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As the inverter-based generation replaces the conventional synchronous generators, it may also need to fill in the missing ancillary service support. One of these ancillary services is dynamic reactive power provision and voltage control. This paper analyzes optimal strategy of reactive and active fault-current support of the inverter-based generation leading to fast voltage recovery of the system. For the purpose of the analysis, new ramping active current controller able to emulate different behavior of active current injection is proposed. By optimizing its parameters for different case studies of the system, the conclusions about optimal behavior of the inverter based generation with respect to system parameters and operating conditions are drawn. It is observed that the optimal combination of active and reactive fault-current is the most sensitive to the dynamic load component penetration levels in the system. With the increasing penetration levels, the significance of active fault-current injection increases. The results show that with higher penetration levels of dynamic load component in the heavy load areas, the ramping down of the inverter-based generation active fault-current results in slower voltage recovery of the system. Following this conclusion, a recommendation on update of current European grid codes is proposed. [ABSTRACT FROM AUTHOR]

Low-inertia microgrids are prone to large frequency fluctuations due to high penetration of inverter-fed renewable sources. A virtual synchronous machine (VSM) is capable of handling these frequency fluctuations. VSM imparts inertial behaviour in grid-connected inverters with the help of an electric storage device. Most of the reported VSM control schemes use a constant inertia parameter that leads to an inadequate dynamic frequency response. In this paper, a fuzzy controller-based self-adaptive VSM control scheme is proposed. Here, the value of the inertia parameter is decided using fuzzy rules which take the penetration index of renewable energy, frequency deviation and rate-of-change-of-frequency as the input parameters. These rules are formulated with the help of the sample data obtained from simulations done in MATLAB/Simulink, where the inertial parameter is optimized using a genetic algorithm for various cases. The proposed method is validated for solar photovoltaic-fed microgrid test systems using power–hardware-in-the loop and the results indicate a reduction in maximum frequency deviation compared to the constant inertia parameter based VSM schemes. The introduction of renewable energy penetration level and fuzzy expert system in modifying the inertia parameter are the novel contributions in this paper. [ABSTRACT FROM AUTHOR]

The grid-connected inverter is a key device in the renewable energy power generation system and large-scale energy storage system, which the operational stability and reliability are the basis for the efficient and safe application of electrical energy. A real-time fault diagnosis method of a three-phase for grid-connected application combining a mixed logic dynamic (MLD) model and finitecontrol set model predictive control (FCS-MPC) is proposed. This paper not only realizes the open circuit fault diagnosis and location of the switching devices in the main power circuit, but also discusses the threshold issues and post-fault operations. The advantage of the proposed method is that it directly uses the control data and measurement signals of the controller without extra sensors and calculation, which will shorter the fault diagnosis time and occupy less calculation resource of the main processor. Simulation results illustrate the quickness of the fault identification and accurate position with robustness to the interference of the diagnosis method. Finally, the effectiveness of the diagnosis method was verified by a 1500 W experimental prototype in a laboratory. [ABSTRACT FROM AUTHOR]

Motor current signature analysis (MCSA) has been widely used for fault diagnosis of induction motors (IMs) because of its non-invasiveness and comparatively low cost. However, the spectral leakage of the fundamental supply frequency can obscure the fault characteristic component when IMs operating at no-load or low-load conditions. In this paper, a broken rotor bar (BRB) fault diagnosis approach based on the third-order energy operator (TOEO) demodulated current signals is presented. The TOEO approach performs effectively in reducing the negative impact of the fundamental supply frequency spectral leakage and enhancing the BRB fault features. Theoretical and experimental results show that the TOEO approach can excellently reveal the fault characteristic frequency, instead of the sideband frequencies. Besides, the results are compared with the Teager-Kaiser energy operator (TKEO). The capability of the presented approach has several advantages over the TKEO, especially when the IMs are fed by an inverter. [ABSTRACT FROM AUTHOR]

This paper presents a different Finite Control Set – Model Predictive Control (FCS-MPC) for grid-connected three-phase bidirectional power inverters. These are typically used in dc or ac renewable-based microgrids (MGs), where bidirectional operation and fast dynamic response is required. The bidirectional grid-connected inverters are an essential part of MG, which inject energy into the ac grid or demand energy from it. The dynamic behavior of the system is a major concern since the current can suddenly change depending on the hierarchical controller. This paper proposes a different cost function using sliding mode theory, which offers a good dynamic response, reduced computational burden, and a parameter-free control model. The operation principle of the proposed controller is given and evaluated using a Hardware-In-the-Loop (HIL) system, but also experimentally with a 1kW laboratory prototype. The final results demonstrate the advantages of using this approach in grid-connected three-phase bidirectional power inverters in terms of dynamic response and reduced computational burden, making this solution technically attractive and viable. [ABSTRACT FROM AUTHOR]

Inverter transformers are one of the key components in a PV plant and their operating patterns are considerably different from that of transformers in a substation. Moreover, these transformers experience higher level of harmonics generated by the inverters. Therefore, investigating the ageing profile of the inverter transformer is a timely requirement. In this paper, the ageing of inverter transformers in a medium-scale solar PV plant is studied by utilizing various real-time measurements including load current, top-oil temperature (TOT), ambient temperature, moisture in top oil, solar irradiance and individual current harmonics. An updated moisture-dependent thermal model (MDTMH), with consideration of current harmonics is developed to estimate the hot-spot temperature (HST). The influence of weather conditions, which directly determine the load current of inverter transformers on the loss of life (LOL) of an inverter transformer is also investigated. Furthermore, the effect of current harmonics levels on the inverter transformer's HST and LOL is explored. Finally, comparisons of HST and LOL of inverter transformers connected to PV arrays with three different tracking technologies, i.e., fixed-tilt, single-axis and Dual-axis PV arrays are performed. [ABSTRACT FROM AUTHOR]

In this paper five different flexibility options are analysed from a techno-economic perspective as alternatives to traditional grid expansion for a specific distribution grid in Germany. The options are: two reactive power control strategies with photovoltaic inverters (as a function of the power feed-in, or of the voltage at the connection point), one residential and two large scale battery storage applications (primary control reserve with autonomous reactive power control or self consumption maximisation strategy with autonomous reactive power control). For the pilot grid located in Southern Germany a photovoltaic expansion pathway is determined. The main goal of this work is to quantify the grid expansion actions that can be avoided by applying these five flexibility options for the assumed expansion pathway, focusing on large scale battery storages. It is shown that the five flexibility options increase the hosting capacity for PV systems, compared to a scenario without, by up to 45%. Furthermore, the results of the economic assessment indicate that the analysed flexibility options might be a viable alternative to traditional grid expansion as all of them show a cost reduction potential for the pilot region. These results could encourage DSOs to consider the integration of additional PV and battery storage systems not as a problem which triggers grid expansion, but as part of the solution reducing future grid expansion costs. [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]

This paper proposes a feedback linearization controller for a grid-following inverter (GFLI) that uses a conventional Phase-Locked Loop (PLL) as the synchronization unit. The proposed controller enhances the GFLI synchronization by expanding the PLL domain of attraction to the whole plane that is limited to a small region around the equilibrium point in a conventional PLL, provided that the grid impedance and voltage are known. Linearizing the overall closed-loop system, the proposed controller provides linear system attributes for the PLL, leading to an infinite domain of attraction and only one equilibrium point. Additionally, a state-feedback controller is integrated within the feedback linearization controller so that it enables the system to have an adjustable dynamic response. Finally, it is shown that the system is robust against parametric uncertainties. The performance of the proposed control design is validated in Matlab/Simulink and experiment. It is verified that the proposed controller expands the domain of attraction and enhances the system dynamic response. [ABSTRACT FROM AUTHOR]

One of the main challenges in the protection of the Inverter-Based Microgrids (IBM) is fault detection. Inverter-based resources have limited fault current; they also adopt different control schemes which make their fault current analysis and estimation more complex. These features make the traditional protection schemes inapplicable in IBMs. In this paper, two parameters are introduced and a protection scheme is proposed based on the introduced parameters to protect feeders and Distributed Generators (DGs) in IBMs. In the proposed scheme, in each sampling window, the last samples of the current waveform are predicted and the difference between the actual samples and the predicted samples are obtained; the voltage phase angle shift between the present and the past sampling windows is also computed; using these parameters and by introducing a new relay characteristic based on Mahalanobis distance the fault is detected in IBM. The effectiveness of the proposed protection scheme is verified through the simulation and experiment results. [ABSTRACT FROM AUTHOR]

The power quality of virtual synchronous generator (VSG) inevitably deteriorates in the presence of local nonlinear loads and distorted grid. In this paper, the conflict involved in the simultaneous elimination of distortion for both the inverter local load voltage and the grid exchanged current is first described. A unified control structure is presented that enables a tunable tradeoff between the two constrained harmonic sources. Then, a hybrid harmonic suppression scheme is proposed to enable the further improvement of the adaptability of VSG, which mainly consists of a local voltage harmonic control loop and an adaptive grid current-controlled loop. The local voltage harmonic control loop aims to scale down the inverter output impedance via a negative feedback loop, while the grid current-controlled compensator is intended to counteract the adverse effects from a weak grid via an additional voltage, which leads to substantially lower total harmonic distortion for both the local load voltage and the grid current at the same time. Small-signal modelling is performed to investigate the system stability and its robustness to parameter perturbations. The effectiveness of the proposed methodology is verified using hardware-in-the-loop simulations. [ABSTRACT FROM AUTHOR]

Unbalanced distribution network/loads and asymmetrical faults are frequent in power systems. Distributed energy resources employing inverter interfaces need to ride through them to abide by emerging grid codes. The balanced grid operation has been a well-researched topic. However, literature lacks a unified framework to model, analyze, and design controllers for unbalanced interfaces. This work develops a nonlinear time-domain state-space model of inverter interfaced to an unbalanced grid. It includes details of frame transformation, synchronization, inner current controller, and filter dynamics. Its linearization around the steady-state trajectory results in a linear time-periodic (LTP) system. The model is then transformed to the dynamic harmonic domain for ease of analysis, consideration of multiple frequency couplings, and controller design. Precise stability boundaries in parameter space are obtained to understand margins for safe operation intuitively. A stability augmentation using output feedback is presented for increasing these margins. Feedback gain is designed using the generalized Nyquist criterion. Proposed augmentation is shown equivalent to the adaptive active damping of unstable periodic trajectories. Simulation test cases confirm the validity of the proposed design. [ABSTRACT FROM AUTHOR]

This article presents a new hybrid three-phase cascaded modular multilevel inverter developed with a reduced switch count concept for harvesting renewable energy. The basic module of the proposed inverter consists of six switches, three asymmetric dc links, and two blocking diodes. The inverter can generate six positive voltage states, which are negatively polarized by the end-side H-bridge unit. As a result, a higher number of voltage levels are obtained with a minimum number of switches that ensure reduced total harmonic distortion in the grid current. Additionally, the proposed inverter can operate at a comparatively lower switching frequency, which decreases the switching losses. Most of the switches in each individual module conduct for a small period resulting in low conduction losses. The reduced switch count with the low switching and conduction losses increases the compactness and the efficiency of the proposed inverter. Multiple high-frequency magnetic links are utilized for the dc sources required for the proposed inverter to achieve inherent dc-link voltage balancing ability and galvanic isolation. The performance of the proposed inverter is validated by simulation and experimental investigations. [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]

In symmetrical grids, single-input single-output (SISO) analysis/design tools are applicable for the analysis and design of grid-tied inverter systems as their average αβ- or dq-frame models are to a high extent decoupled (in the high-frequency range). In this way, designing the control system could be done in a straightforward manner. However, this is not the case under asymmetrical grids. In this condition, the interaction between different phases makes the average model of the grid-tied inverter a multi-input multioutput (MIMO) system, and therefore, complicated to analyze and design. To cope with this challenge, this article deals with deriving the MIMO control system considering the impact of the asymmetrical grid. Then, the MIMO system turns into two decoupled SISO systems. Thus, a simple yet accurate model is developed which intuitively demonstrates the effect of the grid asymmetry on creating coupling between current control loops. In this way, the stability analysis could be carried out irrespective of the MIMO system difficulties. The experimental results validate the accuracy of the proposed approach in asymmetrical grid conditions. [ABSTRACT FROM AUTHOR]

In this article, a new inverter unit is proposed. It is a compact switched-capacitor-based inverter, which is combined of four switches, two diodes, and four capacitors. Since this inverter unit is a mid-point-clamped inverter, it can mitigate the capacitive leakage current in many applications. One of the main applications of the proposed inverter is in grid-tied photovoltaic systems, where the common-mode current brings many challenges. Other than the mentioned feature, the proposed inverter includes the following advantages: 1) spontaneous balancing of the voltage across the capacitors without needing any control system or auxiliary circuits; 2) capability of full reactive power transfer; 3) low voltage stress on the capacitors compared to common three-level inverter topologies (NPC, T-type-NPC, FC, etc.); and 4) symmetric switching sequence, which causes a continuous input current. The feasibility and versatility of the proposed inverter unit is validated by experimental tests, which are conducted by using a lab-scale prototype. [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]

Power filters have been widely used to suppress switching harmonics caused by the modulation of grid-connected inverters. In order to save the total inductance and cost and reduce the volume, different power filters were proposed for the three-phase three-wire grid-connected voltage source inverter. However, in industrial applications, due to the consideration of production cost, the difficulties caused by the deviation of actual parameters have to be addressed. In this article, we propose a new three-phase three-wire power filter with a simple structure, almost the smallest total inductance, and strong resistance to the adverse effects of parameter shifts. The parameters of the proposed filter and the design of the grid-connected system controller are introduced in detail. When studying the influence of parameter offset, a comparison between different types of notch power filters is also given. Simulation and experimental results on a 380 V/50 Hz/6 kW three-phase inverter prototype verify the performance of the proposed power filter. [ABSTRACT FROM AUTHOR]

In this article, a fault-tolerant model-free predictive controller (FT-MFPC) for cascaded H-bridge multilevel inverters (CHMLIs) is proposed. FT-MFPC is based on an association of model-free predictive controller (MFPC) and a proportional--integral (PI) like structure known as current variation controller (CVC). As in MFPC, the dynamic structure (DS) that is used for the prediction of the system output is obtained from past measurements of current variations, but CVC modifies the dynamics before they are stored for prediction. As a consequence, even if the converter operates with bypassed modules, FT-MFPC adapts its DS to represent the real dynamics of the system and enables a closed-loop response with balanced load currents in unbalanced CHMLIs. Simulation and experimental results validate the adaptability of the proposed strategy to operate a seven-level inverter under various configurations of bypassed H-bridge modules. The experimental results show that, for different postfault operations, the maximal current total harmonic distortion and imbalance factor at steady state in the worst case scenario are below 5% and 2%, respectively. [ABSTRACT FROM AUTHOR]

In this article, a multioutput-based transformerless hybrid solar inverter (TLHSI) with no right half-plane zero (RHPZ) and reduced common-mode leakage/ground current is presented for photovoltaic (PV) based microgrid applications. In the proposed multioutput TLHSI, two magnetically coupled inductor coils are placed between the input and dc output sides along with a dc-link capacitor to continuously provide the stored energy to the dc load so that the RHPZ is eliminated and, thus, makes it a minimum phase system. Furthermore, a hybrid unipolar sinusoidal pulsewidth modulation technique is established to nullify the high-frequency components of common-mode voltage across the PV to ground stray capacitor. Also, it normalizes both dc and ac loads at the same instant from a single dc input during each operating state of the proposed TLHSI. The shoot-through issue is resolved in the proposed TLHSI, which increases the system reliability. Furthermore, as the deadtime is not required in between the switching pulses, the quality of the inverter output voltage improves. The performance analysis of the proposed TLHSI is compared with the existing topologies. To validate the performance of TLHSI, a scaled down 340 W laboratory prototype is developed. [ABSTRACT FROM AUTHOR]

In this article, a 13-level switched capacitor multilevel inverters (SC-MLI) based single-phase inverter is proposed in which desired levels of output ac voltage are realized with reduced number of switch counts. The proposed inverter requires 12 switches, three diodes, one input dc source, and three capacitors. The requirement of less number of switches reduces the requirement of gate drivers, which enhances power density of converter. The second challenge in SC-MLI is to maintain self-voltage balancing across the capacitors. The self-voltage balancing capability becomes poor at low values of modulation index. The capacitors connected in proposed SC-MLI are capable to maintain the self-voltage balancing without requiring auxiliary circuits or complex control strategies. The operation of capacitors is self-balanced at all regions of modulation index values. The comparison of the proposed SC-MLI is carried out with state-of-art SC-MLIs discussed in the literature using the parameters like peak inverse voltage and total standing voltage. The switches connected in the proposed 13-level SC-MLI undergoes less voltage stress as compared to the SC-MLI configurations discussed in the literature. The proposed SC-MLI is highly competent to achieve good power quality and is capable to ensure voltage balancing of capacitors even at low values of modulation index. The validation of the proposed inverter topology is carried out using a laboratory prototype. The efficiency of the converter claimed using the simulation results is observed to be 96.2% while the efficiency evaluated using the experimental results is 94.1% at 500 W. [ABSTRACT FROM AUTHOR]

Power electronic converters for integrating renewable energy resources into power systems can be divided into grid-forming and grid-following inverters. They possess certain similarities, but several important differences, which means that the relationship between them is quite subtle and sometimes obscure. In this article, a new perspective based on duality is proposed to create new insights. It successfully unifies the grid interfacing and synchronization characteristics of the two inverter types in a symmetric, elegant, and technology-neutral form. Analysis shows that the grid-forming and grid-following inverters are duals of each other in several ways including a) synchronization controllers: frequency droop control and phase-locked loop (PLL); b) grid-interfacing characteristics: current-following voltage-forming and voltage-following current-forming; c) swing characteristics: current-angle swing and voltage-angle swing; d) inner-loop controllers: output impedance shaping and output admittance shaping; and e) grid strength compatibility: strong-grid instability and weak-grid instability. The swing equations are also derived in dual form, which reveal the dynamic interaction between the grid strength, the synchronization controllers, and the inner-loop controllers. Insights are generated into cases of poor stability in both small-signal and transient/large-signal. The theoretical analysis and simulation results are used to illustrate cases for single-inverter systems, two-inverter systems, and multi-inverter networks. [ABSTRACT FROM AUTHOR]

For the grid-connected inverter, grid voltage sag can be a typical phenomenon. From the prospective of safe and robust operation, the inverter should be stable when the grid voltage sag (or large disturbance) occurs. Although the inverter stability in this case has attracted considerable attentions, few studies have given suggestions on the system stability boundary and parameter design. To fill this gap, this article deeply discusses the large-signal stability of the system under different situations and reveals the quantified impact of the phase-locked loop (PLL) on the large-signal stability of the system based on the fundamental current-source model. The large-signal stability criterion and boundaries for grid voltage sags are established, which are instructive to the design of PLL parameters. Besides, a novel design guideline for the PLL parameters is proposed to ensure the robust operation of the system both under the steady-state and grid voltage sags, as well as to achieve good dynamic performance after large disturbance occurs. The theoretical, simulation, and experimental results show that the proposed stability boundary has certain accuracy and the PLL parameters design guideline can ensure the system stability both under steady-state and grid voltage sag conditions. [ABSTRACT FROM AUTHOR]

Online condition monitoring of winding insulation has been proved to be an effective means to improve the reliability of an inverter-fed machine, but little work has been done on the insulation of line-end coil subjected to much higher transient voltage stress with a higher risk of premature insulation break down. In this article, the inverter self-excited high-frequency switching oscillations are utilized for the line-end coil insulation condition monitoring. To improve the condition monitoring sensitivity and its robustness to dynamic operating conditions, a switching oscillation mode decomposition method is proposed. First, the condition-sensitive mode, i.e., high-frequency common-mode (HFCM) current is selected and separated by the variation mode decomposition algorithm among three dominant modes in switching oscillation current. Then, the amplitude-frequency features of HFCM current are extracted for condition monitoring. Finally, the overall insulation condition of three-phase line-end coils and the degree of asymmetric degradation are quantitatively evaluated by the frequency and amplitude features respectively. Experimental work was carried out and the results demonstrate the good performance of high sensitivity, good robustness, and non-contract safety. [ABSTRACT FROM AUTHOR]

Bipolar dc grid systems are fast gaining attraction for renewable energy source integration, because of their merits of higher reliability, efficiency, and robustness as compared to the unipolar dc grids. However, the progress in multiport converters, resulting into lower cost and more compact design for bipolar microgrid systems, is fairly slow. Therefore, this article proposes a novel family of five nonisolated multiport dc–dc converter topologies with bipolar symmetric outputs. The performance and key operational features of the proposed converters under varying input voltages, duty cycles and loads are numerically verified and experimentally on an in-house test setup to prove the concept of the proposed converters. In the experimental validation, the operation of the converter under simultaneous and arbitrary individual power transfer from two input ports is tested. Furthermore, the easy integration of the proposed converters with a multilevel inverter to achieve high-quality ac voltages is demonstrated. As compared to the few existing counterparts, the proposed converters have a competitive edge in terms of higher number of input ports and voltage gains. Alongside the possibility of arbitrary independent power flow from the input ports, inherently symmetrical outputs require a simple balance control for asymmetrical members of the family. [ABSTRACT FROM AUTHOR]

In an era where high-frequency flying capacitor (FC) multilevel inverters (MLIs) are increasingly gaining attention in energy conversion systems that push the boundaries of power density, the need for a compact, fast, and accurate FC voltage monitoring system is also increasing. In this article, we designed and developed a new FC measurement system, based on precise sampling of the inverter switching node voltage, through a bidirectional clamping circuit. The deviation of FC voltages from their nominal values is extracted by solving a set of linear equations. With a single sensor per phase and no isolation requirements, as opposed to dozens of sensors in traditional FC monitoring, our approach results in significantly lower cost, complexity, and circuit size. Detailed device-level simulations in LTspice and system-scale simulations in MATLAB validate the accuracy and speed of the proposed measurement system and the balancing strategy in steady-state, abrupt load change, and imbalance conditions. Experiments carried out in a three-phase gallium nitride five-level inverter prototype reveal a gain in precision and bandwidth that is more than 30 times that of conventional methods, at a fraction of their cost and footprint. The recorded performance renders the developed sensor an ideal solution for fast MLIs based on wide-bandgap technology. [ABSTRACT FROM AUTHOR]

This article presents a transformerless high step-down dc–dc converter based on a stacked active bridge (SAB) configuration. The SAB converter consists of series-stacked capacitively coupled inverter modules and parallel-connected rectifier modules. The nominal step-down conversion ratio is determined by the number of the inverter modules, while the output current capability scales with the number of the paralleled rectifier modules. SAB operating principles, including phase-shift control and soft switching, achieved through series inductors, are similar to transformer-isolated dual active bridge converters. It is found that the best size versus loss tradeoff is achieved by integration of the series inductors on a custom core. Furthermore, it is shown that how soft switching contributes to natural voltage sharing among the series-stacked inverter modules. The approach is verified by experimental results on a 400–48 V, 3-kW SAB prototype using GaN devices and featuring 400- ${\mathrm{W/}}{\text{in}}^{3}$ power density. A flat efficiency curve is obtained with 99% peak efficiency, 97.5% full-load efficiency, and 98% efficiency at 20% load. [ABSTRACT FROM AUTHOR]

In this article, we introduce a model predictive control with optimal switching sequence (OSS-MPC) for a multilevel static synchronous compensator (STATCOM). The studied STATCOM is based on a hybrid multilevel inverter named HC-FNPC formed by the cascaded connection of H-bridge cells with a full-bridge neutral point clamped cell (FNPC). In this article, the optimal switching sequences are derived for HC-FNPC STATCOM by considering the impact of each switching sequence on the voltage of capacitors. Then, the control objectives of hybrid multilevel STATCOM, i.e., ac current control and capacitors’ voltages balancing, are formulated in terms of OSS-MPC for the first time. In contrast to the conventional MPC, this method provides a fixed-switching frequency. Also, by a combination of optimal switching sequences in MPC, the regulation of capacitors’ voltages with unequal values is realized. The control system's performance is verified on a nine-level (9L) 2.4 Mvar/11 kV HC-FNPC STATCOM in MATLAB/Simulink. Also, the experimental results are provided on a 9L 1.25 kvar/220 V scaled-down hardware prototype. [ABSTRACT FROM AUTHOR]

A charging strategy that transitions from constant current (CC) mode to constant voltage (CV) mode is typical for lithium batteries. This presents design challenges for inductive power transfer (IPT) chargers to achieve high efficiency. In this article, a multiband frequency tracking control is proposed to match CC and CV modes and to mitigate misalignment. As in prior work, two load-independent frequencies fCC and fCV are used for respective CC and CV outputs. Unity power factor can be achieved when fully aligned. For misalignment cases, two narrow frequency bands around fCC are selected for control to deliver target output currents and limit reactive power. The duty ratio of the inverter is controlled to set target output voltages. A single-stage IPT system with a series LCC topology is analyzed. The proposed control has advantages of narrow operating frequency ranges, high misalignment tolerance, direct design procedures, and high efficiency in both CC and CV modes. The method is verified on a 4.4 kW prototype with a 150 mm air gap. Experimental results show that the target CC and CV charging profile can be achieved with up to 120 mm of lateral misalignment. When fully aligned, the maximum system conversion efficiency reaches 97.5%. It stays above 95.5% and 93.3% over the entire CC and CV mode ranges. [ABSTRACT FROM AUTHOR]

An isolated multilevel bidirectional dc–dc converter is presented in this article to interface the low-voltage (LV) battery and the high-voltage (HV) propulsion inverter in electric vehicles. The proposed topology achieves a HV step-up ratio in the motoring mode by reconfiguring two split capacitor half-bridge circuits in series/parallel through the symmetrical modulation of a voltage bidirectional, two quadrant switch. This reconfiguration generates a high-frequency multilevel voltage output at the dc-bus side of the inverter, which reduces the output filter volume. The dc-bus voltage is regulated by the duty cycle modulation of the intermediary switch in the motoring mode, simplifying the sensing and control. During regenerative braking, the converter is operated with phase shift modulation between the HV half bridges and the LV full bridge to regulate the battery voltage. The presented modulation strategies achieve zero voltage switching for the full-bridge devices over a wide load range. A comprehensive analysis of the proposed multilevel converter in motoring and braking modes along with the design constraints and soft switching boundary conditions is presented in this article. Detailed experimental results on a 1.5-kW laboratory prototype are presented to verify the design approach and illustrate the converter performance. [ABSTRACT FROM AUTHOR]

This article introduces a fault-tolerant reconfiguration topology and control strategy for symmetric open-winding multiphase machines. Bidirectional switching devices are proposed to connect the inverter bridge legs on different ends when the open-circuit fault occurs, and the reconfigured circuit can regenerate the missing voltage phasors due to the faults. Furthermore, to prevent overcurrent problems on the reconfigured bridge legs and maximize the torque output capability of the reconstructed system, winding currents are optimized with zero-sequence component injection during fault-tolerant operation. The experiments implemented on the five-phase system validate that the proposed fault-tolerant scheme is effective and has higher torque output capability compared with the traditional fault-tolerant solutions. [ABSTRACT FROM AUTHOR]

Beyond the impacting presence of nonlinear loads, low-voltage microgrids also experience low energy efficiency and resonance phenomena when operating interconnected to a distribution grid that suffers from distorted voltages. This article proposes a model-free control strategy capable of coordinating inverters existing within a dispatchable microgrid, allowing to operate it as a single-controllable entity that behaves like a resistor at selected harmonic frequencies. Such resistive shaping uses a centralized control architecture to steer inverters to distributively compensate reactive and harmonic currents, supporting active current sharing. Consequently, the microgrid point-of-common-coupling operates with a high power factor when the grid imposes distorted voltages. Additionally, if resonant components exist, the strategy supports harmonic resonance damping, which minimizes deterioration of voltage quality. For instance, comparative results show that, for the considered scenario, the proposed resistive shaping damps resonances up to 50% better than a previous approach that compensates harmonics using sinusoidal current synthesis. Simulation results carried out on a three-phase low-voltage microgrid testbench, considering three inverters, demonstrate the abovementioned capabilities of the proposed approach. Experimental results based on a single-phase microgrid prototype comprising two inverters with two linear loads and one nonlinear load validate the applicability of the method to real-life implementations. [ABSTRACT FROM AUTHOR]

To overcome the challenges of large reactive energy transmission loss and high current stress, which ocurr during the operation of auxiliary commutated circuit (ACC) of novel parallel resonant dc-link inverter (PRDCLI), an improved modulation strategy for reactive energy transmission loss is herein proposed based on the discontinuous pulsewidth modulation (DPWM), which adopts sawtooth carrier wave with alternating positive and negative slopes and shunt dead zone. Compared with the original modulation strategy, based on minimizing the ACC operation frequency, the proposed modulation strategy is able to realize all switches soft-switching, and avoid the superposition of resonant current and load current during the operation of ACC, thus, effectively reducing the reactive energy transmission loss and the current stress of auxiliary switches of the ACC. Under the proposed modulation strategy, the equivalent circuits in different operation modes are divided, and the operation principle, voltage/current stress, loss distribution, and optimal parameter design method of novel PRDCLI are researched. Eventually, the proposed modulation strategy is verified by experiments on a 5-kW/40-kHz prototype made by SiC mosfet. [ABSTRACT FROM AUTHOR]

This article presents a ground breaking traction drive for fuel cell-powered light rail vehicles based on a multilevel cascade converter with H-bridge cells. The converter provides dc–ac power conversion with unique buck–boost capability to compensate for the variation of fuel cell terminal voltage with the load power. In comparison to a conventional boost inverter, based on the cascade connection of a boost dc–dc converter and a voltage source inverter, the proposed converter has the significant benefits of much smaller inductor size, lower dc side current ripple, and multilevel voltage waveforms at the ac output. This article presents the converter modeling and design of the boost inductor and submodule capacitors, taking into account the full speed range of the motor. The concept has been fully validated on a bespoke experimental prototype driving an induction motor, showing the suitability of the proposed converter as a traction drive for hydrogen-powered rail vehicles. [ABSTRACT FROM AUTHOR]

This article presents a novel ring-shaped hybrid fractal antenna for detection of partial discharge (PD) in inverter-fed machines. PD is known as a source of degradation and lifetime reduction in the machines winding insulation. PD activity is becoming a major concern in high performance drives, especially due to the increased use of repetitive impulse voltage with short rise-time (high dv/dt) from pulsewidth modulation-controlled inverters. The proposed hybrid fractal antenna is designed and experimentally demonstrated to show a high sensitivity in the ultrahigh-frequency bandwidth of interest for PD detection as well as an ability to discriminate PD signals from other sources of high-frequency disturbances. Additionally, the proposed PD sensing solution features a unique ring-shape geometry that facilitates its physical integration into a live machine, making it suitable as a noninvasive solution for online condition monitoring and PD detection of any inverter-fed electrical machine. [ABSTRACT FROM AUTHOR]

The integration of single-phase microgrids (MG) and unbalanced loads to three-phase MGs results in power quality issues at the point of common coupling (PCC). These issues include unbalanced voltage, unequal power flow, and inaccurate reactive power sharing. In this article, an energy storage system (ESS) has been proposed with novel control methods to help mitigate unbalance issues in these islanded multimicrogrids (MMGs). It presents two novel control methods: modified reverse droop control and multifunction control to compensate the reactive power of the MMG and generate the needed active and reactive power references of the ESS. The developed control methods mitigate PCC voltage and power unbalance of the MMG and also achieve the load power sharing proportionally among different rated photovoltaics (PV) inverters. In addition, the system stability with the proposed control is also presented under different scenarios. The proposed multifunction controller operation is validated using Opal-RT real-time simulator and experimentally using control hardware-in-the-loop. The results demonstrate that the PCC unbalanced voltage and power are reduced significantly by 97.8%, and the proper reactive power sharing is achieved among the different PV inverters in the proposed system. [ABSTRACT FROM AUTHOR]

Direct electrical heating (DEH) is a well-established method for preventing hydrate formation inside subsea oil transfer pipelines. However, poor efficiency and high reactive power requirement of the existing line-frequency DEH systems have necessitated high-frequency power processing along with reactive power compensation. To meet these design objectives, a dc–ac converter using an LCCL resonant tank is presented in this article to function as a high-frequency alternating current source. The LCCL resonant tank is tuned at the frequency of the maximum tank current gain to maximize the heat generation and reduce the VA rating of the tank. The maximum tank current gain operation also ensures zero-voltage switching of the bridge inverter devices. Detailed frequency domain analysis and design guidelines are presented for the proposed LCCL resonant inverter (RI). Experimental results on a 10-A laboratory prototype and controller hardware-in-the-loop results for a 350-A system illustrate the advantages of the LCCL RI in DEH application. [ABSTRACT FROM AUTHOR]

This article proposes a modular approachto the power sharing control of permanent magnet synchronous bearingless machine. The selected machine topology features a winding layout with phases distributed into nonoverlapping three phase groups, a solution whose twofold aim is to increase the fault tolerance and to allow for the radial force generation. The three phase subwindings are supplied by standard three-phase inverter, leading to a modular system architecture. A throughout explanation of the methodology used to develop the control algorithm is presented considering the torque and force control in combination with the power sharing management of the machine. Special emphasis is placed on validating the modeling hypotheses based on a finite element characterization of the machine electro-mechanical behavior. The proposed control strategy is also extended to cater the possibility of one or more inverters failure, thus validating the intrinsic advantage of the redundancy obtained by the modularity of the system. An extensive experimental test campaign is finally carried out on a prototyped multi three-phase permanent magnet synchronous drive. The obtained results validate the bearingless power sharing operation in healthy and faulty scenarios, both at steady state and under extreme transient condition. [ABSTRACT FROM AUTHOR]

Common-mode voltage (CMV) is one of the major concerns in modern motor drive systems using pulsewidth modulation (PWM) controls. To alleviate the CMV issue, a series of different PWM methods have been proposed. However, due to the physical limits of the three-phase structure, none of those reduced-CMV PWMs (RCMV-PWMs) can fully eliminate the CMV. As the motor phases have been extended to even numbers, such as six phases, opportunities to eliminate the CMV by a proper modulation scheme emerge. This article proposes a new modulation method to fully eliminate or reduce the CMV for the full modulation span of a dual-three-phase motor drive system, thereby helping to shrink the CM filter. Mathematical models are built indicating its effectiveness of eliminating the CMV in the linearity range of the modulation index and reducing the CMV in the overmodulation range. Simulation results along with experiments implemented on field-programmable gate array controlled dual inverters validated the effectiveness of the proposed control algorithm. [ABSTRACT FROM AUTHOR]

To guarantee system stability, proportional capacitor-current-feedback (CCF) active damping control has been widely used in LCL-type grid-connected inverter. However, when digital control is adopted in a grid-side current-controlled inverter, proportional CCF active damping will show negative damping characteristic in frequency band higher than one-sixth of the sampling frequency (fs/6), causing poor robustness in weak grid. In this article, an unstable second-order phase lead compensator is proposed to insert into the CCF path for eliminating the negative effect resulting from the digital control delay. In doing so, the positive damping region is extended to almost Nyquist frequency (fs/2), which achieves high robustness against grid impedance variation. To ensure system stability, the discrete rule is investigated for the proposed compensator and system performance is analyzed when the LCL resonance frequency changes in the range of (0, fs/2) in discrete domain, a particularly simple and practical design is further presented for the proposed compensator and the current regulator. Finally, the effectiveness of the proposed strategy and parameters design is verified by the experimental results. [ABSTRACT FROM AUTHOR]

Grid-connected inverters inevitably operate under uncertain system parameters. These uncertain parameters such as inductance and capacitance are caused by device aging or uncertain grid conditions. And they will affect the stability margin of the inverter and bring an unstable operation risk even with a well initial design. The existing research articles mainly focus on robust stability analysis and control strategies while ignoring the influence of parameter distributions on operational risk. How to evaluate the instability risk of the inverter under multiparameter distribution is still an open question. In this article, the structured singular value (SSV) method is proposed to evaluate the robust stability of the inverter with multiparameter distributions. The distribution characteristics of uncertain parameters are briefly reviewed. The system model of a single-phase grid-connected inverter is established with the parameter distributions of the system key components. The probability of stable system operation can be quantified by the SSV method. Experiment and case study results verify the correctness and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The fast short-circuit fault ride-through capability of the inverter is an important indicator for the reliability of an independent power supply system. Fast short-circuit fault ride-through requires the inverter to output the short-circuit fault current quickly and reliably, which is conducive to shortening the duration of short-circuit faults and realizing fast short-circuit protection. In addition, when the single-phase inverter outputs a short-circuit fault current, the third harmonic in the short-circuit current may cause malfunction of the protection circuit. To effectively achieve the fast short-circuit current limiting and third harmonic suppression of the single-phase inverter, this article proposes a short-circuit fault diagnosis strategy and a current-limiting control strategy, respectively. The former is a short-circuit fault diagnosis strategy, and the latter is a fast current limiting control strategy based on adaptive reference feedforward and third harmonic elimination. The proposed strategies can enable the single-phase inverter to reach a current-limiting steady state within a fundamental period, which can greatly improve the response speed of single-phase inverters and achieve fast short-circuit protection. They can also be applied to three-phase inverters. A 35-kVA/50-Hz high-power single-phase inverter prototype is built to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Impedance-source inverters (ZSIs) are getting more attention in renewable energy generation over conventional inverters due to their inherent boosting ability and shoot-through immunity. However, most of the ZSI expansion abilities are somehow limited by the component stresses and poor performance, particularly at a low modulation index. In this article, a new coupled inductor-based switched quasi-ZSI is proposed to accomplish enhanced boost inversion at a high modulation index. It offers a robust boosting feature even with a very low turns ratio, which significantly reduces the passive circuit element's size. The working principle, steady-state analysis, and the design guidelines for selecting circuit parameters are addressed. The proposed inverter significantly improves the input current ripple with enhanced boosting with appropriate control of the impedance-network active switch. A comprehensive comparative analysis with allied topologies is presented to show the proposed inverter's effectiveness. The superior features offered are more suitable for renewable energy generation from low- and variable-voltage renewable sources, such as solar PV and fuel cells. A laboratory prototype is made for experimental verification. Analytical, MATLAB simulation and experimental results are presented to validate the proposed theory. [ABSTRACT FROM AUTHOR]

The frequency-domain passivity theory offers an effective way to assess the stability of inverters in a complex grid. In this article, a unified impedance model, suitable for either inverter current control (ICC) or grid current control (GCC) of LCL-type grid-connected inverters with observer-based capacitor current feedback active damping, is built to facilitate the passivity-based stability assessment and controller parameter design. With the passivity analysis, it is found that when the antiresonant frequency of LCL filter is in certain ranges, i.e., (0.056 $\omega _s$ , 0.20 $\omega _s$) for ICC and (0.046 $\omega _s$ , 0.23 $\omega _s$) for GCC, all frequencies’ passive output admittance of the inverter can be achieved via proposed parameter design guidelines. Due to the utilization of the observer and all frequencies’ passive output admittance property, not only extra current sensors for active damping can be saved, but also the inverter can be connected and stably operated in a grid regardless of the grid impedance. The validity of the theoretical analysis and effectiveness of the proposed method are verified by using experimental results on a laboratory prototype. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) due to high dv/dt pulsewidth modulation has been identified as one of the leading causes of insulation failure in low voltage (LV) variable frequency drive (VFD) fed motors with random wound stator windings. With the recent increase in electrical stress with faster risetime wide bandgap power semiconductor devices and higher dc link voltage for higher efficiency and power density of the system, PD-induced insulation failures are expected to increase. Since LV motor insulation is not resistant to PD, an increasing number of VFD motors are being qualified for PD-free operation. However, the PD inception voltage of qualified motors decreases with insulation aging making LV motors susceptible to failure. In this article, an offline standstill test method is proposed for automated inverter-embedded PD testing under impulse excitation. The main concept is to perform the impulse PD test using the VFD at increased voltage levels for providing advanced warning of PD activity for preventing forced outage of VFD motors. Options for application of increased impulse test voltage and automated detection of PD for implementing the proposed concept are presented. Testing on an 800 V VFD driven motor shows that the proposed method can provide automated impulse PD testing whenever the motor is at standstill to warn the end user of impending PD activity that can lead to failure. [ABSTRACT FROM AUTHOR]

The dynamic performance of a grid-connected inverter in a distributed generation system brings new challenges by affecting the power quality and dynamic stability. The traditional linear control method for the inverter requires complex processes including decoupling and control parameter tuning and relies on a pulsewidth modulation module. The nonlinear control method, e.g., the model predictive control (MPC), can address some of the aforementioned challenges but is incapable of dealing with system parameter changes. A data-driven method using the dynamic Bayesian network-based model predictive control (DBN-MPC) is proposed, which takes advantage of the predictive ability of the DBNs to implement the MPC strategy. A predictive model is built, and the prediction signals are generated based on the DBNs. Then, by constructing the cost function and optimization criteria, the controller generates the optimal switching state combination. Using the proposed DBN-MPC method, the predictive model implements parameter learning online, providing more accurate prediction signals for further optimization and the feedback correction. In addition, the control law of the proposed controller can update over time, which enables the grid-connected inverter system to achieve the optimal control. The case studies using the grid-connected inverter system and IEEE 39-bus benchmark power system integrated with the battery energy storage system demonstrate and verify the superiority of the proposed DBN-MPC method. [ABSTRACT FROM AUTHOR]

Doubly grounded transformerless grid-connected inverters are widely used in the PV application because of no common mode leakage current. A boost converter should be added into the doubly grounded PV inverters under shading condition to have boost function. However, the inverter with boost capability is a two-stage system and all switches are operated at high frequency, which reduce efficiency of the system. In addition, processing power of the boost converter is high. Therefore, a single-stage doubly grounded transformerless PV grid-connected inverter with boost function is proposed. The proposed inverter consists of two boost and one buck converters. A cooperative control method among different periods is proposed to reduce processing power of the boost converter and simplify the two-stage system to a single-stage system in each period. In addition, only one switch is operated at high frequency in each period, so efficiency of the system can be improved. Operating principle of the proposed inverter is illustrated. Control strategy is designed. Filter design guidelines and example are given. Loss calculation of the proposed inverter is given. Simulation and experimental results of the proposed inverter verify the theoretical analysis. Finally, comparison among other inverters and the proposed inverter is provided. [ABSTRACT FROM AUTHOR]

This article presents the design and demonstration of an all silicon carbide (SiC) 2×250 kW dual inverter for heavy-duty traction applications. Through the use of a formal electrothermal-control co-design approach, the maximum power density of the proposed inverter system is 60 kW/L, which is accomplished by. More specifically, to analyze the thermal performance of the inverter, a lumped thermal model is developed based on the characteristics of the power modules and the cold plate. To analyze the complicated interaction between the power losses and module junction temperatures, a closed-loop iterative estimation scheme is proposed to estimate the inverter power loss and module junction temperature. Based on the proposed thermal model and estimation scheme, a detailed optimization procedure on the selection of switching frequency and dc-link capacitor is presented to maximize power density. Furthermore, design optimization of the busbar was conducted to minimize the stray inductance, thus reducing the voltage overshoot during switching transients. Comprehensive experimental studies were performed on a physical converter prototype built based on the optimization results, which validate both the effectiveness of the presented design approach. [ABSTRACT FROM AUTHOR]