Showing papers in "IEEE Journal of Emerging and Selected Topics in Power Electronics in 2020"

Grid-Forming Inverters: A Critical Asset for the Power Grid

Abstract: Increasing inverter-based sources reduces the system’s inertia resulting in possible frequency stability issues. Understanding low-inertia systems and their stability properties is of crucial importance. This article introduces fundamental ways to integrate high levels of renewable energy (RE) and distributed energy resources (DERs) in the power system while creating a more flexible power system. Using RE and DER in the distribution system has many advantages such as reducing the physical and electrical distance between generation and loads, bringing sources closer to loads contributes to the enhancement of the voltage profile, reduction in distribution and transmission bottlenecks, improved reliability, lower losses, and enhances the potential use of waste heat. A basic issue for high penetration of DER is the technical complexity of controlling hundreds of thousands to millions of inverters. This is addressed through autonomous techniques using local measurements eliminating the need for fast control systems. The key issues addressed in this article include using inverter damping to stabilize frequency in systems with low or no inertia, autonomous operation, methods for relieving inverter overload, energy reserves, and their implementation in photovoltaics (PV) systems. This article provides important insight into the interactions between inverter bases sources and the high-power system. The distinction between grid-forming (GFM) inverter and grid-following (GFL) inverter is profound. GFM inverters provide damping to frequency swings in a mixed system, while GFL inverter can aggravate frequency problems with increased penetration. Rather than acting as a source of inertia, the GFM inverter acts as a source of damping to the system. On the other hand, the application of inverters in the power system has two major issues. One is the complexity of controlling hundreds of thousands to millions of inverters. This is addressed through autonomous techniques using local measurements. The other is the potential of high overcurrent in GFM inverters and techniques for explicitly protecting against overloading. To exploit the innate damping of GFM inverters, energy reserves are critical.

Transformerless Inverter Topologies for Single-Phase Photovoltaic Systems: A Comparative Review

Abstract: In photovoltaic (PV) applications, a transformer is often used to provide galvanic isolation and voltage ratio transformations between input and output. However, these conventional iron- and copper-based transformers increase the weight/size and cost of the inverter while reducing the efficiency and power density. It is therefore desirable to avoid using transformers in the inverter. However, additional care must be taken to avoid safety hazards such as ground fault currents and leakage currents, e.g., via the parasitic capacitor between the PV panel and ground. Consequently, the grid connected transformerless PV inverters must comply with strict safety standards such as IEEE 1547.1, VDE0126-1-1, EN 50106, IEC61727, and $\text{A}S/N$ ZS 5033. Various transformerless inverters have been proposed recently to eliminate the leakage current using different techniques such as decoupling the dc from the ac side and/or clamping the common mode (CM) voltage (CMV) during the freewheeling period, or using common ground configurations. The permutations and combinations of various decoupling techniques with integrated voltage buck–boost for maximum power point tracking (MPPT) allow numerous new topologies and configurations which are often confusing and difficult to follow when seeking to select the right topology. Therefore, to present a clear picture on the development of transformerless inverters for the next-generation grid-connected PV systems, this paper aims to comprehensively review and classify various transformerless inverters with detailed analytical comparisons. To reinforce the findings and comparisons as well as to give more insight on the CM characteristics and leakage current, computer simulations of major transformerless inverter topologies have been performed in PLECS software. Moreover, the cost and size are analyzed properly and summarized in a table. Finally, efficiency and thermal analysis are provided with a general summary as well as a technology roadmap.

Transient Stability of Voltage-Source Converters With Grid-Forming Control: A Design-Oriented Study

Abstract: Driven by the large-scale integration of distributed power resources, grid-connected voltage-source converters (VSCs) are increasingly required to operate as grid-forming units to regulate the system voltage/frequency and emulate the inertia. While various grid-forming control schemes have been reported, their transient behaviors under large-signal disturbances are still not fully explored. This article addresses this issue by presenting a design-oriented transient stability analysis of the grid-forming VSCs. First, four typical grid-forming control schemes, namely, the power-synchronization control (PSC), the basic droop control, the droop control with low-pass filters (LPFs), and the virtual synchronous generator (VSG) control, are systematically reviewed, whose dynamics are characterized by a general large-signal model. Based on this model, a comparative analysis on the transient stabilities of different control schemes is then carried out. It reveals that the PSC and the basic droop control can retain a stable operation as long as there are equilibrium points, due to their noninertial transient responses, while the droop control with LPFs and the VSG control can be destabilized even if the equilibrium points exist, due to the lack of damping on their inertial transient responses. With the phase portrait, the underlying stability mechanism is explicitly elaborated, and the quantitative impacts of the controller gains and the virtual inertia are clearly identified. Subsequently, controller design guidelines are proposed to enhance the system damping as well as the transient stability. Finally, experimental results are provided to verify the theoretical analysis.

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.

A Review of SiC Power Module Packaging Technologies: Challenges, Advances, and Emerging Issues

Abstract: Power module packaging technologies have been experiencing extensive changes as the novel silicon carbide (SiC) power devices with superior performance become commercially available. This article presents an overview of power module packaging technologies in this transition, with an emphasis on the challenges that current standard packaging face, requirements that future power module packaging needs to fulfill, and recent advances on packaging technologies. The standard power module structure, which is a widely used current practice to package SiC devices, is reviewed, and the reasons why novel packaging technologies should be developed are described in this article. The packaging challenges associated with high-speed switching, thermal management, high-temperature operation, and high-voltage isolation are explained in detail. Recent advances on technologies, which try to address the limitations of standard packaging, both in packaging elements and package structure are summarized. The trend toward novel soft-switching power converters gave rise to problems regarding package designs of unconventional module configuration. Potential applications areas, such as aerospace applications, introduce low-temperature challenges to SiC packaging. Key issues in these emerging areas are highlighted.

A Survey of EMI Research in Power Electronics Systems With Wide-Bandgap Semiconductor Devices

Abstract: Wide-bandgap (WBG) power semiconductor devices have become increasingly popular due to their superior characteristics compared to their Si counterparts. However, their fast switching speed and the ability to operate at high frequencies brought new challenges, among which the electromagnetic interference (EMI) is one of the major concerns. Many works investigated the structures of WBG power devices and their switching performance. In some cases, the conductive or radiated EMI was measured. However, the EMI-related topics, including their influence on noise sources, noise propagation paths, EMI reduction techniques, and EMC reliability issues, have not yet been systematically summarized for WBG devices. In this article, the literature on EMI research in power electronics systems with WBG devices is reviewed. Characteristics of WBG devices as EMI noise sources are reviewed. EMI propagation paths, near-field coupling, and radiated EMI are surveyed. EMI reduction techniques are categorized and reviewed. Specifically, the EMI-related reliability issues are discussed, and solutions and guidelines are presented.

Impedance Modeling and Stability Analysis of Grid-Connected DFIG-Based Wind Farm With a VSC-HVDC

Abstract: A new type of subsynchronous oscillation (SSO) has been observed recently in double-fed induction generator (DFIG)-based wind farm integrated via voltage source converter-based HVdc (VSC-HVdc) system. However, the mechanism of this emerging oscillation is not entirely understood. In this paper, the impedance models of DFIG with and without considering the phase-locked loop (PLL) dynamics are both derived. Then, the impedance-based simplified equivalent circuit of the multiple DFIGs interfaced with VSC-HVdc system is established. This model can be further represented as the RLC series resonance circuit to quantify the start-oscillating condition intuitively. The theoretical analysis results show that DFIGs behave as an inductance in series with a negative resistance at the resonance point, whose interaction with wind farm side VSC (WFVSC) (regard as a resistance–capacitance) constitutes an equivalent RLC resonance circuit with negative resistance. Therefore, the oscillation tends to occur due to the negative damping. In addition, the impact of various factors including number of grid-connected DFIG-wind turbines (WTs), wind speed, and parameters of PI controllers and PLL on the SSO characteristics is analyzed based on the proposed simplified model. Finally, the correctness of the theoretical analysis is validated by both the time-domain simulation and hardware-in-loop experiments.

Five-Level Transformerless Inverter for Single-Phase Solar Photovoltaic Applications

Abstract: Transformerless inverters are extensively employed in grid-connected photovoltaic (PV) generation systems due to its advantages of achieving low cost and high efficiency. However, the common-mode voltage issues have been motivated the proposition of new topologies, control, and modulation schemes. In common ground PV inverters, the grid neutral line is directly connected to the negative pole of the dc bus. Therefore, the parasitic capacitances are bypassed and the leakage current can be eliminated. In this paper, a five-level common ground transformerless inverter with reduced output harmonic content for PV systems is proposed. In addition, the proposed inverter can process reactive power and it presents a maximum dc-voltage utilization in opposition to half-bridge-based topologies. The operation modes of the proposed inverter, a simple modulation strategy, as well as the design guidelines are analyzed in detail. Finally, experimental results demonstrate the feasibility and good performance of the proposed inverter.

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.

A Comparative Study of Two Widely Used Grid-Forming Droop Controls on Microgrid Small-Signal Stability

Abstract: Historically, two similar grid-forming droop controls are widely reported in literature—the single-loop and multi-loop droop controls. Although being very similar, the authors find that the dynamic performance and stability characteristics of each control method are very different in a microgrid. Compared with the single-loop droop control, the multi-loop droop control is prone to be less damped and loses stability more easily under some circumstances. This article provides a novel insight into the different dynamic responses of the two basic controls. It points out that the two similar controls adjust the angular frequency and voltage magnitude at different locations within the inverter, resulting in different coupling reactances that impact the dynamic response and stability of microgrids differently. The use of the single-loop droop control results in a larger coupling reactance, which helps improve the dynamic response and stability. This novel insight is verified through full-order small-signal analysis, offline electromagnetic transient simulation, and real-time hardware-in-the-loop simulation experiments. The results show that the microgrid has a larger small-signal stability boundary when using single-loop droop control, and this difference increases as the value of an inverter’s inner filter inductance increases.

A Seven-Level Inverter With Self-Balancing and Low-Voltage Stress

Abstract: Based on the switched-capacitor (SC) principle, a seven-level inverter is proposed, which can synthesize seven levels containing a single dc source. Moreover, it can further generate more levels by a cascaded extension. Meanwhile, the proposed topology does not require any sensor due to the use of SC technology. Furthermore, the capacitor voltage is self-balanced without utilizing the complicated control strategy and additional control circuits. The phase disposition pulsewidth modulation is adopted to reduce the total harmonic distortion. The topology can generate different levels with a wide range of modulation index. In addition, the topology can also work in overmodulation. Compared with the traditional SC multilevel inverter, the absence of H-bridge makes low-voltage stress in proposed topology. The voltage stress of all switches is not more than the input voltage. Operational principles, modulation strategy, and voltage stress analysis are discussed. Simulation and experiment are conducted in low power to verify the feasibility of the proposed topology.

Comprehensive Review and Comparison of Single-Phase Grid-Tied Photovoltaic Microinverters

Abstract: The power processing and the presence of the electrical isolation between the PV module and the grid is a very crucial aspect in determining the performance requirement, as well as the utility operator’s specifications for the PV microinverter design. The grid-connected PV microinverter design can be classified into four categories: 1) nonisolated single-stage topologies; 2) isolated single-stage topologies; 3) nonisolated double-stage topologies; and 4) isolated double-stage topologies. Typically, a microinverter’s performance can be enhanced by the use of nonisolated topologies to be more efficient, more compact, less bulky, and less costly than the isolated topologies. Whereas, the use of a transformer in microinverter topologies provides high-power quality as well as galvanic isolation to eliminate the safety issues, which in return meet the grid standards. The power processing (boosting the dc voltage of PV panel, extracting the maximum power and converting it to ac power), which can be achieved either via single stage or double stage, has a significant impact on the microinverter performance. This paper reviews and compares experimentally verified microinverter topologies in terms of their corresponding efficiency, power density, reliability, and cost. The most efficient topology in each category is designed and simulated in comparison with a benchmark. The topologies are then compared in terms of their component count, input voltage range, modular structure, soft-switching implementation, and battery integration.

Critical Clearing Time Determination and Enhancement of Grid-Forming Converters Embedding Virtual Impedance as Current Limitation Algorithm

Abstract: This article deals with the postfault synchronization of a voltage source converter based on the droop control. In the case of large disturbances on the grid, the current is limited via current limitation algorithms such as the virtual impedance. During the fault, the power converter internal frequency deviates, resulting in a converter angle divergence. Thereby, the system may lose the synchronism after fault clearing and which may lead to instability. Hence, this article proposes a theoretical approach to explain the dynamic behavior of the grid-forming converter subject to a three-phase bolted fault. A literal expression of the critical clearing time is defined. Due to the precise analysis of the phenomenon, a simple algorithm can be derived to enhance the transient stability. It is based on adaptive gain included in the droop control. These objectives have been achieved with no external information and without switching from one control to the other. To prove the effectiveness of the developed control, experimental test cases have been performed in different faulted conditions.

Novel K-Type Multilevel Inverter With Reduced Components and Self-Balance

Abstract: This article proposed a single-source multilevel inverter (MLI) based on the novel K-type unit (KTU). The 13-level topology with two KTUs and 1.5 voltage gain is first introduced, and its operation modes at different output levels are described in this article. The two capacitors in series connection with each KTU can get self-voltage balance due to their symmetric operation in a cycle, reducing the complexity of control compared with conventional MLIs. The analysis of self-balance and capacitance calculation are given in detail. Afterward, the single-source generalized structure equipped with more KTUs is presented for increasing output levels. The output levels are significantly increased with additional KTUs, and the voltage gain rises as well. Moreover, through the comparative study against other MLI topologies proposed in recent years, the advantages of the proposed KTU topology are indicated in the aspects for reduced components, self-balance, voltage stress, and overall cost. Finally, the 13-level simulation and 1-kVA experimental prototype with fundamental frequency modulation (FFM) are implemented to verify the feasibility and transient performance of the proposed topology.

Support Vector Machine-Based Islanding and Grid Fault Detection in Active Distribution Networks

Abstract: Many techniques used and still in usage for solving the problem of islanding detection are intrinsically passive, active, or hybrid of both. Each one of them has its own benefits and drawbacks. In this paper, we propose a method, which takes the advantage of a machine learning (ML)-based algorithm, namely, support vector machine (SVM), in order to produce the results more efficiently. The results of the simulations based on the model and experimentally measured parameters of a real-life practical photovoltaic (PV) plant give much better output than the traditional reported methods. During the tests and simulations, an additional problem, namely, grid fault, emerged, posing new challenges for the proposed method. Occurrences of islanding and grid fault are grouped together with the same kernel dimension and no custom hyperplane bordering. Discrimination between islanding and grid fault events is an essential dilemma, which is handled by the proposed SVM-based algorithm to achieve more precision in islanding detection and simultaneously detect the grid faults authentically. Nondetection zones (NDZs) and detection time (DT) are tested using two dimensions, namely, the generated active energy from PV plant (0%–110% of $P_{n}$ ) and distribution network voltage levels (±10% of $U_{n}$ ). Simulations based on the model and parameters of a real-life practical PV power plant are performed in MATLAB/Simulink environment, and several tests are executed for several scenarios. Finally, comparisons with previously reported techniques prove the effectiveness, authenticity, selectivity, accuracy, and precision of the proposed islanding and grid fault detection strategy with allowable impact on power quality according to UL1741 and its superiority over other methods.

A Mode-Adaptive Power-Angle Control Method for Transient Stability Enhancement of Virtual Synchronous Generators

Abstract: The virtual synchronous generator (VSG) is emerging as an attractive solution for interconnecting converter-based resources with the power grid. However, due to the nonlinear power-angle relationship, VSGs are, similar to synchronous generators, prone to the loss of synchronization (LOS) under large grid disturbances. This article, thus, analyzes the large-signal synchronization stability, i.e., the transient stability of grid-connected VSGs, and then proposes a mode-adaptive power-angle control method for enhancing the transient stability. In this approach, the positive-feedback mode of the power-angle control of the VSG is detected and adaptively switched to the negative-feedback mode after large disturbances. Thus, the LOS of the VSG can be avoided when there are equilibrium points after the disturbance. Moreover, during severe grid faults without any equilibrium points, a bounded dynamic response of the power angle can be obtained with the mode-adaptive control, and the VSG can still be stabilized even if the fault-clearing time is beyond the critical clearing time. These superior features prevent the VSG-based system from collapsing due to the delayed fault clearance or the malfunction of protective relays. Finally, the time-domain simulations and experimental tests are performed to verify the effectiveness of the control method.

Review of Packaging Schemes for Power Module

Abstract: SiC devices are promising for outperforming Si counterparts in high-frequency applications due to its superior material properties. Conventional wirebonded packaging scheme has been one of the most preferred package structures for power modules. However, the technique limits the performance of a SiC power module due to parasitic inductance and heat dissipation issues that are inherent with aluminum wires. In this article, low parasitic inductance and high-efficient cooling interconnection techniques for Si power modules, which are the foundation of packaging methods of SiC ones, are reviewed first. Then, attempts on developing packaging techniques for SiC power modules are thoroughly overviewed. Finally, scientific challenges in the packaging of SiC power module are summarized.

A Hybrid Cascaded DC–DC Boost Converter With Ripple Reduction and Large Conversion Ratio

Abstract: This paper presents a hybrid cascaded boost converter, in which the input terminal is interlaced in parallel and the output capacitors embedded in voltage multiplier cells are interlaced in series at the output terminal [input parallel output series boost converter (IPOSB)]. The IPOSB can reduce the input current ripples because two primary windings of coupled inductors are connected in parallel with the cross. The voltage multiplier units combine with diode–capacitor and coupled inductor in the output side are charged and discharged in a series and parallel way. In addition, the leakage inductance of the coupled inductor inhibits the inrush current of the capacitors. Therefore, the IPOSB can attain high gain and lower output voltage ripples under a proper duty cycle, and the leakage energy of coupled inductor can also be recycled to the load. At the same time, the voltage stress of power devices is lowered, so the low voltage level MOSFETs can be adopted to reduce the losses and cost. Meanwhile, the soft switching performance of the zero-current-switching is fulfilled, which reduces effectively switching losses. The operational principle and steady-state performance of the converter are analyzed in detail. The correctness of the theoretical analysis is verified by setting up a 450-W experimental prototype.

A Novel Predictive Fuzzy Logic-Based Energy Management System for Grid-Connected and Off-Grid Operation of Residential Smart Microgrids

Abstract: In this paper, a novel energy management system (EMS) with two operating horizons is proposed for a residential microgrid application. The microgrid utilizes the energies of a photovoltaic, a fuel cell (FC), and a battery bank to supply the local loads through a combination of electric and magnetic buses. The proposed microgrid operates in a large number of grid-connected and off-grid operation modes. The EMS includes a long-term data prediction unit based on a 2-D dynamic programing and a short-term fuzzy controller. The long-term prediction unit is designed to determine the appropriate variation range of the battery state of charge and FC state of hydrogen. The efficiency performance of the microgrid components, predicted energy generation and demand, energy cost, and the system constraints are taken into account. The resultant data then are sent to the short-term fuzzy controller which determines the operation mode of the microgrid based on the real-time condition of the microgrid elements. A prototype of the proposed microgrid including the EMS is developed, and experimental tests are conducted for three different energy management scenarios. The proposed management technique is validated through energy distribution and cost analysis.

Impedance-Based Analysis of Interconnected Power Electronics Systems: Impedance Network Modeling and Comparative Studies of Stability Criteria

Abstract: Impedance is an intuitive and effective way for dynamical representation of power electronics devices [e.g., voltage source converters (VSCs)]. One of its strengths toward others is the natural association with circuits. However, impedances of VSCs are locally evaluated via linearization, a process dependent on the angle of the reference frame; thus, the reference frame transformation (i.e., rotation) is required before connecting them in circuits for the purpose of network analysis. Although this issue was properly treated in the state-space modeling, a counterpart for the impedance-based analysis, particularly the stability impacts of this rotation, has not been thoroughly discussed and worth being clarified. On the other hand, there are fundamental differences in applying the impedance-based stability criteria of a single-VSC system to an interconnected one. Several restrictions as revealed (e.g., sensitivity to partition points of the Nyquist-based analysis), if not properly considered, may lead to inaccurate stability assessments. In this respect, a clarification of three commonly employed impedance-based stability criteria is achieved. At last, the capability of the Nyquist-based analysis in identifying the system’s weak point and in facilitating better network design and planning is presented. All the models and analyses are verified by frequency-scanning and time-domain simulations in PSCAD/EMTDC.

Harmonic Stability Analysis of MMC-Based DC System Using DC Impedance Model

Abstract: The dc impedance model of a modular multilevel converter (MMC) is the basis for analyzing harmonic resonances of MMC-based dc systems. As an MMC typifies a multiple harmonic response system, its internal dynamics and controls significantly influence its external characteristics. In this paper, a dc impedance model of an MMC is developed by harmonic transfer function method that considers the internal dynamics and typical controls of MMCs. The internal dynamics mainly include capacitor voltage fluctuation and multi-harmonic response characteristics, while typical controls consist of dc voltage control, positive–negative sequence separation-based phase current control, circulating current control, and some other linear controls. As a result, the proposed impedance model can be used not only to analyze the harmonic stability of an MMC-based dc system, but also to investigate the influence of additional controls in an MMC on system stability. Furthermore, the proposed model makes up for the deficiencies in harmonic stability analysis of MMC-based dc systems. The results of both the hardware-in-the-loop RT-LAB digital simulation and the physical experimentation validate the proposed impedance models and analyses.

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.

Low-Loss Integrated Inductor and Transformer Structure and Application in Regulated LLC Converter for 48-V Bus Converter

Abstract: In this article, an LLC converter using gallium nitride (GaN) transistors is proposed for a 48-V regulated and isolated bus converter. Compared with pulsewidth modulation (PWM)-based topologies, the soft switching capability of an LLC allows operation at very high frequencies. In addition, the size of the magnetic components is reduced without sacrificing the efficiency. In this article, a novel magnetic structure that integrates a matrix transformer and inductor with minimum winding and a single magnetic core is proposed, to allow a high-density and high-efficiency LLC converter design for a bus converter. A 40—60-V input and regulated 12-V output converter is developed to deliver a 1-kW output power in a quarter brick form factor. The designed converter can achieve a power density of 700 W/in3 with a maximum efficiency of 97.82% at half-load, dropping to 97.7% at full-load operation.

Current Reference Generation Based on Next-Generation Grid Code Requirements of Grid-Tied Converters During Asymmetrical Faults

Abstract: Increased penetration of converter-based power generation has enforced system operators to require ancillary services from distributed generation in order to support the grid and improve the power system stability and reliability. Recent and next-generation grid codes require asymmetrical current provision during unbalanced faults for optimal voltage support. To address this, based on the highly used flexible positive- and negative-sequence control method for current reference generation, this paper presents a general current reference strategy for asymmetrical fault control, where a direct and explicit method is proposed to calculate power references and controller gains while simultaneously complying with converter current limitation and fulfilling the next-generation grid code requirements. The proposed method is tested for three distinct asymmetrical grid faults considering the requirements for dynamic voltage support of the recently revised German grid code as well as the next-generation grid codes. It is shown that the proposed method can improve the fault ride-through performance during asymmetrical faults compared with conventional solutions and comply with modern grid code requirements in a general and flexible manner.

Impedance-Based Stability Analysis of Voltage-Controlled MMCs Feeding Linear AC Systems

Abstract: This paper addresses the small-signal stability of voltage-controlled modular multilevel converters (MMCs) feeding linear ac systems. By using the harmonic state-space (HSS) modeling method, the ac-side impedance matrices (IMs) of the MMC with the open-loop and closed-loop voltage control are derived and their relationship is also explicitly given. It is revealed that the ac voltage regulator has the same effect on the centered diagonal element of the IM of the MMC as that of two-level voltage-source converters (VSCs). Moreover, when the MMC is feeding linear ac loads, the return-ratio matrix of the cascaded system has an eigenvalue that is equal to the ratio between the centered diagonal element of the IM of the MMC and the load impedance, which, consequently, facilitates the stability evaluation by checking that single-input single-output impedance ratio as a necessary condition. These findings also provide physical insights into the subsynchronous oscillation (SSO) of the voltage-controlled MMC with the proportional-resonant (PR) regulator, and a proportional-integral-resonant (PIR) regulator is further introduced to mitigate the SSO. Finally, time-domain simulations verify the effectiveness of the theoretical analysis.

Detection and Identification of Global Maximum Power Point Operation in Solar PV Applications Using a Hybrid ELPSO-P&O Tracking Technique

Abstract: Nonhomogeneous irradiation conditions due to environmental changes introduce multiple peaks in nonlinear $P$ – $V$ characteristics. Hence, to operate photovoltaic at the global power point, numerous algorithms have been proposed in the literature. However, due to the insufficient exploitation of control variables, all the maximum power point tracking (MPPT) methods presented in the literature fail to guarantee global maximum power point (GMPP) operation. In this paper, a new detection technology to identify GMPP zones using hybrid enhanced leader particle swarm optimization (ELPSO) assisted by a conventional perturb and observe (P&O) algorithm is proposed. With inherent mutations, ELPSO applied to MPPT excels in exploring global regions at initial stages to determine the global best leader, whereas P&O is reverted back soon after global solution space is detected. The transition from ELPSO to P&O is mathematically verified and allowed only when ELPSO finds the global optimal zone. Adapting this hybrid strategy, the proposed method has produced interesting results under partial shaded conditions. For further validation, the results of the proposed hybrid ELPSO-P&O are compared with the conventional ELPSO and the hybrid PSO-P&O methods. The experimental results along with energy evaluations confirmed the superiority of the ELPSO-P&O method in obtaining the maximum available power under all shaded conditions.

The Future 5G Network-Based Secondary Load Frequency Control in Shipboard Microgrids

Abstract: This paper presents the applicability of the future fifth-generation network technology for a marine vessel power system with sea wave energy, photovoltaic, and energy storage systems. In this paper, a new optimal structured interval type-2 fractional order fuzzy proportional derivative/fuzzy proportional integral controller is proposed for the secondary load frequency control (LFC) of a networked shipboard multimicrogrid. The effect of the various degradation factors associated with the communication infrastructure such as the time delay and packet loss is modeled and addressed to assess the system performance in the networked control system (NCS) operation. The parameters embedded in the established structure are decisive factors, which significantly affect the quality of control output actions. Accordingly, by employing the concepts of the black-hole optimization algorithm and Levy flight, an enhanced JAYA algorithm is proposed to adjust the setting of the established structured controller. Finally, comprehensive studies and hardware-in-the-loop real-time simulations are conducted to appraise the acceptability of the suggested controller for a secondary LFC problem in the face of the uncertain NCS.

Photovoltaic Synchronous Generator: Architecture and Control Strategy for a Grid-Forming PV Energy System

Abstract: Transforming a conventional photovoltaic (PV) energy system from a grid-following to a grid-forming system is necessary when PV power generation is dominating the generation mix and for replacing traditional synchronous generators (SGs). The grid-forming PV energy system can provide frequency support functionality, which is vital for the stability of the power grid. This article presents a novel ac coupled solution that transforms an existing grid-following PV system to a grid-forming one without any hardware and software modification of the PV inverter. The resulting system, the PVSG, is achieved by an ac coupled supercapacitor-based energy storage system (ESS). The novel control of the PVSG is implemented in the ESS side. The novel control scheme includes fast and slow instantaneous power controls. The fast-instantaneous power flow control is fulfilled by the dc-link voltage control and ac voltage control. The cascaded voltage source controls enable fast-instantaneous power balance, while a slow instantaneous power control is used to implement inertia and grid synchronization. Two important fundamental functions are realized in the PVSG. The first one is the frequency inertia to resist the grid frequency variation where df/dt -based power control is achieved; the second one is the inertia against the PV intermittent power. The correctness and the effectiveness of the proposed PVSG are experimentally validated in a 480 V PVSG prototype with a TMS320F28379D DSP controller.

Design of a High-Efficiency, High Specific-Power Three-Level T-Type Power Electronics Building Block for Aircraft Electric-Propulsion Drives

Abstract: The electric propulsion drives for the more-electric aircraft need lightweight and high-efficiency power converters. Moreover, a modular approach to the construction of the drive ensures reduced costs, reliability, and ease of maintenance. In this article, the design and fabrication procedure of a modular dc–ac three-level t-type single phase-leg power electronics building block (PEBB) rated for 100-kW, 1-kV dc-link is reported for the first time. A hybrid switch (HyS) consisting of a silicon insulated-gate bipolar junction transistor (IGBT) and silicon carbide metal–oxide–semiconductor field-effect transistor (MOSFET) was used as an active device to enable high switching frequencies at high power. The topology and semiconductor selection were based on a model-based design tool for achieving high conversion efficiency and lightweight. Due to the unavailability of commercial three-level t-type power modules, a printed circuit board (PCB) and off-the-shelf discrete semiconductor-based high-power switch was designed for the neutral-point clamping. Also, a nontrivial aluminum-based multilayer laminated bus bar was designed to facilitate the low-inductance interconnection of the selected active devices and the capacitor bank. The measured inductance indicated symmetry of both current commutation loops in the bus bar and value in the range of 28–29 nH. The specific power and volumetric power density of the block were estimated to be 27.7 kW/kg and 308.61 W/in3, respectively. The continuous operation of the block was demonstrated at 48 kVA. The efficiency of the block was measured to be 98.2%.

High Step-Up Interleaved dc–dc Converter With Asymmetric Voltage Multiplier Cell and Coupled Inductor

Abstract: In this article, a novel high step-up interleaved dc–dc converter is presented, which is composed of an interleaved structure, an asymmetric voltage multiplier cell (AVMC), and a passive lossless clamped circuit. Compared to the classical interleaved boost converter, the proposed converter has a higher voltage gain owing to the employment of the AVMC and the coupled inductor. In addition, the input current ripple is limited to low values with the help of the interleaved structure, which gives more lifetime to the input power source. The voltage stresses of main switches are substantially low so that the MOSFETs with low voltage rate and ON-resistance ( $R_{\textrm {DS}(\mathrm{ON})} $ ) can be used. In addition, the voltage stresses and the reverse recovery problems of diodes are improved dramatically. Moreover, the zero current switching (ZCS) turn-on of switches and the ZCS turn-off of the clamp diodes are realized to reduce the switching losses. The leakage inductor energy is recycled and the voltage spikes are improved greatly by the passive lossless clamped circuit so that the efficiency can be upgraded further. Finally, a 400-W, 40-V-input, 400-V-output prototype is established to demonstrate the performance of this converter. The highest efficiency is about 97.3% and the full-load efficiency is approximately 97%.