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Volt-VAR and Volt-Watt functionality in photovoltaic (PV) smart inverters provide mechanisms to ensure system voltage magnitudes and power factors remain within acceptable limits. However, these control functions can become unstable, introducing oscillations in system voltages when not appropriately configured or maliciously altered during a cyberattack. In the event that Volt-VAR and Volt-Watt control functions in a portion of PV smart inverters in a distribution grid are unstable, the proposed adaptation scheme utilizes the remaining and stably-behaving PV smart inverters and other Distributed Energy Resources to mitigate the effect of the instability. The adaptation mechanism is entirely decentralized, model-free, communication-free, and requires virtually no external configuration. We provide a derivation of the adaptive control approach and validate the algorithm in experiments on the IEEE 37 and 8500 node test feeders.

Organic electrochemical transistors (OECTs) based on Poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) are a benchmark system in organic bioelectronics. In particular, the superior mechanical properties and the ionic‐electronic transduction yield excellent potential for the field of implantable or wearable sensing technology. However, depletion‐mode operation PEDOT:PSS‐based OECTs cause high static power dissipation in electronic circuits, limiting their application in electronic systems. Hence, having control over the threshold voltage is of utmost technological importance. Here, PEDOT:PSS‐based dual‐gate OECTs with solid‐state electrolyte where the threshold voltage is seamlessly adjustable during operation are demonstrated. It is shown that the degree of threshold voltage tuning linearly depends on the gate capacitance, which is a straightforward approach for circuit designers to adjust the threshold voltage only by the device dimensions. The PEDOT:PSS‐based dual‐gate OECTs show excellent device performance and can be pushed to accumulation‐mode operation, resulting in a simplified and relaxed design of complementary inverters. [ABSTRACT FROM AUTHOR]

A direct speed control strategy based on finite control set model predictive control (FCS–MPC) with a voltage smoother is presented herein. In the proposed concept, a finite set of smoothed voltage vectors characterized by an adjustable amplitude and a movable origin is introduced as voltage candidates in the FCS–MPC scheme. The controller predicts the future current and speed and outputs the optimal smoothed voltage using pulse-width modulation. Owing to this control scheme, an abrupt change in the output voltage, which causes a large current ripple, is avoided without additional computational costs. Simulation and experimental results obtained using a permanent magnet synchronous motor fed by a two-level three-phase inverter show that the proposed method effectively reduces the current ripple while achieving a high dynamic drive compared with the conventional FCS–MPC scheme. [ABSTRACT FROM AUTHOR]

In islanded ac microgrids, the mismatched impedances of the interfacing feeders between the inverters and the load bus cause poor reactive power sharing when the conventional frequency and voltage droop control technique is employed. Such operation endangers the whole microgrid reliability as it may lead to overloading certain inverters and, consequently, triggering protection relays and causing cascaded failure. Thus, this article proposes an accurate reactive power sharing strategy that considers the mismatched feeder impedances in islanded ac microgrids. It is based on the optimal tuning of the virtual complex impedance for each inverter. The proposed strategy has several advantages. First, it has a physical meaning as it establishes an explicit relationship between the mismatched values of the actual resistive–inductive feeders and the assigned values for the proposed virtual complex impedance to each inverter. Hence, further degradations in the microgrid voltages are prevented. Second, there is no need for prior knowledge about the actual feeder impedances in the design stage as they are estimated online from the available measurements. Finally, the proposed control is reliable and fault-tolerant as it copes with unexpected failures such as failure of (or sudden switching off) some inverters and communication disruptions/delays. It ensures accurate power sharing even under the primary controllers after losing communication links with the secondary controller. The simulation and experimental verification results are presented to validate the performance of the proposed technique. [ABSTRACT FROM AUTHOR]

Multilevel inverters (MLIs) featuring reactive power ability and common ground (CG) have become much popular in transformerless grid-tied photovoltaic applications. A switched-capacitor (SC) based five-level transformerless inverter configuration is thus proposed in this article, which requires only six switches, one diode, three capacitors, and one input dc voltage supply. Notably, only four power switches are operated in high frequency, thus, the switching losses are reduced. Moreover, the presented topology can effectively tackle the leakage current problem using a CG architecture. A multicarrier phase disposition pulsewidth modulation strategy is employed to achieve reactive power regulation and SC voltage self-balancing. To achieve the neutral point voltage balance of the presented five-level inverter, a simple closed-loop voltage-balancing control method is proposed. The operation principles with modulation strategy and voltage self-balancing of SC are discussed in depth. Comparisons between the proposed and the state-of-the-art MLIs are presented in detail. Finally, experimental tests on a single-phase 1.1-kW prototype validate the appropriate performance of the proposed inverter. [ABSTRACT FROM AUTHOR]

This article presents a nonlinear passivity-based control for a grid-connected cascaded H-bridge (CHB) photovoltaic (PV) system. The proposed control system is based on a port-controlled Hamiltonian model extracted from the synchronous reference frame system. Grid current dynamics is included in this model. Compared to existing solutions, the proposed control method has a robust performance and enhances the stability of the CHB-based PV system in the case of disturbances such as unequal irradiance over distinct PV arrays or a sudden sag or swell in the grid voltage. This controller also provides the independent control of each dc-link voltage. Therefore, the individual maximum power point tracking algorithms can be realized in each PV array, and the harvested energy can be maximized. Finally, simulation results and experimental investigations are conducted to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

The switched-capacitor of boost seven-level active neutral-point-clamped inverter has the large voltage ripple due to its inherent long discharging interval. Aiming at reducing the switched-capacitor voltage ripple and subsequently the capacitance, this article proposes a flexible third harmonic voltage modulation method. In specific, the long discharging interval can be divided into short intervals or even avoided by flexibly regulating the amplitude and phase-angle of third harmonic voltage based on various modulation indexes and power factors. An offline particle swarm optimization algorithm is employed to find the corresponding optimal amplitude and phase-angle of third harmonic voltage, which will further be put into the online look-up table to simplify the real time implementation. Experimental results verified the performance of the proposed modulation method. [ABSTRACT FROM AUTHOR]

Space vector pulsewidth modulation (SVPWM) algorithms for cascaded H-bridge multilevel (CHB ML) inverter usually provide the possibility of using several combinations of active voltage vectors to generate the same output voltage vector. For preselected H-bridges, some of them may generate output voltages opposite to the assumed direction. This results in the change of the dc-link voltages of these H-bridges in the opposite direction to the assumed direction in the ordering algorithm. Consequently, these algorithms are characterized by undue constraints and narrow possibilities of dc-link voltage balancing. In the proposed control algorithm, CHB ML inverter is treated as groups of successively activated three-level inverters; depending on the length of the reference voltage vector. These three-level inverters consist of three H-bridges selected from each phase. The proposed extended selection method enables firm-grip control of the dc-link voltages. For a given direction of phase currents, the possibility of using H-bridges with lowest and highest dc-link voltages is simultaneously analyzed. Additionally, each of the three-level inverters is controlled by one of three proposed alternative modulation methods for which both the attainable output voltage vectors and unbalanced dc-link voltages are predicted. Simulation and experimental results confirm the correctness of the algorithm execution. [ABSTRACT FROM AUTHOR]

This article investigates a decoupled vector current control strategy for grid-tied single-phase nine-level Packed E-Cell (PEC) inverters for vehicle-to-grid (V2G) technologies. The control design is based on a loop shaping approach, where performance specifications such as desired closed-loop bandwidth, closed-loop stability, robustness to pure time-delays in control loops/process, and decoupling between direct ($d$) and quadrature ($q$) axis of control signals are specified in the frequency domain. The control design problem is formulated as a convex optimization problem whose solution results in a multivariable controller in the $dq$ framework. The effectiveness of the proposed control strategy in terms of robust stability, current tracking, and improved total harmonic distortion is evaluated by experimental and simulation results under several case studies. The results also show that the designed controller provides the desired transient response as well as decoupling between the $dq$ output currents. [ABSTRACT FROM AUTHOR]

This article reveals that the saturation block (SB) used with the power-angle control loop can enhance the transient stability of grid-forming (GFM) inverters. Three methods of using the SB in the power-angle loop are examined based on power-angle curves. It is found that all can enhance the transient stability of GFM inverters by either decreasing the acceleration area or increasing the deceleration area. At the same time, the threshold value selection of SB in the three schemes is also explained, considering the tradeoff between the transient stability and the response speed. A holistic comparison of the three schemes recommends the scheme with the SB used for power error adopted by the GFM inverters. Finally, experimental tests confirm the effectiveness and correctness of the analysis. [ABSTRACT FROM AUTHOR]

The performance of flux-weakening controllers in the multiphase permanent magnets (PM) machines depends on the accurate fundamental voltage limit derivation, and the minimal copper loss design in the flux-weakening operation region has not been well discussed. In this article, the accurate fundamental voltage limit with considering harmonic current suppression is derived at first. Then, two control concepts are addressed, and the corresponding flux-weakening control strategies are designed with the gradient descent method. In the first control strategy, both the fundamental and harmonic voltage vectors are feasible, resulting in the larger amplitude of the phase currents. The second control strategy obtains higher bus voltage utilization by an additional transition stage. At the same time, the harmonic currents cannot be further suppressed in the flux-weakening operation region. Third, the copper loss per electrical cycle is calculated, and a switching scheme is designed to obtain lower copper loss in the flux-weakening operation region. Finally, both strategies are successfully implemented in a dual three-phase PMSM. Moreover, the proposed switching scheme obtains minimal copper loss in the whole flux-weakening operation region. [ABSTRACT FROM AUTHOR]

Grid-forming inverters (GFMIs) are recognized as a prominent driver toward achieving renewable energy-rich power grids. Unlike grid-following inverters (GFLIs), which are controlled as current sources, GFMIs are controlled as voltage sources. In GFMIs, dynamic control of the magnitude ($V_{\mathrm{c}}$) and angle ($\theta$) of the point of common coupling (PCC) voltage is used to achieve active ($P$) and reactive ($Q$) power transfer across a line. However, independent control of $P$ and $Q$ via $V_{\mathrm{c}}$ and $\theta$ becomes challenging due to the coupling between $P$ and $Q$ loops. The coupling becomes severe as the resistance-to-reactance ratio of the grid impedance and the power angle between the GFMI and the grid voltages are increased. This article proposes a novel multivariable controller to completely decouple $P$ and $Q$ loops in GFMIs. The proposed multivariable controller could be designed based on the prevalent control structures for GFMIs such as droop controller, swing equation-based virtual synchronous generator (VSG) controller, zero steady-state error reactive power controller, and fixed steady-state error reactive power controller. The additional cross-channel decoupling controllers in the proposed multivariable controller provide superior decoupling action over the existing decoupling methods, such as the virtual inductor-based method. An $\mathcal {H}_\infty$ -based method is adopted to tune the proposed multivariable controller parameters, where the straightforward formulation of the desired closed-loop dynamics based on the open-loop system is clearly shown. The decoupling performance of the controller is experimentally validated extensively. The experimental results show that the proposed controller results in superior performance over the prevalent decoupling methods, such as the virtual-inductor decoupling method. [ABSTRACT FROM AUTHOR]

Electroporation is a phenomenon that increases the permeability of cells by applying an electric field. This technique has relevant applications to cancer treatment using both electrochemotherapy and tumor ablation using irreversible electroporation. In order to carry out these treatments, new power converters are needed to apply of output voltages of several kV and output currents of tens of A in pulses of a width of μs. In addition, to achieve safe and relapse-free treatments, it is necessary to generate a controlled and uniform electric field. A challenge in current treatments is to ensure effective treatments despite tissue heterogeneity. In this context, this article proposes a new pulse generator that allows the use of a multielectrode structure that enables the application of a uniform electric field in multiple directions to avoid issues related to tissue heterogeneity. The proposed generator also allows the continuous monitoring of the treatment to adequately control the generator and apply a safer and more effective treatment. An 18-output 1500-V experimental prototype was designed, implemented, and tested using vegetal tissue, proving the feasibility of this proposal. [ABSTRACT FROM AUTHOR]

The power converters currently used in high-power (a few megawatts) medium-voltage PV systems require the use of a line-frequency transformer (LFT), which is bulky and costly. To solve this issue, cascaded H-bridge converter and modular multilevel converter based converters were proposed and investigated in the literature. They can eliminate the LFT but suffer several technical challenges, one of which, for example, is a power imbalance. To address these challenges, extra efforts, such as modified converters, modulations, and controls, are needed. Such extras burden the original converters and introduce new challenges. Therefore, a new power converter is proposed in this work. It eliminates the LFT without these challenges faced by the existing solutions. In addition, it features reliable short-circuit protection, high scalability, simple, and well-proven converters. The operation principle is introduced, and the control scheme is developed. The performance is investigated and verified based on both simulations and lab-scaled experiments. [ABSTRACT FROM AUTHOR]

The traditional direct torque control (DTC) of two-level voltage source inverter-fed motor system can achieve evident reduction of common-mode voltage (CMV) if zero vectors are not used, as the amplitude of the CMV of the zero vector is much larger than that of the active vector. However, due to the influence of dead-time effect, zero vectors can still be generated during inverter commutation, resulting in CMV spikes. To eliminate the CMV spikes caused by dead-time effect, a novel DTC strategy is proposed in this article. Through the analysis of the voltage vector during the dead-time of the commutation process, the process of looking up the DTC switching table is modified, and a transient vector is inserted when necessary, so that no zero vector is generated during commutation. The proposed DTC strategy eliminates CMV spikes in a look-up table way, so that the simple control structure of traditional DTC is maintained, while neither modification of hardware circuit nor detection of current polarity are required. Experiments verified the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Model predictive flux control (MPFC) is an alternative control solution to the classic model predictive torque control (MPTC) for driving induction motors (IMs) without the complexity of tuning the weighting factors. However, when applied in motor drives fed by a two-level converter, it results in higher flux and torque ripples. To improve its performance, this article proposes a virtual three-level MPFC (V3-MPFC) method. The virtual vector method increases the total number of voltage vectors crucial for improving the closed-loop performances of the model predictive control. To reduce the computational burden, direct torque control and novel deadbeat (DB) control strategies are proposed. Compared with existing methods, the proposed DB-based method reduces V3-MPFC to a suboptimization problem and still ensures that the suboptimal solution is equivalent to the global optimal solution provided by V3-MPFC. In addition, the redundant states are exploited to lower the switching frequency (SF) without extending the cost function. The simulation and experiment results show that, compared with existing methods, the proposed strategy effectively reduces the flux and torque ripples, and requires a lower SF and execution time. [ABSTRACT FROM AUTHOR]

This article investigates the impacts of different current limiting strategies on the transient stability of the virtual synchronous generator (VSG). The power–angle curve of the VSG under different operating conditions is theoretically characterized in detail and experimentally verified through tests conducted on hardware-implemented lab-scale VSGs. It is shown that the reference current saturation approaches prioritizing the current vector angle, the d-axis current, and the q-axis current reshape the VSG power–angle curve in different ways. The resulting impacts on the VSG transient stability are comprehensively investigated through time-domain simulation of a battery energy storage system that is operated as a VSG and connected to a medium-voltage Canadian distribution feeder. The transient stability margin of the VSG is evaluated by determining the critical clearing time (CCT) of various fault scenarios. The studies conducted in the PSCAD/EMTDC software environment utilizing a detailed switching model of the VSG indicate that 1) the current limit of the VSG significantly impacts its power output during and after faults; and 2) the q-axis priority current limiting strategy provides a larger transient stability margin (CCT) as compared with the current vector angle and d-axis current priority approaches. [ABSTRACT FROM AUTHOR]

This article presents an improved virtual space vector modulation scheme for the reduced switch count three-level inverter with balanced and unbalanced neutral-point voltage (NPV) conditions. By analyzing the effects of unbalanced NPV conditions on basic voltage vectors, the new virtual vectors are constructed by considering the dc unbalancing coefficient and distribution factor for small vectors simultaneously. To facilitate the calculation of duty cycles, the nonorthogonal coordinate system is employed. In addition, an optimized control scheme is developed to obtain the optimized distribution factor, and the relative durations of P-type and N-type small vectors are adjusted to control the voltages across two dc-link capacitors separately. The satisfactory quality of output currents is maintained with both balanced and unbalanced NPV conditions. The feasibility of the proposed scheme is verified by simulated and experimental results. [ABSTRACT FROM AUTHOR]

The penetration of distributed energy resources (DERs) in smart grids significantly increases the number of field devices owned and controlled by consumers, aggregators, third parties, and utilities. As the interface between DER and power grids, DER inverters are becoming smarter with various grid-support functions and communication capabilities. Meanwhile, the cybersecurity risks of smart inverters are also on the rise due to the extensive utilization of information and communication technologies. The potential negative impacts of cyberattacks on smart inverters have attracted significant attention from scholars and organizations. To advance the research on smart inverter cybersecurity and provide insights into its technical achievements, barriers, and future directions, this article will give a comprehensive review of critical attacks and defense strategies for smart inverters and inverter-based systems like microgrids. We start this survey with an overview of the smart inverter introduction, including device- and grid-level architectures, grid-support functions, and communication protocols. We then review various cyberattacks and defense strategies in different categories and scenarios tailed with discussions including their feasibility and remaining gaps. Finally, we discuss the opportunities and challenges of emerging technologies that can secure smart inverters. We hope this survey can inspire efforts to close research gaps and develop more mature cybersecurity solutions for smart inverters in the smart grid. [ABSTRACT FROM AUTHOR]

Impedance-based approachis generally studied for stability analysis of the power electronic converter-based system. Considering multiple-frequency coupling effects, in this article, we establish the harmonic transfer function (HTF) based αβ-frame impedance models of droop inverters based islanded microgrid with unbalanced loads. Therein, the HTF-based αβ-impedance modeling methods of unbalanced loads, as well as droop inverters in parallel, are proposed. Afterward, the HTF-based αβ-frame impedance models are equivalent to single-input single-output (SISO) impedances for simpler impedance measurement and stability analysis, preserving the αβ-frame coupling and multiple-frequency coupling characteristics. Finally, the experimental measurement results verify the accuracy of the αβ-frame SISO impedances. Besides, the stability analysis based on the SISO impedance models accurately reveals low-frequency oscillations of the islanded microgrid with unbalanced loads, further verifying the effectiveness of the SISO impedance models. [ABSTRACT FROM AUTHOR]

This letter applies a multiagent fuzzy Q-learning (FQL) algorithm, incorporated with a model-free nonlinear controller (MFNC), entitled FQL-MFNC for stabilized controlling of a recently introduced grid-connected nine-level Packed E-Cell (PEC9) inverter under dynamical operation. Unlike previous tuning schemes, which concentrate on extracting mathematical formulation of a controlled plant, this letter investigates a fuzzy Q-learning agent for optimal design of PEC9. In first step, the fuzzy reinforcement learning is adopted to tune the MFNC controller in the simulation environment. In fact, the FQL algorithm finds the optimal policy based on a reward function for adjustment of the MFNC control coefficients to guarantee the grid-connectivity requirements under PEC9 dynamical operation are met. The experimental tests are conducted to assure efficiency and practicability of the designed multi-agent FQL-MFNC scheme on the single-phase grid-connected PEC9 inverter. [ABSTRACT FROM AUTHOR]

Cascaded split-source inverter (CSSI) is a single-stage modular multilevel structure. This article proposes the multidimensional pulsewidth modulation (MD-PWM) for this topology. All multilevel modulations are a particular version of this approach. The principles of this geometrical method are based on selecting the optimal points and determining the switching sequences and time intervals. By considering the operational principles of CSSI, this method is modified to accurately control the voltage of dc links, charge inductors with constant duty cycles, control dc and ac sides of cells independently, and reduce the reverse recovery effects of input diodes. MD-PWM, with a low computational burden, can be implemented for CSSI with any number of voltage levels. The output power can be distributed equally or unequally among cells, and both buck and boost operation modes are realized for each cell. Also, this method is extended for the operation of the converter with unequal input sources. For an asymmetric topology with N cells, harmonics are located at multiples of N times switching frequency. Thus, this strategy eliminates low-frequency distortions from the output voltage of asymmetric CSSI. The simulation and experimental results verify the performance and accuracy of the proposed technique and mathematical relations. [ABSTRACT FROM AUTHOR]

This article proposes a three-phase five-level switched-capacitor active neutral point clamped converter, which can be characterized by voltage boost ability, low-voltage stress across semiconductor devices, and balance of floating capacitor voltage without any auxiliary circuits and/or sensors. Operating principles and modulation scheme of the proposed converter, as well as a comparative analysis with similar topologies are included. Simulation and experimental results for a three-phase prototype are given to demonstrate the feasibility of the proposed five-level converter. [ABSTRACT FROM AUTHOR]

A novel dual motor synchronous control system supplied by four-switch mono-inverter is proposed, where one phase of both motors is connected to the mid-point of dc bus capacitors, and other phases are connected to inverter leg. Since only four power switches are required, the installation space, weight, and cost can be further reduced than conventional mono-inverter. However, it presents control challenges due to the limited control freedom degree. An advanced vector control method is proposed for this problem. Based on predictive model, two motors' reference voltage vectors are calculated independently, and a self-adapting composite vector is selected as the output vector of inverter. The duty cycles of switching signals are obtained based on space vector modulation of the selected vector. The use of unified-model or master–slave strategies in conventional mono-inverter control methods is avoided. The control is simplified and computation is reduced, while a good performance is ensured. Experiment results verify the effectiveness of proposed method. [ABSTRACT FROM AUTHOR]

In order to extend the speed range and reduce power electronics devices of the primary permanent-magnet linear motor traction system, a half-centralized open-end winding topology is studied in this article, in which N movers are supplied by N+1 voltage-source inverters. In order to maximize the speed range, two unified maximum voltage utilization control (UMVUC) methods are proposed for the studied traction system, which can be easily implemented for any number of movers. The first UMVUC is named as the original UMVUC, and it requires a heavy computation burden, which makes the implementation difficult when the number of movers is large. For the linear motor system, the relative positions of all movers are approximately constant. Based on this feature, the second UMVUC named as simplified UMVUC is proposed, which can significantly reduce the computation burden. In order to suppress the zero-sequence current (ZSC) and reduce the switching loss, a zero-sequence polarity assignment method is employed in the proposed UMVUCs. An experiment platform is developed, and the effectiveness of the proposed UMVUCs has been validated by experimental results. [ABSTRACT FROM AUTHOR]

Discontinuous conduction mode (DCM) for inverters can lower the size of filter inductors and can realize the soft switching for switches. However, the current stress under the DCM is high. Aiming to solve this problem, this article uses a T-type three-level circuit to construct an LCL-type grid-connected inverter, which introduces the trapezium conduction mode (TPCM) with the zero-voltage switching to lower the current stress when the grid current is large. Both the DCM and the TPCM can work at a fixed switching frequency, but it is difficult to design controllers using the conventional average state model. To address this challenge, this article proposes a charge quantity model (CQM) that is applicable to any inverter topology and conduction mode, where the output charge quantity of the bridge is used as a control variable. The proposed model represents a linear two-order model, which can greatly simplify controller design. Based on the CQM, a direct charge control method working at a fixed switching frequency is developed, which combines a PID regulator and a feedforward controller of the grid voltage. The excellent steady-state and dynamic control performances of the proposed model and control method are verified by simulations and experiments. [ABSTRACT FROM AUTHOR]

Model predictive current control (MPCC) of a 4-level inversion fed open-end winding permanent magnet synchronous motor (OEW-PMSM) using 37 voltage vectors (VV) suffers from the disadvantage of the increased computation burden. A high computation burden on the processor puts a limit on the selection of the minimum sampling time. In this article, a novel method is presented for reducing the computation burden. The proposed method reduces the number of candidate VVs from 37 to a maximum of 4 and a simple cost function is proposed to select the optimal VV. The proposed method calculates the change in stator current (CSC) needed to obtain the required change in the stator flux. The CSC vector is in the direction of the required change in the stator flux, which is used to shortlist the candidate VV for the reduced control set. The required change in the flux is obtained using the reference stator current of the next sample. The multiple predictions of the d-q currents are eliminated and a simple cost function based on the difference between per-unit CSC and candidate VV is used to simplify the calculations. The proposed method is experimentally tested and compared with existing MPCC. The results confirm the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

This letter proposes an active power ripple mitigation strategy for the three-phase three-wire grid-tied inverter. Under the unbalanced grid voltage scenarios, the double-line frequency oscillation will have occurred on the inverter output power and the dc-link voltage. Traditionally, the three-phase four-wire or the three-phase four-leg topology could be adopted to deal with this issue. Besides, the negative and the zero-sequence current component can be controlled to cancel out the power ripple. However, the circuit cost and control complexity might be increased. Therefore, a virtual phase-current regulation (VPCR) method is developed by adding compensation to the current feedback of the fault voltage. The compensation value is consistent with the voltage drop ratio, which can effectively offset the actual power ripple. The main feature of the VPCR is its simplicity, whereas it can be realized via the digital signal processor without adding extra circuit components. Theoretical analysis and mathematical derivations of the VPCR will also be revealed. Finally, a 3 kW prototype circuit with both simulation and experimental results verify the performance and feasibility of the proposed strategy. The experimental results show that the output power ripple can be eliminated with 64.3% via the proposed VPCR. [ABSTRACT FROM AUTHOR]

Larger battery packs, an efficient battery management system, highly efficient electromechanical conversion, and fast battery charging are the key factors for extending the drive range of electric vehicles and the expansion of electromobility. We present a novel highly stable modular multilevel powertrain design with a variable dc-bus voltage, active inner battery energy flow control, and reduced filter size. The underlying powertrain design replaces conventional hard-wired batteries by a modular multilevel architecture, thus modularizing the battery pack and increasing the energy efficiency, reducing the output filter size, and providing full control over the output voltage and energy flow of the battery cells. We analyse the large-signal stability of the proposed system and compare it to dc/dc-dc/ac conventional multiconverter powertrains, demonstrating better stability and lower filter requirements. Further, we experimentally evaluate the proposed system structure. In the experimental setup, we achieve stable converter operation with the output filter capacitor reduced by a factor of 24 and the filter inductor reduced by a factor of 17 compared to dc-dc-dc-ac reference system, thereby leading to a significant reduction of system cost, size, and weight. [ABSTRACT FROM AUTHOR]

In this letter, a simplified single current loop control scheme for single-phase dual-boost inverter has been developed, combining half cycle modulation and virtual-vector (VV) based model predictive control (MPC). As the control reference can be slipped and allocated into two side boost, only a unified grid current reference is relied on in this proposed control method. Furthermore, with the concept of feedforward control, the global optimal solution space in MPC can be compressed, taking the duty ratio as boundary for the precompacted solution space. Thereby, the improved MPC can select the optimal VV in a smaller and closer subspace. Consequently, much finer control effect can be obtained. Experimental tests are provided to validate the proposed solution and its merits. [ABSTRACT FROM AUTHOR]

In this work, we develop a zero-sequence model to analyze the zero-sequence current, i.e., circulating current, in multiple paralleled power converters. On this basis, we propose and verify a novel and simple solution: predictive zero-sequence current control (PZSC $^2$) to freely mitigate the zero-sequence current. The proposed technique is implemented in a direct model predictive control framework via a synthesized voltage vector, which consists of an optimal shrunken voltage vector and a zero voltage vector. The former is selected to minimize the tracking error of the $\alpha \beta$ -axis current, while the latter is properly selected to mitigate the circulating current. Notably, the zero voltage vector is solely determined by the zero-sequence current direction, without using other converters’ operating knowledge. This permits the proposed PZSC $^2$ to operate in a self-governing manner, fitting the spatially distributed applications. We validate the proposed solution through a lab-constructed hardware test bench. Experimental results verify its effectiveness considering both steady-state and transient performance. [ABSTRACT FROM AUTHOR]

In this article, a nine-level multilevel topology is proposed by stacking of low voltage basic inverter cells and then cascading with H-bridges. The dc-link is split into four equal halves by series-connected capacitors from a common dc-link. Three common coupling points (CCPs) are formed due to the stacking of four capacitors, which are connected across the dc-link. The CCP voltage balancing is ensured by drawing or injecting zero instantaneous currents from each of these points at any instant. A six-phase symmetrical IM is used to generate equal and opposite instantaneous phase currents by applying exactly equal and opposite phase voltages for two opposite phases from the six-phase inverter. Simultaneously, the capacitor voltage balancing of H-bridge inverter cells is also achieved by proper selection of redundancies from the available pole voltage redundancies. The concept is verified with a nine-level inverter laboratory prototype, which consists of a five-level stacked inverter, and followed by two low voltage cascaded H-bridge inverters in each phase. Detailed experimental results of steady-state and transient conditions are included to validate the proposed scheme. [ABSTRACT FROM AUTHOR]

Three-phase four-wire grid-forming (GFM) inverters are promising to interface distributed energy resources into low-voltage networks. However, these inverters are prone to overcurrent under grid faults. Physically increasing the inverter current capacity is not cost-effective to cope with complicated fault conditions. In this article, an adaptive fault ride-through (FRT) scheme based on instantaneous saturators and virtual negative- and zero-sequence resistances is proposed. It features not only overcurrent limitation by modifying voltage references, but also seamless transition between normal and grid fault conditions. The proposed FRT scheme is first analyzed from different aspects, including the virtual sequence resistances, grid short-circuit ratio, fault types, and fault levels. The virtual sequence resistances are then designed to be adaptive to ensure high voltage quality at the healthy phase. The proposed FRT scheme is verified by MATLAB/Simulink simulations under asymmetrical faults. A laboratory platform with a grid-connected 3kW GFM inverter is further constructed to demonstrate its effectiveness (a video of the experimental results under three asymmetrical faults is attached). [ABSTRACT FROM AUTHOR]

In order to improve the control performance of conventional finite-control-set model predictive control (FCS-MPC) method and keep the balance between the steady-state performance and switching frequency of the system, this article proposes an improved MPC method based on the conventional MPC method. First, the reference voltage vector of the next control instant is predicted to determine the candidate voltage vectors of the next two control instants. Then, an optimal vector selection way is proposed, in which candidate voltage vectors selected by the reference voltage vector and designed cost function structure in series. Moreover, the selection process of optimal voltage vector from the candidate voltage vectors is analyzed. Finally, the comparison experiments between the conventional FCS-MPC method and the proposed MPC method are conducted. And experimental results show the validity of the proposed control strategy. [ABSTRACT FROM AUTHOR]

In this article, an improved pulse density modulation (PDM) control for secondary side controlled wireless power transfer (WPT) system is proposed. The LCC-S network is selected for the compensation of the reactive power in the system. In the secondary side, an active rectifier with improved PDM control is used to regulate the output voltage. The output voltage regulation is achieved by changing the pulse density of the rectifier switches. By using the proposed PDM pattern, the input pulse sequence of the active rectifier can be distributed more homogenously compared to the conventional PDM control. Therefore, the output current ripple of the primary side inverter and the output voltage ripple of the active rectifier are reduced. The light load efficiency is also improved compared to conventional PDM control. The soft switching conditions are achieved for inverter and active rectifier switches. The experimental prototype with 150 mm air gap is built to validate the performance of the proposed system. The overall efficiency of the system with the proposed PDM is improved by 1.7% and 9.9% compared to the conventional irregular and regular PDM control, respectively. The output voltage ripple of the active rectifier and the output current ripple of the primary inverter are also reduced at light load conditions with the proposed PDM. [ABSTRACT FROM AUTHOR]

Due to the mismatched line resistance in the modular online uninterruptible power supply (UPS) systems, accurate active power and harmonic power sharing is an arduous work. To tackle this issue, this article proposes a distributed dynamic event-triggered control for the power sharing to accurately compensate the effect of mismatch among the line resistances. Benefiting from the proposed control strategy, the UPS modules adaptively regulate the virtual resistance at the fundamental and harmonic frequencies, and hence, the accurate active power and harmonic power sharing are achieved. In addition, the suggested technique allows the UPS modules to exchange information at only the event-triggered times, which greatly reduces the number of communication among the neighboring units. Furthermore, the system's stability is analyzed using a Lyapunov function. Finally, experimental case studies are provided to evaluate the performance of the proposed control strategy, showing its effectiveness in achieving the accurate power sharing, and plug-and-play capability. [ABSTRACT FROM AUTHOR]

The instantaneous output power of the two-stage single-phase inverter pulsates at double-line frequency, generating a large amount of second harmonic current (SHC) in the front-end dc-dc converter and input dc source. The SHC is harmful to the system performance. To address the issue, a control strategy based on port-impedance editing, where inductor current feedforward (ICFF) and bus voltage feedforward paths are employed, is proposed to reduce the low frequency ripple in the article. Due to only bandpass filter employed in each feedforward path, the proposed control scheme is relatively easy to be implemented. Under the control strategy, the dc-bus port-impedance of dc-dc converter is increased rapidly at double-line frequency, while staying low impedance for other frequencies. What is more, different ICFF paths are taken into insight to obtain optimized path. Meanwhile, the small signal model of front-end dc-dc converter is established and the design principle of key parameters is provided for the derived control strategy. Furthermore, the dynamic response with bandwidth limitation is also discussed in this article. Finally, a two-stage single-phase inverter prototype was fabricated and tested. The experimental results verify the performance of the proposed strategy with SHC reduction. [ABSTRACT FROM AUTHOR]

In the proposed method in this article, to minimize the burden on the switches of the multifunctional grid-connected voltage-source inverter (MFGCVSI), the voltages of dc-link split capacitors are adjusted according to power injection from the MFGCVSI. The MFGCVSI is used to control the active power injection from solar photovoltaic (SPV) power generation to the main grid and loads, load reactive power compensation, harmonic mitigation, and main grid current total harmonic distortion within 5 $\%$. In such a case, the voltages of split capacitors depend on the rated power injection from the MFGCVSI. The power generated from the SPV system and the loads that are connected at the point of common coupling vary with respect to time. It is observed that the selection of rated dc-link voltages during low power injection from the MFGCVSI leads to additional switching stress and higher switching losses. Therefore, to reduce the burden on inverter switches and switching losses, the reference dc voltage is reduced under low power injection from the MFGCVSI. A model predictive controller with a multiconstraint technique is used in the proposed method for balancing the dc-link capacitor voltages and average switching frequency reduction. The proposed method is validated through simulation and hardware setup. [ABSTRACT FROM AUTHOR]

T-type three-level inverters have attracted extensive attention for their utility in many applications, especially photovoltaic power generation. For these inverters, parallel operation offers superior performance in capacity, reliability, and scalability. However, the introduced circulating current (CC) may cause overcurrent in line, which seriously threatens the safety and stability of the system. The mechanism of CC lacks detailed analysis in single-phase modular inverters. Thus, an accurate mathematical model is established and a CC suppression method is proposed in this article for parallel-operated single-phase T-type three-level inverters. First, the characteristics of port degradation are analyzed, and an improved equivalent circuit is introduced based on the findings. Then, according to the improved equivalent circuit, a detailed mathematical model is established. Using this model, excitation sources of CC are analyzed and divided into three parts. After that, a CC suppression method involving carrier-based pulsewidth modulation with offset voltage injection is proposed, and a deadbeat controller is designed to calculate the offset voltage. Moreover, the proposed method can balance the neutral-point voltage across dc-link capacitors as well. Simulation and experimental results validate the theoretical analysis and effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

The article proposes a new vector analysis based model predictive control (VAMPC) method for multilevel converters with four different switch configurations. It is based on track of an error vector using fundamental state vectors, and can achieve deadbeat control of four switching states multilevel converters with multiple control objectives. Compared with conventional finite control set model predictive control (FCS-MPC) method, VAMPC can achieve excellent spectrum characteristics in the case of a slight increase in computational burden. The VAMPC method is experimentally validated on a 100-kHz GaN-based three-level flying capacitor (FC) converter. Compared with the FCS-MPC, VAMPC leads to significant reduction in steady-state control errors. For example, the inductor current total harmonic distortion of the FC converter is now reduced by more than 60%. [ABSTRACT FROM AUTHOR]

Voltage rise caused by high levels of distributed generation is manifesting as voltage regulation challenges for many electricity network service providers. In this environment it would be ideal to reduce supply voltage magnitudes, however, many network operators are hesitant to do so due to concerns related to consumer appliance performance at reduced supply voltage magnitudes. Voltage regulation requirements are defined by network standards and network service providers must ensure voltages remain within specified limits. Through an evaluation of domestic appliance performance when supplied at various voltage magnitudes, this paper examines the impact of varying voltage levels on residential appliances. Equipment energy demand, operation and actuation were monitored for each applied voltage magnitude. While no equipment failures were recorded, appliance behaviour varied significantly with applied voltage magnitude. Individual appliance conservation voltage reduction (CVR) factors have also been established. The results highlight the importance of good voltage regulation and provide substantiated appliance performance figures for future studies. The outcomes of this paper allow electricity network service providers to understand the implications of supply voltage magnitude on domestic appliance performance, whether it be understating of the impact of higher voltage magnitudes caused by distributed generation or implications of reducing voltage magnitudes to provide headroom for distributed generation integration. [ABSTRACT FROM AUTHOR]

Line-commuted converter-based high-voltage direct current (LCC-HVDC) has undergone extensive development in power systems. Commutation failure (CF) in LCC-HVDC poses a considerable threat to the safe and stable operation of power systems. The coupling of electrical quantities, including DC current, DC voltage, AC voltage, and extinction angle (EA) of an inverter station, is a critical factor that makes CF difficult to suppress effectively. Existing methods adopt preset fixed control parameters and a single control target. The demand of CF recovery typically cannot be satisfied. Control even becomes the driving force of subsequent CF (SCF). Accordingly, a new concept of a fault security region (FSR) in an inverter station under the effects of multi-electrical quantities is proposed. The security range under the coupling effects of electrical quantities is described by the active power and exchanged reactive power of the inverter station. EA, active power, and exchanged reactive power are coordinated through the adaptive regulation of DC current. Thus, SCF can be prevented under the condition that the effect of varying active power and reactive power of LCC-HVDC on a power system is balanced. First, the coupling characteristics of multi-electrical quantities of the inverter station are analyzed. Second, the modeling method for the FSR in the inverter station is proposed. Third, an adaptive current control method is developed by analyzing the evolution characteristics of the FSR. Moreover, a calculation method for the DC current order value is presented. Finally, the effectiveness of the proposed method is verified using the CIGRE HVDC standard system. [ABSTRACT FROM AUTHOR]

The voltage rise problem due to the reverse power flow is one of the main obstacles to expanding the photovoltaic systems (PVSs) in distribution networks. In this paper, a decentralized control method for coordinated control of on-load tap changer (OLTC) transformers and PV inverters, is proposed for the voltage regulation of radial distribution networks. In this method, the allowable voltage range is divided into several zones, and appropriate corrective actions are adaptively taken according to each zone. To reduce the tension on the OLTC, first, the reactive power capability of the PV inverters is employed for the voltage regulation, and if it is insufficient, OLTC is used as the last solution. In this regard, the unused capacity of PV inverters is utilized for the reactive power compensation, considering the grid codes reactive power requirements. Furthermore, decentralized control based on a signaling method is used for coordination between OLTC and PVSs, which eliminates the need for communication links. Finally, the effectiveness of the proposed method is validated by the simulation results for the IEEE 33-bus distribution network, and the impact of different reactive power control methods on the number of OLTC operation is investigated. [ABSTRACT FROM AUTHOR]

Nowadays, a large number of inverter-based resources (IBRs) are integrated into the grid at a single connection point as an IBR plant. In this article, a virtual dynamic grid impedance concept is introduced to evaluate the harmonics and stability for grid integration of an IBR plant containing multiple IBRs. First, a detailed theoretical study is conducted to build a foundation of the virtual grid impedance concept, which is a dynamic impedance that changes with the number of IBRs added into an IBR plant. Then, based on the new virtual dynamic grid impedance concept, frequency spectrum analysis is performed to explore harmonic impact at the interconnection point for an IBR plant with IBRs having L, LC, and LCL filters, respectively. An electromagnetic transient (EMT) simulation model of a grid-connected IBR plant is developed to explore the harmonics and stability of the IBR plant connected to the grid as well as the reliable operation of IBRs within the plant from the novel virtual equivalent dynamic grid impedance point of view. Hardware experiments are conducted to validate the EMT simulation evaluation. The results show that the number of IBRs added into an IBR plant influences the grid impedance, i.e., grid strength, and the grid impedance variation has multiple impacts on the IBR plant and IBRs within the plant depending on the grid-connected filters of the IBRs. [ABSTRACT FROM AUTHOR]

At present, the single stage inverters are popular in integrating large-scale solar farms with distribution networks that demand higher dc bus voltage. The elevated dc potentials would degrade the reliability of the solar panels and inverter modules. In this work, the multiboost solar inverter topologies of three variants are presented for grid-connected applications. Since the proposed topologies aim to achieve higher voltage boost at ac side with reduced dc bus potential, it is required to use asynchronous switching strategies, unlike parallel inverter configurations. Although the proposed topologies are advantageous in-terms of improved reliability of solar panels and inverter modules, but instantaneous characteristic impedance imbalance due to asynchronous switching provokes circulating current within the inverter modules. Since the circulating current is undesirable concerning power quality and thermal aspects, in this work, the method of instantaneous impedance balance is ensured with the specially designed switching algorithm for proposed topologies. The eliminated instantaneous circulating current provides the flexibility of operating all inverter modules with the common dc bus. The proposed high gain boost configurations and switching methodologies are demonstrated on hardware prototype by pumping 2.4-kW power to the grid. [ABSTRACT FROM AUTHOR]

The power density can be improved and the system cost can be saved in the reduced dc-link capacitance motor drive system. The line inductance can also be diminished to reduce the system volume further. In previous research works, the active damping control methods mainly concentrated on the drive system equipped with a larger line inductor. In this article, a dc-link voltage estimation feedback-based active damping method is proposed to suppress the grid current harmonics and improve the drive system stability with low line inductance. The dc-link voltage ripple caused by the LC resonance is analyzed to evaluate the direct dc-link voltage feedback-based active damping control method. To improve the damping performance, the resonant component of the dc-link voltage caused by the LC resonance could be obtained through the grid current to establish the feedback control loop. The sampling delay should be compensated to improve the control performance. The stability of the drive system is evaluated by the Nyquist plots of the impedance model, and the grid current is also analyzed, which could satisfy the harmonic requirements of IEC-61000-3-2. Experimental results are performed to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This article proposes a coordinate impedance compression strategy for high frequency inverter systems. Based on a two parallel inverter structure, the strategy adopts two methodologies: 1) a passive LC network to adjust the current and voltage seen by each inverter module, and 2) an active modulation of the inverter input voltage and output voltage phase to adjust the load current distribution. For the proposed strategy, the resistance and reactance can be compressed coordinately, which can help the inverter to operate in high efficiency load range. The Half-Bridge Class D inverter is taken as an example to be analyzed. A 10-MHz prototype is built and the experimental results verify the correctness and feasibility of the proposed strategy. [ABSTRACT FROM AUTHOR]

In the low-speed operating range of sensorless interior permanent magnet synchronous motor (IPMSM), this article proposed an ultrahigh frequency (UHF) sinusoidal voltage injection method supplied by the control power supply, and the corresponding signal injection and detection system topology are designed. The proposed method replaces the traditional high-frequency injection method to continuously detect the rotor position by injecting UHF sinusoidal detection voltage into the three-phase motor windings during the main power supply (MPS) down, thereby avoiding the loss of rotor polarity information. When the MPS is restored, there is no need to wait for the convergence process of the position observer and the rotor polarity determination process. The sensorless IPMSM can immediately resume normal operation at low speed. For applications with low load inertia, the proposed method improves the rapidness of rotating restart, realizes zero-wait operation after MPS is restored, and reduces the speed drop during the power down. For electric vehicle (EV) applications, the proposed method meets the requirements of EVs event data recorder to continuously record the motor speed when the MPS is off. The experimental results verify the correctness of the theoretical analysis and the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

Power grids are evolving toward a highly distributed architecture with power-electronics circuits interfacing a majority of generation, storage, and loads. To ensure stability as synchronous machines are retired, grid-forming (GFM) inverters will be needed across the board. In this article, we investigate whether systems built with interconnected single-phase droop-controlled GFM inverters are capable of self organizing into balanced three-phase systems. We model, analyze, and build a system comprised of three delta-connected droop-controlled single-phase inverters connected across loads. After deriving a model of the angle dynamics for this system, we show that its stable equilibria coincide with balanced conditions, where each phase is offset by $1/3$ of an ac cycle. Furthermore, we observe that the desirable equilibrium with balanced phase offsets is robust against voltage and load imbalances. This demonstrates the feasibility of assembling three-phase systems using fleets of decentralized single-phase GFM inverters. The analytical developments are substantiated experimentally on a system of three delta-connected single-phase inverters. Spontaneous emergence of phase balancing from startup with nonidentical initial conditions are validated in the experimental setup. Empirical observations further illustrate robust operation during unbalanced conditions. [ABSTRACT FROM AUTHOR]