converters based on voltage-lift technique is proposed. In comparison with conventional nonisolated boost dc–dc converters, the proposed converter generates higher voltage gain. In this paper, the relations between voltage and current of all elements in continuous conduction mode and discontinuous conduction mode are calculated as well as voltage gain in each mode. Then, the critical inductance and stress of switch current are extracted. Finally, the validity of given theories is examined by using the experimental results.

An Open-Circuit Fault Diagnosis Approach for Single-Phase Three-Level Neutral-Point-Clamped Converters

Abstract: This paper presents a model-based open-circuit fault diagnosis approach for single-phase three-level neutral-point-clamped (3LNPC) converters in electric railway application. The diagnosis algorithm, which only requires the signals existing in the control system, not only detects open-circuit faults but also identifies the faulty device among the transistors and clamping diodes. The mixed logical dynamic (MLD) model of the converter is built to estimate the grid current. The residual generated from the measured current subtracting the estimated one is analyzed under different open-circuit faults. According to the characteristics of the residual changing rate, the proposed approach allows fault localization. The proposed method is effective both in traction and regenerative braking operation and has fast diagnosis speed. Hardware-in-the-loop (HIL) experiments are carried out to verify the effectiveness of the proposed fault diagnosis algorithm.

Model Predictive Control for DC–DC Boost Converters With Reduced-Prediction Horizon and Constant Switching Frequency

Abstract: The implementation of multistep direct model predictive control (MPC) for dc–dc boost converters overcomes the well-known issue of nonminimum phase behavior. However, it can lead to a high computational burden depending on the prediction horizon length. In this paper, a simple and computationally efficient MPC method for dc–dc boost converters is proposed. The key novelty of the presented control strategy lies in the way dynamic references are handled. The control strategy is capable of providing suitable references for the inductor current and the output voltage, without requiring additional control loops. Moreover, this reference design allows the predictive controller to be implemented with a single-step prediction horizon. Thus, a significant reduction in the required real-time calculations executed in the control hardware is achieved. To obtain constant switching frequency, the power switch commutation instants within a sampling period are considered as control inputs. Therefore, the predictive controller is formulated as a continuous control set MPC. Additionally, the proposed formulation is able to deal with different operation modes of the converter without changing the controller structure. Finally, an observer is used to dynamically modify the reference to provide robustness to system parameter uncertainties. Simulation and experimental results show an accurate tracking of dynamic inductor current and output voltage references, while respecting the restrictions on maximum inductor current levels of the converter.

Power electronics converters: Past, present and future

Abstract: The development of power electronics in the past century and the current state of the art of power electronics converters are briefly reviewed, before giving an insight into the deficiencies of the conventional current-source and voltage-source converters and into the superiority of impedance-source converters and, then, proposing a design methodology for impedance-source converters aimed to replace the traditional tedious, manual and experience-dependent design methods. Some examples for their deployment in renewable-energy applications are discussed, and the direction into which power electronic converters will develop in the future is indicated.

Voltage Balance Control Based on Dual Active Bridge DC/DC Converters in a Power Electronic Traction Transformer

Abstract: In this paper, a voltage balance control strategy based on dual active bridge (DAB) dc/dc converters in a power electronic traction transformer (PETT) is proposed. Based on this strategy, the output-parallel DAB converters can be equivalent to an input-series-output-parallel system. Furthermore, a PETT starting control method is put forward, which can effectively avoid risks of overcurrent and overvoltage in the PETT starting process. In order to carry out the controller design and system stability analysis, three different kinds of mathematical models of DAB converters are set up. The first model is related to a single DAB converter, the second model reflects the equivalent relation between an output-parallel DAB system and a single DAB converter in terms of the output-voltage control loop, and the third model indicates that the voltage balance control system based on DAB converters is a multiinput-multioutput system. Due to the nonzero off-diagonal elements of the controlled plant, there is a mutual effect between different control loops, which is defined as “interaction” in the multivariable feedback control theory. The stability of the voltage balance control system is made up of two parts, including the stability of each single-input-single-output (SISO) control loop and the influence of the interaction on the system stability. The research is carried out to measure the intensity of the interaction in this paper, and a criterion directly based on the controlled plant is proposed to predict the influence of the interaction, which can obviously simplify the system stability analysis. Considering the particular traction onboard application, a new control structure toward the voltage balance controller is introduced. Based on the new structure, the controller is designed and the stability of the SISO system is analyzed. Finally, a five-cell PETT prototype with rated power of 30 kW is taken to carry out further research, and the experiment results verify the effectiveness and correctness of the proposed algorithms.

Synthesis and Comparative Analysis of Very High Step-Up DC–DC Converters Adopting Coupled-Inductor and Voltage Multiplier Cells

Abstract: A synthesis methodology for developing a set of very high step-up dc–dc converters is presented and discussed in this paper. The proposed method makes use of a boost converter as basic topology in which three step-up techniques are combined and incorporated. The studied techniques are the switched capacitors voltage multipliers (VM), the diode VMs, and the coupled-inductors. With the proposed methodology, many well-known converters are identified and two novel converters are proposed. In addition to a detailed analysis of the synthesis of each topology, a comparative analysis among of some important converters is presented. This comparison involves aspects such as voltage gain, voltage stress, component stress factor, component count, and relative cost. By means of these comparisons, the main characteristics and constraints of the analyzed converters are identified. Results from 250 W prototypes, designed according to photovoltaic ac-module specifications, are obtained experimentally to validate the theoretical analyses and point out advantages and limitations of each converter. The results demonstrate that the combination of the three studied techniques provides the best trend off on the comparative analysis carried out in this work.

Stability Improvement for Three-Phase Grid-Connected Converters Through Impedance Reshaping in Quadrature-Axis

Abstract: Three-phase AC−DC and DC−AC power converters have been extensively employed as grid-interfaces in various applications, e.g., distributed generation and energy storage systems. In these applications, power converters should always synchronize with the mains grid so that active and/or reactive power can properly be regulated while maintaining desired waveforms of grid currents. Grid synchronization necessitates accurate information of grid voltages, which is normally obtained through phase-locked-loops (PLLs). However, the employment of PLLs may bring in stability concerns. Previous research revealed that the inclusion of PLLs shapes the impedance of power converters into a negative resistance in the quadrature-axis ( q -axis), and this should be responsible for instability. To resolve the instability issue caused by PLLs, this paper proposes an impedance controller for reshaping the q -axis impedance into a positive resistance in the low-frequency band. Without any extra burden on system hardware, the proposed controller can easily be implemented by directly relating the q -axis voltage to the q -axis current reference. As a result, the presented three-phase power conversion system can operate stably even under a severely weak grid condition, which are verified by simulation and experimental results.

Reactive Power Minimization in Bidirectional DC–DC Converters Using a Unified-Phasor-Based Particle Swarm Optimization

Abstract: This paper deals with an optimized three-level modulated phase shift control using particle swarm optimization (PSO) strategy based on the unified phasor analysis with an aim to improve the efficiency of the bidirectional dual active bridge (DAB) converter for the whole operation range. A unified mathematical model based on Fourier transform is built for the DAB converter. All possible operation states under the three-level modulated phase shift control are covered. Accurate complex mathematical expressions for the inductor current, the transmission power, and the reactive power are obtained. Both modulus and angle variables are illustrated with respect to the inner and outer phase shift angle with the phasor diagram. The proposed method is able to achieve the minimum reactive power under three-level modulated phase shift control by obtaining the optimal phase-shift angles directly. The cumbersome process of the optimal operation mode selection for different voltage conversion ratio and load conditions in conventional methods is overcome successfully, thus greatly simplifying the theoretical calculation and implementation difficulty. Simulation and experimental results in terms of the reactive power, soft-switching range, and efficiency are provided to verify the practical feasibility of the proposed method for the bidirectional DAB converters.

Design and Comparison of Cascaded H-Bridge, Modular Multilevel Converter, and 5-L Active Neutral Point Clamped Topologies for Motor Drive Applications

Abstract: This paper presents the design procedure and comparison of converters currently used in medium-voltage high-power motor drive applications. For this purpose, the cascaded H-bridge (CHB), modular multilevel converter (MMC), and five-level active neutral point clamped (5-L ANPC) topologies are targeted. The design is performed using 1.7-kV insulated gate bipolar transistors (IGBTs) for CHB and MMC converters, and utilizing 3.3- and 4.5-kV IGBTs for 5-L ANPC topology as normally done in industry. The comparison is done between the designed converter topologies at three different voltage levels (4.16, 6.9, and 13.8 kV, with only the first two voltage levels in case of the 5-L ANPC) and two different power levels (3 and 5 MVA), in order to elucidate the dependence of different parameters on voltage and power rating. The comparison is done from several points of view such as efficiency, capacitive energy storage, semiconductor utilization, parts count (for measure of reliability), and power density.

A Decentralized Coordination Control Method for Parallel Bidirectional Power Converters in a Hybrid AC–DC Microgrid

Abstract: In the hybrid ac–dc microgrid, the ac and dc subgrids are connected by bidirectional power converters (BPCs) that play an important role in the load power sharing and power interaction between the ac and dc subgrids. The coordination control and circulating current suppression for the parallel BPC system are very challenging. In this paper, a decentralized coordination control method is proposed for parallel three-phase BPCs, which can suppress the circulating currents, realize proper power interaction, and achieve overall load power sharing in both the grid-connected mode and the islanded mode. The performance of the proposed control methods is verified by the real-time hardware-in-loop tests.

Model Predictive Control With Power Self-Balancing of the Output Parallel DAB DC–DC Converters in Power Electronic Traction Transformer

Abstract: This paper focuses on the output parallel dual-active-bridge (DAB) dc–dc converters in power electronic traction transformers. A model predictive control with current-stress-optimized (MPC-CSO) scheme based on dual-phase-shift (DPS) control is proposed to improve the dynamic performance, balance the transmission power, and realize the current-stress optimization. The dynamic behavior of output voltage in the next horizon is predicted accurately under the input voltage fluctuation and load disturbance conditions by developing the prediction model. In addition, the proposed MPC-CSO scheme can track the output voltage with the desired value directly with no overshoot during the start-up process. Combining the MPC and CSO scheme, the fast dynamic response and high-efficiency performance of DAB converters can be achieved simultaneously, and the transmission power of each DAB cell can be self-balanced. Finally, three control schemes consisting of traditional voltage closed-loop control with single phase shift, traditional CSO control with DPS, and MPC-CSO with DPS schemes are compared in a scale-down three DAB dc–dc converter cells experimental prototype by using TMS320F28335+FPGA_6SLX45 as core controller. Extensive experimental results have verified the excellent performance of the proposed MPC-CSO scheme and associated analysis in this paper.